Development of a H in Feldspar Magmatic Hygrometer

Water exerts a strong influence on the physical and chemical properties of magmas. In order to understand dynamic processes associated with magma chambers and volcanic eruptions, it is therefore crucial to document variations and evolution of magmatic H2O contents through the stages of crystallization, differentiation, degassing, and eruption. This project has as a central goal to develop a new method for estimating magmatic H2O by determining experimentally how the element hydrogen (H) is partitioned between the ubiquitous feldspar minerals and silicate liquids, or magmas.

The new technique to be developed in this study will complement existing methods such as melt inclusion studies, constraints from phase equilibria, and albite-anorthite (Ab-An) hygrometry. The team will conduct experiments over a range of temperatures, bulk H2O contents, and oxygen fugacities. The experiments will focus on plagioclase (An25-An90) and coexisting basaltic-rhyolitic melt compositions. The experiments will be supplemented by analyses of natural plagioclase/melt inclusion pairs from Mt. Hood and Mt. Mazama, Oregon. These samples provide a test case for this method because they have already been studied using melt inclusion and Ab-An hygrometry. They will analyze hydrogen in both experimental and natural samples by SIMS, using a newly developed calibration for feldspars. At the same time, they plan to use new electron microprobe (EPMA) analyses to provide new constraints on thermodynamic models of Ab-An hygrometry. Broader impacts of the study include providing opportunities for direct participation by undergraduates in the research process, collaboration with Phil Ihinger at UW Eau Claire (a principally undergraduate research institution), and development of feldspar standards as part of this project that will be disseminated to SIMS laboratories in the Earth Science community.