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.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2020
• Subjects: Lava Lake; Fennoscandia
• Online Access: https://orca.cardiff.ac.uk/id/eprint/133158/; https://doi.org/10.1016/j.chemgeo.2020.119761; https://orca.cardiff.ac.uk/id/eprint/133158/1/Jerram%20et%20al.%202020.pdf

Data on the chromium stable isotope composition of planetary reservoirs have the potential to provide information about core formation, partial melting and conditions of the Moon formation. In order to detect the small Cr isotopic differences between various reservoirs in the solar system, their compositions need to be precisely constrained. The current BSE value of δ53Cr = −0.11 ± 0.06‰ (Sossi et al., 2018) cannot resolve differences between achondrites, (Vesta δ53Cr = −0.17 ± 0.05‰) and chondrites (carbonaceous δ53Cr = −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 to estimate 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 fractionation during 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 constrain the 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 in the Belingwe Greenstone Belt, Zimbabwe, the Victoria's Lava Lake in Fennoscandia, and komatiites from Gorgona 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 result in Cr isotopic fractionation between the komatiitic melt and mantle residue. However, limited Cr isotopic fractionation is found to be a consequence of komatiite lava differentiation. For the lava flows with high Mg content and high Cr2+/ƩCrTOT (the molar ratio of Cr2+/(Cr2+ + Cr3+)), such as Tony's Flow and ...