Search

co-auteur étranger; International audience; Transition metal isotopes are particularly useful for understanding the conditions under which magmatic processes occur. Moreover, those with more than one oxidation state (e.g. Cr2+, Cr3+ and Cr6+) may also provide powerful constraints on the evolution of the redox state of the mantle over time. By investigating the Cr isotopic compositions in both magnesiochromite and silicate melts during experiments performed at 1300 °C and under controlled redox conditions (−12 −6) are much lighter than those of melts equilibrated with magnesiochromite at lower oxygen fugacity. The observed variations can be explained by changes in bonding environment for Cr under oxidised conditions in the silicate melts and/or in the magnesiochromite grains. Similarly, the second set of experiments designed to study fractional crystallisation (series B) suggest that Cr isotope fractionation is larger under oxidising conditions.

Co-auteur étranger; International audience; Oxygen isotope analysis of zircon, often combined with geochronology and Hf isotope analysis, has been pivotal in understanding the evolution of continental crust. In this contribution, we expand the use of underexplored accessory phases (titanite and apatite) by demonstrating that their oxygen isotope systems can be robust, and by developing geochemical indicators involving O isotopes and trace element concentrations to better constrain magma petrogenesis. These minerals have the advantage over zircon of being present in less evolved magmas and being more responsive to igneous processes and crustal metamorphism. We present new data on titanite, apatite and zircon from carefully-selected granitoids through geological time: the Phanerozoic high Ba-Sr granites (Caledonian province, Scotland), Archean sanukitoids (Karelia province, Finland) and a Neoproterozoic basalt-andesite-dacite-rhyolite suite (BADR; Guernsey, Channel Island). We demonstrate: (i) that d 18 O values of the studied accessory minerals are not affected by crystal fractionation, (ii) a strong correlation between d 18 O in all three accessory minerals, showing that apatite and titanite can faithfully record the magmatic d 18 O; (iii) that these accessory minerals can also record metamorphic and/or fluid circulation events during the syn-to post-magmatic history of granitoids.

Achondrite meteorites are remnants of the earliest planetary differentiation processes in the Solar System. They have been used to anchor short-lived radiochronometers to absolute ages determined from long-lived radiochronometers. More specifically, when comparing the isotopic systematics of the shortlived 26Al-26Mg chronometer anchored to absolute U-corrected Pb-Pb ages, inferences about the distribution of 26Al (half-life of 717 000 yr) in the protoplanetary disk can be evaluated. The ungrouped achondrite Erg Chech (EC) 002 has a distinct mineralogy and more evolved elemental composition compared to basaltic achondrites. In situ and solution 26Al-26Mg chronometry and 53Mn-53Cr chronometry suggest that EC 002 formed within 0.7 to 2.2 Ma after the formation of Ca-Al-rich inclusions (CAIs), making it the oldest known sample of igneous crust in the Solar System (Barrat et al., 2021; Anand et al., 2022; Zhu et al., 2022; Fang et al., 2022). Here we present the U-corrected Pb-Pb age and 26Al-26Mg age obtained by MC-ICPMS solution analysis of the same mineral separate samples of EC 002. In addition, six merrillite grains were analyzed by in-situ SIMS to determine their Pb-Pb individual ages. The U isotope composition of EC 002 exhibits internal heterogeneities between leached pyroxene (238U/235U = 137.766 ± 0.027) and the bulk rock (238U/235U = 137.8190 ± 0.0074). The Pb isotope composition of progressively leached pyroxenes are characterized by radiogenic 206Pb/204Pb ratios (ranging from 41 to 23487). Using the U isotope composition of the leached pyroxenes, the resulting age of the 207Pb/206Pb-204Pb/206Pb isochron is 4565.87 ± 0.30 Ma (2r). The weighted mean of the Pb-Pb ages of seven SIMS analyses of merrillites are 4564.3 ± 5.2 Ma (2r). These similar ages (within uncertainty) indicate rapid cooling and the absence of significant thermal events after 4559 Ma on the parent body of EC 002. The 26Al-26Mg isochron through a bulk rock, pyroxene, fine-rained and four plagioclase fractions defines an initial 26Al/27Al ratio of [8.89 ± 0.79] 106 corresponding to a formation age of 1.83 ± 0.12 Ma after CAIs ([5.23 ± 0.13] 105; Jacobsen et al., 2008). The initial 26Al abundance is consistent with previous MC-ICP-MS 26Al-26Mg reported systematics for EC 002 (Fang et al., 2022), but 0.46 ± 0.13 Myr older than the in situ SIMS 26Al-26Mg age previously reported by Barrat et al. (2021).When anchored to the absolute Pb-Pb age of CV3 CAIs (4567.30 ± 0.16 Ma; Connelly et al., 2012), the Al-Mg model age of EC 002 is 4565.47 ± 0.20 Ma, slightly younger than its U-corrected Pb-Pb age.

Zircon is often used to study granites (sensu lato) and continental crust because it is very resistant and can be analyzed for various isotope systems that provide time and source information about their parental melts. Granites with different petrological histories have distinct bulk-rock silicon isotope compositions but it is unclear if these differences are also detectable in zircon because of superimposed fractionation effects (e.g., related to temperature, silica content, magmatic processes). The present study explores the Si isotope signatures of zircon from various granite types to constrain their isotope fractionation behavior and uses them as igneous petrogenetic tools, and possibly as granite source discriminators when zircon is found in detrital sediments. Our results show that although Si isotope compositions in zircon can be modified by secondary (post-crystallization) processes such as alteration/weathering and metamorphism, they are primarily controlled by zircon-melt isotope fractionation, which depends on both zircon crystallization temperature and magma silica content. Once these fractionation effects are understood and filtered out, a pattern emerges between Si isotope signatures of zircons from different granite types that is consistent with theoretical and experimental results as well as with known Si isotope differences at the bulk-rock scale. Silicon isotope ratios in zircon can track magma evolution (e.g., temperature and SiO2 changes) and, hence, reveal complex processes that involved magma mingling, fractional crystallization, and/or multiple sources. This study, therefore, illustrates that Si isotopes in zircon can be used to investigate magma evolution and represents a useful complement to existing techniques in granite studies involving zircon (e.g., U-Th-Pb, Lu-Hf and O isotopes) provided that it is not used as a stand-alone technique.

International audience; ICP-AES method was used to evaluate the impact of uranium mine dumps on the surface waters from Crucea region (Romania). Speciation modelling indicates that uranyl-carbonateUO2(CO3)2-2 and hydroxy uranyl-carbonate UO2CO3(OH)3- are the main uranium species and that thorium hydroxyl-carbonate Th(OH)3CO3- represents the main thorium species in these waters. Uranium is the most significant trace element in the surface waters nearby the waste rock dumps, sometimes reaching levels op up to 1 mg L-1, well in excess of theRomanian standards limits. A remarkably good correlation exists between dissolved U and the sum of anion concentrations (NO3- + CO32- + SO42- + Cl-) whereas no correlation was found between U and silica, indicating that uranium in these stream waters derived mainly from oxidation of uraniferous bitumen and/or dissolution of carbonates.

This study investigates the partitioning of U and Th between molten metal and silicate liquid (DU and DTh) during Earth’s core-mantle differentiation. We report new Th partition coefficients between chondritic silicate melt and various Fe-rich alloys in the system Fe-Ni-C-S-Si as determined by experiments in a multi-anvil apparatus at 3–8 GPa, 2073–2373 K, and oxygen fugacities from 1.5 to 5 log units below the iron-wustite (IW) buffer. By compiling all existing data on molten metal-silicate liquid partitioning of U and Th, we develop global models of U and Th partitioning between the mantle and core throughout Earth’s accretion. The calculated concentrations in the Bulk Silicate Earth (BSE) are in agreement with previous studies (UBSE = 11.42 ± 0.45 ppb and ThBSE = 43.20 ± 1.73 ppb), whereas the contents of these radioactive elements in the Earth’s core remain negligible. Compared to recent geochemical estimations, the calculated (Th/U)BSE supports EL rather than EH enstatite chondrites as the reduced building blocks of the Earth. Furthermore, we demonstrate that Th is much more sensitive than U to the oxygen content of the metallic phase. To reproduce the Th/U ratio of the BSE within its uncertainties, the oxygen content of the Earth’s core must be lower than 4.0 wt%. By combining other existing constraints, this suggests that the core contains 2.0–4.0 wt% O. The calculations of U and Th concentrations and Th/U in the BSE developed herein can be used as new constraints for determining the concentrations of other refractory lithophile elements in the BSE as soon as their metal-silicate partition coefficients are well constrained over the conditions of core segregation.