Prediction of magmatic water contents via measurement of H₂O in clinopyroxene phenocrysts

Abstract

Water is fundamental to magma genesis, evolution, and eruption. Few direct measurements of magmatic H₂O exist, however, because rocks found at the surface have extensively degassed upon eruption. Olivine-hosted melt inclusions provide a standard approach to measuring volatiles in undegassed magma, but many volcanic deposits do not contain melt inclusions large enough for analysis (>30 μm), or olivine at all. Here we use an Al^IV-dependent partitioning relationship to calculate magmatic H₂O from direct measurements of H₂O in clinopyroxene phenocrysts. We test this approach using phenocrysts from four arc volcanoes (Galunggung, Irazú, Arenal, and Augustine) that span the global range in H₂O contents as measured in olivine-hosted melt inclusions (from 0.1 to 7 wt% H₂O). The average and maximum magmatic H₂O contents calculated from the clinopyroxene measurements agree within 15% of the melt inclusion values for most of the samples. The evolutionary paths recorded in H₂O-Mg# variations overlap in some clinopyroxene and olivine-hosted melt inclusion populations, and in others, the clinopyroxenes record a larger portion of the liquid line of descent or a different portion of the magma system. Thus, the use of phenocrysts to estimate magmatic H₂O contents creates a new and powerful tool in igneous petrology and volcanology.