Abstract

From phase equilibrium studies of silicate systems that are similar but simpler than the Bushveld Complex magma, it seems unlikely that a reversal of the normal fractionation trend of decreasing Mg/Fe for bronzite and olivine could occur in the closed system at constant or increasing pressure. If the pressure on the magma is decreasing, however, such a reversal in the normal direction of compositional change of the precipitating bronzite and olivine is possible, as shown schematically by a model. The reversal is related to a greater steepening of the slope of the solidus curve than of the liquidus curve near the high-temperature end of the temperature-composition phase diagrams when pressure is decreased. A further effect of decreasing pressure is expansion of the olivine primary phase volume, relative to the bronzite volume. A period of sustained pressure decrease during bronzite precipitation could, therefore, lead to the initiation of olivine coprecipitation.

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