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

International audienceData 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 Flow and Gorgona, δ53Crincreases in the evolved portion of the magma during olivine fractionation due to the preferential inclusion oflight Cr into olivine. Other flows with lower MgO content do not show this behaviour because a smaller fractionof the Cr is contained in olivine. The effects of fractional crystallisation must, therefore, be taken into accountwhen calculating the Cr isotopic composition of the source of komatiite lavas.The weighted average of δ53Cr for the komatiite lavas analysed is −0.12 ± 0.04‰ (n = 5) and representsour best estimate for the Cr isotopic composition of the BSE. It agrees with the previous estimates, while providingan improvement to the uncertainty. There is no resolvable difference between this value and that ofchondritic meteorites. Our data also indicate that the δ53Cr of the mantle has been constant since at least theArchean