Magnesium and iron isotopic evidence of inter-mineral diffusion in ultramafic cumulates of the Peridotite Zone, Stillwater Complex

Abstract

To better understand the role and extent of inter-mineral diffusion during subsolidus processes, we investigated the iron and magnesium isotopic compositions of olivine, orthopyroxene, and chromite separates from the Peridotite Zone of the Stillwater Complex. Olivine and orthopyroxene show limited Mg isotopic variations, with δ26Mg values ranging from −0.40 to −0.26‰ and from −0.29 to −0.22‰, respectively. In contrast, chromite displays extremely large Mg isotopic variations, with δ26Mg values ranging from −0.05 to +0.84‰. The δ56Fe values of olivine and orthopyroxene range from 0.00 to +0.17‰ and −0.04 to +0.06‰, respectively, whereas those of chromite range from −0.09 to +0.13‰. Most olivine-orthopyroxene pairs in our samples show no clear Fe-Mg isotopic differences, whereas silicate and coexisting chromite pairs plot away from theoretically predicted equilibrium lines, indicating disequilibrium inter-mineral fractionation. Higher δ56Fe values in silicates than in chromite and significantly higher δ26Mg values in chromite than in silicates indicate that subsolidus re-equilibration is responsible for the observed disequilibrium Fe-Mg isotope fractionation. We develop a quantitative model based on the investigated olivine and chromite to reproduce the observed kinetic Fe-Mg isotope fractionations. Our model results indicate a systematic difference between the Fe and Mg isotope fractionation factors, β, of olivine (βFe ≈ 0.27 and βMg ≈ 0.16) and chromite (0.03 < βFe < 0.08 and 0.05 < βMg < 0.16). This study demonstrates that combined investigations of the Fe-Mg isotopic compositions of cumulus minerals provide more recognizable and more reliable information on inter-mineral diffusion among igneous minerals in magma chambers than elemental analyses alone.