V003-0005
Hydroxyl in Quartz and Feldspar Phenocrysts from the Bishop Tuff, CA, and the Huckleberry Ridge Tuff, Yellowstone

Monday, 7 December 2020
Poster
Rachel Rappoccio1, Elizabeth A Johnson1, Madison Myers2, Paul J Wallace3 and Colin J N Wilson4, (1)James Madison University, Department of Geology and Environmental Science, Harrisonburg, VA, United States, (2)Montana State University, Bozeman, MT, United States, (3)University of Oregon, Department of Earth Sciences, Eugene, OR, United States, (4)Victoria University of Wellington, School of Geography, Environment and Earth Science, Wellington, New Zealand
Abstract:
Quartz-hosted melt inclusions are widely used to determine water concentrations for felsic volcanic systems, but availability and quality of inclusions limit the scope of research on volatiles within these systems. In this study, we determine hydroxyl concentrations [OH] in coexisting sanidine, plagioclase, and quartz phenocrysts within a single sample of the 2.08 Ma Yellowstone Huckleberry Ridge Tuff (HRT) basal fall deposit to explore partitioning of OH among these phases. We also measure [OH] of quartz phenocrysts from the Bishop Tuff, CA, fall deposit and additional HRT deposits to explore quartz-melt partitioning and OH variations among different felsic eruptions. Rb-corrected water concentrations in quartz-hosted melt inclusions from these units are high (3.8-5.6 wt%; Myers et al. 2016, 2019), maximizing the possibility for easily resolvable quartz [OH] bands. When possible, feldspar cleavage fragments and euhedral quartz crystals were used to produce oriented polished slabs for FTIR measurements. Polarized infrared spectra were obtained using the Thermo-Nicolet i550 FTIR Spectrometer at JMU. [OH] was calculated using the modified Beer-Lambert Law and absorption coefficients for feldspars (Mosenfelder et al. 2015) and quartz (Thomas et al. 2009). Quartz [OH] was 16-23 ppm for the Bishop Tuff and 10-24 ppm for the HRT. These values are the same as or slightly higher than the [OH] concentration of 17 ppm reported for the Yellowstone Mesa Falls Tuff, with melt inclusion water concentrations of 2.9-3.3 wt% H2O (Tollan et al. 2019).In the HRT basal fall sample YP287, [OH] concentrations ranged from 58 ppm in sanidine, to 34-57 ppm in plagioclase, and 9-11 ppm in quartz. Corresponding partition coefficients (DH2O(mineral-melt)) using a weighted mean of 3.8 wt% H2O for quartz melt inclusions (Myers et al. 2016) 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). The quartz partition coefficient calculated for BT and additional HRT samples range from 0.0003 to 0.0006. Preliminary observations suggest larger quartz crystals may retain higher [OH]. This study shows the potential for using quartz and feldspar phenocrysts to study water in other silicic volcanic deposits.
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