PREDICTION OF THE COMPOSITION OF CHROME SPINEL ON THE BASIS OF THE SUBSOLIDUS HISTORY OF THE PARENT ULTRAMAFIC ROCKS

Abstract

To interpret the diffusional processes that bring about correlated change in the chemistry of coexisting olivines and chrome spinels, one can use the model of a semiinfinite composite sphere or the novel method of kinetic calculations proposed by A.C. Lasaga. But the complex mathematics of the kinetic models makes them useless for practical purposes. These methods are employed to develop an algorithm for approximate calculation of the chromium and magnesium content of the central parts of chrome spinel grains coexisting with olivine and certain important petrologic implications are discussed. The relationships, so derived were calibrated against the contour chart of the cooling rates of ultramafic rocks in the Kondera pluton. This intrusion was chosen as a standard because its ultramafic rocks exhibit the lowest rates of subsolidus cooling and thus the most pronounced alteration of mineral parageneses.