Influence of Augite on Plagioclase Fractionation

Abstract

The estimated initial plagioclase compositions and plagioclase/liquid partitions for $$X_{An}$$ in the Kiglapait intrusion and Makaopuhi lava lake are the same: $$An_{67}$$ crystal, $$An_{57}$$ liquid. The Kiglapait residuum amounts to 0.01% by volume at $$An_{11.5}$$, whereas the Makaopuhi residuum amounts to 3% at $$An_{6.4}$$. The large difference in secular plagioclase variation can be ascribed only to augite, which is abundant in the lava lake and causes strong depletion of Ca. The effective secular fractionation for $$X_{An}$$ can be described as a function of CIPW-normative di/(di + fsp), here denoted $$F'_{di}$$, by $$D^{eff} = 1.81F'_{di}+ 1.04$$, where $$D^{eff}$$ represents the effective distribution coefficient $$X^{plag}_{An}/X^{lig}_{An}$$. This relation holds for systems ranging at least from olivine tholeiite to high alumina basalt in bulk composition. Plagioclase fractionation alone accounts for a bulk D of only about 1.17. Augite is the major cause of extreme An depletion. These relation...