Chromium isotope heterogeneity in the mantle

To better constrain the Cr isotopic composition of the silicate Earth and to investigate potential Cr isotopic fractionation during high temperature geological processes, we analyzed the Cr isotopic composition of different types of mantle xenoliths from diverse geologic settings: fertile to refractory off-craton spinel and garnet peridotites, pyroxenite veins, metasomatised spinel lherzolites and associated basalts from central Mongolia, spinel lherzolites and harzburgites from North China, as well as cratonic spinel and garnet peridotites from Siberia and southern Africa. The δ⁵³Cr_{NIST 979} values of the peridotites range from $-0.51\pm 0.04‰$ (2SD) to $+0.75\pm 0.05‰$ (2SD). The results show a slight negative correlation between δ⁵³Cr and Al₂O₃ and CaO contents for most mantle peridotites, which may imply Cr isotopic fractionation during partial melting of mantle peridotites. However, highly variable Cr isotopic compositions measured in Mongolian peridotites cannot be caused by partial melting alone. Instead, the wide range in Cr isotopic composition of these samples most likely reflects kinetic fractionation during melt percolation. Chemical diffusion during melt percolation resulted in light Cr isotopes preferably entering into the melt. Two spinel websterite veins from Mongolia have extremely light δ⁵³Cr values of $-1.36\pm 0.04‰$ and $-0.77\pm 0.06‰$, respectively, which are the most negative Cr isotopic compositions yet reported for mantle-derived rocks. These two websterite veins may represent crystallization products from the isotopically light melt that may also metasomatize some peridotites in the area. The δ⁵³Cr values of highly altered garnet peridotites from southern Africa vary from $-0.35\pm 0.04‰$ (2SD) to $+0.12\pm 0.04‰$ (2SD) and increase with increasing LOI (Loss on Ignition), reflecting a shift of δ⁵³Cr to more positive values by secondary alteration.

The Cr isotopic composition of the pristine, fertile upper mantle is estimated as $\delta {}^{53}\mathrm{Cr}=-0.14\pm 0.12‰$, after corrections for the effects of partial melting and metasomatism. This value is in line with that estimated for the BSE ($-0.12\pm 0.10‰$) previously.