The δ53Cr isotope composition of komatiite flows and implications for the composition of the bulk silicate Earth

• Published in: Chemical Geology
• Main Authors: Jerram, Matthew, Bonnand, Pierre, Kerr, Andrew C., Nisbet, Euan G., Puchtel, Igor S., Halliday, Alex N.
• Other Authors: Department of Earth Sciences Oxford, University of Oxford, The Earth Institute, Columbia University, New York, NY, Laboratoire Magmas et Volcans (LMV), Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Jean Monnet - Saint-Étienne (UJM)-Université Clermont Auvergne 2017-2020 (UCA 2017-2020 )-Centre National de la Recherche Scientifique (CNRS)-Observatoire de Physique du Globe de Clermont-Ferrand (OPGC), Institut national des sciences de l'Univers (INSU - CNRS)-Université Clermont Auvergne 2017-2020 (UCA 2017-2020 )-Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Clermont Auvergne 2017-2020 (UCA 2017-2020 )-Centre National de la Recherche Scientifique (CNRS), Cardiff University, Department of Earth Sciences Egham, Royal Holloway University of London (RHUL), University of Maryland College Park, University of Maryland System
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2020
• Subjects: Cr stable isotopes; Komatiites; Mantle melting; Fractional crystallisation; Bulk silicate Earth; [SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry; Lava Lake; Fennoscandia
• Online Access: https://uca.hal.science/hal-03001483; https://uca.hal.science/hal-03001483/document; https://uca.hal.science/hal-03001483/file/1-s2.0-S0009254120303004-main.pdf; https://doi.org/10.1016/j.chemgeo.2020.119761

International audience Data on the chromium stable isotope composition of planetary reservoirs have the potential to provide informationabout core formation, partial melting and conditions of the Moon formation. In order to detect thesmall Cr isotopic differences between various reservoirs in the solar system, their compositions need to beprecisely constrained. The current BSE value of δ53Cr = −0.11 ± 0.06‰ (Sossi et al., 2018) cannot resolvedifferences between achondrites, (Vesta δ53Cr = −0.17 ± 0.05‰) and chondrites (carbonaceousδ53Cr = −0.12 ± 0.05‰; ordinary δ53Cr = −0.11 ± 0.04‰). The composition of the bulk silicate Earth(BSE) is often used as a reference point for comparisons to other planetary reservoirs. However, past attempts toestimate the Cr isotopic composition of the BSE have been unable to provide a well-constrained BSE value.Traditional methods, using mantle peridotites, are affected by the susceptibility of Cr isotopes to fractionationduring metasomatism. More recently, the Cr isotope composition of the BSE has been calculated using komatiites,in addition to mantle peridotites, to produce a more precise value (Sossi et al., 2018). In order to constrainthe BSE composition to a higher precision, the δ53Cr of remarkably fresh komatiite lava flows from three localities,ranging in age from 2.7 Ga to 89 Ma, have been investigated in detail. These included the Tony's Flow inthe Belingwe Greenstone Belt, Zimbabwe, the Victoria's Lava Lake in Fennoscandia, and komatiites fromGorgona Island in Colombia.In the komatiites studied, a range in Cr isotopic compositions was found, from δ53Cr = −0.16 ± 0.02 to−0.01 ± 0.02‰. We show that the high degrees of partial melting that produced the komatiites, did not resultin Cr isotopic fractionation between the komatiitic melt and mantle residue. However, limited Cr isotopicfractionation is found to be a consequence of komatiite lava differentiation. For the lava flows with high Mgcontent and high Cr2+/ƩCrTOT (the molar ratio of Cr2+/(Cr2++Cr3+)), such as Tony's ...