[Show abstract][Hide abstract] ABSTRACT: Recycling of marine volatiles back into the mantle at subduction zones has a profound, yet poorly constrained impact on the geochemical evolution of the Earth’s mantle. Here we present a combined noble gas and halogen study on mantle xenoliths from the Western Antarctic Rift System (WARS) to better understand the flux of subducted volatiles to the sub continental lithospheric mantle (SCLM) and assess the impact this has on mantle chemistry. The xenoliths are extremely enriched in the heavy halogens (Br and I), with I concentrations up to 1 ppm and maximum measured I/Cl ratios (85.2 × 10−3) being ∼2000 times greater than mid ocean ridge basalts (MORB). The Br/Cl and I/Cl ratios of the xenoliths span a range from MORB-like ratios to values similar to marine pore fluids and serpentinites, whilst the 84Kr/36Ar and 130Xe/36Ar ratios range from modern atmosphere to oceanic sediments. This indicates that marine derived volatiles have been incorporated into the SCLM during an episode of subduction related metasomatism. Helium isotopic analysis of the xenoliths show average 3He/4He ratios of 7.5 ± 0.5 RA (where RA is the 3He/4He ratio of air = 1.39 × 10−6), similar to that of MORB. The 3He/4He ratios within the xenoliths are higher than expected for the xenoliths originating from the SCLM which has been extensively modified by the addition of subducted volatiles, indicating that the SCLM beneath the WARS must have seen a secondary alteration from the infiltration and rise of asthenospheric fluids/melts as a consequence of rifting and lithospheric thinning. Noble gases and halogens within these xenoliths have recorded past episodes of volatile interaction within the SCLM and can be used to reconstruct a tectonic history of the WARS. Marine halogen and noble gas signatures within the SCLM xenoliths provide evidence for the introduction and retention of recycled volatiles within the SCLM by subduction related metasomatism, signifying that not all volatiles that survive subduction are mixed efficiently through the convecting mantle. The global SCLM therefore represents a potentially important reservoir for the long term residence of subducted volatiles.
Full-text · Article · Mar 2016 · Geochimica et Cosmochimica Acta
[Show abstract][Hide abstract] ABSTRACT: Over the last ~267 ky, the island of Lipari has erupted magmas ranging in compositions from basaltic andesites to rhyolites, with a notable compositional gap in the dacite field. Bulk geochemical and isotopic compositions of the volcanic succession, in conjunction with major and trace elemental compositions of minerals, indicate that the rhyolites were dominantly generated via crystal fractionation processes, with subordinate assimilation. Radiogenic (Sr, Nd, and Pb) and stable (O) isotopes independently suggest ≤30 % of crustal contamination with the majority of it occurring in mafic compositions, likely relatively deep in the system. Within the rhyolites, crystal-rich, K2O-rich enclaves are common. In contrast to previous interpretations, we suggest that these enclaves represent partial melting, remobilization and eruption of cumulate fragments left-over from rhyolite melt extraction. Cumulate melting and remobilization is supported by the presence of (1) resorbed, low-temperature minerals (biotite and sanidine), providing the potassic signature to these clasts, (2) reacted Fo-rich olivine, marking the presence of mafic recharge, (3) An38–21 plagioclase, filling the gap in feldspar composition between the andesites and the rhyolites and (4) strong enrichment in Sr and Ba in plagioclase and sanidine, suggesting crystallization from a locally enriched melt. Based on Sr-melt partitioning, the high-Sr plagioclase would require ~2300 ppm Sr in the melt, a value far in excess of Sr contents in Lipari and Vulcano magmas (50–1532 ppm) but consistent with melting of a feldspar-rich cumulate. Due to the presence of similar crystal-rich enclaves within the rhyolites from Vulcano, we propose that the eruption of remobilized cumulates associated with high-SiO2 rhyolites may be a common process at the Aeolian volcanoes, as already attested for a variety of volcanic systems around the world.
Full-text · Article · Dec 2015 · Contributions to Mineralogy and Petrology
[Show abstract][Hide abstract] ABSTRACT: Pyroxenites and garnet pyroxenites are mantle heterogeneities characterized by a lower solidus temperature than the enclosing peridotites; it follows that they are preferentially involved during magma genesis. Constraining their origin, composition, and the interactions they underwent during their subsequent evolution is therefore essential to discuss the sources of magmatism in a given area. Pyroxenites could represent either recycling of crustal rocks in mantle domains or mantle originated rocks (formed either by olivine consuming melt-rock reactions or by crystal fractionation). Petrological and geochemical (major and trace elements, Sr-Nd and O isotopes) features of xenoliths from various occurrences (French Massif-Central, Jordan, Morocco and Cameroon) show that these samples represent cumulates crystallized during melt percolation at mantle conditions. They formed in mantle domains at pressures of 1-2 GPa during post-collisional magmatism (possibly Hercynian for the French Massif-Central, and Panafrican for Morocco, Jordan and Cameroon). The thermal re-equilibration of lithospheric domains, typical of the late orogenic exhumation stages, is also recorded by the samples. Most of the samples display a metasomatic overprint that may be either inherited or likely linked to the recent volcanic activity that occurred in the investigated regions.
The crystallization of pyroxenites during late orogenic events has implications for the subsequent evolution of the mantle domains. The presence of large amounts of mantle pyroxenites in old orogenic regions indeed imparts peculiar physical and chemical characteristics to these domains. Among others, the global solidus temperature of the whole lithospheric domain will be lowered; in turn, this implies that old orogenic regions are refertilized zones where magmatic activity would be enhanced.
[Show abstract][Hide abstract] ABSTRACT: The oxygen isotope compositions of minerals of peridotite xenoliths from the Subei basin, Eastern China were investigated to detect a possible crustal signature in mantle rocks. The δ18O values of olivine (ol), orthopyroxene (opx) and clinopyroxene (cpx) vary from 5.10 to 5.98‰, 5.46 to 6.78‰, and 5.06 to 6.31‰, respectively. Olivine and opx have δ18O values similar to-, or above the “expected” mantle range, whereas the δ18O values of cpx range from higher to lower than average mantle. These characteristics are inherited from the peridotite mantle source, and likely result from mantle domains that experienced fluid-assisted metasomatism. Coexisting high δ18O values of ol and opx and low δ18O values of cpx are unlikely to result from selective interactions with the fluids with different δ18O values during a single metasomatic event. The measured O-isotope values can be reconciled with a simple two-stage metasomatic model characterized by an early high-δ18O fluid/peridotite interaction followed by a late low-δ18O fluids infiltration. Model calculations of mineral diffusivities predict that the negative Δ18Ocpx-ol values are unlikely to be preserved at mantle conditions for time interval > 80 Ma. Provided the investigated mantle xenoliths were entrained and brought to surface by ~ 9 Ma basaltic eruptions, this also limits the occurrence of the late metasomatic event, and the subducting Pacific plate is the best candidate to provide such fluids.
[Show abstract][Hide abstract] ABSTRACT: In this study, Mg/Ca, Sr/Ca and Ba/Ca ratios in a Lateglacial to Holocene stalagmite (CC26) from Corchia Cave (central Italy) are compared with stable isotope data to define palaeohydrological changes. For most of the record, the trace element ratios show small absolute variability but similar patterns, which are also consistent with stable isotope variations. Higher trace element-to-calcium values are interpreted as responses to decreasing moisture, inducing changes in the residence time of percolation, producing prior calcite precipitation and/or variations in the hydrological routing. Statistically meaningful levels of covariability were determined using anomalies of Mg/Ca, δ18O and δ13C. Combining these three time series into a single ‘palaeomoisture-trend’ parameter, we highlight several events of reduced moisture (ca. 8.9–8.4, 6.2, 4.2, 3.1 and 2.0 ka), a humid period between ca. 7.9 and 8.3 ka and other shorter-term wet events at ca. 5.8, 5.3 and 3.7 ka. Most of these events can be correlated with climate changes inferred from other regional studies. For both extremities of the record (i.e. before ca. 12.4 ka and after ca. 0.5 ka) Mg/Ca and Sr/Ca are anti-correlated and show the greatest amplitude of values, a likely explanation for which involves aragonite and/or gypsum precipitation (the latter derived from pyrite oxidation) above the CC26 drip point.
Full-text · Article · May 2014 · Journal of Quaternary Science
[Show abstract][Hide abstract] ABSTRACT: The gabbroic bodies from the Jurassic Ligurian ophiolites are structurally and compositionally similar to the gabbroic sequences from the oceanic core complexes of the Mid Atlantic Ridge. Initial cooling of the Ligurian gabbros is associated with local development of hornblende-bearing felsic dykes and hornblende vein networks. The hornblende veining is correlated with the widespread development of hornblende as coronas/pseudomorphs after the clinopyroxene in the host gabbros. In addition, the studied gabbroic body includes a mantle sliver locally containing hornblende gabbros and hornblendite veins. The hornblendes from the felsic dykes and the hornblende-rich rocks within the mantle sliver show a similar geochemical signature, characterized by low Mg#, CaO and Al2O3, negligible Cl, and high TiO2, K2O, REE, Y, Zr and Nb concentrations. The whole-rock Sm–Nd isotopic compositions of the felsic dykes and the hornblende-rich rocks define a Sm–Nd isochron corresponding to an age of 154 ± 20 Ma and an initial εNd of 9.2 ± 0.5. The δ18O of the hornblendes and coexisting zircons from these rocks (about +4.5‰ and +5.8‰, respectively) do not indicate the presence of a seawater component in these melts. The formation of the felsic dykes and of the hornblende-rich rocks within the mantle sliver involved SiO2-rich silicate melts with negligible seawater component, which presumably were derived from high degree fractional crystallization of MOR-type basalts. The vein and the coronitic/pseudomorphic hornblendes show high Mg# and CaO, significant Cl (0.02–0.17 wt%) and low TiO2 and K2O concentrations. The coronitic/pseudomorphic hornblendes have trace element compositions similar to those of the clinopyroxenes from the gabbros and δ18O values (+1.0‰ to 0.7‰) close to seawater, suggesting an origin by reaction between migrating seawater-derived fluids and the host gabbros. The vein hornblendes commonly show slight LREE enrichment, relatively high concentrations of Nb (up to 2.5 ppm) and δ18O ranging from +3.7‰ to +0.8‰. The crystallization of these hornblendes most likely involved both seawater and magmatic components.
[Show abstract][Hide abstract] ABSTRACT: Fluids control the mechanical behavior of fault zones during the seismic cycle. We used geochemical, mineralogical, microstructural, hydrogen isotope compositions and Fourier Transform Infrared (FTIR) investigations to characterize the origin of hydrous fluids involved in ductile and brittle shear zones at the bottom of the seismogenic crust. Natural samples were collected from exhumed mylonitic shear zones and cataclasite-pseudotachylyte bearing faults in the northern Adamello (Italian Southern Alps), which were active at 9-11 km depth. Pseudotachylytes, solidified coseismic friction-induced melts, testify to ancient seismogenic behavior of the faults. Natural pseudotachylytes were compared with artificial pseudotachylytes produced in high velocity friction experiments simulating seismic slip.
Full-text · Article · Jan 2014 · Earth and Planetary Science Letters
[Show abstract][Hide abstract] ABSTRACT: Mantle peridotites from an exposed lithospheric section (Vema Lithospheric Section, VLS), generated during ~ 26 Ma at a ~ 80 km long Mid Atlantic Ridge segment (11° N), have been sampled and studied to understand the evolution of the serpentinization process. The VLS was uplifted due to a 10 Ma transtensional event along the Vema transform. Before the uplift residual mantle rocks were lying beneath a 0.8–1.3 km thick basaltic crustal layer. The major and trace element compositions of the serpentinites, as well as their H, O, Sr, Cl and B isotopic compositions were interpreted based on thermal models of lithospheric spreading from ridge axis. The results suggest that serpentinization occurred mostly near the ridge axis. Serpentinization temperatures, estimated from stable isotopes, are consistent with resetting of the closure temperatures during the tectonic uplift of the lithospheric sliver, reflected by decreasing δ18O and increasing δ11B values. Modeling shows that the thermal influence of the transtensional event affected mainly the region close to the RTI (ridge–transform intersection). Petrological, elemental and isotopic data suggest that, when the ultramafic basal unit of the VLS was uplifted and exposed on the ocean floor, serpentinization became superseded by low temperature water–rock reactions, with Fe–Mn crust formation, which is still progressing, as recorded by δD. Ultramafic mylonites, prevalent in a short stretch of the VLS, show only a partial serpentinization process, together with pervasive contamination by low-temperature Fe–Mn crust.
[Show abstract][Hide abstract] ABSTRACT: Ischia, one active volcano of the Phlegraean Volcanic District, prone to very high risk, is dominated by a caldera formed 55 ka BP, followed by resurgence of the collapsed area. Over the past 3 ka, the activity extruded evolved potassic magmas; only a few low-energy explosive events were fed by less evolved magmas. A geochemical and Sr–Nd–O isotope investigation has been performed on minerals and glass from products of three of such eruptions, Molara, Vateliero, and Cava Nocelle (<2.6 ka BP). Data document strong mineralogical, geochemical, and isotopic heterogeneities likely resulting from mingling/mixing processes among mafic and felsic magmas that already fed the Ischia volcanism in the past. Detailed study on the most mafic magma has permitted to investigate its origin. The mantle sector below Ischia underwent subduction processes that modified its pristine chemical, isotopic, and redox conditions by addition of ≤1 % of sediment fluids/melts. Similar processes occurred from Southeast to Northwest along the Apennine compressive margin, with addition of up to 2.5 % of sediment-derived material. This is shown by volcanics with poorly variable, typical δ18O mantle values, and 87Sr/86Sr progressively increasing toward typical continental crust values. Multiple partial melting of this modified mantle generated distinct primary magmas that occasionally assimilated continental crust, acquiring more 18O than 87Sr. At Ischia, 7 % of Hercynian granodiorite assimilation produced isotopically distinct, K-basaltic to latitic magmas. A SW–NE regional tectonic structure gave these magmas coming from large depth the opportunity to mingle/mix with felsic magmas stagnating in shallower reservoirs, eventually triggering explosive eruptions.
Full-text · Article · Jun 2013 · Contributions to Mineralogy and Petrology
[Show abstract][Hide abstract] ABSTRACT: The Alpine region currently experiences complex climatic conditions
(Efthymidias et al., 2007). Such a complexity is also apparent during
the Holocene, as revealed by the isotopic composition of speleothems
(e.g. Spötl et al., 2010). However, virtually no speleothem data
are available from the western Alps, which are dominated by North
Atlantic synoptic systems. With this in mind, several flowstone cores
were retrieved in Rio Martino Cave (Piemonte, Northern Italy, ca. 1530 m
asl). A large part of the cave's catchment was substantially glaciated
until the end of the Younger Dryas. U/Th dating of three different
flowstones indicates that deposition started at the beginning of the
Holocene and covered a thick deposit of cemented sand and gravels
probably related to the last deglaciation. The combined δ18O
record of different flowstones shows substantial variability through the
Holocene, both on millennial and century time scales. Relatively low
δ18O values last until ca. 6 ka. From 6 to 3 ka, the δ18O
increases gradually before decreasing again from 3 ka to the present.
This long-term trend may be related to changes in the seasonal patterns
of precipitation, as suggested for the lake level record at Ledro (Magny
et al., 2012). Superimposed on this trend there are several short-term
oscillations which may reflect alternating periods of drier and wetter
conditions. Some of these episodes are also in agreement with changes
documented at Ledro. Efthymidias D., et al., 2007. Influence of
large-scale atmospheric circulation on climate variability in the
Greater Alpine Region of Europe. Journal of Geophysical Research, 112,
D12104. Magny M. et al., 2012. Holocene palaeohydrological changes in
the northern Mediterranean borderlands as reflected by the lake-level
record of Lake Ledro, northeastern Italy. Quaternary Research, 77,
382-396 Spötl, C., et al., 2010. Humid climate during deposition
of sapropel 1 in the Mediterranean Sea: Assessing the influence on the
Alps. Global and Planetary Change 71, 242-248.
[Show abstract][Hide abstract] ABSTRACT: We have investigated lava flows representative of the whole eruptive history of the Colli Albani ultrapotassic volcanic district (Central Italy). One of the most intriguing features concerning some of these lava flows is the occurrence of primary, magmatic calcite in the groundmass. The primary, magmatic nature of calcite has been inferred by microtextural investigations showing that it typically occurs i) interstitially, associated with clinopyroxene, nepheline and phlogopite, ii) in spherical ocelli, associated with nepheline, fluorite and tangentially arranged clinopyroxene, and iii) in corona-like reaction zones around K-feldspar xenocrysts. These microtextural features distinctly indicate that calcite crystallized from a carbonate melt in a partially molten groundmass, implying that the temperature of the system was above the solidus of the hosted lava flow (> 850 °C). Geochemical features of calcite crystals (i.e., stable isotope values and trace element patterns) corroborate their primary nature and give insights into the origin of the parental carbonate melt. The trace element patterns testify to a high-temperature crystallization process (not hydrothermal) involving a carbonate melt coexisting with a silicate melt. The high δ18O (around 27‰ SMOW) and wide δ13C (-18 to + 5‰ PDB) values measured in the calcites preclude a mantle origin, but are consistent with an origin in the crust. In this framework, the crystallization of calcite can be linked to the interaction between magmas and carbonate-bearing wall rocks and, in particular, to the entrapment of solid and/or molten carbonate in the silicate magma. The stability of carbonate melt at low pressure and the consequent crystallization of calcite in the lava flows groundmass are ensured by the documented, high activity of fluorine in the studied system and by the limited ability of silicate and carbonate melts to mix at syn-eruptive time scales.
[Show abstract][Hide abstract] ABSTRACT: Obtaining meaningful redox potential determinations is questionable in disequilibrium solutions, like surface waters and groundwater. However, identifying redox processes in a natural ecosystem is a key factor to characterize the chemical quality and, consequently, the sustainability of that environment. To do that, the determination of an exact redox potential is not needed and on many cases it is more important to describe how and if the redox state of a system changes. In particular, the redox status of aqueous systems is assigned based on analysis of samples for redox-sensitive compounds. In general, these analyses do not include measurements of gases dissolved in the waters.Here, we present an innovative method to underline in an easy way changes in redox state of a system using the elemental composition (C, H, S) measured in the headspace gas after the equilibration with water phase. The results allow defining the empirical formula of the gas, the mean number of oxidation of the carbon and the molar ratio between C:H:S. The method is applied to investigate the redox condition in a stratified lake, San Rocchino (Northern Tuscany, Italy) using an instrument (Thermo Scientific FlashEA 1112) equipped for headspace gas analysis. Samples of waters and dissolved gases were collected at five different depths along a vertical profile. Such analytical system is used to determine the mean oxidation number of the carbon gaseous phase in the headspace, which allows distinguishing relatively oxidizing from relatively reducing condition. Further information on the redox state is reached considering the molar ratio C/H and C/S.This innovative method is simple and rapid, and the instrument is robust and it needs low maintenance. Moreover, it could be applied to other research and exploratory fields, such as geothermic and volcanic monitoring, polluted aquifer and contaminated sites.
No preview · Article · Jan 2013 · Journal of Geochemical Exploration
[Show abstract][Hide abstract] ABSTRACT: Crystal-rich lithic clasts occurring in volcanic deposits are key tools to understand processes of storage, cooling, and fractionation of magmas in pre-eruptive volcanic systems. These clasts represent snapshots of the magma-chamber-host-rock interface before eruption and provide information on crystallization, differentiation, and degrees of interaction between magma and wall-rock. In this study we have focused on the petrology of clasts of cumulate and skarn rocks from the Colli Albani Volcanic District with the aim of shedding light on magma-carbonate interaction and CO2 emission in volcanic areas. By means of phase relations, bulk-rock chemistry, mineral compositions and isotope data we have identified different types of cumulates and skarns. Cumulates containing either clinopyroxene +/- olivine associated with Cr-bearing spinel or glass + phlogopite have been classed as primitive and differentiated, respectively. Cumulates originate at the interface between either a primitive or differentiated magma and carbonate-bearing wall-rock characterized by the occurrence of CaO-rich melt. Skarns have been classed as exoskarns, characterized by xenomorphic textures and abundant calcite, and endoskarns, characterized by a hypidiomorphic texture, Ca-Tschermak-rich mineral phases, and interstitial glass. Exoskarns formed by means of solid-state reactions in a dolomite-bearing protolith whereas endoskarns crystallized from a silicate melt that experienced exoskarn assimilation. Our study indicates that magma-carbonate interaction is a multi-step process that proceeds beyond the formation of skarn shells. Magma and carbonate rocks, when in contact, continuously interact leading to the formation of exoskarns, endoskarns, cumulates (primitive and differentiated types), and differentiated melts. The geochemical characteristics of the studied endoskarn and cumulate rocks indicate that crustal contamination of the Colli Albani magmas occurs through the simultaneous assimilation of both solid crustal material (dolomite and/or exoskarns) and partially molten crustal material (CaO-rich melt). The oxygen and carbon isotope compositions of calcite in equilibrium with the skarns suggest that dolostone-limestone assimilation and decarbonation are able to provide the massive CO2 release observed in carbonate-hosted magmatic systems, such as Colli Albani.
No preview · Article · Oct 2012 · Journal of Petrology
[Show abstract][Hide abstract] ABSTRACT: Calcite crystals are relatively common in alkalic hypoabyssalite and kimberlite rocks, but rarely documented in effusive rocks (e.g. alkaline lava flows). Magmatic calcite in effusive rocks has been usually related to mantle-origine carbonate; only in very few cases, it has been explained throughout magma-sediments interaction. The ultrapotassic Colli Albani Volcanic District (Central Italy) represents one of the most peculiar volcanic districts on the Earth because of its liquid line of descent characterized by differentiated, low silica (SiO2≤45wt%), K-foiditic magmas. Field, geochemical, and experimental studies have demonstrated that such a differentiation trend, starting from trachybasaltic parental magma, is mainly due to magma-carbonate sediments interaction. One of the most intriguing question concerning Colli Albani petrology is the occurrence of calcite crystals in the groundmass of some lava flows. In general, Colli Albani lava flows are made up of leucite and clinopyroxene phenocrysts and the groundmass contains leucite, clinopyroxene, and Ti-magnetite. More evolved products may also contain calcite, usually associated with nepheline. A detailed microtextural study of these calcite-bearing lava flows has shown that calcite occurs as follows: i) interstitially, associated with clinopyroxene, nepheline and phlogopite; ii) in ocelli, associated with fluorite and tangentially arranged clinopyroxene; iii) in coronitic reaction zone around K-feldspar xenocrysts. These microtextural features clearly indicate that calcite crystallized under magmatic conditions. Moreover, the high 18 O (25-29‰ SMOW) and low 13 C (down to -19‰ PDB) values of calcite crystals prove the sedimentary origin of the carbonate involved in the process. Finally, the occurrence of limestone fragments in the lava flows accounts for a syn-eruptive assimilation of the carbonate sediments. The high activity of fluorine in the Colli Albani magmas, as demonstrated by the occurrence of F-rich mineral phases (i.e, amphibole and mica), associated with fast crystallization due to the low magma viscosity, can have played a central role on the subsistence of sedimentary carbonate melt (then calcite crystals) at atmospheric pressure. This study may help unravelling the formation processes of the so-called "pseudocarbonatites", i.e. carbonated rocks related to the anatexis of crustal limestone.
[Show abstract][Hide abstract] ABSTRACT: An oxygen isotope record of Adélie penguin (Pygoscelis adeliae) eggshells from Terra Nova Bay (Victoria Land, Antarctica) has been reconstructed in order to provide insights into the palaeoenviroment of this region during the last ~8000 years. The δ18O values measured in eggshell carbonate show a great variability over the course of the studied period, with maximum values consistently around +27.5‰. Adopting a metabolic enrichment relative to water source of ~+28.5‰, eggshell δ18O values can be interpreted in terms of marine water uptake by the penguins. The spread in δ18O of eggshell carbonate at any given time toward more negative values reflects the existence of a low δ18O water end member, most plausibly snow or snowmelt. Samples younger than 2000 years are characterized by a general lack of very negative δ18O values. As also attested by other proxies, this shift toward more positive isotopic values is likely to be related both to a decreasing contribution of snow or meltwater in the seawaters and consequently to an absence or paucity on the boundary of the nesting sites of snow or snowfall and freshwater that could be directly eaten or drunk by penguins.
[Show abstract][Hide abstract] ABSTRACT: Keywords: stable-isotope magma geochemistry CO 2 -degassing Vesuvius Mafic phenocrysts from selected products of the last 4 ka volcanic activity at Mt. Vesuvius were investigated for their chemical and O-isotope composition, as a proxy for primary magmas feeding the system. 18 O/ 16 O ratios of studied Mg-rich olivines suggest that near-primary shoshonitic to tephritic melts experienced a flux of sedimentary carbonate-derived CO 2 , representing the early process of magma contamination in the roots of the volcanic structure. Bulk carbonate assimilation (physical digestion) mainly occurred in the shallow crust, strongly influencing magma chamber evolution. On a petrological and geochemical basis the effects of bulk sedimentary carbonate digestion on the chemical composition of the near-primary melts are resolved from those of carbonate-released CO 2 fluxed into magma. An important outcome of this process lies in the effect of external CO 2 in changing the overall volatile solubility of the magma, enhancing the ability of Vesuvius mafic magmas to rapidly rise and explosively erupt at the surface.
Full-text · Article · Oct 2011 · Earth and Planetary Science Letters
[Show abstract][Hide abstract] ABSTRACT: We present the first Antarctic terrestrial record of climate variations through the Cenozoic, based on the hydrogen isotope composition of hydrothermally altered minerals of intrusive rocks. This new record provides an independent geochemical proxy for continental climatic conditions; whereas, most land surface temperature proxies are biological. The temperature record is consistent with the range predicted by global climate models and proxy records for glacial and pre-glacial conditions in the Ross Sea region of Antarctica. The combined stable isotope (O and H) and age (Ar-40-Ar-39) determinations of hydrous mineral from Cenozoic igneous plutons and dikes show that the protracted time scale of magmatic activity and extensive hydrothermal exchange with local meteoric waters has preserved a semiquantitative climate signal of intervals in which atmospheric temperatures significantly fluctuated. These data also reveal that glacial episodes comparable with current polar conditions occurred repeatedly prior to geographical and thermal isolation of the Antarctic continent.
[Show abstract][Hide abstract] ABSTRACT: Among the abundant mantle xenoliths carried by Cenozoic alkaline basalts of northern Victoria Land (Antarctica), we studied a sub-continuous suite of clinopyroxene-rich cumulates collected at Browning Pass (Mt. Melbourne Volcanic Province), ranging in composition from wehrlites to clinopyroxenites. Clinopyroxenes belonging both Cr-diopside (wehrlites) and Al-augite series (ol-clinopyroxenites and clinopyroxenites) all show convex upward shapes of REE patterns. Modal and cryptic metasomatism has variably affected the xenoliths, accounting for amphibole replacement of clinopyroxenes and/or selective enrichment in incompatible elements. Chemical features, along with O-Sr-Nd isotopic data, indicate that both parental magmas and metasomatizing melts are related to the Cenozoic magmatic activity and imply the role of at least two mantle components with distinct isotopic fingerprints. The positive covariation between the δ18Oolivine and the amount of modal olivine, and between the δ18Oolivine and olivine Fo-content, suggest that during the fractionation of olivine and pyroxene, the source magma experienced a change of O-isotope composition thereby implying that a low-δ18O melt component was added to minerals not only during the metasomatic event but is also involved in the genesis of parental melts. Browning Pass cumulates therefore constrain the origin of the Antarctic Cenozoic magmatism from a heterogeneous mantle source whose depleted end member is the local lithospheric mantle, while the enriched end member is represented by early metasomatic veins or domains emplaced into the depleted mantle during the amagmatic phase of rifting at the beginning of Ross Sea opening. Thermobarometric analysis of the process shows that the respective contribution to the magma generation is related to the change of local thermal regime induced by an “edge effect” in the mantle circulation at the lithospheric step between the thick East Antarctic craton and the thinned Ross Sea crust.
No preview · Article · Feb 2011 · Journal of Petrology