Iron isotope fractionation in subduction-related high-pressure metabasites (Ile de Groix, France)

Characterisation of mass transfer during subduction is fundamental to understand the origin of compositional heterogeneities in the upper mantle. Fe isotopes were measured in high-pressure/low-temperature metabasites (blueschists, eclogites and retrograde greenschists) from the Ile de Groix (France), a Variscan high-pressure terrane, to determine if the subducted oceanic crust contributes to mantle Fe isotope heterogeneities. The metabasites have δ56Fe values of +0.16 to +0.33‰, which are heavier than typical values of MORB and OIB, indicating that their basaltic protolith derives from a heavy-Fe mantle source. The δ56Fe correlates well with Y/Nb and (La/Sm)PM ratios, which commonly fractionate during magmatic processes, highlighting variations in the magmatic protolith composition. In addition, the shift of δ56Fe by +0.06 to 0.10‰ compared to basalts may reflect hydrothermal alteration prior to subduction. The δ56Fe decrease from blueschists (+0.19 ± 0.03 to +0.33 ± 0.01‰) to eclogites (+0.16 ± 0.02 to +0.18 ± 0.03‰) reflects small variations in the protolith composition, rather than Fe fractionation during metamorphism: newly- formed Fe-rich minerals allowed preserving bulk rock Fe compositions during metamorphic reactions and hampered any Fe isotope fractionation. Greenschists have δ56Fe values (+0.17 ± 0.01 to +0.27 ± 0.02‰) similar to high-pressure rocks. Hence, metasomatism related to fluids derived from the subducted hydrothermally altered metabasites might only have a limited effect on mantle Fe isotope composition under subsolidus conditions, owing to the large stability of Fe-rich minerals and low mobility of Fe. Subsequent melting of the heavy-Fe metabasites at deeper levels is expected to generate mantle Fe isotope heterogeneities

High‐spatial‐resolution measurements of iron isotopes in pyrites by secondary ion mass spectrometry using the new Hyperion‐II radio‐frequency plasma source

International audienceIron isotopic signatures in pyrites are considered as a good proxy to reconstruct paleoenvironmental and local redox conditions. However, the investigation of micro-pyrites less than 20µm in size has been limited by the evaluable analytical techniques. The development of the new brighter radio-frequency plasma ion source (Hyperion-II source) enhances the spatial resolution by increasing the beam density 10 times compared with the Duoplasmatron source.Here we present high-spatial-resolution measurements of iron isotopes in pyrites using a 3 nA–3 μm primary 16O− beam on two Cameca IMS 1280-HR2 ion microprobe instruments equipped with Hyperion sources at CRPG-IPNT (France) and at SwissSIMS (Switzerland). We tested analytical effects, such as topography and crystal orientation, that could induce analytical biases perceptible through variations of the instrumental mass fractionation (IMF).Results: The δ56Fe reproducibility for the Balmat pyrite standard is ±0.25‰ (2 standard deviations) and the typical individual internal error is ±0.10‰(2 standard errors). The sensitivity on 56Fe+ was 1.2 × 107 cps/nA/ppm or better. Tests on Balmat pyrites revealed that neither the crystal orientation nor channeling effects seem to significantly influence the IMF. Different pyrite standards (Balmat and SpainCR) were used to test the accuracy of the measurements. Indium mounts must be carefully prepared with a sample topography less than 2 μm, which was checked using an interferometric microscope. Such a topography is negligible for introducing change in the IMF. This new source increases the spatial resolution while maintaining the high precision of analyses and the overall stability of the measurements compared with the previous Duoplasmatron source.Conclusions: A reliable method was developed for performing accurate and highresolution measurements of micrometric pyrites. The investigation of sedimentary micro-pyrites will improve our understanding of the processes and environmental conditions during pyrite precipitation, including the contribution of primary (microbial activities or abiotic reactions) and secondary (diagenesis and/or hydrothermal fluid circulation) signatures

Immiscibilité entre liquides silicatés dans les mésostases et les inclusions vitreuses des andésites basiques de Santorin (Arc Egéen)

Immiscibility textures have been observed in basic andesites from the Skaros series, Santorini (Greece). They occur both in the groundmass as Fe-rich brown glassy globules enclosed in a Si-rich clear glass, and in plagioclase phenocrysts as minute Si-rich glassy globules within a Fe-rich brown glass. The clear glass exhibits a granitic normative composition with high Si, Al, Na and K contents whereas the brown glass is characterized by high Fe, Ti, Mg, Ca and Mn contents with a pyroxenitic composition. The present study suggests that silicate liquid immiscibility is controlled in plagioclase phenocrysts by concentration gradients at the liquid-mineral interface, and in the groundmass by liquid composition (especially Ti).L'étude pétrographique des andésites basiques du centre éruptif de Skaros (Santorin) permet de mettre en évidence l'existence de textures d' immiscibilité dans le verre résiduel de la mésostase (globules de verre brun enrichi en Fe disséminés dans un verre clair enrichi en Si), ainsi que dans les inclusions vitreuses des phénocristaux de plagioclase (le verre brun enrichi en Fe englobe des globules de verre clair enrichi en Si). Le verre clair présente une composition normative typiquement granitique tandis que le verre brun se caractérise par une composition normative pyroxénitique. Cette étude suggère que le développement de liquides immiscibles dans les inclusions vitreuses de plagioclase résulterait du gradient de concentration à l'interface liquide-minéral. L' immiscibilité dans la mésostase serait principalement contrôlée par la composition chimique du liquide magmatique (essentiellement par les teneurs en Ti).Luais Béatrice. Immiscibilité entre liquides silicatés dans les mésostases et les inclusions vitreuses des andésites basiques de Santorin (Arc Egéen). In: Bulletin de Minéralogie, volume 110, 1, 1987. pp. 93-109

Temporal changes in Nd isotopic composition of Piton de la Fournaise magmatism (Réunion Island, Indian Ocean)

International audienceA Nd isotopic study of twelve historical samples and fifty-seven core samples covering the last 530 ka of volcanic activity of the Piton de la Fournaise volcano of Réunion Island was undertaken, with additional new Sr isotopic analyses of historical samples. The core samples were obtained from the cliffs of the deep (ca.1000 m) canyons of the Remparts and Langevin Rivers. Four types of rocks, namely basalts (MgO = 5-9 wt.%), high-Mg basalts (MgO = 10-20 wt.%), plagioclase-rich basalts, and differentiated samples (SiO2 = 48-53 wt.%) occur randomly throughout the lava sections. The overall variations in Nd isotopic ratios (0.512751 to 0.512902) are small, but significant. The range of Nd isotopic variations is larger than previously described in the literature, and extends toward lower values. Nd isotopic ratios increase through time and are broadly negatively correlated with the alkalinity index. The older alkaline Remparts samples (527-290 ka) (average ɛNd = +3.26) exhibit lower 143Nd/144Nd ratios than the younger tholeiitic Langevin samples (70-40 ka) (average ɛNd = +4.17) and the historical lavas (1901-1956) (average ɛNd = +4.74). Comparison of 143Nd/144Nd ratios with available 87Sr/86Sr ratios on the same set of samples (Remparts and Langevin series [Albarède et al., 1997]; historical series, this study) shows no correlation between Sr and Nd isotopes. It is suggested that the high Nd isotopic ratios of historical tholeiitic samples would be representative of the isotopic composition of the Réunion plume. The increase in Nd isotopic ratios over 530 ka reflects the effect of decreasing contribution of an enriched component to the Réunion plume through time. This enriched component would occur as isolated blobs in the mantle source

Early Planet formation pictured from Germanium isotopes: the Earth-Mars and Moon-Vesta dichotomy.

International audienceThe distribution of siderophile and volatile elements between metallic and mantle reservoirs of terrestrial planets and the Moon points to a strong depletion in their silicate reservoirs under P-T-fO2 conditions of planetary body early evolution. A major concern is to identify and quantify what combination of processes - metal-silicate segregation, volatile loss during magma ocean stages, and accretion- could explain the variability in moderately siderophile (MSE) and volatile (MVE) element abundances, for example in germanium, between Earth-Mars (0.7-2ppm) and the Moon-Vesta (HED) (≤0.05ppm). The Germanium isotopes have also shown to be strongly fractionated between chondrites [1,2], iron meteorites [3,4], and terrestrial crust-mantle samples with a positive ∆74/70Geirons-silicate Earth fractionation [4]. Completing the picture using Ge isotopes in silicate reservoirs of Mars, Vesta, and the Moon will add complementary constraints on their formation and evolution.We present first-ever high precision germanium isotopic data obtained on shergottite-nackhlite-chassignite Martian meteorites, eucrite-diogenite meteorites thought to originate from Vesta, and one lunar basalt. Bulk Ge concentration and isotopic measurements were performed at CRPG-Nancy using solution ICP-MS (SARM facilities), and HG-MC-ICPMS techniques (δ74/70GeNIST3120a ≤0.1‰, 2σ SD) [4], respectively.The Germanium isotopic signatures of planetary silicate reservoirs confirm the fundamental dichotomy based on Ge abundances, at different scales: (1) on a planetary scale: Mars and the Earth display overlapping δ74/70Ge values (+0.35 to 0.84‰) that are lighter than CI-chondrites and iron meteorites, emphazing a positive ∆74/70Geirons-silicate. It is opposed to highly Ge-depleted HEDs and the Moon having similar to higher values (+1.07 to +1.74‰) than CI-chondrite and irons. Negative [Ge]-δ74/70Ge correlation for Earth-Mars and HED-Moon would indicate distinct volatility mechanisms superimposed to core formation [5]; (2) on a Mars scale: higher δ74/70Ge and lower [Ge] (+0.84‰, 0.73 ppm) in shergottites than in nakhlites (+0.35‰, 2.2 ppm) emphasize outgassing processes [6]. Forthcoming δ74/70Ge data of Chassigny can constrain the nakhlite-chassignite filiation.References: [1] Luais et al. (2022), 85th MetSoc, #57. [2] Florin et al. (2020), GCA 269, 270-291. [3] Luais (2007), EPSL 262, 21-36. [4] Luais (2012), Chem. Geol. 334, 295-311. [5] Sossi et al. (2019), GCA 260, 204-231. [6] Humayun et al. (2016) 47th LPSC, #245

Germanium Isotope Geochemistry

International audienc

Mass dependent δ74/70 Ge variations within chondrites: Insights on inner-outer Solar System dichotomy

International audienceThe moderately siderophile and volatile elements are strong tracers of early solar nebula processes, including condensation and accretion-collision of meteorite parent bodies. Among them, the germanium (T50% condensation=825K) shows significant isotopic fractionation between metal and silicate phases in undifferentiated chondrites and in differentiated planetesimal reservoirs (i.e. mantle-core) [1,2]. Additionally, germanium isotopic data (δ 74/70 Ge‰) correlate with oxygen anomalies (Δ 17 O) in ordinary chondrites [1], demonstrating its capacity to trace oxidizing processes during accretion as well as genetic links between parent bodies. Here we present new high precision δ 74/70 Ge data obtained on bulk carbonaceous (CC) and ordinary (OC) chondrites, and an enhanced version of the δ 74/70 Ge-Δ 17 O correlation to assess NC-CC dichotomy. Bulk CC have positive δ 74/70 Ge values, showing exceptional large variations of ≈1‰, from CI (Orgueil) with the heaviest composition (δ 74/70 Ge=+0.901±0.060‰) toward lighter composition in CV (Allende) (δ 74/70 Ge=+0.096±0.120‰), whereas bulk ordinary chondrites display negative δ 74/70 Ge [1]. The δ 74/70 Ge values and matrix fraction (%) of OCs and CCs are positively correlated and describe a mixing line between CI composition and a [Ge]-depleted-δ 74/70 Ge-light component. In addition, OC and CC type chondrites present fundamental stable δ 74/70 Ge dichotomy that follow O, Ti, and Cr isotopic anomalies [3]. Within CC, the mass dependent δ 74/70 Ge compositions are exceptionally well correlated with D 17 O, e 54 Ti, and e 54 Cr, questioning the origin and processes that lead to isotopic signature dichotomy between the inner and outer the Solar System

Condensation and evaporation processes during CB chondrite formation: Insights from Ge isotopes and highly siderophile element abundances

International audienceWe analyzed the highly siderophile element (HSE) contents and bulk Ge isotopic compositions of large metal grains in the CB chondrites Bencubbin (CBa), Gujba (CBa), and HaH 237 (CBb). Our results suggest that the large grains were formed by the aggregation of smaller condensed grains, and the two Benccubinite groups are distinguishable based on their bulk metal δ74/70Ge mass-dependent isotopic values of 0.99 ± 0.30‰ (CBa) and −0.65 ± 0.10‰ (CBb). Based on our observations of these three samples, the isotopic compositions of metal in CBa chondrites are best explained by condensation at slow cooling rates in the center of an impact plume, whereas the metal in CBb chondrites formed under fast cooling rates along the plume edges. We also analyzed the Ge contents and isotopic compositions of the core, intermediate, and rim fractions of two Gujba metal grains, which were separated by sequential digestion. These results show a gradual decrease in δ74/70Ge and [Ge] from core to rim. We suggest that these δ74Ge zonations result from near-equilibrium condensation and evaporation processes in a heterogeneous plume. We propose a model for their formation in which (1) small grains (to become grain cores) condensed at equilibrium; (2) these grains were transported to a warmer region of the plume where they reached temperatures lower than that of Fe-Ni condensation, but high enough for the rapid evaporation of Ge; (3) Ge evaporation followed by slow cooling enriched the grains in heavy Ge isotopes and the surrounding gas in light Ge isotopes; and (4) equilibrium recondensation of metal from the gas and around the small grains formed the light Ge isotopic zonations observed in grain rims

Influence of redox processes on the germanium isotopic composition of ordinary chondrites

International audienceOrdinary chondrites (OCs) are classified into three groups, according to their oxidation state,which increases from the H to L to LL groups. This is demonstrated by the decrease in metalcontent (H=~8 vol%, L=~4 vol%, and LL=~2 vol%), and by a positive correlation between Δ17Oand %Fa through the OC sequence. Compared to other chondrites, OCs exhibit the largestvariation in oxidation state but there is an ongoing debate on the processes that control thisvariation. To constrain the causes of the variations in the oxidation state with respect to theassociated nebular versus parent bodies processes, we investigated the elemental and isotopicvariations of germanium (moderately siderophile and volatile) in the bulk sample, as well as inthe metal, silicate and sulfide phases, over a range of petrographic types for the H, L, and LLordinary chondrites.We found that δ74/70Gemetal is a proxy for the δ74/70Gebulk composition and that each OC group isdistinguishable by their δ74/70Gemetal, which increases from –0.51±0.09‰ for H chondrites, –0.31±0.06‰ for L chondrites, and, finally, to –0.26±0.09‰ for LL chondrites (2σSD).Additionally, the OC sequence exhibited a positive correlation, from H to L to LL, betweenδ74/70Gemetal and %Fa, as well as oxygen isotopes (δ17O, δ18O and Δ17O), that was not aconsequence of a “size sorting effect” on chondrules (i.e. chondrule mixing) or metamorphicprocesses in the parent bodies but, rather, was the result of nebular processes. We propose thatthe correlation between the δ74/70Ge values and %Fa, Δ17O, δ18O can be explained by anincreasing proportion of accreted hydrated phyllosilicates, from the H, L to LL groups, with highδ74/70Ge and Δ17O. We found that 10 to 15% of phyllosilicates, with a composition of [Ge]=4–7ppm and δ74/70Ge=3–2.5‰, is needed to change the δ74/70Ge from H to LL, which corresponds toa Δ17O≈8-7‰. This value agrees with the Δ17O≈7‰ composition of the accreted nebularcomponent reported by Choi et al. (1998). During thermal metamorphism, phyllosilicatesdestabilize, liberating germanium that will be incorporated in the metal, then leading to its highδ74/70Ge signature.High-temperature metamorphism can explain the lack of δ74/70Gemetal variation with the petrologictype in the OC, even for the type 3 chondrites (T≈675°C), implying a complete reaction even atlow petrologic types. In addition, metal-silicate re-equilibration in response to thermalmetamorphism results in a decrease in Δ74/70Gemetal-silicate from 0.33‰ to 0.06‰, within the Hchondrite group, which is interpreted as the result of δ74/70Gesilicate variation. The mean positiveΔ74/70Gemetal-silicate fractionation factor of +0.22±0.36‰ (error propagation on individual error) alsodisplays a remarkable similarity to the direction of isotopic fractionation with other germaniumisotopic metal-silicate datasets, such as the magmatic iron meteorites, the Earth silicatereservoirs. We propose that the Δ74/70Gemetal-silicate and the negative δ74/70Ge values of OCs areinherited from metal-silicate melting and partial exchange before planetesimal accretion in a lightisotope-enriched gas. Finally, the δ74/70Gemetal-Δ17Osilicate correlation between the IIE ironmeteorites and OCs, provides new evidence for the existence of a highly reduced HH group