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

The oxidation state of Cr, Cr2+/Sigma Cr (where Sigma Cr = Cr2+ + Cr3+ = 0.35 wt%), in Fe-free silicate glasses quenched from melts equilibrated as a function of pressure to 3.5 GPa at 1500 degrees C, and as a function of temperature to 1500 degrees C at atmospheric pressure, and at oxygen fugacities (fO(2), in log units relative to the quartz-fayalite-magnetite (QFM) buffer) between Delta QFM = - 1 and - 2, was determined by XANES spectroscopy. Increasing temperature stabilises Cr2+ and increasing pressure stabilises Cr3+. A general expression for Cr3+/Cr2+ in silicate melts was derived: log(Cr3+/Cr2+) = 1/4(Delta QFM + 8.58 - 25,050/T + 940P/T - 0.02P) + 9770/T - 7.69 + 6.22 Lambda + (900P - 172P(2))/T, where P is pressure in GPa, T is temperature in K, and Lambda is the optical basicity of the composition. This equation reproduces 213 Cr2+/Sigma Cr values reported here and in the literature with an average Delta Cr2+/Sigma Cr of 0.02. A MORB melt at 1400 degrees C and QFM is predicted to have Cr2+/Sigma Cr similar to 0.35 at the surface but Cr2+/Sigma Cr similar to 0 at a depth of similar to 60 km. Although Cr2+ is an important oxidation state in silicate melts it is not preserved at low temperatures due to an electron exchange reaction with Fe3+.