[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.
Journal of Quaternary Science 05/2014; 29(4):381-392. · 2.66 Impact Factor
[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.
Geochimica et Cosmochimica Acta 04/2014; 130:188–211. · 4.25 Impact Factor
[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.
Mylonites have mineralogical, elemental and hydrogen isotope compositions (−80‰<δD<−78‰−80‰<δD<−78‰) similar to the host tonalite (−77‰<δD<−73‰−77‰<δD<−73‰), within the analytical error of ±5‰±5‰. Cataclasites have instead mineralogical (chlorite, epidote, K-feldspar, no biotite), major and trace elements (enrichment in K2O, Ba, Rb; depletion in CaO, Na2O, SiO2) and hydrogen isotope (−69‰<δD<−60‰−69‰<δD<−60‰) compositions suggesting interactions with a crustal metamorphic fluid. Pseudotachylytes are composed of high temperature minerals (plagioclase, biotite, dmisteinbergite, cordierite, and scapolite) and have elemental compositions resulting from mixing of tonalite and cataclasite. Pseudotachylytes have complex microstructures, including: (i) microlitic domains, with well crystallized micrometric biotite, which have hydrogen isotope composition (−81‰<δD<−59‰−81‰<δD<−59‰) similar to cataclasites and tonalite; and (ii) cryptocrystalline domains, with poorly crystallized biotite, which have very high water content, release water upon heating at T>50°C and have low δD value (−93‰−93‰). The hydrogen isotope composition of bulk samples is dominated by the composition of cryptocrystalline domains (−103‰<δD<−88‰−103‰<δD<−88‰), where most of the water is hosted. Their hydrogen isotope composition is compatible with adsorption of present day rainfall water (δD=−95‰δD=−95‰). Artificial pseudotachylytes have the same hydrogen isotope compositions of the starting tonalite (−76‰<δD<−74‰−76‰<δD<−74‰) or cataclasite (−68‰<δD<−62‰−68‰<δD<−62‰), with a slight decrease of the δD values in some samples (−85‰<δD<−81‰−85‰<δD<−81‰).
The first ingression of a crustal metamorphic fluid occurred in cataclastic faults. Natural pseudotachylytes, when not contaminated by present day rainfall water, have a hydrogen isotope composition similar to tonalite and cataclasite, as reproduced in dry high velocity friction experiments. The fluids dissolved in coseismic melts are most likely derived from the breakdown of hydrous minerals of cataclasite and tonalite undergone melting, and we could not identify the infiltration of an external fluid during earthquakes.
[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.
Contributions to Mineralogy and Petrology 06/2013; 165(6):1173-1194. · 3.48 Impact Factor
[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.
Journal of Geochemical Exploration 01/2013; 124:14–21. · 2.43 Impact Factor
[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.
The Holocene 01/2012; 22(1):63-69. · 3.79 Impact Factor
[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.
Earth and Planetary Science Letters 10/2011; · 4.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: INTRODUCTION Climate changes dramatically affect the environment and, as such, they are recorded in different geological materials. World climate evolution and temperature variations of oce-anic bottom water throughout the past 50 m.y. have been reconstructed primarily using the oxygen and carbon isotope records of foramin-ifera from deep-sea cores (Miller et al., 1987; Zachos et al., 2001), as the δ 18 O value of sea-water is primarily controlled by atmospheric temperatures and global volume of ice. Much less is known in terms of continental climate evolution prior to the widespread ice sheet ini-tiation and expansion at the Eocene-Oligocene transition. The absence or poor preservation of terrestrial outcrops able to preserve a tem-perature record means that it is has not been possible to verify independently the marine isotope record. In this paper we evaluate the paleoclimate signal derived from a sequence of magmatic intrusions emplaced between 50 and 25 Ma along the Ross Sea coastline, northern Victoria Land, Antarctica. Our work applies a well-established technique for determining the oxygen and isotope composition of surface meteoric waters in the geologic past, in order to explore the continental climatic conditions and the glacial state of Antarctica during the Cenozoic. Combining stable isotope ratios with 40 Ar/ 39 Ar ages of hydrous minerals from Tertiary plutons from the Ross Sea coastline, northern Victoria Land, we trace the stable isotope composition of local meteoric waters at the times the rocks were emplaced (from 50 to 25 Ma), and semiquantitatively estimate the variation of land-based temperature. Earlier studies reported low 18 O/ 16 O ratios of hydrothermally altered Mesozoic and Ceno-zoic plutons from North Atlantic, South Pacifi c, and polar regions, demonstrating how altered plutonic rocks may be used to develop a cli-mate and environmental proxy record (Nevle et al., 1994; Blattner et al., 1997; Dallai et al., 2001). However, all rocks previously studied were intruded over relatively short time spans, limiting their use for systematic paleoclimate reconstructions. Our new paleoclimate record has been constructed on the basis of more than 40 samples covering a time interval of ~25 m.y. These new data appear to hold a low temporal resolution continental record of paleotempera-ture oscillations in Antarctica, the fi rst terrestrial record to infer long-term (>1 m.y. resolution) events where high-latitude deep-sea cores are absent and/or sequences of Cenozoic continen-tal deposits are discontinuous. GEOLOGICAL SETTING AND ROCK TYPES Alkaline magmatism related to the rifting in the Ross Sea Embayment intruded the north-eastern region of the Wilson terrane, northern Victoria Land, throughout the Cenozoic (Fig. 1; Muller et al., 1991; Rocchi et al., 2002). Most of these plutonic rocks were emplaced at shal-low crustal depths (pressure, P <3 kbar) in frac-tured permeable country rocks, thereby allow-ing development of meteoric-hydrothermal systems. The dominant lithotype is an amphi-bole-bearing syenite often associated with volu-metrically subordinate monzonite and mon-zogabbros, and bimodal (trachytic to basaltic) dikes. Mineralogically, the rocks are composed of large K-feldspar crystals, abundant quartz, amphibole, minor biotite, and accessory phases (opaques, apatite, sphene). Amphibole is Mg-hastingsitic to Fe-edenitic (mineral grains are slightly zoned with Al-, Fe-, and Na-enriched rims), and kaersutitic in mafi c dikes. A few
[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.
Journal of Petrology 02/2011; 52(4):665-690. · 4.49 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The origin and evolution of CO2 inclusions and calcite veins in peridotite xenoliths of the Pannonian Basin, Hungary, were investigated by means of petrographic investigation and stable isotope analyses. The fluid inclusions recovered in paragenetic olivine and clinopyroxene belong to distinct populations: type A (texturally early) inclusions with regular shapes (often with negative crystal forms) forming intragranular trails; type B (texturally late) inclusions defining randomly oriented trails that reach grain boundaries. Type B inclusions are often associated with silicate melt (type C) inclusions. Stable carbon isotope compositions in inclusion-hosted CO2 were obtained by vacuum crushing followed by conventional dual inlet as well as continuous flow mass spectrometry in order to eliminate possible lab artifacts. Olivines, clino- and orthopyroxenes of the host peridotite have oxygen isotope compositions from 5.3 to 6.0‰ (relative to V-SMOW), without any relationship with xenolith texture. Some of the xenoliths contained calcite in various forms: veins and infillings in silicate globules in veins, secondary carbonate veins filling cracks and metasomatic veins with diffuse margins. The former two carbonate types have δ13C values around –13‰ (relative to V-PDB) and low Sr contents (< 0.5 wt.%), whereas the third type,veins with high-temperature metasomatic features have a δ13C value of –5.0‰ and high Sr contents up to 3.4 wt.%. In spite of the mantle-like δ13C value and the unusually high Sr content typical for mantle-derived carbonate, trace element compositions have proven a crustal origin. This observation supports the conclusions of earlier studies that the carbonate melt droplets found on peridotite xenoliths in the alkaline basalts represent mobilized sedimentary carbonate. The large δ13C range and the 12C-enrichment in the carbonates can be attributed to devolatilization of the migrating carbonate or infiltration of surficial fluids containing 12C-rich dissolved carbon.Carbon isotope compositions of inclusion-hosted CO2 range from –17.8 to –4.8‰ (relative to V-PDB) with no relation to the amount of CO2 released by vacuum crushing. Low-δ13C values measured by stepwise heating under vacuum suggest that the carbon component is pristine and not related to surficial contamination, and that primary mantle fluids with δ13C values around –5‰ were at least partly preserved in the xenoliths. Tectonic reworking and heating by the basaltic magma resulted in partial CO2 release and local 13C-depletion.
[Show abstract][Hide abstract] ABSTRACT: In the study of the ancient and modern subduction systems geologists and geophysicis have noticed that the plate boundary is a fault of variable thickness where sediment subduction, mélange formation, and exhumation occur. The subduction-channel model (Shreve and Cloos, 1986) is the appropriate tool to describe processes focused along the plate boundary where the subduction-driven deformation is largely concentrated in a relatively thin layer of relatively rapid shearing. From geological data we know that viscosity, density and permeability of sediments in the channel vary not only along strike and down dip, but also through the channel itself. Bedding anisotropy, large unconformities, lithification and variable pore-fluid pressure produce heterogeneities that can influence the flow in the channel. The late Eocene-middle Miocene erosive plate boundary between the European plate and the Adriatic plate is exhumed in the Northern Apennines of Italy. The fossil fault zone is about 500 m thick and the outcrops reach portions of the megathrust where we can infer there were about 5 km of overriding material. At this plate boundary basal and frontal tectonic erosion incorporated unlithified, fluid-rich sediments into the fault zone along a subduction channel. Here we couple a detailed structural analysis of the Apennine subduction channel, focusing, in particular, on calcite veins, with a stable isotope analysis to characterize the fluid regime along an active subduction channel. Structural observations indicate fluid flow along and across the subduction channel. At the shallowest levels deformation occurred while portions of the subducting material were still unlithified. In the field the shallow portion of the subduction channel shows a highly heterogeneous pattern of deformation concentrated within unlithified material that separates well lithified ``rigid'' rocks. The unlithified material shows pervasive extensional deformation involving both failure and compaction, that occurred with no associated veining. At deeper levels calcite veins are associated to faults within a regime where discrete deformation prevails. Deformation occurred in a more homogeneous and well lithified material. Shear structures parallel to the channel boundaries and normal faults are mutually cross cutting. Extensional veins are present. The deformation shows evidence for cyclic hydrofracturing, vein precipitation and Pf build up and release. The 13C and 18O composition of calcite vein and host rock samples within the fault zone indicates that there is a deep source of fluids migrating upward along the subduction channel, in addition to locally derived fluid components.
[Show abstract][Hide abstract] ABSTRACT: Nominally anhydrous minerals in 46 peridotite xenoliths hosted by Cenozoic basalts from five localities (Fangshan, Penglai, Qixia, Changle, and Hebi) of the eastern part of the North China Craton (NCC) have been investigated by Fourier transform infrared spectrometry (FTIR). The water contents (H2O wt %) of clinopyroxene (cpx), orthopyroxene (opx), and olivine (ol) range from 27 to 223 ppm, 8 to 94 ppm, and ˜0 ppm, respectively. On the basis of (1) the homogenous H2O content within single pyroxene grains and (2) the equilibrium partitioning of H2O between cpx and opx, it is suggested that the pyroxenes largely preserve the H2O content of their mantle source, although possible H loss during xenolith ascent cannot be excluded for ol. The recalculated whole-rock H2O contents, using mineral modes and assuming a partition coefficient of 10 for water between cpx and ol, range from 6 to 56 ppm (average of 23 ± 13 ppm). In combination with previously reported data, the recalculated whole-rock water contents of peridotite xenoliths (105 samples from 9 localities) hosted by Cenozoic basalts from the eastern part of the NCC range from 6 to 85 ppm (average of 25 ± 18 ppm). The Cenozoic lithospheric mantle of the eastern part of the NCC is therefore characterized by a low water content compared to continental lithospheric mantle worldwide represented by typical cratonic and off-cratonic peridotites (normally 40-180 ppm, with average values of 119 ± 54 ppm and 78 ± 45, respectively) and to oceanic mantle values (>50 ppm) inferred from MORB and OIB. Peridotite xenoliths have low-to-moderate spinel Fe3+/SigmaFe (0.02-0.34) and whole rock DeltaFMQ values (from -4.2 to 2.2, normally between -2.5 and 1.5), which are not correlated with pyroxene H2O contents. Therefore, the low water contents cannot have resulted from oxidation of the mantle xenoliths and may have been caused instead by heating from an upwelling asthenosphere flow that acted in concert with NCC lithospheric thinning during the late Mesozoic to early Cenozoic. If so, the present eastern NCC lithospheric mantle represents essentially relict ancient lithospheric mantle after the thinning event, rather than newly accreted and cooled asthenospheric mantle.
Journal of Geophysical Research Atmospheres 10/2010; 115(B07207):1-22. · 3.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Adélie penguin (Pygoscelis adeliae) modern and fossil eggshells and guano samples collected from ornithogenic soils in Terra Nova Bay (Victoria Land, Ross Sea) were processed for carbon and nitrogen isotopic ratios with the aim of detecting past penguin dietary changes. A detailed and greatly expanded Adélie penguin dietary record dated back to 7,200 years BP has been reconstructed for the investigated area. Our data indicate a significant dietary shift between fish and krill, with a gradual decrease from past to present time in the proportion of fish compared to krill in Adélie penguin diet. From 7,200 to 2,000 years BP, δ(13)C and δ(15)N values indicate fish as the most eaten prey. The dietary contribution of lower-trophic prey in penguin diet started becoming evident not earlier than 2,000 years BP, when the δ(13)C values reveal a change in the penguin feeding behavior. Modern eggshell and guano samples reveal a major dietary contribution of krill but not a krill-dominated diet, since δ(13)C values remain much too high if krill prevail in the diet. According to the Holocene environmental background attested for Victoria Land, Adélie penguin dietary shifts between fish and krill seem to reflect penguin paleoecological responses to different paleoenvironmental settings with different conditions of sea-ice extension and persistence. Furthermore, Adélie penguin diet appears to be particularly affected by environmental changes in a very specific period within the breeding season, namely the egg-laying period when penguin dietary and feeding habit shifts are clearly documented by the δ(13)C of eggshell carbonate.