A COMPARISON OF HYDROXYL ABUNDANCES IN COEXISTING FELDSPAR AND QUARTZ PHENOCRYSTS WITHIN THE INITIAL FALL DEPOSIT OF THE HUCKLEBERRY RIDGE TUFF, YELLOWSTONE

The 2.08 Ma Huckleberry Ridge Tuff (HRT) is a silicic pyroclastic deposit produced by the first caldera-forming eruption at Yellowstone. Volatile concentrations in quartz-hosted melt inclusions have been well-studied in this unit (Myers et al. 2016). While melt inclusions are typically used to determine water concentrations, availability and quality of inclusions limit research on other felsic volcanic systems. In this study, we determine hydroxyl (OH) concentrations in coexisting sanidine, plagioclase, and quartz phenocrysts from a sample (YP287: Myers et al. 2016) from the HRT basal fall deposits and estimate water partition coefficients for these minerals. Polarized infrared spectra were obtained in three mutually perpendicular directions on 4-sided prisms using a Continuμm microscope on the Thermo-Nicolet i550 FTIR Spectrometer at JMU. OH concentrations were calculated using the modified Beer-Lambert Law and absorption coefficients for feldspars from Mosenfelder et al. (2015) and for quartz from Thomas et al. (2009) and had 10% relative error. OH concentrations ranged from 58 ppm in sanidine, to 34-57 ppm in plagioclase, and 9-11 ppm in quartz. Sanidine OH concentration is slightly higher than the previously reported range (35-43 ppm; Ruefer et al. 2018). Quartz OH concentrations from this study are lower than the OH concentration of 17 ppm reported for the Yellowstone Mesa Falls Tuff, which had melt inclusion water concentrations of 2.9-3.3 wt% H₂O (Tollan et al. 2019). We determined partition coefficients (D_H2O(mineral-melt)) using the weighted mean of 3.8 wt% H₂O for quartz melt inclusions in sample YP287 as reported by Myers et al. (2016). Calculated water partition coefficients are 0.002 for sanidine, 0.001 for plagioclase, and 0.0003 for quartz. The plagioclase partition coefficient is similar to the value of 0.002 in previous work (Mosenfelder et al. 2018). This case study shows that quartz crystals can be used as a proxy to determine magmatic water concentration in other silicic volcanic eruptions.