Nickel and Chromium Stable Isotopic Composition of Ureilites: Implications for the Earth's Core Formation and Differentiation of the Ureilite Parent Body

We report the first Ni and Cr stable isotope data for ureilite meteorites that are the mantle residue of a carbon-rich differentiated planet. Ureilites have similar Ni stable isotope compositions as chondrites, suggesting that the core-mantle differentiation of ureilite parent body (UPB) did not fractionate Ni isotopes. Since the size of Earth is potentially larger than that of UPB; with diameter >690 km), resulting in higher temperatures at the core-mantle boundary of Earth, it can be predicted that the terrestrial core formation may not directly cause Ni stable isotope fractionation. On the other hand, we also report high-precision Cr stable isotope composition of ureilites, including one ureilitic trachyandesite (ALM-A) that is enriched in lighter Cr stable isotopes relative to the main-group ureilites, which suggests that the partial melting occurred on UPB. The globally heavy Cr in the UPB compared to chondrites can be caused by sulfur-rich core formation processes

Constraining P–T conditions during thrusting of a higher-pressure unit over a lower-pressure one (Gran Paradiso, Western Alps)

International audienceIdentifying higher-pressure units overlying lower-pressure ones is a first order argument to determine the presence of large-scale thrusting. For the first time petrology is used to quantify the pressure difference between two stacked units in the Western Alps. In the Gran Paradiso Massif, the Money unit crops out as a tectonic window below the Gran Paradiso unit. The reconstruction of the Alpine evolution of these two units and the history of their tectonic contact has been achieved using a multidisciplinary approach that combines meso- and micro-structural analysis and pseudosection calculations. In both units, four stages of deformation and metamorphism have been identified. Stage 1 reflects the phase of continental crust subduction and P–T conditions of ~18-20 kbar, 480-520 °C and of ~13-18 kbar, 500-530 °C have been estimated for the Gran Paradiso and the Money units, respectively. This yields a maximum difference of ~20 km in the depth reached by these two units during the early Alpine history. Thrusting of the Gran Paradiso unit over the Money unit (stage 2) led to the development of the main foliation and occurred in the high-pressure part of the albite stability field at P–T conditions of ~12.5-14.5 kbar and 530-560 °C, identical in both units. The thrust contact was folded during stage 3 together with the entire Money unit, and then both units were exhumed together (stage 4). During this polyphase evolution, detrital garnet has been partially dissolved, while the earliest Na-bearing phases (glaucophane, paragonite) have been overprinted by the low-pressure mineral associations. The uncertainties on derived pressures between the two units are unfortunately larger than hoped, and this is attributed to the muscovite solid-solution model not incorporating a pyrophyllite component

Experimental constraints on metasomatism of mantle wedge peridotites by hybridized adakitic melts

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Suseinargiuite, (Na<SUB>0.5</SUB>Bi<SUB>0.5</SUB>)MoO<SUB>4</SUB>, the Na-Bi analogue of wulfenite, from Su Seinargiu, Sardinia, Italy

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A window into an older orogenic cycle: P‐T conditions and timing of the pre‐Alpine history of the Dora‐Maira Massif (Western Alps)

International audienceDeciphering the pre-orogenic evolution of subducted continental basement is challenging due to pervasive reworking of crust during subduction and exhumation. Survival of such polycyclic basement may occur locally in low strain domains bounded by intensely overprinted rocks. The Palaeozoic history of basement involved in Alpine continental subduction is investigated in the northern Dora-Maira Massif where a kilometre-scale domain of low strain preserves a pre-Alpine amphibolite-facies foliation in garnet-biotite orthogneiss and garnet-staurolite micaschist. By contrast, a first generation garnet is the only pre-Alpine relict in pervasively reworked domains surrounding the low-strain domain. Thermodynamic modelling based on garnet isopleths in micaschist constrains the pre-Alpine P–T evolution from 4–5 kbar and ~ 500 °C to 6–7 kbar and ~ 650 °C, which is consistent with Barrovian metamorphism up to the staurolite zone. In this micaschist, monazite included in garnet rims provide an age of 324 ± 6 Ma (95% confidence interval; c.i.). On the basis of textural and chemical data, this is interpreted as recording peak Barrovian metamorphic conditions. Low Th/U metamorphic zircon overgrowths and crystals yield an age of 304 ± 2 Ma (95% confidence interval). On the basis of the trace element concentrations and REE patterns measured in garnet and metamorphic zircon, the latter is tentatively interpreted as having grown during early exhumation or cooling, involving garnet consumption and fluid infiltration. The reconstructed Variscan Barrovian metamorphism of the northern Dora-Maira basement is consistent with that documented in the External Crystalline Massifs and in the Austroalpine domain of the Alps. The Palaeozoic basement of the Dora-Maira Massif likely represents upper crustal material, later involved in Alpine continental subduction under high- to ultra-high-pressure conditions

Nature and evolution of the lithospheric mantle beneath the passive margin of East Oman: evidence from mantle xenoliths sampled by Cenozoic alkaline lavas

International audienceCenozoic alkaline lavas from the Al Ashkharah area, facing the Indian ocean along the North-East Oman coastline, contain numerous small (‹ 2cm) mantle xenoliths. They provide a unique opportunity to investigate the nature and evolution of the upper mantle beneath the Oman passive margin, bordering the Owen Basin. All studied xenoliths are porphyroclastic to equigranular spinel lherzolites and harzburgites They are all devoid of amphibole and phlogopite. The composition of their clinopyroxenes, orthopyroxenes, olivines and spinels indicate that these samples are witnesses of a typical subcontinental lithospheric upper mantle and are quite distinct from the peridotites cropping out in the nearby Oman ophiolite. The clinopyroxene major element composition record an evolution from fertile lherzolites (Mg#: 89 and Al2O3: 7.5 wt%) to refractory harzburgites (Mg#: 93.5 and Al2O3: 2.5 wt%). The clinopyroxene of most samples are characterised by REE patterns evolving continuously from spoon-shaped to LREE-enriched with almost flat HREE spectra (LaN/YbN: 2.5-30; LaN/SmN: 3.2-24; SmN/YbN: 0.25-4.6; HoN/LuN : 0.88-1.15) and strong negative Ba, Nb, Zr, Hf and Ti anomalies. We propose that these geochemical fingerprints can be accounted for in the frame of a two stages; (1) a - likely ancient - decompression melting event characterised by a degree ranging from 1 to a maximum of 19 % and unrelated to the recent tectonic evolution of the Oman margin, followed by (2) metasomatic transformation possibly related to the circulation of carbonate-rich silicate melt during the Cenozoic rifting event that led to the opening of the Owen basin

Tin concentration in hydrothermal sulphides related to ultramafic rocks along the Mid-Atlantic Ridge: a mineralogical study

Hydrothermal polymetallic sulphides collected along the Mid-Atlantic Ridge and hosted in ultramafic rocks (Ashadze, Logatchev and Rainbow ore fields) are enriched in Sn compared to sulphides associated with volcanic rocks. At Logatchev, the averaged Sn ore concentration reaches 2000 ppm. The distribution of Sn among sulphides was studied using scanning electron microscopy and electron-microprobe analysis. The Sn concentration can reach up to 6 wt.% in sphalerite and 2 wt.% in chalcopyrite. Raman micro-spectroscopy investigation suggests that most of Sn is carried by stannite micro-inclusions in sulphides. According to the mineralogical and chemical studies, the following paragenetic sequence is proposed: (1) Sn (<1 wt.%) first precipitates as solid solution in low-temperature sphalerite; (2) at high temperature (300 C) and low pH (∼3) sphalerite is replaced by chalcopyrite; Sn, previously contained in the low-temperature sphalerite, contributes to the formation of stannite, as micro-inclusions in the replacement front; (3) eventually Sn is distributed within newly formed chalcopyrite (<1 wt.%). Tin enrichment of Zn-Cu ores appears as an indicator of hydrothermal reworking of ultramafic sulphide deposits

Diffusion parameters of grain-growth inhibitors in WC based hardmetals with Co, Fe/Ni and Fe/Co/Ni binder alloys

The diffusion behaviour of the grain-growth inhibitors (GGI) Cr and V during early sintering stages from 950-1150 °C was investigated by means of diffusion couples of the type WC-GGI-binder/WC-binder. Besides Co, also alternative Fe/Ni and Fe/Co/Ni binder alloys were investigated. It was found that the diffusion in green bodies differs significantly from sintered hardmetals. Diffusivities in the binder phase were determined from diffusion couples prepared from model alloys and were found to be almost equal for Co and alternative binder alloys. The diffusion parameters determined from green bodies allowed to estimate the GGI distribution in a hardmetal during heat up. This was subsequently used to estimate an appropriate grain size of VC and Cr3C2 in hardmetals, which is required to ensure a sufficient GGI distribution during sintering before WC grain-growth initiates