INSIGHTS INTO MAGMATIC WATER CONTENTS AT THE ONSET OF A YELLOWSTONE SUPERERUPTION FROM HYDROXYL CONTENTS IN FELDSPAR

Supereruptions differ from other explosive volcanic eruptions in their sheer size, ejecting around 1000 km³ of material over vast areas, and lasting anywhere from days to months (Wilson 2008), defining their source as a supervolcano. A typical supervolcano is Yellowstone, with the earliest major eruption (2500 km³ of material) forming the Huckleberry Ridge Tuff (HRT). The HRT began with episodic activity, producing fall deposits over a period of days to weeks before changing into generating voluminous pyroclastic flows (Myers et al. 2016). To help determine if the HRT supereruption was internally (e.g. gas overpressure) or externally triggered (e.g. by faulting), we measured the OH (hydroxyl) concentrations in plagioclase phenocrysts from the basal, middle, and upper parts of the initial fall deposit, to give a stratigraphic profile of the changes within the magma chamber. Using a Nicolet Magna­750 Fourier Transform Infrared Spectrometer (FTIR) at JMU, we analyzed absorption bands of the plagioclase in three mutually perpendicular crystallographic directions to determine OH concentrations using the calibration of Mosenfelder et al. (2015). Calculated OH concentrations in plagioclase are 57-59 ppm for the basal, 43-44 ppm for the middle, and 40-43 ppm H₂O for the upper parts of the fall deposit. These values linearly correlate with magmatic water concentrations determined from melt inclusions (Myers et al. 2016) of 4.6 wt% (basal), 4.0 wt% (middle), and 3.6 wt% (upper). The OH concentration in plagioclase is apparently retained during eruption and can be used as a proxy for magmatic water concentration, even in cases where melt inclusions are absent or have devitrified.