Stable chromium isotopic variations in peridotite mantle xenoliths: Metasomatism versus partial melting

Abstract

Chromium isotope compositions have been determined for 42 mantle-derived xenoliths from Cameroon, France, Mongolia, Russia and the USA. The samples are mainly spinel and garnet lherzolites, or harzburgites, and yield δ53CrNIST979 from −0.33 to 0.43‰, within the range of previously reported compositions, −0.51 to 0.74‰ (Xia et al., 2017). Partial melting has been proposed as a process that alters the δ53Cr of the mantle residue. New non-modal melting models show that the compatible nature of Cr is inconsistent with large changes in the residue’s isotopic composition. The fractionation during melting is constrained by calculated Δ53Crmantle-melt-values <0.09 and the measured Δ53/52Cr mantle-melt ≈ 0.07‰, which is too small to generate the observed range in δ53Cr at low to medium degrees of melting. Fertile to melt-depleted, unmetasomatised peridotite xenoliths show no correlations between δ53Cr and indicators of partial melting. Almost all LREE-enriched samples studied here define a correlation between δ53Cr and (La/Yb)N. This is consistent with alteration of Cr isotopic compositions by some process that is related to interactions with low degree partial melts. The low Cr content of small degree partial melts renders metasomatism via such liquids a difficult explanation for the isotopic effects found. A possible explanation is some form of disequilibrium loss of isotopically light Cr into such liquids during their percolation through the mantle. This is similar to a previously proposed mechanism of kinetic isotopic fractionation of Cr associated with interactions between silicate melts and peridotitic mantle (Xia et al., 2017).