Chromium isotope variations in garnet-facies mantle rocks and their minerals: Implications for Cr isotope behavior in high-temperature processes

Abstract

Previous work on Cr isotope composition of the lithospheric mantle focused on shallow spinel-facies peridotites and invoked the effects of partial melting and metasomatism. However, little is known on the Cr isotope composition and behavior in the deeper and more voluminous garnet-facies mantle, notably during generation and passage of magmas and fluids. Here, we present Cr isotope data for 26 garnet and ten spinel whole-rock peridotite xenoliths (mainly harzburgites) from the Udachnaya kimberlite on the Siberian craton and minerals from five of them, as well as for minerals of 14 eclogites and four garnet clinopyroxenites from the Dabie orogen in eastern China. Garnet (Gt) and clinopyroxene (Cpx) are major Cr hosts in the mantle. Our ionic modeling suggests the following order of 53Cr/52Cr values in coexisting minerals: garnet ≈ spinel > Cpx > olivine. Cr isotopes are equally partitioned between garnet and Cpx in four peridotites, while garnet is enriched in heavy Cr in one peridotite and all the eclogites (Δ53CrGt-Cpx 0.01‰ to 0.27‰, av. 0.10‰), but depleted in heavy Cr in the pyroxenites (Δ53CrGt-Cpx −0.03‰ to −0.16‰, av. −0.10‰). The Δ53CrGt-Cpx variations are attributed to effects of chemical composition, equilibration temperature and oxygen fugacity on equilibrium Cr isotope fractionation. Further, the anomalous Δ53CrGt-Cpx values in the Dabie pyroxenites and one peridotite may indicate disequilibrium inter-mineral isotope fractionation via kinetic diffusion during phase transitions as a result of pressure-temperature (P-T) changes and metasomatism.The whole-rock δ53Cr range in the garnet peridotites (−0.27 to 0.13‰; average −0.08 ± 0.18‰, 2 SD, n = 25, excluding an outlier with −0.86‰) overlaps bulk silicate Earth estimates whereas the spinel peridotites tend to have higher δ53Cr (−0.12 to 0.16‰; av. 0.02 ± 0.20‰, 2 SD, n = 10). While these rocks are residues of high degrees (30–40%) of melt extraction, their range contrasts with limited equilibrium Cr isotope fractionation we calculated for melting residues of both spinel and garnet peridotites (Δ53Crresidue-source < 0.03‰). The δ53Cr values in the spinel peridotites show correlations with modal abundances of Cpx, garnet and spinel, as well as major and trace element composition. We posit that the Cr isotope variations in these peridotites are somehow linked to solid–melt and inter-mineral diffusive exchanges during P-T changes, deformation and ingress of metasomatic media previously documented in the Udachnaya peridotites. The average δ53Cr value (−0.09 ± 0.16‰, 2 SD, n = 31) for garnet peridotites (excluding U9) in this and previous studies is similar to those previously obtained for (mainly off-craton) spinel peridotites and komatiites, and appears to be representative of the Cr isotope composition for bulk mantle. Numerical modeling for mantle magmas derived at different depths and by different melting degrees of garnet and spinel peridotites and eclogites yields a narrow δ53Cr range, possibly affected by oxygen fugacity.