Factors effecting the stability field of Ca-poor pyroxene and the origin of the Ca-poor minimum in Ca-rich pyroxenes from tholeiitic intrusions

Abstract

Ca-poor pyroxene ceases to crystallise towards the end of fractionation in tholeiitic intrusions and is usually replaced by Fe-rich olivine. Using the data of Nicholls et al. (1971), the $$a_{{\text{SiO}}_2 }$$ at which olivine and pyroxene can coexist has been calculated at different temperatures and pressures. From these calculations it is clear that the Fe/Mg ratio of the last Ca-poor pyroxene to crystallise from a melt is increased by raising the temperature or pressure of crystallisation. The Ca-poor pyroxene-Fe-rich olivine relationship is also dependent on the $$a_{{\text{SiO}}_2 }$$ of the melt. In magmas which crystallise Fe-rich olivine before quartz, inicreasing their $$a_{{\text{SiO}}_2 }$$ will raise the Fe/Mg ratio of the last Ca-poor pyroxene to crystallise. If the $$a_{{\text{SiO}}_2 }$$ of the magma is so high that SiO2 saturation is reached before the appearance of cumulus Fe-rich olivine, any further increase in the $$a_{{\text{SiO}}_2 }$$ of the melt will not influence the stability field of Ca-poor pyroxene. The replacement of Ca-poor pyroxene by Fe-rich olivine requires the magma to reach a high level of a FeO late in its fractionation. If a magma fractionates with an FeO depletion trend, Ca-poor pyroxene is replaced by Ca-rich pyroxene. The reaction is initiated by the appearance of cumulus K-feldspar which results in a marked reduction in the amount of anorthite crystallising from the magma. This increases the a CaO of the melt so that Ca-poor pyroxene is replaced by Ca-rich pyroxene.