The coordination of Cr 2+ in silicate glasses and implications for mineral-melt fractionation of Cr isotopes

Abstract

The Cr2+-O and Cr3+-O bond lengths in CaO-MgO-Al2O3-SiO2 glasses containing ~0.3 wt% Cr with Cr2+/ΣCr = ~ 1 or 0 (where ΣCr = Cr2+ + Cr3+) were determined by extended X-ray absorption fine structure (EXAFS) spectroscopy to be 2.034(5) and 1.967(7) Å, respectively. The Cr2+-O bond length is consistent with square planar coordination and the Cr3+-O bond length with octahedral coordination. These bond lengths were used to calculate force constants of 1206(9) Nm−1 for Cr2+-O and 2001(21) Nm−1 for Cr3+-O in silicate melts. The value for Cr3+-O is similar to previous estimates but that for Cr2+-O is almost 20% lower. The force constants were used to calculate the difference in Cr isotopic composition (Δ53Cr) between both olivine and melt and spinel and melt for MORB with equal amounts of Cr2+ and Cr3+ at 1150 °C. The resulting values, Δ53Crol-mlt = −0.050(4) and Δ53Crspl-mlt = 0.087(4), are larger by ~0.03 than those obtained using Cr-O bond lengths estimated from ionic radii. For Δ53Crspl-mlt this difference is equivalent to a change in temperature of over 300 °C at constant Cr2+/ΣCr. The fractionation of Cr isotopes in ocean island basalts should be larger for fractional crystallisation in the crust than partial melting in the mantle due to the strong negative effect of pressure on Cr2+/ΣCr. The new bond length data for Cr2+-O provide an additional constraint for modelling and interpreting Cr isotope fractionation during igneous petrogenesis.