• Oneill, Hugh St C.
• Foran, Garry J.

Abstract

<p>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, Δ⁵³Cr_ol-mlt = −0.050(4) and Δ⁵³Cr_spl-mlt = 0.087(4), are larger by ~0.03 than those obtained using Cr-O bond lengths estimated from ionic radii. For Δ⁵³Cr_spl-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.</p>

Topics

• impedance spectroscopy
• mineral
• melt
• glass
• glass
• fractionation
• extended X-ray absorption fine structure spectroscopy