The coordination of Cr2+ in silicate glasses and implications for mineral-melt fractionation of Cr isotopes

The Cr2+-O and Cr3+-O bond lengths in CaO-MgO-Al2O3-SiO2 glasses containing similar to 0.3 wt% Cr with Cr2+/Sigma Cr = similar to 1 or 0 (where Sigma Cr = Cr2+ + Cr3+) were determined by extended X-ray absorption fine structure (EXAFS) spectroscopy to be 2.034(5) and 1.967(7) angstrom, respectively. The Cr 2+ -0 bond length is consistent with square planar coordination and the Cr2+-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 Cr 3+ -0 is similar to previous estimates but that for Cr(2+ )0 is almost 20% lower. The force constants were used to calculate the difference in Cr isotopic composition (Delta Cr-5(3)) between both olivine and melt and spinel and melt for MORB with equal amounts of Cr2+ and Cr3+ at 1150 degrees C. The resulting values, Delta Cr-53(ol-mlt )= -0.050(4) and Delta Cr-53(spl-mlt) = 0.087(4), are larger by similar to 0.03 than those obtained using Cr-O bond lengths estimated from ionic radii. For Delta Cr-53(spl-mlt) this difference is equivalent to a change in temperature of over 300 degrees C at constant Cr2+/ECr. 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 Cr-2 +/-/Sigma Cr. The new bond length data for Cr2+-O provide an additional constraint for modelling and interpreting Cr isotope fractionation during igneous petrogenesis.