Olivine phenocryst growth in Hawaiian tholeiites: Evidence for supercooling

Abstract

The MgO contents of primitive tholeiitic magmas are estimated using the olivine geothermometer. This application assumes that olivine phenocrysts form by equilibrium crystallization. It is shown here that tholeiitic magmas crystallize by supercooled crystallization, and a representative degree of supercooling is estimated to be about 40°C. A degree of supercooling of 40°C results in an underestimate of the MgO content of a tholeiitic magma by 2 wt % MgO. When supercooling occurs, the assumption of equilibrium crystallization excludes true primary tholeiitic compositions from being considered. © The Author 2011. Published by Oxford University Press. All rights reserved.