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The Jurassic Chon Aike Silicic Large Igneous Province (Patagonia and the Antarctic Peninsula) is dominated by voluminous, crust-derived magmas (235,000 km3) that erupted as predominately explosive silicic material over ~40 m.y. In this study, we combine petrological descriptions and bulk-rock major- and trace-element compositions with quartz oxygen-isotope measurements from multiple silicic units (primarily ignimbrites and some rhyolitic flows) from two of the five silicic formations in Patagonia. We have identified that quartz oxygen-isotope values are high (>9‰–12‰). Quartz phenocrysts analyzed by secondary ion mass spectroscopy (SIMS) are also homogeneous at the microscale with no measurable change in isotope value with respect to internal and often complex zoning textures. The ubiquity of widespread high δ18O rhyolites and their trace-element compositions support their origin from melting of a metasedimentary source with a similarly high δ18O value. Mass balance calculations require that an average of >75% melt derived from partial melting of the dominant basement lithology is needed to explain the isotopic and chemical composition of the rhyolites. The ideal P-T environment was identified by thermodynamic models for fluid-absent melting of graywackes at 900 °C and 5 kbar. Regional-scale crustal melting occurred during a widespread, high heat-flux environment within an extensional setting during the break- up of the Gondwanan supercontinent. The overlap of a unique tectonic and igneous environment, combined with a fertile crust dominated by graywacke and pelitic compositions in southern Patagonia, generated large volumes of some of the highest δ18O silicic magmas documented in the geologic record.

Deep incised glacial valleys surrounded by high peaks form the modern topography of the Southern Patagonian Andes. Two Miocene plutonic complexes in the Andean retroarc, the cores of the Fitz Roy (49°S) and Torres del Paine (51°S) massifs, were emplaced at 16.7±0.3 Ma and 12.5±0.1 Ma, respectively. Subduction of ocean ridge segments initiated at 54°S, generating northward opening of an asthenospheric window with associated mantle upwelling and orogenic shortening since 16 Ma. Subsequently, the onset of major glaciations at 7 Ma caused drastic changes in the regional topographic evolution. To constrain the respective contributions of tectonic convergence, mantle upwelling and fluvio-glacial erosion to rock exhumation, we present inverse thermal modeling of a new dataset of zircon and apatite (U-Th)/He from the two massifs, complemented by apatite 4He/3He data for Torres del Paine. Our results show rapid rock exhumation recorded in the Fitz Roy massif between 10.5 and 9 Ma, which we ascribe to mantle upwelling and/or crustal shortening due to ridge subduction at 49°S. Both massifs record a pulse of rock exhumation between 6.5 and 4.5 Ma, which we interpret as the result of the onset of Patagonian glaciations. After a period of erosional quiescence during the Miocene/Pliocene transition, increased rock exhumation since 3-2 Ma to present day is interpreted as the result of alpine glacial valley carving promoted by reinforced glacial-interglacial cycles. This study demonstrates that along-strike thermochronological studies provide us with the means to assess the spatio-temporal variations in tectonic, mantle, and surface processes forcing on rock exhumation.

The diffusivity and diffusion mechanisms of hydrogen together with with deuterium and lithium, parallel to the c axis of quartz, were investigated experimentally at 800°C, 0.1 GPa with the activity of H2O or 2H2O ≈ 1 [2H is used throughout this work to describe deuterium rather than D, to avoid confusion with the diffusion coefficient, D]. The pH was set using mixtures of H2O (or 2H2O) and HCl. Three types of experiment were conducted: (1) H-in/Li-out; (2) 2H-in/H-out; and (3) 2H-in/H + Li out, using three different natural quartz crystals as starting materials. Profiles of H, 2H and Li were measured using Fourier-transform infrared (FTIR) spectroscopy and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). H, 2H and Li are charge-compensated by Al3+ replacing Si4+, or by excess O2–. The total atomic concentration of monovalent cations appears to remain constant over the duration of the experiments. The resulting diffusion profiles are different for the three experimental designs and three starting materials, and some show complex shapes inconsistent with simple diffusion. A multi-site diffusion–reaction model is developed, with the theory based on previous models that have been derived mainly on the basis of conductivity measurements. In these models, the monovalent cations move away from their charge-balancing ion then diffuse rapidly to another site. The mobility of the monovalent cations is described by both a diffusion coefficient and an equilibrium constant that enables dissociation of the immobile charge-balanced defects. This model can describe complex step-shaped profiles formed in H-in/Li-out experiments, profiles with local maxima ('humped’ profiles) in 2H-in/H + Li out experiments, and error function-shaped profiles in 2H-in/H-out and previously published Li-in/H-out experiments. Our data provide support for models previously proposed for quartz. Studies of the lengths and forms of diffusion profiles from such experiments provide a useful complement to assertions from conductivity experiments.

The metamorphic rocks from the Torres del Paine contact aureole (Patagonia, Chile) show field, petrographic, and geochemical evidence for small amounts of igneous fluid infiltration due to the solidification of the granite complex. Hydrogen isotope ratios (D/H) in the contact aureole first decrease while approaching the intrusion and subsequently increase toward the granite contact. Initial decrease with metamorphic grade is due to preferential loss of the 2H isotopes due to Rayleigh fractionation during prograde dehydration reactions. The infiltration of high-δD fluids from the intrusion increases δD within the last 150 m. In contrast, (18O/16O) ratios show no systematic changes, indicating that neither oxygen loss by Rayleigh fractionation nor oxygen exchange by fluid infiltration was significant enough to dominate original variations seen in the oxygen isotope ratio of the protolith. Calculated volume of fluid using the position of the hydrogen isotope exchange front gives a relatively low time-integrated fluid flux of about 4 m3/m2 at the contact for the igneous fluid. These small amounts of fluid flux are in agreement with whole rock oxygen isotope data that are not affected in the contact aureole. Chlorine content of metamorphic biotite crystals, in contrast to oxygen isotopes, supports infiltration of igneous fluids. Indeed, relatively high-Cl concentrations in biotite were measured in some samples close to the intrusion (up to 0.2 wt%), while chlorine concentrations in biotite are constant everywhere else in the entire contact aureole, having low concentrations (0.01–0.06 wt%). The absence of a well-marked Rayleigh fractionation trend in Cl concentrations with increasing metamorphism is surprising since chlorine strongly fractionates into the fluid. This is best explained by slow diffusive exchange of chlorine in biotite in the cooler outer aureole. Hence recrystallization of biotite would be required to modify its Cl composition. Biotite grains from samples close to the intrusion with high-Cl content also have lower Ti content (0.4 pfu) than biotite (0.5 pfu) from other samples containing biotite with lower Cl content located at the same distance from the contact. Since Ti content in biotite is a function of temperature, this is a good indication that magmatic fluid infiltration started post peak, early during cooling of the metamorphic rocks. Episodes of fluid flow appear to have been nearly continuous during cooling as evidenced by numerous retrogression textures, such as secondary muscovite (above 470 °C) or chlorite + muscovite intergrowth after cordierite or biotite (slightly below 470 °C). This might be related to crystallization of subsequent batches of granites or the onset of minor fluid convection during cooling of the aureole. Nevertheless, only minor secondary muscovite has been found, and fresh cordierite is present throughout the aureole confirming small amounts of fluid infiltration.The time-integrated fluid flux computed from the hydrogen isotope exchange front is two orders of magnitude lower than values computed for metacarbonate in many other contact aureoles, suggesting low permeabilities of pelitic rocks. In conclusion, Cl contents and hydrogen isotope compositions of hydrous minerals provide a sensitive tool to identify small fluid-rock interaction events, much more sensitive than oxygen isotope compositions of the whole rock or minerals.

The mixing and mingling of magmas of different compositions are important geological processes. They produce various distinctive textures and geochemical signals in both plutonic and volcanic rocks...

International audience; The tectono-metamorphic evolution of the European Alps is still contentious. The Monte Rosa tectonic unit is a prominent nappe in the Central European Alps and estimates of its peak Alpine pressure (P) and temperature (T) conditions are essential for reconstructing its tectonometamorphic evolution. However, the reported peak Alpine P and T estimates vary considerably between 1.2 and 2.7 GPa and 490 and 640 °C for a variety of lithologies. Here, we show petrology and pseudosection modelling of metapelitic assemblages from the western portions of the Monte Rosa nappe (upper Ayas valley, Italy). We present newly discovered staurolite-chloritoid bearing metapelitic assemblages. These assemblages exhibit an Alpine high-P metamorphic overprint of a former contact-metamorphic mineral assemblage generated by post-Variscan granitic intrusions. Staurolite contains major amounts of Zn (up to 1.0 atoms per formula units), which is currently, in contrast to Fe-and Mg-staurolite end-members, not considered in any thermodynamic database. We employ two end-member mixing models for Zn in staurolite, site mixing and molecular mixing. Both models enlarge the P and T stability range for the observed assemblage, where site mixing has the largest influence of  0.2 GPa and  20 C. Our results for three metapelite assemblages, with and without staurolite, indicate peak Alpine P of 1.6 ± 0.2 GPa and peak T of 585  20 C. These peak P estimates agree with previously published estimates for metagranites in the nappe, and are in stark contrast with peak P obtained from talc, chloritoid, phengite and quartz bearing lithologies termed 'whiteschists' (>2.2 GPa). Our results confirm a variation of peak Alpine P of 0.6 ± 0.2 GPa between metagranite/metapelite lithologies and a nearby whiteschist lens (>2.2 GPa) within the metagranite. Field observations indicate that the studied region is structurally coherent and that the whiteschist is not a tectonic slice formed by tectonic mélange. We suggest that the consistent peak P for metapelite and metagranite assemblages represents the regional peak P and that the higher pressure recorded in the whiteschist lens is likely due to dynamic pressure, possibly resulting from tectonic and/or reaction-induced stresses. If the calculated P of 1.6 ± 0.2 GPa represents regional peak Alpine conditions, then the Monte Rosa nappe was exhumed from a significantly shallower depth than previously assumed, based on peak P estimates >2.2 GPa for whiteschist lithologies.

Fluids released within subduction zones affect many fundamental Earth processes, but very little is understood about processes of fluid transport within subduction zones and the chemistry of the fluids. Centimeter-scale variations in bulk-rock Li isotope compositions in subduction-related metamorphic rocks are observed associated with fluid-related features. The small spatial scales of these variations suggest very short duration fluid infiltration events – on the order of months to centuries. However, bulk-rock measurements record a time-integrated record, while in situ measurements in metamorphic porphyroblasts such as garnet have the potential to record individual events experienced by the rock. In this study, traverses across garnet in Franciscan eclogite and amphibolite were analyzed for Li isotopes in situ using SIMS to determine the potential for inferring the duration of metamorphic fluid flow and for deciphering fluid chemistry. The Li concentrations of these garnet crystals are very low, ∼5 µg/g. The measured range of δ7Li within some Franciscan garnets is 19‰, which falls well outside the uncertainty of the measurement (

Ocean paleotemperatures have been reconstructed for almost the entirety of the Phanerozoic using the oxygen isotope compositions of calcium carbonates formed by marine organisms and preserved in ocean sediments. However, the isotopic composition of these calcitic tests and shells can be substantially altered through diagenetic processes. Here, we used 18O as an isotopic tracer in controlled experiments designed to simulate early diagenesis of modern benthic foraminifera tests to investigate how fluids penetrate into and exchange oxygen isotopes with these biogenic calcites. Initially pristine tests of Ammonia sp., Haynesina germanica, and Amphistegina lessonii were immersed in an 18O-enriched artificial seawater at 90 °C for hours to days. High-resolution SEM images of the tests before and after the experiments were indistinguishable yet the bulk oxygen isotope compositions of reacted tests revealed rapid and species-dependent isotopic exchange with the water. Correlated SEM, TEM and NanoSIMS imaging of 18O intra-test distributions showed that fluid penetration and exchange is ubiquitous yet heterogenous, and is intimately tied to test ultrastructure and associated organic matter. Species level differences in ultrastructure, quantified through image analysis, explained the observed species-dependent rates of isotopic exchange. Consequently, even calcitic skeletons considered texturally pristine for paleo-climatic reconstruction purposes may have experienced substantial isotopic exchange and hence a critical re-examination of the paleo-temperature record is warranted.

We report measurements of experimentally induced diffusion profiles in minerals using local electrode atom probe (LEAP) tomography, and demonstrate that this is a viable technique for use in experimental diffusion studies, with distinct advantages over current state-of-the-art techniques such as secondary ion mass spectrometry and Rutherford backscattering spectroscopy. For this purpose, we have investigated Ca diffusivity in synthetic forsterite, which we measured parallel to [1 0 0] from 750 to 1300 °C, at 1 atm. pressure in air, with silica activity buffered by forsterite-enstatite or forsterite-periclase assemblages. We observe no dependence of Ca diffusion on silica activity, and argue that this may be due the strong preference of Ca for the M2 site, while M-site vacancies in olivine are more favorably located in M1 sites. Taken together, our data yield the following Arrhenius relation for Ca diffusion parallel to [1 0 0] in synthetic forsterite at 1 atm. pressure: log D Ca _ [ 100 ] m 2 s - 1 = - 8.42 ± 0.36 - 250 ± 9 kJ mo l - 1 2.303 RT . Time series experiments at 750 °C reveal that the shortest profiles measured by secondary ion mass spectrometry depth profiling (using an oxygen primary ion beam) are compromised by ion-beam mixing, while LEAP tomography yields accurate diffusion coefficients from the same samples. Because LEAP tomography does not suffer from the type of analytical artefacts that complicate the measurement of extremely short profiles via secondary ion mass spectrometry depth profiling, and also typically produces an order of magnitude better sensitivity than Rutherford backscattering spectroscopy, this technique offers a valuable new tool for quantifying the diffusivities of slow-diffusing trace elements in crystalline materials.

A series of tourmaline reference materials are developed for in situ oxygen isotope analysis by secondary ion mass spectrometry (SIMS), which allow study of the tourmaline compositions found in most igneous and metamorphic rocks. The new reference material was applied to measure oxygen isotope composition of tourmaline from metagranite, meta-leucogranite, and whiteschist from the Monte Rosa nappe (Western Alps). The protolith and genesis of whiteschist are highly debated in the literature. Whiteschists occur as 10 to 50 m tube-like bodies within the Permian Monte Rosa granite. They consist of chloritoid, talc, phengite, and quartz, with local kyanite, garnet, tourmaline, and carbonates. Whiteschist tourmaline is characterized by an igneous core and a dravitic overgrowth (XMg > 0.9). The core reveals similar chemical composition and zonation as meta-leucogranitic tourmaline (XMg = 0.25, δ18O = 11.3–11.5‰), proving their common origin. Dravitic overgrowths in whiteschists have lower oxygen isotope compositions (8.9–9.5‰). Tourmaline in metagranite is an intermediate schorl-dravite with XMg of 0.50. Oxygen isotope data reveal homogeneous composition for metagranite and meta-leucogranite tourmalines of 10.4–11.3‰ and 11.0–11.9‰, respectively. Quartz inclusions in both meta-igneous rocks show the same oxygen isotopic composition as the quartz in the matrix (13.6–13.9‰). In whiteschist the oxygen isotope composition of quartz included in tourmaline cores lost their igneous signature, having the same values as quartz in the matrix (11.4–11.7‰). A network of small fractures filled with dravitic tourmaline can be observed in the igneous core and suggested to serve as a connection between included quartz and matrix, and lead to recrystallization of the inclusion. In contrast, the igneous core of the whiteschist tourmaline fully retained its magmatic oxygen isotope signature, indicating oxygen diffusion is extremely slow in tourmaline. Tourmaline included in high-pressure chloritoid shows the characteristic dravitic overgrowth, demonstrating that chloritoid grew after the metasomatism responsible for the whiteschist formation, but continued to grow during the Alpine metamorphism. Our data on tourmaline and quartz show that tourmaline-bearing white-schists originated from the related meta-leucogranites, which were locally altered by late magmatic hydrothermal fluids prior to Alpine high-pressure metamorphism.

Fluid-rock interaction occurred in hydrothermal veins in dolomitic wall pendants during cooling of the Bruffione granodiorite (Adamello batholith, Italy). These veins formed through fluid infiltration along a central fracture, and the fluid reacted with the dolomite to precipitate forsterite + calcite in cm-wide replacement zones through SiO2 metasomatism. Textural observations have been coupled to in-situ stable isotope and chemical analysis of dolomite, calcite and forsterite to show that stable isotope exchange occurs only through mineral reaction and recrystallization. We identified two distinct reaction regimes; (1) a regime dominated by net transfer reactions, which is characterized by the complete dissolution of dolomite and the formation of the new vein minerals (i.e. forsterite + calcite) close to the open fracture. The driver of the reaction is the large chemical disequilibrium introduced by infiltration of fluid of magmatic origin. And (2) a regime dominated by the dissolution and reprecipitation of dolomite more distal to the central fracture. The dolomite protolith is partially dissolved along grain boundaries, but also trough the grains which are being fragmented, and reprecipitation of newer generations of dolomite rims occur. A progressive change in fluid composition is detected with increasing distance from the vein. In comparison to the first regime, the system is rock-dominated, and only small amounts of fluids and small chemical disequilibrium at the trace element level are suggested to be the drivers for this reaction. A fundamental understanding of mineral reactions in the presence of fluids is necessary for many areas of science and industry, and recrystallization reactions driven by very small chemical disequilibrium as described here are important to be identified and integrated into reactive transport models. In fact, for this case study, the fluid-rock interaction volume would be largely underestimated if only the net transfer reaction zone is considered. This could be crucial for safety evaluations in technical applications.

In situ analysis of a garnet porphyroblast from a granulite facies gneiss from Sor Rondane Mountains, East Antarctica, reveals discontinuous step‐wise zoning in phosphorus and large δ18O variations from the phosphorus‐rich core to the phosphorus‐ poor rim. The gradually decreasing profile of oxygen isotope from the core (δ18O = ~15‰) to the rim (δ18O = ~11‰) suggests that the 18O/16O zoning was originally step‐wise, and modified by diffusion after the garnet rim formation at ~800°C and 0.8 GPa. Fitting of the 18O/16O data to the diffusion equation constrains a duration of the high‐T event (~800°C) to c. 0.5–40 Ma after the garnet rim formation. The low δ18O value of the garnet rim, together with the previously reported low δ18O values in metacarbonates, indicates regional infiltration, probably along a detachment fault, of low δ18O fluid/melt possibly derived from meta‐mafic to ultramafic rocks.

Our refined ability to estimate metamorphic conditions incurred by rocks has increased our understanding of the dynamic earth. Calculating pressure (P), temperature (T) and time (t) histories of these rocks is vital for reconstructing tectonic movements within subduction zones. However, large disparities in peak P within a structurally coherent tectonic unit poses difficulties when attempting to resolve a tectono-metamorphic history, if a depth dependant lithostatic P is assumed. However, what is clear is that pressure, or mean stress, in a rock cannot exactly be lithostatic during an orogeny due to differential stress, required to drive rock deformation or to balance lateral variations in gravitational potential energy. Deviations from lithostatic P is commonly termed tectonic pressure, and both its magnitude and impact on metamorphic reactions in disputed.For the ‘Queen of the Alps’ (the Monte Rosa massif), estimates for the maximum P recorded during Alpine orogenesis remain enigmatic. Large disparities in published estimates for peak P exist, ranging between 1.2 and 2.7 GPa. Moreover, the highest P estimates (2.2 - 2.7 GPa) are for rocks that comprise only a small percentage (< 1%) of the total volume of the nappe (whiteschist bodies and eclogitic mafic boudins). We present newly discovered whiteschist lithologies that persistently exhibit higher P conditions (c. 2.2 GPa) compared to metagranitic and metapelitic lithologies (c. 1.4 - 1.6 GPa). Detailed mapping and structural analysis in these regions lack evidence for tectonic mixing. Therefore, we suggest that a ΔP 0.6 ± 0.2 GPa during peak Alpine metamorphism could potentially represent tectonic pressure. Furthermore, we outline possible mechanisms that facilitate ΔP, namely mechanically- and/or reaction-induced. We present data from numerical models that exhibit significant ΔP (c. 0.4 GPa) during a transient period of high differential stress prior to buckling and subsequent exhumation of viscous fold nappes, similar to exhumation mechanisms suggested for the Monte Rosa nappe. As well as this, we present new routines for calculating metamorphic facies distribution within numerical models of subduction zones that agree with natural distributions within orogens.The maximum burial depth of the Monte Rosa unit was likely significantly less than 80 km (based on the lithostatic pressure assumption and minor volumes of whiteschist at c. 2.2 GPa). Rather, the maximum burial depth of the Monte Rosa unit was presumably equal to or less than c. 60 km, estimated from pressures of 1.4 - 1.6 GPa recorded frequently in metagranite and metapelitic lithologies. In order to understanding, more completely, a rocks metamorphic history, consideration of the interplay between tectonic and metamorphic processes should not be overlooked.

In the accreting Earth and planetesimals, carbon was distributed between a core forming metallic melt, a silicate melt, and a hot, potentially dense atmosphere. Metal melt droplets segregating gravitationally from the magma ocean equilibrated near its base. To understand the distribution of carbon, its partitioning between the two melts is experimentally investigated at 1.5–6.0 GPa, 1300–2000 °C at oxygen fugacities of −0.9 to −1.9 log units below the iron-wuestite reference buffer (IW). One set of experiments was performed in San Carlos olivine capsules to investigate the effect of melt depolymerization (NBO/T), a second set in graphite capsules to expand the data set to higher pressures and temperatures. Carbon concentrations were analyzed by secondary ionization mass spectrometry (SIMS) and Raman spectra were collected to identify C-species in the silicate melt. Partition coefficients are governed by the solubility of C in the silicate melt, which varies from 0.01 to 0.6 wt%, while metal melts contain ∼7 wt% C in most samples. C solubility in the silicate melt correlates strongly with NBO/T, which, in olivine capsules, is mostly a function of temperature. Carbon partition coefficients DCmetal/silicate at 1.5 GPa, 1300–1750 °C decrease from 640(49) to 14(3) with NBO/T increasing from 1.04 to 3.11. For the NBO/T of the silicate Earth of 2.6, DCmetal/silicate is 34(9). Pressure and oxygen fugacity show no clear effect on carbon partitioning. The present results differ from those of most previous studies in that carbon concentrations in the silicate melt are comparatively higher, rendering C to be about an order of magnitude less siderophile, and the discrepancies may be attributed to differences in the experimental protocols. Applying the new data to a magma ocean scenario, and assuming present day mantle carbon mantle concentrations from 120 to 795 ppm, implies that the core may contain 0.4–2.6 wt% carbon, resulting in 0.14–0.9 wt% of this element for the bulk Earth. These values are upper limits, considering that some of the carbon in the modern silicate Earth has very likely been delivered by the late veneer. © 2020 Elsevier B.V., Earth and Planetary Science Letters, 554, ISSN:0012-821X, ISSN:1385-013X

Zircon occasionally crystallizes in evolved melt pockets in mafic large igneous province (LIP) magmas, and in these cases, it is used to provide high-precision age constraints on LIP events. The precision and accuracy of high-precision ages from LIPs are crucially important, because they may be implicated in mass extinctions. However, why zircon crystallizes in these magmas is not clearly understood, since their mafic compositions should limit zircon saturation. Here, we investigate the occurrence of zircon (and baddeleyite) in intrusive and extrusive mafic rocks from Central Atlantic Magmatic Province (CAMP) using petrography, trace-element analysis, Ti temperatures, Hf and oxygen isotopes, and high-precision U–Pb geochronology, along with petrological and thermal modeling. We provide new ages for CAMP sills that intruded into Paleozoic sediments in Brazil, indicating that the high and low Ti magmatism in this area occurred synchronously over 264 ± 57 ka. We show that upper crustal assimilation, especially of shales, during the emplacement of the CAMP likely led to zircon saturation. Assimilation of upper crustal sediments is also supported by high δ18O values and some rare negative εHf values in the zircon crystals. The only extrusive sample analyzed was the North Mountain basalt in Nova Scotia, Canada. This sample contains a large age variation in its zircon crystals (up to 4 Ma), and the older crystals have slightly more negative εHf values suggesting the presence of small (micron scale) xenocrystic cores associated with very late-stage sediment assimilation. However, the CAMP dataset as a whole suggests that the presence of xenocrystic cores is rare. Assuming no xenocrystic cores, and considering the zircon undersaturated nature of LIP mafic melts, the oldest zircon age clusters in a population should record the magma emplacement (or time when assimilation occurred), and the younger ages in a population are more likely to reflect Pb loss, especially given the high U concentrations of LIP zircon. Our identification of heterogeneous isotopic and elemental compositions in LIP zircon indicates that zircon in these magmas saturate in isolated minute melt pockets just before the system cools below its solidus.

Ore precipitation in mineral deposits formed in the upper parts of a porphyry system, at shallow crustal level (< 1.5 km), such as epithermal Au-Ag-(Cu)-(As) and polymetallic deposits is often triggered by fluid cooling and/or mixing between fluids from different sources. Commonly, in such deposits, two main fluid sources are identified a deeply sourced magmatic fluid and a shallow meteoric water stored in a surficial aquifer. Oxygen and hydrogen isotope compositions of gangue and alteration minerals using conventional bulk isotopic methods support the existence of mixing between these two fluid types. However, bulk isotope analysis provides only limited information on the exact mixing mechanisms and on the changing proportions of the involved fluids. Due to their high spatial resolution, SIMS in-situ oxygen isotope and LA-ICP-MS trace element analyses, in transects across growth zones of single crystals are adequate tools to trace the dynamics of this fluid mixing. In this study, in-situ SIMS oxygen isotope and LA-ICP-MS trace element analyses were performed on 10 selected quartz crystals from the giant Cerro de Pasco porphyry-related epithermal polymetallic deposit in central Peru. The results, combined with previous microthermometric and LA-ICP-MS fluid inclusion studies on the same or equivalent crystals, allow quantifying and documenting the mixing between different types of fluids that formed the large Cerro de Pasco epithermal polymetallic deposit. The δ 18 O quartz values range between 4‰ and 20‰ and display variations up to 11.5‰ inside single crystals that cannot be only, nor mainly ascribed to fluid temperature changes. Rather, these variations record variable mixing proportions of a rising moderate-salinity magmatic fluid with a δ 18 O H2O around 10‰ and a low-salinity fluid with a δ 18 O H2O between 0 and 4‰, the latter stored below the paleo-water table. Each analyzed quartz crystals also display important variation of their trace element content, with Al (43 to 2098 ppm), Li (0.7 to 18 ppm), Ge (1.1 to 24ppm) and Ti (0.8 to 10 ppm). These variations do not systematically correlate with oxygen isotope compositions. This suggests that quartz trace element content is controlled by a complex interplay of fluid composition, temperature, pressure, and growth rate. Application of published Ti-in-quartz geothermometers on quartz grains from which the precipitation temperature is well constrained by fluid inclusion microthermometry, shows that it can lead to overestimation or underestimation of precipitation temperatures by more than 50 • C. The obtained δ 18 O quartz patterns measured along profiles in the studied quartz crystals, and less clearly the in-situ trace element compositions, reveal abrupt changes and suggest that mixing between magmatic and surface-derived low-salinity fluids was not a continuous process. It rather took place through the influx of multiple short-lived pulses of magmatic fluid into the surface-derived low-salinity fluid surface aquifer.

We present a high-resolution in situ study of oxygen and boron isotopes measured in tourmaline from the world-class San Rafael Sn (–Cu) deposit (Central Andean tin belt, Peru) aiming to trace major fluid processes at the magmatic-hydrothermal transition leading to the precipitation of cassiterite. Our results show that late-magmatic and pre-ore hydrothermal tourmaline has similar values of δ 18 O (from 10.6‰ to 14.1‰) and δ 11 B (from −11.5‰ to −6.9‰). The observed δ 18 O and δ 11 B variations are dominantly driven by Rayleigh fractionation, reflecting tourmaline crystallization in a continuously evolving magmatic-hydrothermal system. In contrast, syn-ore hydrothermal tourmaline intergrown with cassiterite has lower δ 18 O values (from 4.9‰ to 10.2‰) and in part higher δ 11 B values (from −9.9‰ to −5.4‰) than late-magmatic and pre-ore hydrothermal tourmaline, indicating important contribution of meteoric groundwater to the hydrothermal system during ore deposition. Quantitative geochemical modeling demonstrates that the δ 18 O- δ 11 B composition of syn-ore tourmaline records variable degrees of mixing of a hot Sn-rich magmatic brine with meteoric waters that partially exchanged with the host rocks. These results provide thus direct in situ isotopic evidence of fluid mixing as a major mechanism triggering cassiterite deposition. Further, this work shows that combined in situ δ 18 O and δ 11 B analyses of tourmaline is a powerful approach for understanding fluid processes in dynamic magmatic-hydrothermal environments.

Whiteschists from the Monte Rosa Nappe were examined in the field as well as with petrographic, geochemical, and isotopic methods to constrain the controversial origin of these rocks in their Alpine metamorphic context. Whiteschists occur as ellipsoidal-shaped, decametric-sized bodies, within a Permian metagranite, and consist mainly of chloritoid, talc, phengite, and quartz. The transition from whiteschist to metagranite is marked by multiple sharp mineralogical boundaries defining concentric zones unrelated to Alpine deformation. The development of reaction zones, as well as the geometry of the whiteschist suggest a pervasive fluid infiltration, facilitated and canalized by reaction fingering. Whole-rock compositions of whiteschists and metagranites indicate an enrichment in MgO and H2O and depletion of Na2O, CaO, Ba, Sr, Pb, and Zn in the whiteschist relative to the metagranite. Trace- and rare-earth elements, together with all other major elements, notably K2O and SiO2, were within uncertainty not mobile. Such a K and Si saturated, Na undersaturated fluid is not compatible with previous interpretations of fluids derived from ultramafic rocks, evaporites, or Mg-enriched seawater due to mantle interactions. Together with the large variations in δD and δ18O values, this indicates large fluid fluxes during metasomatism. Calculated δD and δ18O values of fluids in equilibrium with the whiteschist support a magmatic–hydrothermal fluid source, as does the chemical alteration pattern. Bulk rock 87Sr/86Sr ratios in whiteschists confirm a pre-Alpine age of fluid infiltration. The 87Sr/86Sr ratios in whiteschists were subsequently partially homogenized in a closed system during Alpine metamorphism. In conclusion, the granite was locally affected by late magmatic–hydrothermal alteration, which resulted in sericite–chlorite alteration zones in the granite. The entire nappe underwent high-pressure metamorphism during the Alpine orogeny and the mineralogy of the whiteschist was produced during dehydration of the metasomatic assemblage under otherwise closed-system metamorphism. While each whiteschist locality needs to be studied in detail, this in-depth study suggests that many whiteschists found in granitic bodies in the Alps might be of similar origin.

International audience; Reactions in the Earth's crust occur through a dissolution-precipitation process in the presence of fluid. Dissolution releases aqueous species which are transported to the locus of precipitation. This replacement process generates creep deformation (i.e., dissolution-precipitation creep) due to volume change during reaction and stress-controlled mass re-distribution in the rock. Reaction under stress also modifies the rock microstructure and the pressure record during metamorphism. A quantitative model for dissolution-precipitation creep is developed here by considering both dissolution and precipitation at grain interfaces to simulate replacement reactions under stress. A new creep law is obtained for pressure solution, allowing for the reaction- and the diffusion-controlled cases to be modeled with a single expression. It is extended to replacement reactions by introducing volume change during reaction. Deformation mechanism maps are generated with the new creep law, indicating that, when fluid is present, dissolution-precipitation creep is the dominant deformation mechanism in the Earth's crust. Numerical model reveals that grain shape preferred orientation only develops near thermodynamic equilibrium. This is consistent with measurements of porphyroblasts preferred orientation in rocks from the Nufenen Pass (Switzerland) having experienced prograde metamorphism. Kinetics play a key role on the thermodynamic pressure of metamorphic reaction. Near the equilibrium, reaction is controlled by either σ1 or σ3 depending on the total volume change during reaction whereas it is controlled by the mean stress far from the equilibrium.

We report experimental data for Y, La, Lu and Hf diffusion in garnet, in which diffusant concentrations and silica activity have been systematically varied. Experiments were conducted at 950 and 1050 °C, at 1 atm pressure and oxygen fugacity corresponding to the quartz–fayalite–magnetite buffer. At Y and REE concentrations below several hundred ppm we observe both slow and fast diffusion mechanisms, which operate simultaneously and correspond to relatively high and low concentrations, respectively. Diffusivity of Y and REE is independent of silica activity over the studied range. General formulae for REE diffusion in garnet, incorporating data from this and previous studies, are logDREE(f)(m2 s−1)=−10·24(±0·21)−221057(±4284)2·303RT(K) for the ‘fast’ REE diffusion mechanism at 1 atm pressure, and logDREE(s)(m2 s−1)=−9·28(±0·65)−265200(±38540)+10800(±2600)×P(GPa)2·303RT(K) for the ‘slow’ REE diffusion mechanism. These slow and fast diffusion mechanisms are in agreement with previous, apparently conflicting, datasets for REE diffusion in garnet. Comparison with high-pressure experiments suggests that at high pressures (>∼1 GPa minimum) the fast diffusion mechanism no longer operates to a significant degree. When Y and/or REE surface concentrations are greater than several hundred ppm, complex concentration profiles develop. These profiles are consistent with a multi-site diffusion–reaction model, whereby Y and REE cations diffuse through, and exchange between, different crystallographic sites. Diffusion profiles of Hf do not exhibit any of the complexities observed for Y and REE profiles, and can be modeled using a standard (i.e. single mechanism) solution to the diffusion equation. Hafnium diffusion in garnet shows a negative dependence on silica activity, and is described by logDHf(m2 s−1)=−8·85(±0·38)−299344(±15136)+12500(±900)×P(GPa)2·303RT(K)−0·52(±0·09)×log⁡10aSiO2. In many natural garnets, diffusion of both Lu and Hf would be sufficiently slow that the Lu–Hf system can be reliably used to date garnet growth. In cases in which significant Lu diffusion does occur, preferential retention of 176Hf/177Hf relative to 176Lu/177Hf will skew isochron relationships such that their apparent ages may not correspond to anything meaningful (e.g. garnet growth, peak temperature or the closure temperature of Lu or Hf). Late-stage reheating events are capable of causing larger degrees of preferential retention of 176Hf/177Hf relative to 176Lu/177Hf and partial to full resetting of the Sm–Nd system within garnet, thus increasing the separation between garnet Lu–Hf and Sm–Nd isochron dates, owing to the fact that these systems are more significantly disturbed through diffusion as more radiogenic 176Hf and 143Nd have accumulated.

International audience; Serpentinization plays a key role on the evolution of the physicochemical properties of the mantle lithosphere. The rate of serpentinization reactions is controlled by the transport of fluid, which itself depends on volume change during reaction. Element transfer can strongly modify the magnitude and sign of volume change. Here, we measure solid volume change and element transport perpendicular to a serpentine vein in a serpentinized dunite collected at depth during the Oman Drilling Project. The sample is extensively replaced (extent of reaction > 80%) by a serpentine/brucite mixture parallel to a main serpentine vein network. The Mg content of serpentine and brucite indicates reaction with a small amount of fluid at temperatures below 100°C. Concentrations of fluid‐mobile trace elements (Na, Ca, Sr, Rb, and Ba) decrease perpendicular to the main vein. Primary olivine contains parallel platelets of a clinopyroxene/magnetite symplectite. Tomography at the nanoscale reveals that these inclusions do not react during serpentinization but are cracked and displaced. We use these inert markers to measure a 59% to 74% positive volume change that is close to the 52% expected for reaction in a closed system. Chemical data indicate no change in major element composition during reaction except for the addition of water. The initial olivine zoning in Al, Ti, V, Sc, and Cr is still preserved in serpentine and brucite. Serpentinization can thus be a local replacement process during which the solid volume homogeneously increases at the micrometer scale and the transport of aqueous species is limited.

The densification of the lower crust in collision and subduction zones plays a key role in shaping the Earth by modifying the buoyancy forces acting at convergent boundaries. It takes place through mineralogical reactions, which are kinetically favored by the presence of fluids. Earthquakes may generate faults serving as fluid pathways, but the influence of reactions on the generation of seismicity at depth is still poorly constrained. Here we present new petrological data and numerical models to show that in the presence of fluids, densification reactions can occur very fast, on the order of weeks, and consume fluids injected during an earthquake, which leads to porosity formation and fluid pressure drop by several hundreds of megapascals. This generates a mechanically highly unstable system subject to collapse and further seismic-wave emission during aftershocks. This mechanism creates new pathways for subsequently arriving fluids, and thus provides a route for self-sustained densification of the lower crust.

Metamorphic olivine formed by the reaction of antigorite + brucite is widespread in serpentinites that crop out in glacier-polished outcrops at the Unterer Theodulglacier, Zermatt. Olivine overgrows a relic magnetite mesh texture formed during ocean floor serpentinization. Serpentinization is associated with rodingitisation of mafic dykes. Metamorphic olivine coexists with magnetite, shows high Mg# of 94–97 and low trace element contents. A notable exception is 4 µg/g Boron (> 10 times primitive mantle), introduced during seafloor alteration and retained in metamorphic olivine. Olivine incorporated 100–140 µg/g H2O in Si-vacancies, providing evidence for low SiO2-activity imposed by brucite during olivine growth. No signs for hydrogen loss or major and minor element diffusional equilibration are observed. The occurrence of olivine in patches within the serpentinite mimics the former heterogeneous distribution of brucite, whereas the network of olivine-bearing veins and shear zones document the pathways of the escaping fluid produced by the olivine forming reaction. Relic Cr-spinels have a high Cr# of 0.5 and the serpentinites display little or no clinopyroxene, indicating that they derive from hydrated harzburgitic mantle that underwent significant melt depletion. The enrichment of Mg and depletion of Si results in the formation of brucite during seafloor alteration, a pre-requisite for later subduction-related olivine formation and fluid liberation. The comparison of calculated bulk rock brucite contents in the Zermatt-Saas with average IODP serpentinites suggests a large variation in fluid release during olivine formation. Between 3.4 and 7.2 wt% H2O is released depending on the magnetite content in fully serpentinized harzburgites (average oceanic serpentinites). Thermodynamic modelling indicates that the fluid release in Zermatt occurred between 480 °C and 550 °C at 2–2.5 GPa with the Mg# of olivine varying from 68 to 95. However, the majority of the fluid released from this reaction was produced within a narrow temperature field of

Syntectonic veins commonly have been used to assess the composition and source of fluids involved in fault zone activity. Such veins also provide information on the ambient stress conditions during deformation and mineralization. Based on bulk sampling and bulk O- and C-isotope analysis, combined with fluid inclusion microthermometry, many studies have demonstrated that syntectonic veins provide snapshots of fluid composition and stress conditions over the course of fault history. This is widely acknowledged for mode I extension veins that develop in the damage zones of faults. However, an important and unanswered question is the extent to which such veins record a more detailed fault history at the micron scale. In this study, we present new detailed in-situ micron-scale δ 18 O data, measured using Secondary Ion Mass Spectrometry (SIMS), combined with detailed fluid inclusion microthermometry and Δ47 clumped isotope thermometry to document the fluid and temperature conditions during mode I vein growth related to deformation along the Cotiella thrust fault (Pyrenees). All the studied veins show three distinct episodes of vein opening, recording a complex history of varying fluid composition and temperature. Results show that the studied fault portion passed from a hydrological rock-buffered system, in which formation waters were in isotopic equilibrium with the host sediments, to a fluid-buffered system involving meteoric water. However, such information is only achievable at the micron scale using Secondary Ion Mass Spectrometry. This study demonstrates the potential of these new micro-beam techniques for investigating fault behavior in more detail than previously available, particularly with respect to the nature of the fluids involved and the P–T conditions extant during fault activity.

ISSN:0263-4929 ISSN:1525-1314

We measured the extensional‐mode attenuation and Young's modulus in a porous sample made of sintered borosilicate glass at microseismic to seismic frequencies (0.05–50 Hz) using the forced oscillation method. Partial saturation was achieved by water imbibition, varying the water saturation from an initial dry state up to ∼99%, and by gas exsolution from an initially fully water‐saturated state down to ∼99%. During forced oscillations of the sample effective stresses up to 10 MPa were applied. We observe frequency‐dependent attenuation, with a peak at 1–5 Hz, for ∼99% water saturation achieved both by imbibition and by gas exsolution. The magnitude of this attenuation peak is consistently reduced with increasing fluid pressure and is largely insensitive to changes in effective stress. Similar observations have recently been attributed to wave‐induced gas exsolution–dissolution. At full water saturation, the left‐hand side of an attenuation curve, with a peak beyond the highest measured frequency, is observed at 3 MPa effective stress, while at 10 MPa effective stress the measured attenuation is negligible. This observation is consistent with wave‐induced fluid flow associated with mesoscopic compressibility contrasts in the sample's frame. These variations in compressibility could be due to fractures and/or compaction bands that formed between separate sets of forced‐oscillation experiments in response to the applied stresses. The agreement of the measured frequency‐dependent attenuation and Young's modulus with the Kramers–Kronig relations and additional data analyses indicate the good quality of the measurements. Our observations point to the complex interplay between structural and fluid heterogeneities on the measured seismic attenuation and they illustrate how these heterogeneities can facilitate the dominance of one attenuation mechanism over another.

The identification of markers of subduction zones in orogenic belts requires the estimation of paleo-geothermal gradients through pressure-temperature-time (P-T-t) estimates in mafic rocks that potentially derive from former oceanic units once. However, such markers are rare in supracrustal sequences specially in deeply eroded and weathered Precambrian orogens, and reconstructing their metamorphic history is challenging because they are commonly retrogressed and only preserve a few mineral relicts of high-pressure metamorphism. Metamorphosed mafic rocks from Pouso Alegre region of the Neoproterozoic Southern Brasilia Orogen outcrop as rare lenses within continental gneisses. They have previously been classified as retrograde eclogites, based on the presence of garnet and the characteristic symplectitic texture replacing omphacite. These rocks were interpreted to mark the suture zone between the Paranapanema and Sao Francisco cratons. To test the possible record of eclogitic conditions in the Pouso Alegre mafic rocks, samples including the surrounding felsic rocks have been investigated using quantitative compositional mapping, forward thermodynamic modeling and in-situ dating of accessory minerals to refine their P-T-t history. In the metamorphosed mafic rocks, the peak pressure assemblage of garnet and omphacite (Jd20, reconstructed composition) formed at 690 ± 35 °C and 13.5 ± 3.0 kbar, whereas local retrogression into symplectite or corona occurred at 595 ± 25 °C and 4.8 ± 1.5 kbar. The two reactions were coupled and thus took place at the same time. A zircon U-Pb age of 603 ± 7 Ma was obtained for metamorphic rims and linked to the retrogression stage. Monazite and metamorphic zircon U-Th-Pb ages for the surrounding rocks are at ca. 630 Ma and linked to peak pressure conditions similar to the one recorded by the mafic rocks. The low maximal pressure of 14 kbar and the high geothermal gradient do not necessarily support subduction process-related metamorphism but, more likely, metamorphism related to continental collision.

International audience; Studies of lithospheric mantle from (ultra)slow spreading ridges have shown that meltextraction at mid-ocean ridges may be incomplete, producing metasomatism/refertilizationof the shallow lithospheric mantle. However, it remains unclear whether similar processesoperate off axis and could affect the cooling lithosphere. Here, we report the discovery of agarnet xenocryst in a petit-spot lava sampled on the top of the downgoing Pacific plate infront of Japan. The trace-element composition of this garnet xenocryst, in particular thelow chromium, excludes a peridotitic origin, while the flat mid– to heavy rare earth elementpattern does not support direct crystallization from melt percolating through the oceaniclithosphere. Garnet formation is therefore interpreted as formed by a subsolidus reactionof a plagioclase-bearing cumulate during the progressive off-axis cooling of the lithosphere.Combining lithosphere cooling models and the specific physical conditions required for sub-solidus formation of garnet in tholeiitic systems (0.7–1.2 GPa) indicates that melt percolationto produce plagioclase-bearing cumulate occurs >150 km off axis. These conditions supportthat some low-degree melts produced off axis are not collecting to form mid-oceanic ridgebasalt (MORB), but percolate and crystallize during the cooling and thickening of the litho-spheric mantle. The demonstration of mantle refertilization affecting Pacific lithosphere offaxis is critical because such a process could explain the presence of metasomatic domainswith distinct physical and chemical properties in the depleted oceanic lithosphere.

Sri Lanka is a crucial Gondwana fragment mostly composed of granulitic rocks in the Highland Complex surrounded by rocks with granulite to amphibolite grade in the Vijayan and Wanni Complex that were structurally juxtaposed during Pan-African orogeny. Fluids associated with granulite-facies metamorphism are thought to have controlled various lower crustal processes such as dehydration/hydration reactions, partial melting, and high-temperature metasomatism. Chemical disequilibrium in the hybrid contact zone between a near peak post-tectonic ultramafic enclave and siliceous granulitic gneiss at Rupaha within the Highland Complex produced metasomatic reaction zones under the presence of melt. Different reaction zones observed in the contact zone show the mineral assemblages phlogopite + spinel + sapphirine (zone A), spinel + sapphirine + corundum (zone B), corundum (~ 30%) + biotite + plagioclase zone (zone C) and plagioclase + biotite + corundum (~ 5%) zone (zone D). Chemical potential diagrams and mass balance reveal that the addition of Mg from ultramafic rocks and removal of Si from siliceous granulitic gneiss gave rise to residual enrichment of Al in the metasomatized mineral assemblages. We propose that contact metasomatism between the two units, promoted by melt influx, caused steady state diffusional transport across the profile. Corundum growth was promoted by the strong residual Al enrichment and Si depletion in reaction zone whereas sapphirine may have been formed under high Mg activity near the ultramafic rocks. Modelling also indicated that metasomatic alteration occurred at ca. 850 °C at 9 kbar, which is consistent with post-peak metamorphic conditions reached during the initial stage of exhumation in the lower crust and with temperature calculations based on conventional geothermometry.

Pressure recorded in metamorphic rocks is typically assumed to represent a hydrostatic stress and thus depends linearly on depth. Recently, work in the Monte Rosa nappe in the western Alps has challenged this lithostatic assumption. Observable pressure differences of 0.8 ± 0.3 GPa between chloritoid, talc, and phengite-bearing lithologies (locally known as ‘whiteschists’) at ca. 2.2 - 2.5 GPa, and metagranite lithologies at 1.4 – 1.6 GPa have been recorded. These pressure variations, rather than being attributed to variable rock kinetics, partial retrogression, or tectonic mixing, have been interpreted to be mechanically induced. As part of this ongoing investigation, we will present work undertaken on newly discovered staurolite-chloritoid bearing metapelites belonging to the Monte Rosa basement, in order to constrain the peak pressure and temperature conditions during burial within the Alpine orogeny and the resulting tectono-metamorphic and geodynamic implications.Metapelitic samples from the Monte Rosa basement show a rich polymetamorphic history from high-T Variscan garnet growth through to high-P Alpine equilibration and decompression. Extensive phase petrology calculations have been undertaken on staurolite + chloritoid + phengite + paragonite assemblages, as well as garnet + chlorite + phengite + paragonite assemblages, representing equilibration at peak Alpine conditions. Various mixing models were employed due to non-negligible amounts of ZnO recorded in staurolite (~5% wt% and ~1 a.p.f.u) and the lack of available solution models. These result in peak Alpine conditions of 1.6 ± 0.2 GPa and 580 ± 15 ºC. These findings confirm the presence of significant disparities in pressure of 0.6 ± 0.2 GPa within the coherent Monte Rosa nappe.Vital for the reconstruction and tectonic history for the western Alps is the maximum burial depth of units involved. We argue that the maximum burial depth of the Monte Rosa unit was significantly less than 80 km (based on the lithostatic pressure assumption and minor volumes of whiteschist at > 2.2 GPa). Rather, the maximum burial depth of the Monte Rosa unit was presumably equal or less than ca. 60 km, estimated from pressures of 1.4 - 1.6 GPs recorded frequently in metagranite and metapelitic lithologies. This depth is compatible with burial and exhumation within an orogenic wedge, rather than a complex exhumation mechanism such as within a weak and long subduction channel. Equally, the relatively slower exhumation rates from shallower crustal depths fit more reasonable tectonic velocities.