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

Jennifer A. Wade^*,1, Terry Plank^,1, Erik H. Hauri², Katherine A. Kelley³, Kurt Roggensack⁴ and Mindy Zimmer¹

^{1 1} Boston University, 685 Commonwealth Avenue, Boston, Massachusetts 02215, USA
^{2 2} Carnegie Institution of Washington, 5424 Broad Branch Road, Washington, D.C. 20005, USA
^{3 3} Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882, USA
^{4 4} School of Earth and Space Exploration (SESE), Arizona State University, Tempe, Arizona 85287, USA

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.

Key Words: clinopyroxene • partition coefficient • water • melt • subduction zones

This article has been cited by other articles:

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