Article

Constraints on the nature of the subvolcanic reservoir at South Sister volcano, Oregon from U-series dating combined with sub-crystal trace-element analysis of plagioclase and zircon

January 2012
Earth and Planetary Science Letters 313(1)

DOI:10.1016/j.epsl.2011.10.035

Authors:

Mark E. Stelten

United States Geological Survey, Menlo Park, CA, USA

Kari M. Cooper

University of California, Davis

We present sub-crystal-scale 238U–230Th zircon ages and 238U–230Th–226Ra plagioclase ages of bulk mineral separates from the Holocene (2.0–2.3ka) eruptions of the Rock Mesa (RM) and Devil's Hills (DH) rhyolites at South Sister volcano, Oregon. We link these age data with sub-crystal trace-element analyses of zircon and plagioclase to provide insight into the subvolcanic system at South Sister, as an example of a small-volume continental arc volcano. Our results document the presence of coeval yet physically-distinct regions within the magma reservoir and constrain the timescales over which these heterogeneities existed.Zircons from the RM and DH dominantly record ages from 20 to 80ka, with some grains recording ages >350ka, whereas plagioclase records 230Th–226Ra ages of 2.3–6.8ka (RM) and 4.0–9.6ka (DH-3) and a 238U–230Th age of 10±34ka (DH-3). We interpret zircons with ages

Zircon Constraints on the Eruptive Sequence and Magma Evolution of Rhyolites at South Sister Volcano, Oregon

Article

Full-text available

Aug 2024
GEOCHEM GEOPHY GEOSY

We present ²³⁰Th‐²³⁸U crystallization ages and trace element compositions for zircons spanning the late Pleistocene to Holocene rhyolite eruptive record at South Sister volcano in the central Oregon Cascade Range. Most zircon ages are between 100 and 20 ka, with very few in secular equilibrium (>350 ka). The weighted mean of zircon ages for the two oldest South Sister rhyolites, 31.5 ± 2.1 and 39.1 ± 2.4 ka, are significantly younger than the associated ⁴⁰Ar/³⁹Ar ages, 47.4 ± 9.7 and 51.4 ± 9.7 ka. We propose that these ⁴⁰Ar/³⁹Ar dates, performed on plagioclase separates, are compromised by a subtle amount of excess Ar and therefore the younger weighted mean zircon ages yield more reliable eruption ages. These results imply that the interval of rhyolite eruption at South Sister during the late Pleistocene was both shorter and more productive than previously thought and that eruption at South Sister initiated after Middle Sister. Compositionally, zircons from the Pleistocene rhyolites are broadly similar and show down‐temperature zircon and plagioclase crystallization trends. However, we argue that destabilized amphibole and titanite in a common mush also exert leverage on the Pleistocene zircon trace element compositions. Divergence in the Eu/Eu* ratio between the Pleistocene and Holocene lavas implies chemically distinct magma reservoirs originating from the Pleistocene rhyolite eruptive sequence and the Holocene eruptive sequence. This work suggests a higher flux of rhyolite volcanism than previously thought and characterizes magmatic storage distinctions between the Pleistocene and Holocene rhyolites, aiding in the assessment of future eruptive hazards at South Sister volcano.

Divergent T–ƒO2 paths during crystallisation of H2O-rich and H2O-poor magmas as recorded by Ce and U in zircon, with implications for TitaniQ and TitaniZ geothermometry

Article

Full-text available

Nov 2018
CONTRIB MINERAL PETR

During solidification of magma chambers as systems closed to chemical exchange with environs, the residual siliceous melt may follow a trend of rising, constant, or decreasing oxidation state, relative to reference buffers such as nickel + nickel oxide (NNO) or fayalite + magnetite + quartz. Titanomagnetite–hemoilmenite thermometry and oxybarometry on quenched volcanic suites yield temperature versus oxygen fugacity arrays of varied positive and negative slopes, the validity of which has been disputed for several years. We resolve the controversy by introducing a new recorder of magmatic redox evolution employing temperature- and redox-sensitive trace-element abundances in zircon. The zircon/melt partition coefficients of cerium and uranium vary oppositely in response to variation of magma redox state, but vary in tandem as temperature varies. Plots of U/Pr versus Ce⁴⁺/Ce³⁺ in zircon provide a robust test for change in oxidation state of the melt during zircon crystallisation from cooling magma, and the plots discriminate thermally induced from redox-induced variation of Ce⁴⁺/Ce³⁺ in zircon. Temperature-dependent lattice strain causes Ce⁴⁺/Ce³⁺ in zircon to increase strongly as zircon crystallises from cooling magma at constant Ce⁴⁺/Ce³⁺ ratio in the melt. We examine 19 zircon populations from igneous complexes in varied tectonic settings. Variation of zircon Ce⁴⁺/Ce³⁺ due to minor variation in melt oxidation state during crystallisation is resolvable in 11 cases but very subordinate to temperature dependence. In many zircon populations described in published literature, there is no resolvable change in redox state of the melt during tenfold variation of Ce⁴⁺/Ce³⁺ in zircons. Varied magmatic redox trends indicated by different slopes on plots of zircon U/Pr versus Ce⁴⁺/Ce³⁺ are corroborated by Fe–Ti-oxide-based T–ƒO2 trends of correspondingly varied slopes. Zircon and Fe–Ti-oxide compositions agree that exceptionally, H2O-rich arc magmas tend to follow a trend of rising oxidation state of the melt during late stages of fluid-saturated magmatic differentiation at upper-crustal pressures. We suggest that H2 and/or SO3 and/or Fe²⁺ loss from the melt to segregating fluid is largely responsible. Conversely, zircon and Fe–Ti-oxide compositions agree in indicating that H2O-poor magmas tend to follow a T–ƒO2 trend of decreasing oxidation state of the melt during late stages of magmatic differentiation at upper-crustal pressures, because the precipitating mineral assemblage has higher Fe³⁺/Fe²⁺ than coexisting rhyolitic melt. We present new evidence showing that the Fe–Ti-oxide oxybarometer calibration by Ghiorso and Evans (Am J Sci 308(9):957–1039, 2008) retrieves experimentally imposed values of ƒO2 in laboratory syntheses of Fe–Ti-oxide pairs to a precision of ± 0.2 log unit, over a large experimental temperature range, without systematic bias up to at least log ƒO2 ≈ NNO + 4.4. Their titanomagnetite–hemoilmenite geothermometer calibration has large systematic errors in application to Ti-poor oxides that precipitate from very oxidised magmas. A key outcome is validation of Fe–Ti-oxide-based values of melt TiO2 activity for use in Ti-in-zircon thermometry and Ti-in-quartz thermobarometry.

Mechanisms and Timescales of Generating Eruptible Rhyolitic Magmas at Yellowstone Caldera from Zircon and Sanidine Geochronology and Geochemistry

Article

Full-text available

Aug 2015
J PETROL

We constrain the physical nature of the magma reservoir and the mechanisms of rhyolite generation at Yellowstone caldera via detailed characterization of zircon and sanidine crystals hosted in three rhyolites erupted during the (c. 170–70 ka) Central Plateau Member eruptive episode—the most recent post-caldera magmatism at Yellowstone. We present 238 U– 230 Th crystallization ages and trace-element compositions of the interiors and surfaces (i.e. unpolished rims) of single zircon crystals from each rhyolite. We compare these zircon data with 238 U– 230 Th crystallization ages of bulk sanidine separates coupled with chemical and isotopic data from single sanidine crystals. Zircon age and trace-element data demonstrate that the magma reservoir that sourced the Central Plateau Member rhyolites was long-lived (150–250 kyr) and genetically related to the preceding episode of magmatism, which occurred c. 256 ka. The interiors of most zircons in each rhyolite were inherited from unerupted material related to older stages of Central Plateau Member magmatism or the preceding late Upper Basin Member magmatism (i.e. are antecrysts). Conversely, most zir-con surfaces crystallized near the time of eruption from their host liquids (i.e. are autocrystic). The repeated recycling of zircon interiors from older stages of magmatism demonstrates that sequentially erupted Central Plateau Member rhyolites are genetically related. Sanidine separates from each rhyolite yield 238 U– 230 Th crystallization ages at or near the eruption age of their host magmas, coeval with the coexisting zircon surfaces, but are younger than the coexisting zircon interiors. Chemical and isotopic data from single sanidine crystals demonstrate that the sanidines in each rhyolite are in equilibrium with their host melts, which considered along with their near-eruption crystallization ages suggests that nearly all Central Plateau Member sanidines are autocrystic. The paucity of antecrystic sanidine crystals relative to antecrystic zircons requires a model in which eruptible rhyolites are generated by extracting melt and zircons from a long-lived mush of immobile crystal-rich magma. In this process the larger sanidine crystals remain trapped in the locked crystal network. The extracted melts (plus antecrystic zircon) amalgamate into a liquid-dominated (i.e. eruptible) magma body that is maintained as a physically distinct entity relative to the bulk of the long-lived crystal mush. Zircon surfaces and sanidines in each rhyolite crystallize after melt extraction and amalgamation, and their ages constrain the residence time of eruptible magmas at Yellowstone. Residence times of the large-volume rhyolites ($40–70 km 3) are 1 kyr (conservatively <6 kyr), which suggests that large volumes of rhyolite can be generated rapidly by extracting melt from a crystal mush. Because the lifespan of the crystal mush that sourced the Central Plateau Member rhyolites is two orders of magnitude longer than the residence time of eruptible magma bodies within the reservoir, it is apparent that the Yellowstone magma reservoir spends most of its time in a largely crystalline (i.e. uneruptible) state, similar to the present-day magma reservoir, and that eruptible magma bodies are ephemeral features.

Timescales of crustal magma reservoir processes: Insights from U-series crystal ages

Article

May 2015

Kari M. Cooper

The dynamic processes operating within crustal magma reservoirs control many aspects of the chemical composition of erupted magmas, and crystals in volcanic rocks provide a temporally constrained archive of these changing environments. In this review, I compile ²³⁸ U– ²³⁰ Th ages of accessory phases and ²³⁸ U– ²³⁰ Th– ²²⁶ Ra ages of bulk mineral separates of major phases. These data document that crystals in individual samples can have ages spanning most of the history of a volcanic centre. Age populations for accessory phases show protracted pre-eruptive crystal residence times but few crystals predate magmatic activity at a given centre. These data have been interpreted in the context of residence times of the host magmas or timescales of the storage of crystals within a largely crystalline portion of the reservoir system. In contrast, less than half of the bulk separate ²³⁸ U– ²³⁰ Th– ²²⁶ Ra ages for major phases are more than 10 kyr older than the eruption. Many of these apparently conflicting observations of ages of major and accessory phases can be reconciled within the context of a model where a crystal mush was remobilized during processes leading to eruption. Overall, the compiled data show that crystals contain rich archives of magmatic processes in crustal reservoirs, especially when combined with other crystal-scale geochemical data. Supplementary material Compilation of U–Th–Pb ages of accessory phases and associated references are available at www.geolsoc.org.uk/SUP18820

Piggy-back Supervolcanoes--Long-Lived, Voluminous, Juvenile Rhyolite Volcanism in Mesoproterozoic Central Australia

Article

Full-text available

May 2015
J PETROL

ABSTRACT The Talbot Sub-basin is one of several bimodal volcanic depositional centres of the Mesoproterozoic Bentley Basin in central Australia. It is dominated by rocks of rhyolitic composition and includes ignimbrites, some forming large to super-eruption size deposits. Ferroan, incompatible trace element enriched, A-type compositions, anhydrous mineralogy and clear evidence for local rheomorphism indicate high eruption temperatures, with apparent zircon-saturation temperatures suggesting crystallization at >900�C. Comagmatic basalt is of mantle origin with minor Proterozoic basement contamination. The rhyolites cover the same range of Nd isotope compositions (eNd(1070) þ1�24 to –0�96) and La/Nb ratios (1�2–2�1) as the basalts (eNd(1070) þ2�1 to –1�1: La/Nb 1�2–2�3) and are compositionally far removed from all older basement and country-rock components (average eNd(1070)¼–4, La/Nb¼10). The rhyolites and basalts are cogenetic through a process probably involving both fractional crystallization of mafic magmas and partial melting of recently crystallized mafic rock in a lower crustal intraplate, extraction of dacitic magmas to a voluminous upper crustal chamber system, and separation of rhyolite by processes involving rejuvenation and cannibalization of earlier chamber material. More than 230 000km3 of parental basalt is required to form the >22 000km3 of preserved juvenile rhyolite in the Talbot Sub-basin alone, which represents one of the most voluminous known felsic juvenile additions to intracontinental crust. Zircon U–Pb age components are complex and distinct from those of basement and country rock and contain antecrystic components reflecting dissolution–regrowth processes during periodic rejuvenation of earlier-emplaced chamber material without any significant interaction with country rock. The overall duration of magmatism was >30 Myr but can be divided into between two and four separate intervals, each probably of a few hundred thousand years’ duration and each probably reflecting one of the distinct lithostratigraphic groups defined in the sub-basin. Neither the composition nor style of felsic and mafic volcanism changes in any significant way from one volcanic event to the next and the range of zircon U–Pb ages indicates that each period utilized and cannibalized the same magma chamber. This volcanism forms a component of the 1090–1040Ma Giles Event in central Australia, associated with magmadominated extension at the nexus of the cratonic elements of Proterozoic Australia. This event cannot be reasonably reconciled with any putative plume activity but rather reflects the >200 Myr legacy of enhanced crustal geotherms that followed the final cratonic amalgamation of central Australia.

Repeated Rhyolite Eruption From Heterogeneous Hot Zones Embedded Within a Cool, Shallow Magma Reservoir

Article

Full-text available

Mar 2019

Despite the hazard posed by explosive silicic eruptions, the magma storage conditions and dynamics that precede these events remain controversial. The Laguna del Maule volcanic field, central Chile, is an exceptional example of postglacial (younger than ca. 20,000 years) rhyolite volcanism and sustained unrest driven by a large, shallow, active silicic magma system. New zircon petrochronologic data reveal that compositionally distinct domains developed concurrently within the Laguna del Maule magma reservoir, which produced two episodes of concentrated rhyolitic eruptions at 23–19 and 8–2 ka. Zircon crystallization ages record 160 kyr of magma emplacement resulting in a several hundreds of cubic kilometers reservoir that has been imaged geophysically. The average magma emplacement rate inferred from the zircon geochronology and tomographically defined magma volume is consistent with those required by thermal models to maintain a shallow silicic system. Ti‐in‐zircon temperatures of crystal cores and rims and hiatuses in crystal growth indicates most of this volume persisted in a near‐solidus state. However, consistent patterns of trace element zoning in crystal interiors and crystallization rates derived from a model of diffusion‐limited zircon growth suggest the erupted rhyolite magma batches originated from long‐lived hot zones of extractable mush embedded within the larger, cool reservoir of rigid mush. These contrasting, coeval magma storage conditions obviate a simple hot versus cold storage dichotomy for large silicic magma systems.

Eruptive history of Middle Sister, Oregon Cascades, USA—Product of a late Pleistocene eruptive episode

Article

Aug 2018
GEOSPHERE

New mapping, geochemistry, and argon geochronology illuminate a brief, remarkably silicic episode set in a mafic segment of the Cascade arc. Middle Sister was constructed during a 35-k.y. episode in the late Pleistocene from mafic, intermediate, and silicic eruptions adjacent to the primarily rhyolitic South Sister. Eruptions in the Three Sisters volcanic cluster prior to 50 ka were exclusively mafic (<57 wt% SiO2), and several basaltic andesite lava flows can be traced to Middle Sister or a predecessor volcano (prior to 150 ka). Lava flows erupted 50-37 ka at Middle Sister and on its periphery were chemically diverse, with abundant basaltic andesite, a high-silica rhyolite flow, and an andesite produced from mixing of a rhyolite and mafic magma. Abundant rhyolite and rhyodacite erupted in this interval also at South Sister. Eruptive activity paused at Middle Sister 37-27 ka but continued at South Sister with large volumes of dacite and andesite lavas. Middle Sister erupted mafic, intermediate, and silicic lava flows 27-15 ka and then ceased to erupt. Calculated eruptive rates for the entire Three Sisters volcanic cluster quadrupled from ~0.2 to ~0.8 km3/k.y. between 50 and 15 ka, largely owing to the eruptions focused at Middle and South Sisters, and the cluster has now returned to its modest eruptive output, mainly away from the stratovolcanoes. Time-volume results for the volcanic cluster are compared to studies of other well-mapped, well-dated stratovolcanoes. Nearly all centers record similar eruptive-volume behavior with long histories of relatively constant output punctuated by short episodes of voluminous eruptions. In addition to the Three Sisters, two of these centers (Mt. Mazama, Crater Lake, Oregon, and Puyehue/Cordon Caule in the southern Andes) record significant compositional changes associated with the voluminous eruptive episodes.

Localized Rejuvenation of a Crystal Mush Recorded in Zircon Temporal and Compositional Variation at the Lassen Volcanic Center, Northern California

Article

Full-text available

Dec 2014
PLOS ONE

Zircon ages and trace element compositions from recent silicic eruptions in the Lassen Volcanic Center (LVC) allow for an evaluation of the timing and conditions of rejuvenation (reheating and mobilization of crystals) within the LVC magmatic system. The LVC is the southernmost active Cascade volcano and, prior to the 1980 eruption of Mount St. Helens, was the site of the only eruption in the Cascade arc during the last century. The three most recent silicic eruptions from the LVC were very small to moderate-sized lava flows and domes of dacite (1915 and 27 ka eruptions of Lassen Peak) and rhyodacite (1.1 ka eruption of Chaos Crags). These eruptions produced mixed and mingled lavas that contain a diverse crystal cargo, including zircon. 238U-230Th model ages from interior and surface analyses of zircon reveal ages from ∼17 ka to secular equilibrium (>350 ka), with most zircon crystallizing during a period between ∼60-200 ka. These data support a model for localized rejuvenation of crystal mush beneath the LVC. This crystal mush evidently is the remnant of magmatism that ended ∼190 ka. Most zircon are thought to have been captured from "cold storage" in the crystal mush (670-725°C, Hf >10,000 ppm, Eu/Eu* 0.25-0.4) locally remobilized by intrusion of mafic magma. A smaller population of zircon (>730°C, Hf <10,000 ppm, Eu/Eu* >0.4) grew in, and are captured from, rejuvenation zones. These data suggest the dominant method to produce eruptible melt within the LVC is small-scale, local rejuvenation of the crystal mush accompanied by magma mixing and mingling. Based on zircon stability, the time required to heat, erupt and then cool to background conditions is relatively short, lasting a maximum of 10 s-1000 s years. Rejuvenation events in the LVC are ephemeral and permit eruption within an otherwise waning and cooling magmatic body.

Melt inclusions in zircon: a window to understanding the structure and evolution of the magmatic system beneath the Laguna del Maule volcanic field

Article

Full-text available

May 2024
CONTRIB MINERAL PETR

Explosive silicic eruptions pose a significant threat to society, yet the development and destabilization of the underlying silicic magmatic systems are still controversial. Zircons provide simultaneous information on the trace element composition and age of silicic magmatic systems, while melt inclusions in quartz and plagioclase yield important constraints on their volatile content as well as magma storage depth. Melt inclusions in zircons (MIZs) combine these data from a single mineral grain, recording the age, storage depth, temperature, and composition of magmas, and thus provide unique constraints on the structure and evolution of silicic magmatic systems. We studied MIZs from the Laguna del Maule (LdM) volcanic field in the southern Andes that is among the most active Pleistocene-Holocene rhyolitic volcanic centers worldwide and a potentially hazardous system displaying inflation rates in excess of 25 cm/yr. The host zircon ages suggest that the LdM MIZ record extends to ~ 30 kyr before eruption, in contrast to the melt inclusions in LdM plagioclase and quartz crystals that formed only decades to centuries before eruption. The major element compositions of MIZs are minimally affected by post-entrapment crystallization, and agree well with the LdM rhyolitic whole rock data. The MIZs record long-term differences in zircon-saturated melt composition between two eruptive units (rdm: Rhyolite of the Laguna del Maule vs. rle: Rhyolite of Los Espejos). The more evolved major element composition of rle MIZs than rdm MIZs, suggests a long-term deeper connection of the rdm crystal mush to a more primitive magma body than that of the rle. The evidence of slow H diffusion observed in MIZs suggest that their H2O contents are not significantly affected by diffusion of H through the host zircon. The magma storage pressures of 1.1 to 2.8 kbars recorded by the H2O contents of rdm and rle MIZs are consistent with the optimal emplacement window (2.0 ± 0.5 kbar) of silicic magma reservoir growth, storage, and eruptibility based on thermomechanical modeling (Huber et al. 2019).

Daly Gaps at South Sister, Oregon, USA, generated via partial melting

Article

Full-text available

Jul 2021
CONTRIB MINERAL PETR

We conducted a detailed petrologic study on six crystal-poor (< 12 vol%) dacites from South Sister volcano (OR) to determine the conditions that favor the formation and extraction (i.e., eruption) of intermediate magmas in a system that features prominent Daly Gaps. We present whole rock compositions for six dacites (63.2–65.1 wt% SiO2) that form the base of South Sister and six mafic flows (51.3–56.1 wt% SiO2) that erupted in the peripheral volcanic field. The dacites are saturated in plagioclase + orthopyroxene + clinopyroxene + ilmenite + titanomagnetite; hornblende is not observed. Evaluation of trace element concentrations in whole rock analyses reveals that end member mixing (basalt with rhyolite) is not the primary petrogenetic mechanism for generating the dacites, and comparison of compositions of minerals in the dacites with those equilibrated in experiments suggests that the majority of crystals in the dacites are plausible phenocrysts. Application of multiple thermometers to pyroxene and Fe–Ti oxide compositions in the dacites shows that crystallization in the dacites initiated at elevated temperatures (980–1045 °C) and that modest zonation in the phenocryst assemblage can be attributed to the effects of minor closed system crystallization ± degassing ± cooling. When pre-eruptive temperatures, plagioclase and whole rock compositions from the dacites are incorporated into the plagioclase hygrometer, we obtain maximum pre-eruptive H2O contents that range from 2.1 to 3.3 wt%; these H2O contents are consistent with those associated with plagioclase-in curves determined from phase equilibrium experiments. Through review of glasses equilibrated in experimental run products in the literature, we find that dacitic melt compositions with temperatures similar to the South Sister dacites (i.e., 980–1045 °C) are restricted to partial melting experiments on MgO-rich (> 5 wt% MgO) basalts and that dacitic melt compositions most common in partial melting experiments conducted under low H2O fugacities. Partial melting of basalt resolves the compositional gaps between the basalts and dacites in the South Sister volcanic record because, during partial melting, dacites (1) can be generated without requiring the formation of a compositional intermediate, (2) are produced in eruptible melt fractions (e.g., 20–25%), (3) are favorably erupted if they occur in fluid undersaturated conditions (owing to the positive Clapeyron slope in the absence of a fluid phase). Though the efficiency of partial melting is debated because of the amount of heat required to generate melt, it is a viable process beneath South Sister because of the elevated geothermal gradient, high pre-eruptive temperatures of primitive basalts erupting in the surrounding areas and the recent history of volcanism in the region. Our results suggest that eruption of dacites saturated in two pyroxenes, absent hornblende, may signal partial melting.

Magmatic Processes at Snæfell Volcano, Iceland, Constrained by Zircon Ages, Isotopes, and Trace Elements

Article

Full-text available

Mar 2021
GEOCHEM GEOPHY GEOSY

We present the first zircon‐based U‐Pb geochronology, trace element concentrations, and O and Hf isotope compositions for Snæfell, an off‐rift volcano in eastern Iceland. These data provide constraints on the longevity and petrogenetic conditions of the Snæfell magmatic system. U‐Pb zircon ages range from 545 ± 59 to 266 ± 16 ka (2SE), but differences between grain core and mantle ages within each sample reveal zircon residence times of 100–200 kyr—far longer than observed at other Neovolcanic systems in Iceland. Zircon δ¹⁸O is restricted to ∼3.5–4‰, and zircon εHf ranges ∼+13 to ∼+17, which is substantially more radiogenic than Snæfell basalts. This combined O and Hf isotopic perspective suggests rhyolite petrogenesis at Snæfell can be attributed to fractional crystallization of mantle‐derived basaltic magmas with limited influence of pre‐existing crustal material. Trace element evidence further characterizes the magmatic source material: Sc/Yb <∼0.01 suggests an amphibole‐free petrogenetic environment, and Ti concentrations in zircon <5 ppm suggest a cool, near‐solidus, crystallization environment for the majority of the zircon's pre‐eruptive history, with elevated Ti in zircon surfaces suggesting a late thermal perturbation, perhaps a mafic input that remobilized (to the point of eruption triggering) near‐solidus magmas. These zircon‐based conclusions are broadly consistent with previous interpretations of rhyolite petrogenesis conditions at Snæfell but provide a multi‐faceted perspective with more detailed resolution of source materials, magma generating processes, system longevity, and pre‐eruptive conditions.

Architecture and dynamics of magma reservoirs

Article

Full-text available

Jan 2019

This introductory article provides a synopsis of our current understanding of the form and dynamics of magma reservoirs in the crust. This knowledge is based on a range of experimental, observational and theoretical approaches, some of which are multidisclipinary and pioneering. We introduce and provide a contextual background for the papers in this issue, which cover a wide range of topics, encompassing magma storage, transport, behaviour and rheology, as well as the timescales on which magma reservoirs operate. We summarize the key findings that emerged from the meeting and the challenges that remain. The study of magma reservoirs has wide implications not only for understanding geothermal and magmatic systems, but also for natural oil and gas reservoirs and for ore deposit formation. This article is part of the Theo Murphy meeting issue ‘Magma reservoir architecture and dynamics’.

Evidence of Young Crystal Ages in Andesitic Magmas from a Hyperactive Arc Volcano— Volcan de Colima, Mexico

Article

Full-text available

May 2017
J PETROL

The residence time of crystals in magmatic systems is an essential parameter to describe the dynamics of these systems and to evaluate the temporal representativeness of the mineral data used to document the physical conditions in the magmas. Uranium-series disequilibria in mineral separates from young volcanic rocks with a known eruption date provide unique insights into these residence times. We present ²³⁸U-²³⁰Th-²²⁶Ra measurements for plagioclase separates and groundmass from magmas erupted at Volcán de Colima, Mexico in 2004 and 2007. The (²³⁰Th/²³²Th) activity ratios in plagioclase show limited variations within the range measured in groundmasses and previously documented for whole-rocks. The (²³⁰Th/²³²Th) and (²³⁸U/²³²Th) activity ratios in plagioclase are predominantly controlled by glass present as inclusions in the crystals or adhering to their rims, even in high-purity crystal separates with less than 2% glass. Variations in these activity ratios are best explained by crustal assimilation during crystallization and do not require ageing of the crystals. One plagioclase separate with very limited contamination by glass impurities has a demonstrable zero ²³⁰Th-²³⁸U crystal age. Precise ²²⁶Ra-²³⁰Th model ages are difficult to obtain owing to the strong influences of uncertainties in the partition coefficients, the temperature of crystallization and the correction for glass impurities in crystal separates. Nevertheless, if these uncertainties are taken into account, ²²⁶Ra-²³⁰Th model ages in the range 0-2000 years are obtained for the plagioclase separates. More complex models elaborated on the basis of the compositional zoning observed in plagioclase phenocrysts suggest that the cores of the crystals are at most 8500 years old. Overall, these results indicate that the plagioclase phenocrysts are relatively young and are not sourced from a long-lived near-solidus crystal mush as is often suggested for arc volcanoes. A more dynamic system in which crystallization predominantly occurs in response to degassing during ascent of the melts is conceivable at Volcán de Colima.

Construction and preservation of batholiths in the northern U.S. Cordillera

Article

Aug 2016

There is a nearly continuous record of magmatism through the Late Cretaceous-early Paleogene in Idaho and adjacent areas of Oregon and Montana, including the various phases of the Idaho batholith. We suggest that much of this magmatic record, however, has been obscured by subsequent tectonic deformation, erosion, and magmatic disruption and cannibalization, the latter of which can be tracked by zircon inheritance. Specifically, a mid-Cretaceous magmatic arc was significantly deformed by the western Idaho shear zone and intruded by the 83-67 Ma Atlanta peraluminous suite of the Idaho batholith. The northern part of the Atlanta peraluminous suite was, in turn, intruded by the 65-55 Ma Bitterroot lobe of the Idaho batholith. Consequently, the present age distribution of magmatism is strongly biased toward the youngest phases of plutonism; much of the older phases were destroyed by tectonic, magmatic, and erosional processes. The destruction of granitic batholiths may characterize Cordilleran-style orogens worldwide, which can lead to significant underestimates of magmatic fluxes.

U-Series Dating

Chapter

Nov 2014

Bernard Bourdon

Radioactive disequilibrium in the uranium decay series (either²³⁸U or²³⁵U) is caused by fractionation processes that can occur between isotopes within the chain based on differences in their nuclear or chemical properties. Because the intermediate daughter products are themselves radioactive, the U-series nuclides will tend to return to a state of secular equilibrium at rates that depend on the decay constants of each isotope. As a result, a number of geochronometers have been developed that can be used to date a wide variety of materials at timescales ranging from decades to over a million years. These methods have been devised to date near-surface processes including deposition of authigenic and biogenic precipitates, development of soils, rates of rock weathering and rates of mineral comminution, deposition of young sediments, and sediment transport times, as well as processes occurring at depth within the crust and mantle including dating of young volcanic rocks or dating of crystal residence times in magma chambers. Emerging studies have taken advantage of advances in analytical capabilities that allowed analysis of low-level samples with greater precision or higher spatial resolution. The field of U-series dating has now become a mature technique for dating Quaternary geological or archeological materials of increasing diversity.

Timescales of storage and recycling of crystal mush at Krafla Volcano, Iceland

Article

Full-text available

May 2016
CONTRIB MINERAL PETR

Processes in upper-crustal magma reservoirs such as recharge, magma mixing, recycling of previously crystallized material, and eruption affect both the physical state and the chemical composition of magmas. A growing body of evidence shows that crystals in intermediate or silicic volcanic rocks preserve records of these processes that may be obscured due to mixing in the liquid fraction of magmas. Fewer studies have focused on crystals in basaltic lavas, but these show evidence for a more subtle, but still rich record of magmatic processes. We present new 238U–230Th–226Ra data for plagioclase, combined with δ18O and trace-element measurements of the same crystal populations, from basalts erupted at Krafla Volcanic Center, Iceland. These data document the presence of multiple crystal populations within each sample, with chemical and oxygen isotope heterogeneity at a variety of scales: within individual crystals, between crystals in a given population, between crystal populations within the same sample, and between crystals in lavas erupted from different vents during the same eruption. Comparison to whole-rock or groundmass data shows that the majority of macroscopic crystals are not in trace-element or oxygen isotope equilibrium with their host liquids. The most likely explanation for these data is that the macroscopic crystals originated within a highly heterogeneous crystal mush in the shallow magma reservoir system. U-series and diffusion data indicate that the crystals (and therefore the mush) formed recently (likely within a few thousand years of eruption, and with a maximum age of 8–9 ka), and that the crystals resided in their host magma prior to eruption for decades to a few centuries at most. These data, in conjunction with other recent studies, suggest a model where erupted Icelandic magmas are the result of diverse magmas entering the crust, followed by complex interactions between melts and previously crystallized material at all crustal levels.

No effect of H2O degassing on the oxidation state of magmatic liquids

Article

Aug 2016
EARTH PLANET SC LETT

The underlying cause for why subduction-zone magmas are systematically more oxidized than those formed at mid-ocean spreading ridges is a topic of vigorous debate. It is either a primary feature inherited from the subduction of oxidized oceanic crust into the mantle or a secondary feature that develops because of H2O degassing and/or magma differentiation. Low total iron contents and high melt H2O contents render rhyolites sensitive to any effect of H2O degassing on ferric–ferrous ratios. Here, pre-eruptive magmatic Fe2+ concentrations, measured using Fe–Ti oxides that co-crystallized with silicate phenocrysts under hydrous conditions, are compared with Fe2+ post-eruptive concentrations in ten crystal-poor, fully-degassed obsidian samples; five are microlite free. No effect of H2O degassing on the ferric–ferrous ratio is found. In addition, Fe–Ti oxide data from this study and the literature show that arc magmas are systematically more oxidized than both basalts and hydrous silicic melts from Iceland and Yellowstone prior to extensive degassing. Nor is there any evidence that differentiation (i.e., crystal fractionation, crustal assimilation) is the cause of the higher redox state of arc magmas relative to those of Iceland/Yellowstone rhyolites. Instead, the evidence points to subduction of oxidized crust and the release of an H2O-rich fluid and/or melt with a high oxygen fugacity ( ), which plays a role during H2O-flux melting of the mantle in creating basalts that are relatively oxidized.

Zircon Xenocryst Resorption and Magmatic Regrowth at El Chichón Volcano, Chiapas, Mexico

Article

Jan 2016
J VOLCANOL GEOTH RES

Changes in magma storage conditions following caldera collapse at Okataina Volcanic Center, New Zealand

Article

Full-text available

Dec 2015
CONTRIB MINERAL PETR

Large silicic volcanic centers produce both small rhyolitic eruptions and catastrophic caldera-forming eruptions. Although changes in trace element and isotopic compositions within eruptions following caldera collapse have been observed at rhyolitic volcanic centers such as Yellowstone and Long Valley, much still remains unknown about the ways in which magma reservoirs are affected by caldera collapse. We present 238U–230Th age, trace element, and Hf isotopic data from individual zircon crystals from four eruptions from the Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand, in order to assess changes in trace element and isotopic composition of the reservoir following the 45-ka caldera-forming Rotoiti eruption. Our data indicate that (1) mixing of magmas derived from crustal melts and mantle melts takes place within the shallow reservoir; (2) while the basic processes of melt generation likely did not change significantly between pre- and post-caldera rhyolites, post-caldera zircons show increased trace element and isotopic heterogeneity that suggests a decrease in the degree of interconnectedness of the liquid within the reservoir following collapse; and (3) post-caldera eruptions from different vents indicate different storage times of the amalgamated melt prior to eruption. These data further suggest that the timescales needed to generate large volumes of eruptible melt may depend on the timescales needed to increase interconnectedness and achieve widespread homogenization throughout the reservoir.

New Thorium Isotope Ratio measurements in Silicate Reference Materials: A-THO, AGV-2, BCR-2, BE-N, BHVO-2 and BIR-1

Article

Sep 2015
GEOSTAND GEOANAL RES

A two-step Th isolation protocol, involving microcolumns of TRU-Spec extraction chromatography material and AG1 resin, is evaluated. The MC-ICP-MS procedure includes 232Th tailing characterisation and correction, and calibrator bracketing using an in-house standard solution (ThS1) to correct for instrumental mass bias and Faraday cup to secondary electron multiplier relative gain. Repeated analyses of reference solutions (UCSC Th “A”, WUN, OU Th”U”, IRMM-36) are consistent with published data. Six reference materials (A-THO, BCR-2, AGV-2, BHVO-2, BE-N and BIR-1) were processed. The average 230Th/232Th values obtained for these samples are in excellent agreement with published data. In addition, we report the first 230Th/232Th values for BE-N and BIR-1. The intermediate precisions for rock samples ranged from ± 0.24 to ± 0.49% (2 RSD), and were similar to those achieved for synthetic solutions, thereby supporting the overall validity of the chemical separation, data acquisition and reduction procedures. Counting statistics on the 230Th isotope is the most significant source of uncertainty. The intermediate precision of the mean 230Th/232Th for the Th-depleted BIR-1 (5.64 × 10-6 ± 0.27%, 2 RSD) is in the range of the analyses of other reference materials analysed in this study.This article is protected by copyright. All rights reserved.

Constraints on the timing of Quaternary volcanism and duration of magma residence at Ciomadul volcano, east-central Europe, from combined U-Th/He and U-Th zircon geochronology

Article

Full-text available

May 2015
J VOLCANOL GEOTH RES

High-spatial resolution zircon geochronology was applied to constrain the timescales of volcanic eruptions of the youngest, mostly explosive volcanic phase of Ciomadul volcano (Carpathian-Pannonian region, Romania). Combined U-Th and (U-Th)/He zircon dating demonstrates that intermittent volcanic eruptions occurred in a time range of 56-32 ka. The reliability of the eruption dates is supported by concordant ages obtained from different dating techniques, such as zircon geochronology, radiocarbon analysis, and infrared stimulated luminescence dating for the same deposits. The new geochronological data suggest that volcanism at Ciomadul is much younger (< ca. 200 ka) than previously thought (up to 600 ka). A dominantly explosive volcanic phase occurred after an apparent lull in volcanism that lasted for several 10’s of ka, after a period of lava dome extrusion that defines the onset of the known volcanism at Ciomadul. At least four major eruptive episodes can be distinguished within the 56-32 ka period. Among them, relatively large (sub-plinian to plinian) explosive eruptions produced distal tephra covering extended areas mostly southeast from the volcano. The 38.9 ka tephra overlaps the age of the Campanian Ignimbrite eruption and has an overlapping dispersion axis towards the Black Sea region. The wide range of U-Th model ages of the studied zircons indicates prolonged existence of a low-temperature (< 800 °C) silicic crystal mush beneath Ciomadul. Crystallization of zircons within a crustal magma chamber started > 100 ka before the onset of volcanic activity. The main zircon crystallization period was between ca. 100 and 200 ka, coeval with the older, mostly extrusive lava dome building stage of volcanism. Even the youngest U-Th model ages obtained for the outermost 4 μm rim of individual zircon crystals predate the eruption by several 10’s of ka. The zircon age distributions suggest re-heating above zircon saturation temperatures via injection of hot mafic magmas prior to eruption, but intermittent intrusions of fresh magma could also play a significant role in keeping the intrusive silicic magmatic reservoir in a partially melted state. The previous history of Ciomadul suggests that melt-bearing crystal mush resided beneath the volcano, and was rapidly remobilized after a protracted (several 10’s of ka) lull in volcanism to trigger several eruptions in a comparatively short time window. This classifies Ciomadul as a volcano with ‘Potentially Active Magma Storage’ (PAMS) which we propose to be common among the seemingly inactive volcanoes in volcanic arc regions. The potential for reactivation of these systems should be included into volcanic hazard assessments.

Episodic Holocene eruption of the Salton Buttes rhyolites, California, from paleomagnetic, U-Th, and Ar/Ar dating

Article

Mar 2015
GEOCHEM GEOPHY GEOSY

In the Salton Trough, CA, five rhyolite domes form the Salton Buttes: Mullet Island, Obsidian Butte, Rock Hill, and North and South Red Hills, from oldest to youngest. Results presented here include 40Ar/39Ar anorthoclase ages, 238U-230Th zircon crystallization ages, and comparison of remanent paleomagnetic directions with the secular variation curve, which indicate that all domes are Holocene. 238U-230Th zircon crystallization ages are more precise but within uncertainty of 40Ar/39Ar anorthoclase ages, suggesting that zircon crystallization proceeded until shortly before eruption in all cases except one. Remanent paleomagnetic directions require three eruption periods: (1) Mullet Island, (2) Obsidian Butte, (3) Rock Hill, North Red Hill, and South Red Hill. Borehole cuttings logs document up to two shallow tephra layers. North and South Red Hill likely erupted within 100 years of each other, with a combined 238U-230Th zircon isochron age of: 2.83 ± 0.60 ka (2 sigma); paleomagnetic evidence suggests this age predates eruption by hundreds of years (1800 cal BP). Rock Hill erupted closely in time to these eruptions. The Obsidian Butte 238U-230Th isochron age (2.86 ± 0.96 ka) is nearly identical to the combined Red Hill age, but its Virtual Geomagnetic Pole position suggests a slightly older age. The age of aphyric Mullet Island dome is the least well constrained: zircon crystals are resorbed and the paleomagnetic direction is most distinct; possible Mullet Island ages include ca. 2300, 5900, 6900, and 7700 cal BP. Our results constrain the duration of Salton Buttes volcanism to between ca. 5900 and 500 years. This article is protected by copyright. All rights reserved.

Iceland is not a magmatic analog for the Hadean: Evidence from the zircon record

Article

Nov 2014
EARTH PLANET SC LETT

Tangible evidence of Earth's earliest (Hadean; >4.0 Ga) crust, and the processes and materials that contributed to its formation, exists almost entirely in a record of detrital zircon from Jack Hills, Western Australia, and a few other locations. Iceland, with its thick, juvenile, basaltic crust and relatively abundant silicic rocks, is considered a potential modern analog for the Hadean magmatic environment where >4 Ga zircon formed. We present the first extensive dataset for Icelandic zircon, with trace element and oxygen isotope compositions from samples that span the island's history and full range of tectonic settings. This statistically robust zircon-based comparison between Iceland and the early Earth reveals distinctions in chemistry that suggest fundamental differences in magmatic environments. Whereas the δ 18O signature of Hadean zircons generally exceed that of zircons equilibrated with mantle-derived magma (85%≥5.3‰85%≥5.3‰; median 6‰), almost all Icelandic zircons are characterized by a “light” oxygen signature (98%≤5.3‰98%≤5.3‰; median 3‰). Deviations from “juvenile” oxygen values indicate that many Hadean zircons and almost all Icelandic zircons grew from magmas with substantial contributions from materials that had interacted with surface waters. In the Hadean case, the interaction occurred at low temperatures, while in Iceland, it was a high-temperature interaction. Icelandic and Hadean zircons are also distinct in their Ti concentrations (Icelandic median concentration 12 ppm, Hadean median 5 ppm). Titanium in zircon correlates positively with temperature of crystallization, and this difference in median Ti concentration suggests a temperature difference of at least 50 °C. Other differences in trace elements compositions are consistent with the interpretation that Icelandic and Hadean zircons grew in magmas with very different origins and histories (e.g., the heavy rare earth element Yb is almost an order of magnitude higher in Icelandic zircon). A comparison with elemental data for Phanerozoic zircon from different environments demonstrates that the Hadean population is unusually depleted in Ti, but otherwise similar to zircons from continental arc settings. Zircons from Iceland, and from modern evolving rift environments where oceanic lithosphere and upwelling asthenosphere are replacing continental lithosphere, are compositionally intermediate between mid-ocean ridge and continental arc zircon populations. The elemental distinctions are consistent with fractionation of zircon-bearing magmas under hotter and drier conditions in Icelandic, mid-ocean ridge, and evolving rift environments and cooler and wetter conditions in arc and, especially, Hadean environments.

Rapid remobilisation of magmatic crystals kept in cold storage

Article

Feb 2014

The processes involved in the formation and storage of magma within the Earth's upper crust are of fundamental importance to volcanology. Many volcanic eruptions, including some of the largest, result from the eruption of components stored for tens to hundreds of thousands of years before eruption. Although the physical conditions of magma storage and remobilization are of paramount importance for understanding volcanic processes, they remain relatively poorly known. Eruptions of crystal-rich magma are often suggested to require the mobilization of magma stored at near-solidus conditions; however, accumulation of significant eruptible magma volumes has also been argued to require extended storage of magma at higher temperatures. What has been lacking in this debate is clear observational evidence linking the thermal (and therefore physical) conditions within a magma reservoir to timescales of storage--that is, thermal histories. Here we present a method of constraining such thermal histories by combining timescales derived from uranium-series disequilibria, crystal sizes and trace-element zoning in crystals. At Mount Hood (Oregon, USA), only a small fraction of the total magma storage duration (at most 12 per cent and probably much less than 1 per cent) has been spent at temperatures above the critical crystallinity (40-50 per cent) at which magma is easily mobilized. Partial data sets for other volcanoes also suggest that similar conditions of magma storage are widespread and therefore that rapid mobilization of magmas stored at near-solidus temperatures is common. Magma storage at low temperatures indicates that, although thermobarometry calculations based on mineral compositions may record the conditions of crystallization, they are unlikely to reflect the conditions of most of the time that the magma is stored. Our results also suggest that largely liquid magma bodies that can be imaged geophysically will be ephemeral features and therefore their detection could indicate imminent eruption.

Frequency and magnitude of volcanic eruptions controlled by magma injection and buoyancy

Article

Full-text available

Jan 2014
NAT GEOSCI

Super-eruptions are extremely rare events. Indeed, the global frequency of explosive volcanic eruptions is inversely proportional to the volume of magma released in a single event1,2. The rate of magma supply, mechanical properties of the crust and magma, and tectonic regime are known to play a role in controlling eruption frequency and magnitude3–7, but their relative contributions have not been quantified. Here we use a thermomechanical numerical model of magma injection into Earth’s crust and Monte Carlo simulations to explore the factors controlling the recurrence rates of eruptions of different magnitudes. We find that the rate of magma supply to the upper crust controls the volume of a single eruption. The time interval between magma injections into the subvolcanic reservoir, at a constant magma-supply rate, determines the duration of the magmatic activity that precedes eruptions. Our simulations reproduce the observed relationship between eruption volume and magma chamber residence times and replicate the observed correlation between erupted volumes and caldera dimensions8,9. We also find that magma buoyancy is key to triggering super-eruptions, whereas pressurization associated with magma injection is responsible for relatively small and frequent eruptions. Our findings help improve our ability to decipher the long-term activity patterns of volcanic systems.

Zircon trace element geochemical constraints on the evolution of the Ediacaran (600–614 Ma) post-collisional Dokhan Volcanics and Younger Granites of SE Sinai, NE Arabian–Nubian Shield

Article

Dec 2013
CHEM GEOL

Constraining volcanic inflation at Three Sisters Volcanic Field in Oregon, USA, through microgravity and deformation modeling

Article

Oct 2012
GEOCHEM GEOPHY GEOSY

Microgravity data were collected between 2002 and 2009 at the Three Sisters Volcanic Complex, Oregon, to investigate the causes of an ongoing deformation event west of South Sister volcano. Three different conceptual models have been proposed as the causal mechanism for the deformation event: (1) hydraulic uplift due to continual injection of magma at depth, (2) pressurization of hydrothermal systems and (3) viscoelastic response to an initial pressurization at depth. The gravitational effect of continual magma injection was modeled to be 20 to 33 μGal at the center of the deformation field with volumes based on previous deformation studies. The gravity time series, however, did not detect a mass increase suggesting that a viscoelactic response of the crust is the most likely cause for the deformation from 2002 to 2009. The crust, deeper than 3 km, in the Three Sisters region was modeled as a Maxwell viscoelastic material and the results suggest a dynamic viscosity between 1018 to 5 × 1019 Pa s. This low crustal viscosity suggests that magma emplacement or stall depth is controlled by density and not the brittle ductile transition zone. Furthermore, these crustal properties and the observed geochemical composition gaps at Three Sisters can be best explained by different melt sources and limited magma mixing rather than fractional crystallization. More generally, low intrusion rates, low crustal viscosity, and multiple melt sources could also explain the whole rock compositional gaps observed at other arc volcanoes.

Magma mixing and the generation of isotopically juvenile silicic magma at Yellowstone caldera inferred from coupling 238U–230Th ages with trace elements and Hf and O isotopes in zircon and Pb isotopes in sanidine

Article

Full-text available

Aug 2013

The nature of compositional heterogeneity within large silicic magma bodies has important implications for how silicic reservoirs are assembled and evolve through time. We examine compositional heterogeneity in the youngest (~170 to 70 ka) post-caldera volcanism at Yellowstone caldera, the Central Plateau Member (CPM) rhyolites, as a case study. We compare 238U–230Th age, trace-element, and Hf isotopic data from zircons, and major-element, Ba, and Pb isotopic data from sanidines hosted in two CPM rhyolites (Hayden Valley and Solfatara Plateau flows) and one extracaldera rhyolite (Gibbon River flow), all of which erupted near the caldera margin ca. 100 ka. The Hayden Valley flow hosts two zircon populations and one sanidine population that are consistent with residence in the CPM reservoir. The Gibbon River flow hosts one zircon population that is compositionally distinct from Hayden Valley flow zircons. The Solfatara Plateau flow contains multiple sanidine populations and all three zircon populations found in the Hayden Valley and Gibbon River flows, demonstrating that the Solfatara Plateau flow formed by mixing extracaldera magma with the margin of the CPM reservoir. This process highlights the dynamic nature of magmatic interactions at the margins of large silicic reservoirs. More generally, Hf isotopic data from the CPM zircons provide the first direct evidence for isotopically juvenile magmas contributing mass to the youngest post-caldera magmatic system and demonstrate that the sources contributing magma to the CPM reservoir were heterogeneous in 176Hf/177Hf at ca. 100 ka. Thus, the limited compositional variability of CPM glasses reflects homogenization occurring within the CPM reservoir, not a homogeneous source.

Constraints on crystal storage timescales in mixed magmas: Uranium-series disequilibria in plagioclase from Holocene magmas at Mount Hood, Oregon

Article

Feb 2012
EARTH PLANET SC LETT

Uranium-series crystal ages, interpreted within a textural and geochemical framework, can provide insight into crystal storage timescales, especially in cases where crystals may derive from multiple sources. We report here 230Th–226Ra model ages of two distinct populations of plagioclase from low silica dacites from Mount Hood, Oregon, a volcano where previous studies show that the compositions of erupted magmas are controlled by magma recharge, mixing, and incorporation of plagioclase derived from mafic and silicic end-member magmas. We have measured trace element concentrations and 238U–230Th–226Ra disequilibria in four plagioclase size fractions from the Timberline (1500 a) and Old Maid (215 a) eruptive sequences. After correction for groundmass and apatite contamination, average 230Th–226Ra model ages of large (> 500 μm) plagioclase are > 4.5 ka (Timberline) and > 5.5 ka (Old Maid), with ages of cores that are > 10 ka in each case, indicating that plagioclase derived from silicic magmas crystallized thousands of years before eruptions. These model ages are longer than timescales of repose between eruptions, indicating that these crystals resided in the sub-surface over multiple eruptions, likely stored in a silicic crystal mush zone that periodically interacts with mafic recharge magmas, remobilizing a fraction of the large plagioclase crystals during each eruptive event. After correction for large plagioclase contamination, small (< 500 μm) plagioclase, derived from mafic magmas, have high (226Ra)/Ba relative to equilibrium with liquid proxies (groundmass and mafic inclusion), leading to 230Th–226Ra model ages that are <~3 ka for Old Maid and undefined for Timberline separates. However, the preservation of significant 230Th–226Ra disequilibria require that the majority of crystals in the separate are young (<<10 ka). The high (226Ra)/[Ba] could potentially be explained by rapid crystallization immediately prior to and/or during mixing events, consistent with evidence of rapid crystallization of rims. Rapid crystallization of mafic intrusions may trigger eruption at Mount Hood by producing a partially-crystalline mafic magma capable of mixing with a reheated silicic crystal mush.

Insights Into Magma Reservoir Dynamics From a Global Comparison of Volcanic and Plutonic Zircon Trace Element Variability in Individual Hand Samples

Article

Full-text available

Nov 2024
GEOCHEM GEOPHY GEOSY

Trace element compositional trends in zircons separated from single hand samples have been used to infer dynamic processes in magma reservoirs. Here, we compile published zircon trace element chemistry to quantify any systematic difference between the range of compositions observed in zircon from individual volcanic and plutonic hand samples and compare these results with geochemical modeling to derive implications for magma reservoir dynamics. We find that both rock types span a wide range of hand‐sample scale variability (i.e., wide range of coefficients of variation), but there is no systematic difference in the average variability between plutonic and volcanic samples (i.e., no difference in the mean coefficient of variation). This indicates that dynamic processes related to eruption are not necessarily required as a fundamental process to create hand sample‐scale compositional heterogeneity beyond what is present due to dynamic processes in the reservoir recorded in plutonic samples. Modeling of felsic systems (>68.5 wt.% SiO2) indicates that the similar average variability in felsic volcanic and plutonic hand samples cannot be reproduced by closed‐system crystallization of compositionally distinct melts locally within a magma reservoir (i.e., isolated melt pockets in a crystal mush) but requires mixing of at least two felsic melt compositions at a small spatial scale. This study provides a framework for focused studies on individual volcanic‐plutonic systems exploring how plutonic and volcanic zircon compositional variability records the time and length scales of magma reservoir processes.

Machine-learning oxybarometer developed using zircon trace-element chemistry and its applications

Article

Jul 2024

Magmatic oxygen fugacity (fO2) is a fundamental property to understanding the long-term evolution of the Earth’s atmosphere and the formation of magmatic-hydrothermal mineral deposits. Classically, the magmatic fO2 is estimated using mineral chemistry, such as Fe-Ti oxides, zircon, and hornblende. These methods, however, are only valid within certain limits and/or require a significant amount of a priori knowledge. In this contribution, a new oxybarometer, constructed by data-driven machine learning algorithms using trace elements in zircon and their corresponding independent fO2 constraints, is provided. Seven different algorithms are initially trained and then validated on a data set that was never utilized in the training processes. Results suggest that the oxybarometer constructed by the extremely randomized trees model has the best performance, with the largest R2 value (0.91 ± 0.01), smallest RMSE (0.45 ± 0.03), and low propagated analytical error (~0.10 log units). Feature importance analysis demonstrates that U, Ti, Th, Ce, and Eu in zircon are the key trace elements that preserve fO2 information. This newly developed oxybarometer has been applied in diverse systems, including arc magmas and mid-ocean ridge basalts, fertile and barren porphyry systems, and global S-type detrital zircon, which provide fO2 constraints that are consistent with other independent methods, suggesting that it has wide applicability. To improve accessibility, the oxybarometer was developed into a software application aimed at enabling more consistent and reliable fO2 determinations in magmatic systems, promoting further research.

Contrasting magmatic evolutions of the Three Sister Volcanoes reflect increased heat flow, crustal melting and silicic magmatism in the Central Oregon Cascade Arc

Article

Dec 2022
CHEM GEOL

Zircon trace‐element compositions in Miocene granitoids in Japan: Discrimination diagrams for zircons in M‐, I‐, S‐, and A‐type granites

Article

Oct 2022

Detrital zircons demonstrate high resistance to alteration and, as such retain information about their formation ages and parental magmas for a long period of time. Geochemical researchers have proposed a wide variety of discrimination diagrams applicable to detrital zircons. In our research, we focused on the conventional classification scheme for granites (Mantle, M; Igneous, I; Sedimentary, S; and Alkaline, A types) and sought to characterize zircon trace‐element compositions that are sensitive to differences among these granite types. To accomplish this, we examined trace‐element compositions of zircons extracted from granitoids in the Ohmine granitic rocks and the Ashizuri plutonic complex in southwestern Japan. The zircons showed systematic differences in Nb/P, Ta/P, Ce/P, Ce/Nd, Y/P, Th/U, and Sc/Yb ratios and the Eu anomaly. Zircons in A‐type granite are rich in Nb, Ta, Ce, and Y, and their signatures clearly reflect those elements in their parental bodies. Sc/Yb ratios of zircons in A‐type granites are <0.1, which is similar to those of ocean‐island‐type zircons. Despite their high abundance at the whole‐rock level, zircons in S‐type granite are characterized by low Nb/P, Ta/P, and Th/U ratios. This is attributable to the depletion of Nb, Ta, and Th in the magma by ilmenite and monazite prior to zircon crystallization. In general, S‐type granitic magmas exhibit reducing environments, which decrease the proportions of Ce4+ and Eu3+. These effects lead to a low Ce/Nd ratio and a large negative Eu anomaly in S‐type zircons. On the basis of these findings, we recommend the combined use of Nb/P–Ce/P or Ta/P–Ce/P crossplots and of Sc/Yb ratios to discriminate zircons in M‐, I‐, S‐, and A‐type granites. Although the crossplots are created using data from Miocene granitoids in Japan, the discrimination diagrams are based on the general features of each type of granite.

Thermal Storage Conditions and Origin of Compositional Diversity of Plagioclase Crystals in Magmas from the 1980 and 2004–2005 Eruptions of Mount Saint Helens

Article

Jul 2022

The thermal histories of upper crustal magma reservoirs place important constraints on the formation, evolution, and remobilization of crustal magmas, yet quantifying thermal histories remains challenging. We report new in-situ plagioclase trace-element data, Sr in plagioclase diffusion timescales, and plagioclase 238U-230Th-226Ra disequilibria data from Mount Saint Helens (MSH) 1980 cryptodome and 2004-2005 dacite domes to evaluate the thermal storage conditions and compositional diversity of recent MSH magmas. This approach allows us to more directly link the thermal (and therefore physical) conditions within the MSH magma reservoir to timescales of storage, thereby constraining the fraction of time 1980-2005 MSH magmas have spent in a mobile state. Plagioclase trace-element data and U-series characteristics reveal a compositionally heterogenous magmatic system beneath MSH and also require multi-stage plagioclase growth histories. The data also show that 2004-05 dacites contain a different plagioclase population relative to the 1980 cryptodome dacite, comprised of either a compositionally (and possibly temporally) distinct plagioclase component or composed of the same plagioclase components but in significantly different proportions. Discordant plagioclase 238U-230Th and 230Th-226Ra apparent ages require a mixture of young (likely eruption related) and old (>20-40 ka) plagioclase crystals in both 1980 and 2004-05 dacites. The low (230Th)/(232Th) in all measured 1980 and 2004-05 plagioclase requires a significant fraction (>10-30%) of the plagioclase to be old (>10s kyr). Maximum modeled Sr diffusion timescales at 750oC range from decades to centuries for both eruptions, with a maximum of ~600 years found in 1980 cryptodome plagioclase. However, partially equilibrated Sr in the innermost exposed parts of some crystals found in both 1980 and 2004-05 plagioclase indicate that a fraction (>20%) of plagioclase may have experienced a total of >10 kyrs at temperatures ≥750oC. Coupling Sr diffusion timescales with U-series measurements indicates that significant fraction (at least ~40%) of modeled MSH plagioclase spent <5% of their storage time at temperatures ≥750oC and thus in an easily mobilized rheological state. In contrast, the fraction of partially equilibrated plagioclase possibly spent >25-50% of their storage time at hotter conditions. Our data combined with data from other arc magmatic systems (e.g., Mt. Hood) imply that the process of remobilizing magma in arc systems towards successive eruptions requires thermally rejuvenating largely crystalline material, as opposed to sequential tapping of a persistent liquid-dominated magma body, even if successive eruptions are spaced closely in time (decades). In addition, rejuvenation events responsible for successive eruptions may sample spatially localized, but potentially overlapping, portions of the broader magma reservoir.

Trace‐element composition of zircon in Kofu and Tanzawa granitoids, Japan: Quantitative indicator of sediment incorporated in parent magma

Article

Jan 2021

Zircon is one of the most important minerals in geochronologic research. Isotopic ratios and trace elements in zircons are expected to reflect those of their parent magmas. Many geochemical researchers have proposed various discrimination diagrams for zircon to indicate tectonic setting and to identify source rock. Because most detrital zircons accumulated at river mouths are derived primarily from granitoids, the classification of zircon within granitoids is potentially meaningful. In our research, we focused on sediment involvement during granitoid formation and tried to identify trace‐element compositions in zircon that are sensitive to variation in sediment incorporation. To accomplish this, we examined trace‐element compositions of both the granitoids and the included zircons in the Kofu granitic complex and the Tanzawa tonalitic plutons in Japan. Among the high‐field‐strength elements (Th, U, Ta, Nb, Hf, and rare earth elements), only Nb and Ta concentrations in the granitoids increased as the rate of sediment contribution increased. However, the zircon did not show such trends in Nb and Ta content. Zircon Y and P contents exhibited a positive correlation, indicating that xenotime substitution occurs to some extent. Because P exists as pentavalent ions in igneous systems, its presence likely affects the concentrations of pentads in zircon. When we divided the Nb and Ta contents by the P content, it became clear that zircon Nb/P and Ta/P ratios increase depending on sediment involvement. While some exceptions exist, we found that zircon Yb/Gd ratios also respond to sediment involvement. Our data further demonstrated that zircons in granitoids with significant sediment incorporation are characterized by low Ce/P contents, which is partly attributable to monazite crystallization before zircon saturation. This study demonstrates that combining these element ratios is useful for indicating sediment incorporation.

Garnet petrochronology reveals the lifetime and dynamics of phonolitic magma chambers at Somma-Vesuvius

Article

Full-text available

Jan 2022

Somma-Vesuvius is one of the most iconic active volcanoes with historic and archeological records of numerous hazardous eruptions. Petrologic studies of eruptive products provide insights into the evolution of the magma reservoir before eruption. Here, we quantify the duration of shallow crustal storage and document the evolution of phonolitic magmas before major eruptions of Somma-Vesuvius. Garnet uranium-thorium petrochronology suggests progressively shorter pre-eruption residence times throughout the lifetime of the volcano. Residence times mirror the repose intervals between eruptions, implying that distinct phonolite magma batches were present throughout most of the volcano’s evolution, thereby controlling the eruption dynamics by preventing the ascent of mafic magmas from longer-lived and deeper reservoirs. Frequent lower-energy eruptions during the recent history sample this deeper reservoir and suggest that future Plinian eruptions are unlikely without centuries of volcanic quiescence. Crystal residence times from other volcanoes reveal that long-lived deep-seated reservoirs and transient upper crustal magma chambers are common features of subvolcanic plumbing systems.

Oxygen Fugacity Across Tectonic Settings

Chapter

Sep 2021

Millennial storage of near-Moho magma

Article

Jul 2019

Millennial magma reserves Lava erupts after being stored as magma deeper underground. Silica-rich magmas characteristic of volcanoes like Pinatubo are stored for thousands to hundreds of thousands of years. However, we have little understanding of the time scales for deeper basaltic magma like that supplying Icelandic volcanoes. Mutch et al. used diffusion of chromium and aluminum atoms to show that these magmas are stored for hundreds to thousands of years at the crust-mantle boundary. This discovery elucidates the time scale for understanding how magma is created and stored and how it erupts. Science , this issue p. 260

The application of abundance sensitivity filters to the precise and accurate measurement of uranium series nuclides by plasma mass spectrometry

Article

Nov 2018
INT J MASS SPECTROM

Application of retardation filters has revolutionized the use of multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) for the measurement of U-series disequilibria. Here, we present an evaluation of the application of retardation energy filters coupled with ion counting detectors for the measurement of U-series disequilibria by MC-ICPMS. We show that: 1) The RPQ minimizes backgrounds caused by ions experiencing irregular flight paths at high masses of uranium series nuclides. This reduced background is important for low count rate measurements and has enabled us to develop an improved method for the determination of 226Ra/228Ra ratios using a dual SEM/RPQ collector block. 2) Systematic biases exist for isotope ratios of 234U/238U and 230Th/232Th without the use of RPQs; thus, the RPQ is important for measurement of both of these isotope ratios. 3) That the RPQ makes possible 210Pb mea- surements in geologic samples using MC-ICPMS, but that this application is limited to samples that have low common Pb, such as carbonates and silica sinter. Additionally, we present mass spectrometric and gamma counting data for U-Th-Ra disequilibria in rock standard reference materials including AGV-2, AThO, BCR-1, BCR-2, BHVO-2, BIR-1a, DTS-1, PCC-1, RGM-2, TML, and W-2a. Comparing this new data with values from published literature enables us to provide consensus or recommended values for U, Th, and 226Ra concentrations and 230Th/232Th ratios for each of these standards. We note that this is the first compilation of recommended radium concentrations for rock standard reference materials.

Zircon as Magma Monitor: Planetary Records Down to Atom Scale

Chapter

Dec 2017

Analysis of natural mineral/host glass pairs provides robust zircon-melt partition coefficients applicable to natural systems. We analyzed zircon rims (outer ~15 µm of grain interiors) or surfaces (1–2 µm deep pits on crystal faces) and glasses in tholeiitic, calc-alkaline, and alkaline dacites and rhyolites from diverse settings (continental extension, AZ-NV, USA; hot spot/spreading center, Iceland; continental arc, Mount St. Helens (MSH), WA, USA). MSH Kds are based on eruption-age surfaces with adhering glass, which should closely approach crystal-melt equilibrium. We parameterize trivalent rare earth element (REE) Kds by X*[Ti] y for Sm to Lu, Nb, Th, and U, where X = 2.5–3600 and y = −0.73 to −1.3 for Sm to Lu. Kds for all elements span more than an order of magnitude but are highly coherent. REE Kds fit lattice strain model parabolas well, and all Kds show strong negative correlations with T indicators. Useful Kds for zircon can be estimated from Tizircon-Kdelement correlations. MSH Kds based on surface analyses are consistent with those from conventional rim analysis. When paired with zircon ages, modeled compositions of MSH melts corroborate and strengthen previous conclusions regarding history and evolution of the MSH magmatic system through time.

U-Series Dating

Chapter

Jan 2015

Bernard Bourdon

Magmatic lulls in the Sierra Nevada captured in zircon from rhyolite of the Mineral King pendant, California

Article

Feb 2014
GEOSPHERE

The Mineral King pendant in the Sierra Nevada batholith (California, USA) contains at least four rhyolite units that record high-silica volcanism during magmatic lulls in the Sierran magmatic arc. U-Th-Pb, trace element (single crystal spot analyses via sensitive high-resolution ion microprobe-reverse geometry, SHRIMP-RG), and bulk oxygen isotope analyses of zircon from these units provide a record of the age and compositional properties of the magmas that is not available from whole-rock analysis because of intense hydrothermal alteration of the pendant. U-Pb spot ages reveal that the Mineral King rhyolites are from two periods, the Early Jurassic (197 Ma) and the Early Cretaceous (134-136 Ma). These two rhyolite packages have zircons with distinct compositional trends for trace elements and delta O-18; the Early Jurassic rhyolite shows less evidence of crustal influences on the rhyolites and the Early Cretaceous rhyolite shows evidence of increasing crustal influences and crystal recycling. These rhyolites capture evidence of magmatism during two periods of low magmatic flux in the Sierran Arc; however, they still show that magmas were derived from interactions of maturing continental crust, increasing from the Early to Late Jurassic. This finding likely reflects the transition of the North America margin from one of docking island arcs in the Early Jurassic to one of a more mature continental arc in the Early Cretaceous. This also shows the utility in examining zircon spot ages combined with trace element and bulk isotopic composition to unlock the petrogenetic history of altered volcanic rocks.

Sub-micron-scale trace-element distributions in natural zircons of known provenance: implications for Ti-in-zircon thermometry

Article

Sep 2014

NanoSIMS measurements of Ti, P, Y, Ce, and Hf in zircon separates from the Youngest Toba Tuff, the Bishop Tuff, the Quottoon Igneous Complex, the Sierra Nevada batholith, and an Adirondack migmatite show that micron-scale oscillatory zoning of Ti is common. The zircons we have studied typically display banded concentration gradients, having between 1 and 7 peaks in Ti abundance over length scales of 10-20 m-the beam diameter commonly used for SIMS trace-element zircon analyses-with amplitudes of up to 4.5 ppm Ti (baseline values are similar to 1-4 ppm) and widths (measured at half-height) of between similar to 0.2 and 4 m. Spatial correlations between concentrations of Ti and other trace elements (P, Y, and Ce) are also common, but variable in character, ranging from oscillatory co-variation of Ti, P, Y, and Ce to cases where only a subset of peaks for a given element is spatially correlated with peaks in Ti. There are also longer length-scale, generally positive correlations among concentrations of Ti, P, Y, and Ce (i.e., gradients on which narrower peaks are superimposed). In contrast, Hf concentrations are either uncorrelated or inversely correlated with these longer length-scale variations in Ti concentrations. The wide range in Ti concentrations over distances of less than 1 m and the various correlations between Ti concentrations and those of the other analyzed elements suggest that on the micron scale and at temperatures between similar to 700 and 800 degrees C, zircon-liquid Ti partitioning is not controlled by bulk or lattice equilibrium. Treating our NanoSIMS Ti ion maps as though they were conventional SIMS analyses (i.e., generating an average Ti concentration for each map), we evaluate the hypothesis that while micron-scale variations in Ti concentrations might be kinetically controlled, when averaged over 100-400 mu m(2), such variations capture the thermal state of the growing zircon. Using these average Ti concentrations, independent petrologic pressure and temperature constraints, estimates of silica and titania activities based on phase assemblages as well as calculations using rhyolite-MELTS, we show that crystallization temperatures predicted by Ti-in-zircon geothermometry generally do not agree with the independently constrained temperatures for the samples.

Cogenetic late Pleistocene rhyolite and cumulate diorites from Augustine Volcano revealed by SIMS 238U-230Th dating of zircon, and implications for silicic magma generation by extraction from mush

Article

Nov 2014
GEOCHEM GEOPHY GEOSY

Augustine Volcano, a frequently active andesitic island stratocone, erupted a late Pleistocene rhyolite pumice fall that is temporally linked through zircon geochronology to cumulate dioritic blocks brought to the surface in Augustine's 2006 eruption. Zircon from the rhyolite yield a 238U-230Th age of ca. 25 ka for their unpolished rims, and their interiors yield a bimodal age populations at ca. 26 ka and a minority at ca. 41 ka. Zircon from dioritic blocks, ripped from Augustine's shallow magmatic plumbing system and ejected during the 2006 eruption, have interiors defining a ca. 26 ka age population that is indistinguishable from that for the rhyolite; unpolished rims on the dioritic zircon are dominantly younger (≤12 ka) indicating subsequent crystallization. Zircon from rhyolite and diorite overlap in U, Hf, Ti, and REE concentrations, though diorites also contain a second population of high-U, high temperature grains. Andesites that brought dioritic blocks to the surface in 2006 contain zircon with young (≤9 ka) rims and a scattering of older ages, but few zircon that crystallized during the 26 ka interval. Both the Pleistocene-age rhyolite and the 2006 dioritic inclusions plot along a whole-rock compositional trend distinct from mid-Holocene–present andesites and dacites, and the diorites, rhyolite, and two early Holocene dacites define linear unmixing trends often oblique to the main andesite array and consistent with melt (rhyolite) extraction from a mush (dacites), leaving behind a cumulate amphibole-bearing residue (diorites). Rare zircon antecrysts up to ca. 300 ka from all rock types indicate that a Quaternary center has been present longer than preserved surficial deposits. This article is protected by copyright. All rights reserved.

Geostandards and Geoanalytical Research Bibliographic Review 2012

Article

Full-text available

Dec 2013
GEOSTAND GEOANAL RES

Corrigendum: Rapid remobilization of magmatic crystals kept in cold storage

Article

Apr 2014
NATURE

The processes involved in the formation and storage of magma within the Earth's upper crust are of fundamental importance to volcanology. Many volcanic eruptions, including some of the largest, result from the eruption of components stored for tens to hundreds of thousands of years before eruption. Although the physical conditions of magma storage and remobilization are of paramount importance for understanding volcanic processes, they remain relatively poorly known. Eruptions of crystal-rich magma are often suggested to require the mobilization of magma stored at near-solidus conditions; however, accumulation of significant eruptible magma volumes has also been argued to require extended storage of magma at higher temperatures. What has been lacking in this debate is clear observational evidence linking the thermal (and therefore physical) conditions within a magma reservoir to timescales of storage-that is, thermal histories. Here we present a method of constraining such thermal histories by combining timescales derived from uranium-series disequilibria, crystal sizes and trace-element zoning in crystals. At Mount Hood (Oregon, USA), only a small fraction of the total magma storage duration (at most 12 per cent and probably much less than 1 per cent) has been spent at temperatures above the critical crystallinity (40-50 per cent) at which magma is easily mobilized. Partial data sets for other volcanoes also suggest that similar conditions of magma storage are widespread and therefore that rapid mobilization of magmas stored at near-solidus temperatures is common. Magma storage at low temperatures indicates that, although thermobarometry calculations based on mineral compositions may record the conditions of crystallization, they are unlikely to reflect the conditions of most of the time that the magma is stored. Our results also suggest that largely liquid magma bodies that can be imaged geophysically will be ephemeral features and therefore their detection could indicate imminent eruption.

Zircon trace element chemistry at sub-micrometer resolution for Tarawera volcano, New Zealand, and implications for rhyolite magma evolution

Article

Apr 2014

Zoned crystals can be important recorders of magmatic processes in space and time. However, in most situations, the temporal dimension is difficult to quantify. Here, we have employed secondary ion mass spectrometry depth profiling to excavate parallel pits into non-polished crystal faces of zircon to obtain ~5 μm resolution U–Th disequilibrium ages (one pit) that can be correlated with trace element zoning at sub-μm resolution derived from a second pit. Data from 17 crystals representing each of the four rhyolite eruptions of Tarawera volcano, an intra-caldera edifice within the Okataina Volcanic Centre, reveal diverse zircon growth conditions over time. Most crystals display rimward depletions in Zr/Hf and Ti, broadly consistent with cooling and crystallization. However, a significant fraction of crystals lacks these patterns and displays rimward trace element variations consistent with isothermal or prograde crystallization. Oscillatory zonation patterns in Y, Th, and U are superimposed on the Zr/Hf and Ti trends. Despite the limited number of crystals analyzed in this way, the striking lack of ubiquitous trace element zoning patterns in crystals from the same hand sample implies that fractional crystallization upon cooling was punctuated by magma recharge and crystal mixing affecting different parts of the magma reservoir. By combining data from all crystals, a systematic change to more heterogeneous trace element abundances is revealed by zircon crystal domains 100 ka prior to caldera formation and is best explained by the post-caldera system consisting of small, isolated melt pockets that evolved independently. An important conclusion is that the zircon ‘cargo’ in volcanic rocks reflects thermally and compositionally divergent processes that act near simultaneously in a magma storage region and not exclusively the conditions in the eruptible magma.

Erratum to: An experimental study of Ti and Zr partitioning among zircon, rutile, and granitic melt

Article

Jul 2013

In order to evaluate the effect of trace and minor elements (e.g., P, Y, and the REEs) on the high-temperature solubility of Ti in zircon (zrc), we conducted 31 experiments on a series of synthetic and natural granitic compositions [enriched in TiO2 and ZrO2; Al/(Na + K) molar ~1.2] at a pressure of 10 kbar and temperatures of ~1,400 to 1,200 °C. Thirty of the experiments produced zircon-saturated glasses, of which 22 are also saturated in rutile (rt). In seven experiments, quenched glasses coexist with quartz (qtz). SiO2 contents of the quenched liquids range from 68.5 to 82.3 wt% (volatile free), and water concentrations are 0.4–7.0 wt%. TiO2 contents of the rutile-saturated quenched melts are positively correlated with run temperature. Glass ZrO2 concentrations (0.2–1.2 wt%; volatile free) also show a broad positive correlation with run temperature and, at a given T, are strongly correlated with the parameter (Na + K + 2Ca)/(Si·Al) (all in cation fractions). Mole fraction of ZrO2 in rutile

\left( {\mathop X\nolimits_{{{\text{ZrO}}_{ 2} }}^{\text{rt}} } \right)

in the quartz-saturated runs coupled with other 10-kbar qtz-saturated experimental data from the literature (total temperature range of ~1,400 to 675 °C) yields the following temperature-dependent expression:

{\text{ln}}\left( {\mathop X\nolimits_{{{\text{ZrO}}_{ 2} }}^{\text{rt}} } \right) + {\text{ln}}\left( {a_{{{\text{SiO}}_{2} }} } \right) = 2.638(149) - 9969(190)/T({\text{K}})

, where silica activity

a_{{{\text{SiO}}_{2} }}

in either the coexisting silica polymorph or a silica-undersaturated melt is referenced to α-quartz at the P and T of each experiment and the best-fit coefficients and their uncertainties (values in parentheses) reflect uncertainties in T and

\mathop X\nolimits_{{{\text{ZrO}}_{2} }}^{\text{rt}}

. NanoSIMS measurements of Ti in zircon overgrowths in the experiments yield values of ~100 to 800 ppm; Ti concentrations in zircon are positively correlated with temperature. Coupled with values for

a_{{{\text{SiO}}_{2} }}

and

a_{{{\text{TiO}}_{2} }}

for each experiment, zircon Ti concentrations (ppm) can be related to temperature over the range of ~1,400 to 1,200 °C by the expression:

\ln \left( {\text{Ti ppm}} \right)^{\text{zrc}} + \ln \left( {a_{{{\text{SiO}}_{2} }} } \right) - \ln \left( {a_{{{\text{TiO}}_{2} }} } \right) = 13.84\left( {71} \right) - 12590\left( {1124} \right)/T\left( {\text{K}} \right)

. After accounting for differences in

a_{{{\text{SiO}}_{2} }}

and

a_{{{\text{TiO}}_{2} }}

, Ti contents of zircon from experiments run with bulk compositions based on the natural granite overlap with the concentrations measured on zircon from experiments using the synthetic bulk compositions. Coupled with data from the literature, this suggests that at T ≥ 1,100 °C, natural levels of minor and trace elements in “granitic” melts do not appear to influence the solubility of Ti in zircon. Whether this is true at magmatic temperatures of crustal hydrous silica-rich liquids (e.g., 800–700 °C) remains to be demonstrated. Finally, measured

D_{\text{Ti}}^{{{\text{zrc}}/{\text{melt}}}}

values (calculated on a weight basis) from the experiments presented here are 0.007–0.01, relatively independent of temperature, and broadly consistent with values determined from natural zircon and silica-rich glass pairs.

High-resolution tephrochronology of the Wilson Creek Formation (Mono Lake, California) and Laschamp event using 238U-230Th SIMS dating of accessory mineral rims

Article

Dec 2012
EARTH PLANET SC LETT

Sediments of the Wilson Creek Formation surrounding Mono Lake preserve a high-resolution archive of glacial and pluvial responses along the eastern Sierra Nevada due to late Pleistocene climate change. An absolute chronology for the Wilson Creek stratigraphy is critical for correlating the paleoclimate record to other archives in the western U.S. and the North Atlantic region. However, multiple attempts to date the Wilson Creek stratigraphy using carbonates and tephras yield discordant results due to open-system effects and radiocarbon reservoir uncertainties as well as abundant xenocrysts. New ion microprobe 238U-230Th dating of the final increments of crystallization recorded by allanite and zircon autocrysts from juvenile pyroclasts yield ages that effectively date eruption of key tephra beds and delimit the timing of basal Wilson Creek sedimentation to the interval between 26.8±2.1 and 61.7±1.9 ka. Tephra (Ash 15) erupted during the geomagnetic excursion originally designated the Mono Lake excursion yields an age of 40.8±1.9 ka, indicating that the event is instead the Laschamp excursion. The new ages support a depositional chronology from magnetostratigraphy that indicates quasi-synchronous glacial and hydrologic responses in the Sierra Nevada and Mono Basin to regional climate change, with intervals of lake filling and glacial-snowpack melting that are in phase with peaks in spring insolation.

Magma Generation at a Large, Hyperactive Silicic Volcano (Taupo, New Zealand) Revealed by U-Th and UPb Systematics in Zircons

Article

Full-text available

Sep 2004

Young (<∼65ka) explosive silicic volcanism at Taupo volcano, New Zealand, has involved the development and evacuation of several crustal magmatic systems. Up to and including the 26·5 ka 530 km 3 Oruanui eruption, magmatic systems were contemporaneous but geographically separated. Subsequently they have been separated in time and have vented from geographically overlapping areas. Single-crystal (secondary ionization mass spectrometry) and multiple-crystal (thermal ionization mass spectrometry) zircon model-age data are presented from nine representative eruption deposits from ∼ 45 to ∼3·5 ka. Zircon yields vary by three orders of magnitude, correlating with the degrees of zircon saturation in the magmas, and influencing the spectra of model ages. Two adjacent magma systems active up to 26·5 ka show wholly contrasting model-age spectra. The smaller system shows a simple unimodal distribution. The larger system, using data from three eruptions, shows bimodal model-age spectra. An older ∼100 ka peak is interpreted to represent zircons (antecrysts) derived from older silicic mush or plutonic rocks, and a younger peak to represent zircons (phenocrysts) that grew in the magma body immediately prior to eruption. Post-26·5 ka magma batches show contrasting age spectra, consistent with a mixture of antecrysts, phenocrysts and, in two examples, xenocrysts from Quaternary plutonic and Mesozoic-Palaeozoic metasedimentary rocks. The model-age spectra, coupled with zircon-dissolution modelling, highlight contrasts between short-term silicic magma generation at Taupo, by bulk remobilization of crystal mush and assimilation of metasediment and/or silicic plutonic basement rocks, and the longer-term processes of fractionation from crustally contaminated mafic melts. Contrasts between adjacent or successive magma systems are attributed to differences in positions of the source and root zones within contrasting domains in the quartzo-feldspathic (<15 km deep) crust below the volcano.

Experimental determination of Ra mineral/melt partitioning for feldspars and 226Ra-disequilibrium crystallization ages of plagioclase and alkali-feldspar

Article

Full-text available

Apr 2009
EARTH PLANET SC LETT

Radium and other trace element partition coefficients have been experimentally determined for plagioclase and alkali-feldspar in equilibrium with silicate melts. In anorthitic plagioclase, alkalis (e.g. Na, K, Rb, Cs) and the heavy earth alkalis (e.g. Ba, Ra) are incompatibles, in albitic plagioclase Na is compatible, whereas Ca and Sr are always compatible in plagioclase. DRaplagioclase/melt was determined to be 0.017±0.006 (An91), 0.025±0.009 (An81) and 0.47±0.08 (An34) for three different An contents, the ratio DRa/DBa increasing with decreasing An component. In alkali-feldspar, K, Rb, Ba, Ra, and Sr are compatible, whereas Na, Cs, and Ca are incompatible. DRaalkali-feldspar/melt was determined to 2.54±0.04 for a Or75Ab21An4 composition, the ratio DRa/DBa is 0.55±0.01. For plagioclase, DRa and XAn are related as RT lnDRa [kJ/mol]=−52.54(±5.75)XAn−4.25(±1.51) and DRa/DBa=exp((−9.47(±6.21)XAn−13.66(±1.52))/RT) [kJ/mol], allowing calculation of DRa from the An content of magmatic plagioclase. For alkali-feldspar we propose the equation DRa/DBa=exp((−16.06(±0.58)+15.52(±0.67) XOr)/RT) [kJ/mol]. The experimentally determined partition coefficients are then used to recalculate Th–Ra–Ba model ages, leading to generally shorter feldspar residence times. The variations encompass 20 to 90% shorter crystallization ages with respect to the originally calculated ones, in two cases feldspar crystallization ages increase slightly. For several cases the measured plagioclase and melt have either never been in equilibrium or the Th–Ra–Ba system has been disturbed after crystallization.

Compositional Zoning of the Bishop Tuff

Article

Full-text available

Mar 2007

Compositional data for >400 pumice clasts, organized according to eruptive sequence, crystal content, and texture, provide new perspectives on eruption and pre-eruptive evolution of the >600 km3 of zoned rhyolitic magma ejected as the Bishop Tuff during formation of Long Valley caldera. Proportions and compositions of different pumice types are given for each ignimbrite package and for the intercalated plinian pumice-fall layers that erupted synchronously. Although withdrawal of the zoned magma was less systematic than previously realized, the overall sequence displays trends toward greater proportions of less evolved pumice, more crystals (0·5–24 wt %), and higher FeTi-oxide temperatures (714–818°C). No significant hiatus took place during the 6 day eruption of the Bishop Tuff, nearly all of which issued from an integrated, zoned, unitary reservoir. Shortly before eruption, however, the zoned melt-dominant portion of the chamber was invaded by batches of disparate lower-silica rhyolite magma, poorer in crystals than most of the resident magma but slightly hotter and richer in Ba, Sr, and Ti. Interaction with resident magma at the deepest levels tapped promoted growth of Ti-rich rims on quartz, Ba-rich rims on sanidine, and entrapment of near-rim melt inclusions relatively enriched in Ba and CO2. Varied amounts of mingling, even in higher parts of the chamber, led to the dark gray and swirly crystal-poor pumices sparsely present in all ash-flow packages. As shown by FeTi-oxide geothermometry, the zoned rhyolitic chamber was hottest where crystal-richest, rendering any model of solidification fronts at the walls or roof unlikely. The main compositional gradient (75–195 ppm Rb; 0·8–2·2 ppm Ta; 71–154 ppm Zr; 0·40–1·73% FeO*) existed in the melt, prior to crystallization of the phenocryst suite observed, which included zircon as much as 100 kyr older than the eruption. The compositions of crystals, though themselves largely unzoned, generally reflect magma temperature and the bulk compositional gradient, implying both that few crystals settled or were transported far and that the observed crystals contributed little to establishing that gradient. Upward increases in aqueous gas and dissolved water, combined with the adiabatic gradient (for the ∼ 5 km depth range tapped) and the roofward decline in liquidus temperature of the zoned melt, prevented significant crystallization against the roof, consistent with dominance of crystal-poor magma early in the eruption and lack of any roof-rind fragments among the Bishop ejecta, before or after onset of caldera collapse. A model of secular incremental zoning is advanced wherein numerous batches of crystal-poor melt were released from a mush zone (many kilometers thick) that floored the accumulating rhyolitic melt-rich body. Each batch rose to its own appropriate level in the melt-buoyancy gradient, which was self-sustaining against wholesale convective re-homogenization, while the thick mush zone below buffered it against disruption by the deeper (non-rhyolitic) recharge that augmented the mush zone and thermally sustained the whole magma chamber. Crystal–melt fractionation was the dominant zoning process, but it took place not principally in the shallow melt-rich body but mostly in the pluton-scale mush zone before and during batchwise melt extraction.

Chronology and Evolution of Caldera-forming and Post-caldera Magma Systems at Okataina Volcano, New Zealand from Zircon U-Th Model-age Spectra

Article

Full-text available

May 2010

U-Th disequilibrium model-age data are presented for zircons from four young eruptive units from Okataina volcano, New Zealand. These data highlight contrasts in the time-scales over which eruptible silicic magma bodies are generated and accumulated below a highly active rhyolite volcano prior to and following the ~ 61 ka caldera-forming Rotoiti eruption (80 -120 km3 magma). The Rotoiti event was followed by ≥12 explosive eruptions of the Mangaone Subgroup between ~ 45 and 30 ka. A change in eruptive styles between ~ 30 and 25 ka brackets the major Oruanui eruption of Taupo volcano ~ 80 km away; subsequently eight rhyolitic, lava-dominated and two basaltic explosive eruptions occurred between 25 ka and 1886 CE. We present (238U-230Th) zircon model-age data determined by secondary-ion mass spectrometry (SIMS) and thermal ionization mass spectrometry (TIMS) from the Rotoiti, Ngamotu (Unit B) (~45 ka), Mangaone (Unit I) (33 ka) and Rotorua (15·4 ka) eruptive units.We couple these data with published and new compositional information to trace magma crystallization and storage patterns. Population density curves of SIMS model ages from zircons in the two pumice types (biotite-free and biotite-bearing) from the Rotoiti eruption differ. Zircons from the former yield a model-age peak coincident with eruption age, whereas zircons from the latter show a peak at 70-90 ka and little variation inTIMS model-age values from different crystal size fractions. Concentration weighted means of model ages from the two pumices, however, are the same within 1 SD error, and their Sr isotopic values overlap at 2 SD precision, suggesting that they are genetically linked. Model-age spectra for the Ngamotu, Mangaone and Rotorua pumices are complex, indicating recycling of crystals from multiple older populations that largely pre-date the 61 ka caldera-forming eruption. Superimposed on the older age spectra are variably developed younger pre-eruptive suites of ages reflecting varying amounts of crystallization prior to each eruption. A lack of commonality in these younger peaks in the post-caldera eruptive rocks, along with compositional and isotopic differences between (and sometimes within) the eruptive units, collectively precludes their origin from a single melt-bearing mush system. Okataina contrasts with Taupo, where comparable-age eruption deposits have simpler age spectra, consistent with larger-scale crystallization cycles and thermal events in the magma chambers there. When compared with other caldera-related silicic systems for which suitable data are available, Okataina differs in lacking a simple pre-eruptive history prior to its caldera-forming event and having a complex, non-coherent magmatic history of post-caldera eruptions.

Experimental study of radium partitioning between anorthite and melt at 1 atm

Article

Full-text available

Aug 2007

We present the first experimental radium mineral/melt partitioning data, specifically between anorthite and a CMAS melt at atmospheric pressure. 226Ra disequilibria are an important chronometer of recent magmatic activity. Ion microprobe measurement of coexisting anorthite and glass phases produces a molar DRa = 0.040 ± 0.006 and DRa/DBa = 0.24 ± 0.05 at 1400 °C. Our results indicate that lattice strain partitioning models fit the divalent (Ca, Sr, Ba, Ra) partition coefficient data of this study well, supporting previous work on crustal melting and magma chamber dynamics that has relied on such models to approximate radium partitioning behavior in the absence of experimentally determined values.

Trace element composition of igneous zircon: A thermal and compositional record of the accumulation and evolution of a large silicic batholith, Spirit Mountain, Nevada

Article

Full-text available

Oct 2010

Hafnium, U, Th, and REE content of zircons from the Spirit Mountain batholith in southern Nevada correlate with calculated temperatures from the Ti-in-zircon thermometer to support field and petrologic evidence of rejuvenation of crystal mush and melt extraction events during the 2-million year accumulation of the granitoid batholith. Marked variation in zircon composition from sample to sample, from grain to grain within individual samples, and from zone to zone within individual grains documents in detail a history of fluctuating conditions with repeated episodes of replenishment, reheating, crystal mush rejuvenation, fractional crystallization, and melt segregation. The zircons exhibit compositional and thermal variability indicative of variations in host melt composition due to (1) melt rejuvenation, mixing, and fractionation (2) coeval growth of other REE-rich accessory minerals, and possibly (3) fluctuation in fO2. KeywordsZircon-Trace elements-Granite-Crystal mush-Fractionation-Rejuvenation-Ti-in-zircon thermometer

Rapid Rates of Magma Generation at Contemporaneous Magma Systems, Taupo Volcano, New Zealand: Insights from U-Th Model-age Spectra in Zircons

Article

Full-text available

Jun 2009

New and/or enlarged datasets of U–Th disequilibrium model ages from secondary ionization mass spectrometry (SIMS) analyses of zircons in eight eruptive units from the area of Taupo volcano, New Zealand, highlight the behavioural contrasts of two closely adjacent, contemporaneous but independent magma chambers. One yielded closely similar crystal-poor (‘Oruanui-type’) rhyolites, sampled in three small precursor eruptions (Tihoi, ‘New plinian’, Okaia) from ∼45 to 30 ka, then the major 27 ka Oruanui eruption. Three of the four eruptions had vents within the modern Lake Taupo, whereas the fourth (‘New plinian’) was sourced ∼20 km NNE of the other vents, fed by lateral magma migration. Samples from all four eruptions share a common model-age peak at ∼95 ka of antecrystic zircons. However, three of the four differ in younger pre-eruptive model-age peaks that require their parental melt-dominant bodies to have been physically extracted independently from a common mush zone represented by the ∼95 ka peak. A sample from a fifth eruption (‘New phreatoplinian’, also at ∼45 ka) shares an older 80–100 ka peak but has numerous older grains and distinctly contrasting Sr-isotopic characteristics to the ‘Oruanui-type’ magmas. The 530 km3 Oruanui melt-dominant body was produced in at most ∼3000 years as shown by differences in zircon model-age spectra and average ages between it and the 30 ka Okaia eruption, despite their coincidence in vent locations. The second suite of eruptions at ∼47, 28 and 16 ka ejected moderately crystal-rich biotite rhyolites from a second source chamber, which vented over a 15 km wide area NE of Taupo (overlapping with Maroa volcano). This second chamber is inferred to have comparable horizontal dimensions to the vent spacing. The three biotite rhyolites show unimodal model-age spectra that peak at 30, 15–25 and 6 kyr prior to each eruption, respectively, and underwent single cycles of melt generation and eruption with no recycling of significantly older antecrysts or xenocrysts (< 1% equiline grains). Crystallization peaks defined by probability density function curves are not in phase between the two magma chambers and they had wholly independent thermal and chemical histories, despite their close geographical proximity. Post-Oruanui activity involved recycling of Oruanui-age zircons, but these crystals are xenocrystic, as the host melts show no lineage towards or mixing with the Oruanui compositions. Magma chambers at Taupo accumulated melt-dominant bodies as rapidly as > 5 m3/s (Oruanui) and effectively drained the mush of melt in doing so (Oruanui vs post-Oruanui activity), probably mediated by active rifting processes and tectonic disruption of the mush pile. Comparisons of ‘magma residence times’ and discussion of the growth histories of large silicic chambers represented by volcanic or plutonic rocks are self-limited by the uncertainties in the respective SIMS analyses. Growth times of Miocene and older plutons dated by SIMS U–Pb techniques are comparable with the 2 Myr lifetime of the whole Taupo Volcanic Zone, and the associated 1σ SIMS analytical uncertainties exceed the lifetime of a volcano such as Taupo. Subtle details that indicate the rapidity of magma accumulation and recycling of crystals in the young Taupo system cannot be discerned in most pre-300 ka silicic systems. Averaging of SIMS model-age data further obscures subtle details that would allow discrimination of newly crystallized versus recycled zircons. Discussions of volcano–plutonic relationships and accumulation rates for large silicic melt-dominant bodies cannot rely on age data in isolation, but require knowledge of the stratigraphic and compositional settings.

Rhyolites and their Source Mushes across Tectonic Settings

Article

Full-text available

Dec 2008

Evolved magmas, including highly explosive rhyolites, are mainly generated by extraction of viscous melts from solid residues either in (1) partial melting zones within the crust (dominantly up-temperature evolution with newly formed silicic melt), or in (2) long-lived crystallizing mush zones fed by mafic to intermediate magmas (dominantly down-temperature evolution with residual silicic melt). Although both processes undoubtedly occur and are generally coupled, allowing for mixing between mantle and crustal components, we argue that combined field, thermal, geochemical, and geophysical observations favor residual melt extraction from crystalline mushes as the likely scenario in all tectonic settings. Depending on the main melting process in the mantle, two end-member differentiation trends occur: (1) a dry lineage leading to hot-reduced rhyolites and granites in magmatic provinces fueled by decompression melting of the mantle; (2) a wet lineage leading to cold-oxidized rhyolites and granites in subduction zones dominated by flux melting of the mantle.

Zircon saturation revisited: temperature and composition effects in variety of crustal magma types

Article

Jan 1983

Holocene rhyodacite eruptions on the flanks of South Sister volcano, Oregon

Article

Jan 1987

William E. Scott

Almost 0.9 km3 (dense-rock equivalent) of rhyodacite tephra, lava domes, and lava flows erupted from numerous vents on the southwest, southeast, and northeast flanks of South Sister volcano during late Holocene time. Eruptions occurred during two brief episodes between 2300 and 2000 1 4C yr B.P., separated by a dormant interval of as long as several centuries. The eruptions of each episode were probably fed by dikes, on the basis of the following: the alignment of vents, the chemical uniformity of eruptive products, and stratigraphic evidence that the eruptions of each episode occurred over a short interval of time. Each eruptive episode began with the explosive eruption of air-fall tephra. Small pyroclastic flows and hot pyroclastic surges erupted from a few vents and traveled as far as 3 km. Rapid snowmelt accompanied the early phase of each episode and triggered small lahars. Each episode culminated with the extrusion of lava domes and flows. The distribution of late Quaternary mafic vents around the area of Holocene rhyodacite vents suggests that a magma chamber with a maximum area) extent of 30 km2 may lie beneath the south flank of South Sister. The chemical uniformity of the eruptive products of each episode is consistent with each having tapped a relatively small homogeneous portion of a compositionally zoned magma chamber of much greater volume than the erupted products. Alternately, if this chemical uniformity reflects the generation and rapid ascent and eruption of a crustal partial melt, then a large magma chamber need not be present.

The half lives of uranium-234 and thorium-230

Article

Jan 2000
CHEM GEOL

Quaternary magmatism in the Cascades - Geologic perspectives

Article

Jan 2007
U S Geol Surv Prof Pap

Wes Hildreth

Squid (1.13b), A user's manual: Berkeley Geochronology Center Special Publication 2. -2003, ISOPLOT 3.0: A geochronological toolkit for Microsoft Excel 855

Article

Jan 2001

K.R. Ludwig

Isoplot, rev. 3.75. A geochronological toolkit for microsoft excel

Article

Jan 2003

K.R. Ludwig

238U-230Th-226Ra disequilibria in dacite and plagioclase from the 2004-2005 eruption of Mount St. Helens

Article

Jan 2008
U S Geol Surv Prof Pap

Uranium-series disequilibria in whole-rock samples and mineral separates provide unique insights into the time scales and processes of magma mixing, storage, and crystallization. We present 238U-230Th- 226Ra data for whole-rock dacite and gouge samples and for plagioclase separated from two dacite samples, all erupted from Mount St. Helens between October 2004 and April 2005. We also present new 238U- 230Th disequilibria for a suite of four reference samples from the 1980-86 eruption of Mount St. Helens. We use the U-series data to evaluate the origin of the 2004-5 magma, its relation to the 1980-86 magma, and the relation of 2004-5 phenocrysts to their host magmas. Dacite samples from 2004-5 show variable (230Th)/(238U), ranging from 238U- enriched to 230Th-enriched. (230Th)/(232Th) ratios in 2004-5 dacite and gouge samples do not vary outside of analytical error and are within the range of (230Th)/(232Th) measured for the 1980s reference suite. However, (230Th)/(232Th) for plagioclase separates for dome samples erupted during October and November 2004 are significantly different from corresponding whole-rock values, which suggests that a large fraction (>30 percent) of crystals in each sample are foreign to the host liquid. Furthermore, plagioclase in the two 2004 samples have U-series characteristics distinct from each other and from plagioclase in dacite erupted in 1982, indicating that (1) the current eruption must include a component of crystals (and potentially associated magma) that were not sampled by the 1980-86 eruption, and (2) dacite magmas erupted only a month apart in 2004 contain different populations of crystals, indicating that this foreign component is highly heterogeneous within the 2004-5 magma reservoir.

Zircon reveals protracted magma storage and recycling beneath Mount St. Helens

Article

Oct 2010
GEOLOGY

Current data and models for Mount St. Helens volcano (Washington, United States) suggest relatively rapid transport from magma genesis to eruption, with no evidence for protracted storage or recycling of magmas. However, we show here that complex zircon age populations extending back hundreds of thousands of years from eruption age indicate that magmas regularly stall in the crust, cool and crystallize beneath the volcano, and are then rejuvenated and incorporated by hotter, young magmas on their way to the surface. Estimated dissolution times suggest that entrained zircon generally resided in rejuvenating magmas for no more than about a century. Zircon elemental compositions reflect the increasing influence of mafic input into the system through time, recording growth from hotter, less evolved magmas tens of thousands of years prior to the appearance of mafic magmas at the surface, or changes in whole-rock geochemistry and petrology, and providing a new, time-correlated record of this evolution independent of the eruption history. Zircon data thus reveal the history of the hidden, long-lived intrusive portion of the Mount St. Helens system, where melt and crystals are stored for as long as hundreds of thousands of years and interact with fresh influxes of magmas that traverse the intrusive reservoir before erupting.

New thermodynamic models and revised calibrations for the Ti-in-zircon and Zr-in-rutile thermometers

Article

Oct 2007

The models recognize that ZrSiO4, ZrTiO4, and TiSiO4, but not ZrO2 or TiO2, are independently variable phase components in zircon. Accordingly, the equilibrium controlling the Zr content of rutile coexisting with zircon is ZrSiO4 = ZrO2 (in rutile) + SiO2. The equilibrium controlling the Ti content of zircon is either ZrSiO4 + TiO2 = ZrTiO4 + SiO2 or TiO2 + SiO2 = TiSiO4, depending whether Ti substitutes for Si or Zr. The Zr content of rutile thus depends on the activity of SiO2 (a_{text{SiO}2}) as well as T, and the Ti content of zircon depends on a_{text{SiO}2} and a_{text{TiO}2} as well as T. New and published experimental data confirm the predicted increase in the Zr content of rutile with decreasing a_{text{SiO}2}, and unequivocally demonstrate that the Ti content of zircon increases with decreasing a_{text{SiO}2}. The substitution of Ti in zircon therefore is primarily for Si. Assuming a constant effect of P, unit a_{text{ZrSiO}4}, and that a_{text{ZrO}2} and a_{text{ZrTiO}4} are proportional to ppm Zr in rutile and ppm Ti in zircon, [log(ppm Zr-in-rutile) + loga_{text{SiO}2}] = A1 + B1/T(K) and [log(ppm Ti-in-zircon) + loga_{text{SiO}2} - loga_{text{TiO}2}] = A2 + B2/T, where the A and B are constants. The constants were derived from published and new data from experiments with a_{text{SiO}2} buffered by either quartz or zircon + zirconia, from experiments with a_{text{SiO}2} defined by the Zr content of rutile, and from well-characterized natural samples. Results are A1 = 7.420 ± 0.105; B1 = -4,530 ± 111; A2 = 5.711 ± 0.072; B2 = -4,800 ± 86 with activity referenced to α-quartz and rutile at P and T of interest. The zircon thermometer may now be applied to rocks without quartz and/or rutile, and the rutile thermometer applied to rocks without quartz, provided that a_{text{SiO}2} and a_{text{TiO}2} are estimated. Maximum uncertainties introduced to zircon and rutile thermometry by unconstrained a_{text{SiO}2} and a_{text{TiO}2} can be quantitatively assessed and are ≈60 to 70°C at 750°C. A preliminary assessment of the dependence of the two thermometers on P predicts that an uncertainty of ±1 GPa introduces an additional uncertainty at 750°C of ≈50°C for the Ti-in-zircon thermometer and of ≈70 to 80°C for the Zr-in-rutile thermometer.

PRECISION MEASUREMENT OF HALF-LIVES AND SPECIFIC ACTIVITIES OF Â²Â³âµU AND Â²Â³â¸U

Article

Magmatic perturbations in the Okataina Volcanic Complex, New Zealand at thousand-year timescales recorded in single zircon crystals

Article

May 2011
EARTH PLANET SC LETT

Using single crystals to trace the chemical evolution of a magmatic system has long been a goal of igneous petrology. Crystals utilized for in situ dating hold great potential to link compositional and temporal information to better understand the evolution of a magmatic system. If micron-scale zoning of trace elements within a single zircon can be directly associated with volcanic events in magmatic systems, then new insights into long-term maintenance and storage of eruptible magma can be unlocked. This study presents new data that directly links the geochemical history recorded in individual zircon crystals with over 50,000 yrs of changing physical and chemical conditions within a magma reservoir. Trace elements (Hf and Y) in zircon have not diffusively equilibrated with their host melt so that they store information about the melt when that zone crystallized. Thus, different stages of growth can be associated with discrete time periods in the magmatic system.Zircon from the two most recent rhyolite eruptions (Kaharoa and Whakatane) within the Okataina Volcanic Complex (OVC), New Zealand, have both isotopic (age) and trace element signatures that correspond with distinct changes in the temperature and phase assemblage of pulses of erupted magma within the caldera system. This discrete change is associated with caldera collapse and reflects a change from cold, wet source rhyolite to a relatively hotter and drier source rhyolite and back. The Kaharoa and Whakatane eruptions are separated by 4000 yrs and 15 km distance, yet the zircon populations record the same distinct thermal and chemical pulse that occurred following caldera collapse, suggesting an interconnected magmatic system that houses at least small volumes of rhyolite melt for timescales of 103 to 105 yrs that is periodically extracted during eruption.

Uranium-Series Crystal Ages

Article

Jan 2008

As the collected papers in this volume aptly demonstrate, chemical and textural records contained within crystals have become increasingly useful tools for understanding the evolution of magmatic systems. An essential component of studies of the dynamics of magmatic systems is the ability to place the thermal, physical, and compositional evolution of magmas in a temporal context. Studies of the kinetics of crystal growth and reaction, and of diffusive re-equilibration of crystals and melts, can provide information about the duration of magmatic processes such as magma ascent (Rutherford 2008), crystal growth (Hammer 2008), and residence of crystals in melts other than the ones in which they crystallized (Costa 2008). However, the only method of extracting information from young crystals about the absolute age of magmatic processes is isotopic dating. The timescales of many sub-volcanic magmatic processes such as magma transport, differentiation, crystallization, and storage within the crust, appear to be commensurate with the half-lives of uranium-series (U-series) nuclides, and therefore this isotopic system offers the greatest potential to quantify timescales and rates of magmatic processes. We focus here on crystal dating using the decay products of 238U and 235U, including 238U-230Th-226Ra-210Pb disequilibria, 235U-231Pa disequilibria, and U-Pb dating of young crystals. In this chapter, we review the principles of U-series dating of crystals, the types of information that have been gained so far from U-series dating, and the potential for combining U-series crystal ages with other crystal-scale and magma-scale information to unravel the dynamics of magmatic systems. ### Principles of U-series dating U-Pb dating is based on the accumulation of 206Pb and 207Pb over time due to decay of 238U and 235U, respectively, and is conceptually similar to most isotopic dating techniques. U-Pb dating of zircon, both by TIMS and in …

Rutile saturation in hydrous siliceous melts and its bearing on Ti-thermometry of quartz and zircon

Article

Jun 2007
EARTH PLANET SC LETT

The TiO2 solubility of rutile-saturated hydrous siliceous melts has been investigated at P=1 GPa and T=650–1000 °C for several representative felsic compositions. The dissolution of a rutile crystal into a TiO2 undersaturated melt provides information on both TiO2 solubility and Ti diffusion. Results of this study confirm that TiO2 solubility is strongly dependent on both temperature and melt composition, but not on the amount of H2O present. For a given T, TiO2 content decreases as the melts become more felsic. The solubility of TiO2 is given by:log⁡(Ti,ppm)=7.95−5305T+0.124FMwhere T is in K and FM is a melt composition parameter,FM=1Si.Na+K+2(Ca+Mg+Fe)Alin which the chemical symbols represent cation fractions.Results of dissolution experiments yield an activation energy (E) for Ti transport in a hydrous felsic melt of 186±27 kJ/mol and a frequency factor, Do, of 3.6±1.2 m2/s. These results suggest an activation energy similar to that established for Zr diffusion in similar melts, but with Ti diffusion rates 2–3 orders of magnitude faster.Both TiO2 solubility and Ti diffusion have important applications in geothermometry, particularly in light of new thermometers calibrated for the incorporation of Ti into quartz and zircon. Rutile saturation is improbable in the types of melts where these thermometers are most likely to be useful, and therefore it is important that rutile solubility behavior in these melts to be well-constrained. TiO2 activities in silicic melts at typical magmatic temperatures are generally 0.6 or higher, implying that Ti thermometry of out-of-context zircons will rarely underestimate zircon crystallization temperature by more than ∼50 °C.

Petrology and Fe-Ti oxide reequilibration of the 1991 Mount Unzen mixed magma

Article

Apr 1999
J VOLCANOL GEOTH RES

A dacitic magma (64.5 wt.% SiO2), a mixture of phenocryst-rich rhyodacite and an aphyric mafic magma, was erupted during the recent 1991–1995 Mount Unzen eruptive cycle. The experimental and analytical results of this study reveal additional details about conditions in the premixing and postmixing magmas, and the nature of the mixing process. The preeruption rhyodacitic magma was at a temperature of 790±20°C according to Fe–Ti oxide phenocryst cores, and at a depth of 6 to 7 km (160 MPa) according to Al-in-hornblende geobarometry. The mafic magma that mixed with the rhyodacite is found as andesitic (54 to 62 wt.% SiO2) enclaves in the erupted magma and was essentially aphyric when intruded. Phase equilibria indicate that an aphyric andesite at 160 MPa is >1030°C (H2O-saturated) and possibly as high as 1130°C (2 wt.% H2O). The composition of the rhyodacite which was mixed with the andesite is estimated to lie between 67 and 69 wt.% SiO2. Using these compositions and temperatures, the temperature of the Unzen magma after mixing is estimated to be at least 850° to 870°C. The groundmass Fe–Ti oxide microphenocrysts and those in pargasite-bearing reaction zones around biotite phenocrysts both give 890±20°C temperatures; the oxide–oxide contacts give temperatures of 910±20°C. The 900±30°C postmixing temperatures are consistent with phase-equilibria experiments which show that the magma was not above 930°C at 160 MPa. Our Fe–Ti oxide reequilibration experiments suggest that the mixing of the two magmas began within a few weeks of the eruption, which is a shorter time than is calculated using available diffusion data. There is also evidence that some mixing took place much closer to the time of extrusion based on the presence of unrimmed biotite phenocrysts in the magma.

Compositional evolution of the zoned calcalkaline magma chamber of Mount Mazama

Article

Users manual for Isoplot/Ex: a geochronological toolkit for Microsoft Excel

Article

Jan 2000

Kenneth R Ludwig

Total half-lives for selected nuclides

Article

Jan 1990
PURE APPL CHEM

N. E. Holden

Measurements of the half-lives of {sup 3}H, {sup 10}Be, {sup 14}C, {sup 26}Al, {sup 40}K, {sup 39}Ar, {sup 53}Mn, {sup 87}Rb, {sup 92}Nb, {sup 129}I, {sup 138}La, {sup 147}Sm, {sup 176}Lu, {sup 174}Hf, {sup 180}Ta, {sup 187}Re, {sup 186}Os, {sup 190}Pt, {sup 204}Pb, {sup 210}Pb, {sup 210}Po, {sup 222}Rn, {sup 224}Th, {sup 226}Ra, {sup 227}Ac, {sup 228}Ra, {sup 228}Th, {sup 230}Th, {sup 232}Th, {sup 231}Pa have been compiled and evaluated. The effect of the {sup 14}C half-life value on carbon dating ages is discussed as well as the stability of {sup 204}Pb. 237 refs., 30 tabs.

The origin of composition gaps at South Sister volcano, central Oregon: implications for fractional crystallization processes beneath active calc-alkaline volcanoes

Article

Nov 2000
J VOLCANOL GEOTH RES

South Sister is an active, calc-alkaline strato-volcano located in west-central Oregon in the high Cascades. South Sister contains lavas that range from basalt (50% SiO2) to rhyolite (75% SiO2). Within this compositional spectrum the lavas are restricted to three distinct populations: basalt/basaltic-andesite, andesite and rhyolite. Compositional, textural and mineralogic criteria indicate that the basaltic-andesites are hybrid mixtures of basalt and andesite magma. This implies that the system is characterized by basalt, andesite and rhyolite magma populations that are separated from one another by composition gaps. Major, minor and trace element modeling suggests that the three magma populations are related to one another primarily through fractional crystallization. We propose a standard fractionation model for generating compositional diversity amongst erupted lavas wherein in situ fractional crystallization occurs within a downward descending solidification front. Extraction and eruption of the interstitial fractionated liquid, now separated from the parental liquid by a composition gap, occurs only after the percentage of crystallization within the solidification front has exceeded the appropriate critical crystallinity value for the given parental magma. We envision a magmatic plumbing system characterized by multiple, independently solidifying bodies of magma. Thus, at any given time, discrete bodies of basalt, andesite and rhyolite magma may exist. Interaction and mixing among these bodies could generate the continuous range of lava compositions that characterize most calc-alkaline volcanic complexes.

Mineral-Melt Partitioning of Uranium, Thorium and Their Daughters

Article

Jan 2003

Jon Blundy

Contrasting punctuated zircon growth in two syn-erupted rhyolite magmas from Tarawera volcano: Insights to crystal diversity in magmatic systems

Article

Jan 2011
EARTH PLANET SC LETT

Two mineralogically and chemically distinct rhyolite magmas (T1 and T3) were syn-erupted from the same conduit system during the 21.9ka basalt intrusion-triggered Okareka eruption from Tarawera volcano, New Zealand. High spatial resolution U–Th disequilibrium dating of zircon crystals at the ~3–5μm scale reveals a protracted yet discontinuous zircon crystallization history within the magmatic system. Both magma types contain zircon whose interiors predate the eruption by up to 200ka. The dominant age peak in the T1 magma is ~30ka with subordinate peaks at ~45, ~75, and ~100ka, whereas the T3 magma has a dominant zircon interior age peak at ~90ka with smaller modes at ~35 and ~150ka. These patterns are consistent with isolated pockets of crystallization throughout the evolution of the system. Crystal rim analyses yield ages ranging from within error of the eruption age to at least ~90ka prior to eruption, highlighting that zircon crystallization frequently stalled long before the eruption. Continuous depth profiling from crystal rims inward demonstrates protracted growth histories for individual crystals (up to ~100ka) that were punctuated by asynchronous hiatuses of up to 30ka in duration. Disparate zircon growth histories can result from localized thermal perturbations caused by mafic intrusions into a silicic reservoir. The crystal age heterogeneity at hand-sample scale requires considerable crystal transport and mixing. We propose that crystal mixing was achieved through buoyancy instabilities caused by mafic magma flow through crystal mush. A terminal pre-eruptive rejuvenation event was capable of mobilizing voluminous melts that erupted, but was too short (

Precision Measurement of Half-Lives and Specific Activities of^{235} U and^{238} U

Article

Nov 1971

New determinations of the half-lives of 235U and 238U have been made. Improved techniques have allowed the half-life values to be measured with greater accuracy than has been heretofore achieved. Samples were prepared by molecular plating and counted in a intermediate-geometry α-proportional counter with an extremely flat pulse-height plateau. The small amount of residual nonplated uranium was counted in a 2π counter. Energy analysis with a silicon-junction detector was used to measure the presence of "foreign" activities. For 235U, the measured specific activity was (4798.1±3.3) (dis/min)/(mg 235U), corresponding to a half-life of (7.0381±0.0048) × 108 yr. For 238U, the specific activity was measured as (746.19±0.41) (dis/min)/(mg 238U), corresponding to a half-life of (4.4683±0.0024) × 109 yr. Errors quoted are statistical (standard error of the mean), based upon the observed scatter of the data. This scatter exceeds that expected from counting statistics alone. We believe that systematic errors, if present, will no more than double the quoted errors.

Uranium Series Accessory Crystal Dating of Magmatic Processes

Article

Apr 2011

Axel K. Schmitt

Complex and protracted crystallization histories over geologic timescales are recorded in accessory minerals (e.g., zircon, allanite). Although magmatic crystallization was traditionally assumed to occur essentially instantaneously for the purposes of interpreting mineral geochronometers with low absolute time resolution for ancient samples, it emerged relatively recently that magmatic crystallization can occur over extended durations. This discovery arose from applying high-spatial-resolution accessory mineral dating techniques for uranium series isotopes to young volcanic and cognate plutonic rocks. The emerging pattern from these studies is that individual crystals and crystal populations record crystallization episodes lasting from

Eruptive history of South Sister, Oregon Cascades

Article

Oct 2011
J VOLCANOL GEOTH RES

South Sister is southernmost and highest of the Three Sisters, three geologically dissimilar stratovolcanoes that together form a spectacular 20km reach along the Cascade crest in Oregon. North Sister is a monotonously mafic edifice as old as middle Pleistocene, Middle Sister a basalt–andesite–dacite cone built between 48 and 14ka, and South Sister is a basalt-free edifice that alternated rhyolitic and intermediate modes from 50ka to 2ka (largely contemporaneous with Middle Sister). Detailed mapping, 330 chemical analyses, and 42 radioisotopic ages show that the oldest exposed South Sister lavas were initially rhyolitic ~50ka. By ~37ka, rhyolitic lava flows and domes (72–74% SiO2) began alternating with radially emplaced dacite (63–68% SiO2) and andesite (59–63% SiO2) lava flows. Construction of a broad cone of silicic andesite–dacite (61–64% SiO2) culminated ~30ka in a dominantly explosive sequence that began with crater-forming andesitic eruptions that left fragmental deposits at least 200m thick. This was followed at ~27ka by growth of a steeply dipping summit cone of agglutinate-dominated andesite (56–60.5% SiO2) and formation of a summit crater ~800m wide. This crater was soon filled and overtopped by a thick dacite lava flow and then by >150m of dacitic pyroclastic ejecta. Small-volume dacite lavas (63–67% SiO2) locally cap the pyroclastic pile. A final sheet of mafic agglutinate (54–56% SiO2) – the most mafic product of South Sister – erupted from and drapes the small (300-m-wide) present-day summit crater, ending a summit-building sequence that lasted until ~22ka. A 20kyr-long-hiatus was broken by rhyolite eruptions that produced (1) the Rock Mesa coulee, tephra, and satellite domelets (73.5% SiO2) and (2) the Devils Chain of ~20 domes and short coulees (72.3–72.8% SiO2) from N–S vent alignments on South Sister's flanks. The compositional reversal from mafic summit agglutinate to recent rhyolites epitomizes the frequently changing compositional modes of the South Sister locus throughout its lifetime. South Sister is part of a reach of the Cascades unusually active in the last 50kyr, characterized by high vent density, N–S vent alignments, and numerous eruptive units of true rhyolite (≥72% SiO2) that distinguishes it from much of the Quaternary Cascade arc; these are eruptive expressions of the complex confluence of arc and intraplate magmatic–tectonic regimes.

Dissolution, growth and survival of zircons during crustal fusion: Kinetic principles, geological models and implications for isotopic inheritance

Article

Jan 1996
EARTH ENV SCI T R SO

Edward Bruce Watson

Finite difference numerical simulations were used to characterise the rates of diffusion-controlled dissolution and growth of zircon in melts of granitic composition under geologically realistic conditions. The simulations incorporated known solubility and Zr diffusivity relationships for melts containing 3 wt% dissolved H 2 O and were carried out in both one and thre dimensions under conditions of constant temperature, linearly time-dependent temperature and for a variety of host system thermal histories. The rate of zircon dissolution at constant temperature depends systematically on time (t½−12;), temperature (exp T ⁻¹ ) and degree of undersaturation of the melt with respect to zircon (in ppm Zr). Linear dissolution and growth rates fall in the range 10 ⁻¹⁹ 10 ⁻¹⁵ cm s ⁻¹ at temperatures of 650-850°C. Radial rates are strongly dependent on crystal size (varying in inverse proportion to the radius, r ): for r>30 μm, dissolution and growth rates fall between 10 ⁻¹⁷ and 10 ⁻¹³ cm s ⁻¹ . During crustal magmatism, the chances of survival for relict cores of protolith zircons depend on several factors, the most important of which are: the initial radius of the zircon; the intensity and duration of the magmatic event; and the volume of the local melt reservoir with which the zircon interacts. In general, only the largest protolith zircons (>120 μm radius) are likely to survive magmatic events exceeding 850°C. Conversely, only the smallest zircons (<50 μm radius) are likely to be completely consumed during low-temperature anatexis (i.e. not exceeding ≍700°C). The effects of stirring the zircon-melt system are unimportant to dissolution and growth behaviour; except under circumstances of extreme shearing (e.g. filter pressing?), zircon dissolution is controlled by diffusion of Zr in the melt.

Time scales of magma storage and differentiation of voluminous high-silica rhyolites at Yellowstone caldera, Wyoming

Article

Dec 2002

Ion microprobe dating of zircons from post-collapse rhyolites at Yellowstone caldera reveals the time scales of crystallization and storage of silicic magma in a differentiating magma reservoir, the role of recycling of crystals from the caldera-forming magmatism, and the timing and efficacy of crystal-melt separation. Zircons in the voluminous (~900 km3) Central Plateau Member lavas, which progressively erupted between 70 to 160 ka, yield 238U-230Th disequilibrium ages dominantly spanning the range from those of their respective eruptions to ~200 ka; mean zircon ages range to ca. 60,000 years before eruption. When considered together with the trace element and Sr- and Nd-isotope compositions of their host melts, the age distributions of the CPM zircons show that the rhyolites are cogenetic and differentiated tens of thousands of years prior to eruption from an evolving magma reservoir. Thus, the post-caldera CPM rhyolites were not erupted from a long-standing body of rhyolitic magma left over from the caldera-forming eruption, nor do they represent significant remobilization of the plutonic roots of the caldera. Rather, the CPM magma was generated and differentiated by episodes of effective crystal-melt separation at ~200 and ~125 ka and, sustained by thermal inputs, stored for timescales on par with estimates for other voluminous caldera-related rhyolites.

Compositional evolution of the zoned calcalkaline magma chamber of Mount Mazama, Crater Lake, Oregon

Article

Feb 1988

The climactic eruption of Mount Mazama has long been recognized as a classic example of rapid eruption of a substantial fraction of a zoned magma body. Increased knowledge of eruptive history and new chemical analyses of ∼350 wholerock and glass samples of the climactic ejecta, preclimactic rhyodacite flows and their inclusions, postcaldera lavas, and lavas of nearby monogenetic vents are used here to infer processes of chemical evolution of this late Pleistocene — Holocene magmatic system. The 6845±50 BP climactic eruption vented ∼50 km3 of magma to form: (1) rhyodacite fall deposit; (2) welded rhyodacite ignimbrite; and (3) lithic breccia and zoned ignimbrite, these during collapse of Crater Lake caldera. Climactic ejecta were dominantly homogeneous rhyodacite (70.4±0.3% SiO2), followed by subordinate andesite and cumulate scoriae (48–61% SiO2). The gap in wholerock composition reflects mainly a step in crystal content because glass compositions are virtually continuous. Two types of scoriae are distinguished by different LREE, Rb, Th, and Zr, but principally by a twofold contrast in Sr content: High-Sr (HSr) and low-Sr (LSr) scoriae. HSr scoriae were erupted first. Trace element abundances indicate that HSr and LSr scoriae had different calcalkaline andesite parents; basalt was parental to some mafic cumulate scoriae. Parental magma compositions reconstructed from scoria wholerock and glass data are similar to those of inclusions in preclimactic rhyodacites and of aphyric lavas of nearby monogenetic vents. Preclimactic rhyodacite flows and their magmatic inclusions give insight into evolution of the climactic chamber. Evolved rhyodacite flows containing LSr andesite inclusions were emplaced between ∼30000 and ∼25000 BP. At 7015±45 BP, the Llao Rock vent produced a zoned rhyodacite pumice fall, then rhyodacite lava with HSr andesite inclusions. The Cleetwood rhyodacite flow, emplaced immediately before the climactic eruption and compositionally identical to climactic rhyodacite (volatile-free), contains different HSr inclusions from Llao Rock. The change from LSr to HSr inclusions indicates replenishment of the chamber with andesite magma, perhaps several times, in the latest Pleistocene to early Holocene. Modeling calculations and wholerock-glass relations suggest than: (1) magmas were derived mainly by crystallization differentiation of andesite liquid; (2) evolved preclimactic rhyodacite probably was derived from LSr andesite; (3) rhyodacites contain a minor component of partial melt from wall rocks, and (4) climactic and compositionally similar rhyodacites probably formed by mixing of evolved rhyodacite with HSr derivative liquid(s) after replenishment of the chamber with HSr andesite magma. Density considerations permit a model for growth and evolution of the chamber in which andesite recharge magma ponded repeatedly between cumulates and rhyodacite magma. Convective cooling of this andesite resulted in rapid crystallization and upward escape of buoyant derivative liquid which mixed with overlying, convecting rhyodacite. The evolved rhyodacites were erupted early in the chamber's history and(or) near its margins. Postcaldera andesite lavas may be hybrids composed of LSr cumulates mixed with remnant climactic rhyodacite. Younger postcaldera rhyodacite probably formed by fractionation of similar andesite and assimilation of partial melts of wallrocks. Uniformity of climactic rhyodacite suggests homogeneous silicic ejecta from other volcanoes resulted from similar replenishment-driven convective mixing. Calcalkaline pluton compositions and their internal zonation can be interpreted in terms of the Mazama system frozen at various times in its history.

Petrogenesis and 230Th-238U disequilibrium at Mt. Shasta, California, and in the Cascades

Article

Jun 1986

Petrogenesis at Mt. Shasta is dominated by mixing of magmas and/or assimilation of wall rock, as is shown by petrographic, major and trace element chemistry, and 238U-230Th disequilibrium data. At least three end- members are involved in these mixing processes. Lavas of very young Cascades lavas, from Mt. Garibaldi in the north to Lassen Peak in the south, are characterized by a large range of thorium isotopic ratios, although series of samples from single volcanoes are characterized by approximately constant (230Th/232Th). There is a monotonic decrease in this ratio from Crater Lake south through Lassen Peak, perhaps reflecting increasing thickness of the underlying crust. Th/U fractionation in Cascades lavas, as evidenced by (230Th/238U) 1, is in the opposite sense to that in most island arc lavas. This trend suggests that fluid transport, which is thought to produce uranium enrichment in island arc, is lacking or somehow modified in the petrogenesis of the Cascades lavas.

Silicic recharge of multiple rhyolite magmas by basaltic intrusion during the 22.6 ka Okareka Eruption Episode, New Zealand

Article

Jul 2008
LITHOS

Constraints on the nature of the subvolcanic reservoir at South Sister volcano, Oregon from U-series dating combined with sub-crystal trace-element analysis of plagioclase and zircon

Abstract

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