The δ53Cr isotope composition of komatiite flows and implications for the composition of the bulk silicate Earth
Résumé
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 Gorgona, δ53Cr
increases in the evolved portion of the magma during olivine fractionation due to the preferential inclusion of
light Cr into olivine. Other flows with lower MgO content do not show this behaviour because a smaller fraction
of the Cr is contained in olivine. The effects of fractional crystallisation must, therefore, be taken into account
when 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 represents
our best estimate for the Cr isotopic composition of the BSE. It agrees with the previous estimates, while providing
an improvement to the uncertainty. There is no resolvable difference between this value and that of
chondritic meteorites. Our data also indicate that the δ53Cr of the mantle has been constant since at least the
Archean.
Domaines
GéochimieOrigine | Fichiers éditeurs autorisés sur une archive ouverte |
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