The Fractional Crystallization of Plagioclase in the Hat Creek Basalt: Observations and Theory

Abstract

Plagioclase in the Hat Creek high alumina olivine tholeiite decreases in anorthite (An) component with crystallization. Petrography reveals the composition of plagioclase that formed at various stages of crystallization represented by microporphyritic glassy tops of the flow and by segregation veins and their residual glasses. The plagioclase ranges from An79 in the cores of microphenocrysts to An9 rims on crystals in glass-bearing segregation veins as the fraction of crystallization increases from 0 to 0.9. The temperatures of crystallization corresponding to the various fractions of crystallization are estimated on the basis of olivine/liquid and magnetite/ilmenite geothermometry and linear interpolation. The temperatures ranged from 1237°C to 980°C. Supercooling was less than 20°C initially. Most of the plagioclase microphenocrysts have An consistent with the plagioclase geothermometer. Plagioclase in most of the segregation veins differs from that expected. The derived histogram of amount of plagioclase versus its composition reveals that the mode lies at the An-rich end of the spectrum with successively smaller amounts formed in more sodic intervals. The curve for composition versus crystallization extrapolates to pure albite at about 100% crystallized. Except for the anomalous segregation veins, the observations agree with a modified theory for perfect fractional crystallization of plagioclase in dry basaltic magma. The anomalous segregation veins apparently lost or gained residual liquid and some inherited microphenocrysts which were carried in during vein formation. The compositional spectrum of plagioclase in basaltic and gabbroic rocks can be used to substantiate closed system fractional crystallization.