The effects of temperature and pressure on the oxidation state of chromium in silicate melts

The oxidation state of Cr, Cr 2+ /ΣCr (where ΣCr = Cr 2+ + Cr 3+ = 0.35 wt%), in Fe-free silicate glasses quenched from melts equilibrated as a function of pressure to 3.5 GPa at 1500 °C, and as a function of temperature to 1500 °C at atmospheric pressure, and at oxygen fugacities ( f O 2 , in log units relative to the quartz-fayalite-magnetite (QFM) buffer) between ∆QFM = − 1 and − 2, was determined by XANES spectroscopy. Increasing temperature stabilises Cr 2+ and increasing pressure stabilises Cr 3+ . A general expression for Cr 3+ /Cr 2+ in silicate melts was derived: log(Cr 3+ /Cr 2+ ) = 1/4(∆QFM + 8.58 − 25,050/ T + 940 P / T − 0.02 P ) + 9770/ T − 7.69 + 6.22Λ + (900 P − 172 P 2 )/ T , where P is pressure in GPa, T is temperature in K, and Λ is the optical basicity of the composition. This equation reproduces 213 Cr 2+ /ΣCr values reported here and in the literature with an average ΔCr 2+ /ΣCr of 0.02. A MORB melt at 1400 °C and QFM is predicted to have Cr 2+ /ΣCr ~ 0.35 at the surface but Cr 2+ /ΣCr ~ 0 at a depth of ~ 60 km. Although Cr 2+ is an important oxidation state in silicate melts it is not preserved at low temperatures due to an electron exchange reaction with Fe 3+ .