Petrology of crystal clots in the pumice of Mount St. Helens' March 19, 1982 eruption; significant role of Fe-Ti oxide crystallization

Abstract

A petrographic, mineralogical and chemical description of the dacite pumice, and of the contained mafic crystal clots, from the March 19, 1982, eruption of Mount St. Helens is presented. The crystal clots are interpreted as cumulates and the phenocryst phases in the pumice are not fully representative of phases fractionating at depth. The crystallization sequence is inferred to be olivine plus plagioclase followed by pyroxenes and late-stage amphibole. Magnetite and ilmenite crystallized early, providing a mechanism for minimal iron enrichment. As estimated from Fe-Ti oxide and two-pyroxene geothermometers, crystallization temperatures were approximately 1000°C at oxygen fugacities approximately one order of magnitude higher than the NNO buffer. The parental magma was more mafic than dacite and had a comparatively low water content. As the magma evolved to dacite, the water content increased to a value in excess of 4 wt.% and amphibole began to crystallize.