The distribution of chromium among orthopyroxene, spinel and silicate liquid at atmospheric pressure

Abstract

A series of experiments has been carried out in which a synthetic silicate melt, of composition equivalent to a “U-type” Bushveld Complex parent liquid, was equilibrated with bronzitic orthopyroxene and chromite spinel between 1334 and 1151°C over a range of oxygen fugacities between the nickel-nickel oxide and iron-wüstite buffers. The partition coefficient for Cr between bronzite and melt increases with falling temperature along a given oxygen buffer, and decreases with falling oxygen fugacity at a given temperature, showing an overall range from 1.1 to 11.7. The Cr content of the melt in equilibrium with spinel (Cr solubility) decreases with falling temperature and increases with lower oxygen fugacity. This variation may be quantified in terms of temperature-dependent solubility of Cr 3+ combined with a changing ratio of Cr 3+ to Cr 2+ in the melt. Iso-oxidation curves for Cr are approximately parallel to Fe-Si-O buffer curves and to curves for equal Fe 3+/Fe 2+ determined by Hill and Roeder (1974), and agree within experimental error with the results of Schreiber and Haskin (1976). When the changing oxidation state of Cr is allowed for, the orthopyroxene partition coefficient may be expressed as the sum of the temperature-dependent coefficient for Cr 3+ and a partition coefficient of about 1 for Cr 2+. The experimental results are used to construct a series of model curves for liquid and bronzite compositional variations during fractional crystallization of a Bushveld parent liquid. Trends for Cr variation are shown to depend critically upon oxygen fugacity, and on whether the liquid is saturated with chromite. The position of the peritectic between chromite and orthopyroxene is shown to be very sensitive to oxygen fugacity within one and a half log units of the QFM buffer. This observation may explain the contrasting distribution of chromite seams in the Bushveld Complex, where chromite occurs within bronzitite-norite sequences, and in the Stillwater Complex in which chromite is restricted to olivine cumulate layers.