The stable chromium isotope composition of different mantle reservoirs

The stable chromium isotope composition of Earth’s mantle has been suggested to be heterogeneous, with mantle-derived chromitites slightly enriched in heavier Cr isotopes than mantle-derived silicate rocks. To better understand such heterogeneity and to search for a potentially complementary light Cr isotope reservoir in the mantle, we analyzed the stable Cr isotope compositions (δ53Cr expressed as deviation of sample 53Cr/52Cr from NIST SRM 979) of whole-rock pyroxenite and peridotite xenoliths from the North China Craton using a double spike technique. The δ53Cr values of the peridotites vary from −0.21‰ to −0.07‰ with an average value of −0.14 ± 0.10‰ (2SD, N = 5), which overlaps with the range of the Bulk Silicate Earth (−0.12 ± 0.10‰). In contrast, pyroxenites in this study yield lower δ53Cr values (−0.30‰ to −0.14‰ with an average value of −0.21 ± 0.08‰, 2SD, N = 20). The variation in the δ53Cr values of the pyroxenites can be explained by a Rayleigh distillation model during partial melting of the primitive mantle. The model provides minimum Cr isotope fractionation factors during partial melting ranging from 0.01‰ to 0.12‰, with light isotopes enriched in the melt. Combined with literature data, our results suggest that both peridotites and basalts are indistinguishable from the Bulk Silicate Earth in terms of δ53Cr. However, pyroxenites and chromitites are isotopically lighter and heavier than the Bulk Silicate Earth, respectively. Future work is needed to investigate the petrogenetic relationship between chromitites and pyroxenites in order to understand why they seem to complement each other in δ53Cr.