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Introduction: High pressure minerals from meteoritic shock melt veins are key to understand the collisional history of the Solar System. The L chondrites, the most shocked meteorites [1], present abundant shock melt veins from which we can retrace their history. To this day, their family of origin is still debated. The shock timescale of 7 chondrites was measured to deduce their parent body diameters. Moreover, we use tuite, a high pressure phosphate mineral to perform U-Pb datation by SIMS. We find a bimodal distribution of ages, which match closely the ages previously obtained for Creston and Novato, two other shocked L6 chondrites [2]. Samples and Methods: Seven samples were first studied by optical microscopy and Raman spectroscopy to identify high pressure (HP) polymorphs. Scanning Transmission Electron Microscopy (STEM) was then used to study microstructures and transformation/growth mechanisms. The EDX was used to locate eligible minerals for datation (phosphates). Combined STEM-EDX (UMET, Lille, France) and NanoSIMS (MNHN, Paris, France) chemical maps were finally collected on the same FIB sections in order to compare these two analytical approaches and produce chemical maps. The U and Pb isotope concentrations where then measured on the identified tuite grains with the help of a Cameca IMS 1280 LG SIMS (CRPG, Nancy France) for datation. Results: Multiple high pressure textures were observed in all seven samples. Polycristalline assemblages of ringwoodite are typically the dominant texture, but more exotic textures were also found. Some meteorites present ringwoodite as lamellae inside olivine crystals, whereas others seem to present an assemblage of MgSiO3 glass with akimotoite crystallites. Both these textures allowed us to investigate elemental diffusion induced by structural changes. We therefore calculated shock timescales following the methods described in [3,4]. For all our samples, assuming a temperature of 2400K [5], shock timescales ranging between 0.5 and 20 seconds were derived. The meteorites that do not contain ringwoodite lamellae have significantly higher shock timescales, between 11 and 16 seconds. These larger shock timescales were caused by an impact between larger bolides, including a parent body of at least 150km wide. This is significantly higher than parent-bodies with diameters around 70km required by the other group. In almost all meteorites, we find tuites inside the shock melt veins. The meteorites that do not contain tuite have at least shocked apatites and whitlockites, with shifted Raman spectra indicating a change in structure. We date both the host rock apatites and the shock vein phosphates. This allows us to obtain conclusive collision ages from a normal concordia diagram for two of our samples, one of each group. The tuites record collisions ages of 461+-57Ma for the group with the largest parent body, and 650+-160Ma for the group with the smaller parent body. The host rock minerals record upper intercepts of 4481Ma in both meteorites. Conclusion: Shocked L chondrites seem to define two groups, which are texturally different and appear to have a completely different history. A smaller, 70km-wide parent body exploded first (650 Ma ago) and yielded most of the shocked meteorites, and a larger one exploded in the cataclysmic collision known today as the "L chondrite parent body breakup" 470Ma ago. The upper intercept of 4481Ma could date the early separation of the original L chondrite parent body into different families. [ABSTRACT FROM AUTHOR]

The analysis of texture, major element and oxygen isotope compositions of cloudy garnet crystals from a metapelite sampled on Ikaria Island (Greece) is used to assess the model of growth and re-equilibration of these garnet crystals and to reconstruct the pressure-temperature-fluid history of the sample. Garnet crystals show complex textural and chemical zoning. Garnet cores (100-200 μm) are devoid of fluid inclusions. They are characterized by growth zoning demonstrated by a bell-shaped profile of spessartine component (7-3 mol.%), an increase in grossular from 14 to 22 mol.% and δO values between 9.5 ± 0.3‰ and 10.4 ± 0.2‰. Garnet inner rims (90-130 μm) are fluid inclusion-rich and show a decreasing grossular component from 22 to 5 mol.%. The trend of the spessartine component observed in the inner rim allows two domains to be distinguished. In contrast to domain I, where the spessartine content shows the same trend as in the core, the spessartine content of domain II increases outwards from 2 to 14 mol.%. The δO values decrease towards the margins of the crystals to a lowest value of 7.4 ± 0.2‰. The outer rims (<10 μm) are devoid of fluid inclusions and have the same chemical composition as the outermost part of domain II of the inner rim. Garnet crystals underwent a four-stage history. Stage 1: garnet growth during the prograde path in a closed system for oxygen. Garnet cores are remnants of this growth stage. Stage 2: garnet re-equilibration by coupled dissolution-reprecipitation at the temperature peak (630 < T < 650 °C). This causes the creation of porosity as the coupled dissolution-reprecipitation process allows chemical (Ca) and isotopic (O) exchange between garnet inner rims and the matrix. The formation of the outer rim is related to the closure of porosity. Stage 3: garnet mode decreases during the early retrograde path, but garnet is still a stable phase. The resulting garnet composition is characterized by an increasing Mn content in the inner rim's domain II caused by intracrystalline diffusion. Stage 4: dissolution of garnet during the late retrograde path as garnet is not a stable phase anymore. This last stage forms corroded garnet. This study shows that coupled dissolution-reprecipitation is a possible re-equilibration process for garnet in metamorphic rocks and that intra-mineral porosity is an efficient pathway for chemical and isotopic exchange between garnet and the matrix, even for otherwise slow diffusing elements. [ABSTRACT FROM AUTHOR]

We present in situ microanalyses of Fe3+/ΣFe in mantle-derived kaersutites as measured by electron probe microanalysis (EMP) based on the "self absorption induced FeLα peak shift" method. The EMP results are not in agreement with bulk (wet chemistry) data. The heterogeneities revealed for some kaersutite megacrysts, when comparing bulk and EMP Fe3+/ΣFe results, cannot explain the differences with the EMP measurements. It is thus proposed that any EMP overestimation of Fe3+/ΣFe results from a beam-induced dehydrogenation and a subsequent oxidation of Fe2+ to Fe3+ according to the known relation: Fe2+ + OH- = Fe3+ + O2- + 1/2 H2. To demonstrate this phenomenon, H losses were measured by secondary ion mass spectrometry (SIMS) after EMP irradiation at different beam currents on two amphiboles with 1.1 and 1.7 wt% H2O, respectively. In both amphiboles, H losses were observed under high beam currents (240 and 100 nA). No dehydrogenation is observed under lower beam currents for the 1.1 wt% H2O amphibole, but still occurs, down to at least 50 nA, for the amphibole with the greatest H2O contents. Amphiboles with low H2O contents (below ~0.5 H+) are less affected by beam damage. For amphiboles with higher H2O contents, the electron beam current density should be reduced with consideration given to the resulting high statistical errors. [ABSTRACT FROM AUTHOR]

Detailed electron- and ion-microprobe analysis of diopsides extracted from a block of marble collected in the Mt. Marcy anorthosite massif (Adirondack Highlands, New York) shows that single crystals have preserved Al-Si zoning features from early contact metamorphism later modified by regional metamorphism. Diopsides show Al and Si complementary zoning from core to rim; average Al concentration in rims is constant (0.11 pfu), but values in the cores vary with crystals from 0.06 to 0.14 pfu. A δ18O profile measured with the ion microprobe shows no zoning. Modeling of the Al-diffusion profiles affected by regional metamorphism of the Grenville orogeny shows that original Al zoning was sharp and corresponds to a rapid change of crystal-growth conditions during early contact metamorphism: (1) diopside cores crystallized with variable Al contents due to restricted fluid circulation or differing sedimentary composition, (2) then pervasive fluid infiltration crystallized the homogeneous rim of diopside, and (3) core to rim Al zoning was later smoothed by granulite-facies metamorphism. Homogeneity of δ18O in core and mantle (near rim) of diopsides suggests that the crystals were isotopically homogeneous prior to and during regional metamorphism. Values of Al-Si diffusion coefficient deduced from modeling are in agreement with low-temperature extrapolation of experimental data. [ABSTRACT FROM AUTHOR]

Abstract: The sodium solubility in silicate melts in the CaO–MgO–SiO2 (CMS) system at 1400°C has been measured by using a closed thermochemical reactor designed to control alkali metal activity. In this reactor, Na(g) evaporation from a Na2O–xSiO2 melt imposes an alkali metal vapor pressure in equilibrium with the molten silicate samples. Because of equilibrium conditions in the reactor, the activity of sodium-metal oxide in the molten samples is the same as that of the source, i.e., aNa2O(sample) = aNa2O(source). This design also allows to determine the sodium oxide activity coefficient in the samples. Thirty-three different CMS compositions were studied. The results show that the amount of sodium entering from the gas phase (i.e., Na2O solubility) is strongly sensitive to silica content of the melt and, to a lesser extent, the relative amounts of CaO and MgO. Despite the large range of tested melt compositions (0

Abstract: Growth of zircon with respect to that of garnet has been studied using a combination of petrography, U–Pb dating and oxygen isotope analysis. The aim is to document the mechanism and pressure–temperature conditions of zircon growth during metamorphism in order to better constrain the Tertiary metamorphic history of Naxos, Greece. Two metamorphisms are recognised: (1) an Eocene Franciscan metamorphism (M1) and (2) a widespread Miocene Barrovian metamorphism (M2) that increases from greenschist facies up to partial melting. An amphibolite sample contains zircon crystals characterised by a magmatic core and two metamorphic rims, denoted as A and B, dated at 200–270, 42–69, and 14–19 Ma, respectively. The first metamorphic rim A (δ 18O=7±1‰) preserves the δ 18O value of the magmatic core (6.2±0.8‰), whereas rim B is characterised by higher δ 18O values (7.8±1.8‰). These observations indicate the formation of A rims by solid-state recrystallisation in a closed system with regard to oxygen and those of B in an open system. Compositional zoning in garnet is interpreted as the result of decompressional heating. Zircon B rims and garnet rims display similar δ 18O values which indicates a contemporaneous growth of garnet and zircon rims during the Miocene Barrovian event (M2). Calcic gneiss and metapelite samples contain zircon crystals with single metamorphic overgrowths aged 41–57 Ma. δ 18O values measured in zircon overgrowths (11.8±1.4‰) from the calcic gneiss are similar to those measured in garnet rims (11.4±1.1‰) from the same rock. This suggests that garnet rims and zircon overgrowths grew during the high pressure–low temperature event in equilibrium with prograde fluids. In the metapelite sample, δ 18O values are similar in garnet cores (14.8±0.2‰) and in zircon metamorphic overgrowths (14.2±0.5‰). As zircon overgrowths have been dated at ca. 50 Ma by U–Pb, garnet cores and zircon overgrowths are interpreted to have grown during the high pressure event. As demonstrated here for the island of Naxos, correlating the crystallisation of zircon with that of metamorphic index minerals such as garnet using stable isotope composition and U–Pb determination is a powerful tool for deciphering the mechanism of zircon growth and pin-pointing zircon crystallisation within the metamorphic history of a terrain. This approach is potentially hampered by an inability to verify the degree of textural equilibrium of zircon with other mineral phases, and the possible preservation (in metamorphic rims) of isotopic signatures from pre-existing zircon when they form by recrystallisation. Nevertheless, this study illustrates the application of this approach in providing key constraints on the timing and mechanism of growth of minerals important to understanding metamorphic petrogenesis. [Copyright &y& Elsevier]

Diffusion modeling of zoning profiles in eclogite garnets from three different tectonic units of Mt. Dabie, UHPM unit, HPM unit and northern Dabie, was used to estimate the relative time span and cooling rates of these rocks. Modeling result for the Huangzhen eclogite garnet shows that the maximal time span for the diffusion-adjustment process is about 22 Ma since the peak-temperature metamorphism, which is the maximum time span from amphibolite facies metamorphism to greenschist facies metamorphism. The Bixiling eclogites had subjected to a cooling process at a rate of ∼ 10°C/Ma from 750°C to 560°C during 20 Ma. The second cooling stage of the Raobazhai eclogite following granulite-facies metamorphism is an initial fast cooling process at a rate of about 25°C/Ma and then slowed down gradually. All these belong to a cohereni Dabie collision orogen with differences in subduction depth and exhumation/uplifting path. [ABSTRACT FROM AUTHOR]

Presents information on a study which investigated the ages and chemical compositions of zircons from pyroxenites in northern Dabie Terrain in China. Methodology; Results and discussion; Conclusions.

The dynamics of magmatic processes at large mafic ocean island volcanoes control the likely locations (central caldera versus flanks) and timing of their eruptions. Crystals and their melt inclusions are key witnesses of these processes but are rarely studied in detail and in the same samples. Here we report the crystal and melt inclusion compositions of the April 2007 caldera-forming eruption of Piton de la Fournaise volcano and discuss how they relate to geophysical unrest monitoring data. Olivine crystals show mainly normal zoning (decrease in Mg/Fe) towards the rims, and also around some melt inclusions. Many crystals also show fine-scale skeletal structures defined by high phosphorus concentrations. Melt inclusions contain 53–205 ppm CO 2 and 0.25–1.1 wt% H 2 O, and δ D (δ D values expressed as δ D VSMOW) ranges from −135 to 62‰. Monitoring data show that inflation of the edifice started about a month before the first 2007 eruption: magma intrusion occurred at ≈3 km below sea level, and quickly migrated towards shallower depths (about 1 km above sea level). Such a time frame of magma movement is recorded in the chemical zoning of the olivine crystals that massively and quickly crystallized when reaching shallow depth, without significant interactions between resident and intruding magmas. The intrusion was followed by lateral flank eruption and caldera collapse. The chemical zoning of the olivine crystal rims and around the melt inclusions indicates that the newly created crystal-mush moved laterally towards the surface in matter of days to 3 months. Post-caldera samples show significant H+ loss, likely due to the depressurization of the magmatic system stored at shallow level. Our findings are different from other mafic ocean island volcanoes or stratovolcanoes (e.g. Kilauea, Canary Islands, and Etna), where crystals commonly record magma mixing between evolved and shallow melts and intruding mafic melts. We speculate that the difference between our findings and those of similar mafic ocean island volcanoes is due to the variety of magma supply rates from depth. • Olivine and their hosted melt inclusions reveal the dynamics of the plumbing system. • Timescales of days to months for magma ascent, lateral transport and eruption. • Correlation between the calculated timescales and the duration of the unrest. • Olivine zoning reveal that crystal-mush was formed "real-time" during the unrest. • H+ diffusion in the melt inclusions is linked with the caldera collapse episode. [ABSTRACT FROM AUTHOR]

Abstract The goal of this paper is to use the structural, metamorphic and geochronological record from the migmatitic core of the Naxos dome (Greece) and its associated subdomes to address the internal dynamics of a partially molten orogenic root. U-Pb ages from ca. 24 to 16 Ma and textures of zircon in the migmatites suggest successive dissolution and precipitation cycles with a period of 1 to 2 Ma , interpreted as the timescale of convective instabilities in a ca. 20 km thick partially molten layer. Dimensional analysis indicates that convection of this root requires a viscosity lower than 1018 Pa·s, consistent with viscosity values expected for partially molten felsic rocks. Structural analysis and U-Pb geochronology of deformed granitic dikes rooting in the migmatites record the subsequent development of the Naxos dome by diapirism from ca. 16 to 13 Ma. The size of the first order migmatite dome on Naxos (5 × 12 km) requires that the unstable layer at the onset of diapirism was 5 to 10 km thick and presented a moderate viscosity contrast with its envelope. From this analysis we propose that the Naxos migmatite dome documents a two stage dynamic evolution for the partially molten root of the Aegean belt characterized by (1) crustal scale convection for at least 8 Ma and (2) diapirism for about 3 Ma during progressive thinning of the collapsing orogenic crust. Graphical abstract Unlabelled Image Highlights • Naxos migmatite domes and subdomes denote a gravitationally unstable orogenic root. • U-Pb zircon ages are consistent with dissolution-precipitation cycles of 1 to 2 Ma. • U-Pb ages of dikes indicate that dome development occurred in about 3 Ma. • Convection of a 20 km thick molten crust requires a viscosity lower than 1018 Pa·s. • Diapirism involves a 5–10 km unstable layer and occurred during orogenic collapse. [ABSTRACT FROM AUTHOR]

A significant portion of the Earth's water is found at trace levels in nominally anhydrous rock forming minerals in the crust and mantle. Its distribution has far-reaching implications in melt-generating processes, mantle rheology and the stability of the lithosphere. We report petrographic, major and trace element, Fourier transform infrared spectroscopy and secondary ion mass spectrometry data on four eclogite xenoliths from the Obnazhennaya kimberlite, Siberia that contain hydrous minerals at upper mantle conditions. Al-rich (>9.9 wt% Al 2 O 3) omphacitic clinopyroxene, a major mineral of eclogites, contains crystallographically controlled exsolutions of CaO-rich (up to 23.6 wt%) garnet (coronae, lenses), garnet-zoisite (lamellae) or amphiboles (needle/blade-like exsolutions). The omphacite hosting the exsolved lamellae shows enrichment or depletion in major (Al, Si, Mg) and trace (e.g., HREE) elements compatible with garnet and zoisite, proportional to the abundance of exsolutions. We argue that zoisite likely formed concomitantly with garnet exsolution from a water- and Al-rich precursory omphacite, without any fluid addition. The compositional gradients are consistent with diffusion-controlled partitioning in a closed system, and their preservation indicates the exsolution likely took place shortly prior to eclogite entrapment by the kimberlite. Large, non-exsolved, omphacite grains contain minimum ~870–1500 ppm wt. H 2 O (determined as OH). We infer that intertwined lamellae of hydrous garnet (600–960 ppm wt. H 2 O) and minor (<1%) zoisite (2 wt% H 2 O) formed at mantle conditions due to progressive exsolution, without external fluids. Recalculated whole rock water contents range from ~320 to 970 ppm wt. H 2 O. These estimates exceed by far those for the surrounding peridotitic mantle and suggest that, although a minor component, eclogites may locally be an important water reservoir in the cratonic mantle. • Rare eclogites from Obnazhennaya kimberlite contain water-rich exsolutions. • Zoisite stability at mantle conditions is demonstrated for the first time in natural samples. • Bulk water concentrations of eclogites exceed those of the surrounding peridotites. • Eclogites may be unique carriers and storage for water in the lithospheric mantle. [ABSTRACT FROM AUTHOR]

The 2007 caldera-forming eruption of Piton de la Fournaise (PdF) erupted the largest volume of magma (210 Mm3) recorded at this volcano in at least three centuries. Major and trace element and Sr–Nd isotope data for bulk-rocks, groundmasses and olivine phenocrysts have been combined with melt inclusion data (major, trace and volatile elements) to track magma evolution over the whole eruptive sequence. We show that each eruptive phase had a distinctive geochemical and petrological signature and that caldera collapse on 5 April was preceded by a marked shift in bulk magma composition and crystal content and size. Aphyric basalt erupted at the beginning of the sequence (February 2007) had relatively high Sr isotope ratio (87Sr/86Sr = 0·70420–0·704180) and low Nd isotopic ratio (143Nd/144Nd = 0·51285–0·51286). Olivine-basalts extruded on 2–5 April just before caldera collapse are less enriched in radiogenic Sr (87Sr/86Sr = 0·70412–0·70416), but characterized by the same Nd isotopic composition. This magma is interpreted as a new deep input, which pressurized the shallow PdF plumbing system and triggered the 2007 activity. Post-collapse oceanite lavas represent the main volume of magma extruded in 2007. Their bulk-rocks and groundmasses have 87Sr/86Sr (∼0·70418) intermediate between those of February and 5 April, and similar to those of the March 2007 and 2001–2006 lavas. We show that the Steady State Basalts (SSB) commonly erupted at PdF are hybrid melts, which result from multistep mixing between ‘alkaline’ and ‘transitional’ end-members. Our results lead us to propose a new model of the PdF plumbing system to reconcile the petrological, geochemical and geophysical observations: (1) the shallow portion (above sea level) of the PdF plumbing system hosts several small sills, in which magma experiences variable degrees of degassing, cooling and crystallization; (2) oceanite lavas result from the withdrawal of shallow harrisitic mushes stored at low pressures (<48 MPa; <1800–2400 m depth) below both the volcano summit and its eastern flank; (3) water degassing plays a major role in fast magma crystallization at shallow depths. Multistep ascent and periodic extrusion of the shallow magmas is promoted by injections of deeper and hotter basaltic magma, containing up to 1·3 wt % H2O and 1630 ppm S. In 2007, the new deep input was the ultimate source of the large excess in sulfur degassing detected by satellites. Lateral draining and intrusion of magma below the eastern flank of the volcano are the cause of major volcano deformation, flank sliding and summit caldera collapse. [ABSTRACT FROM AUTHOR]