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Unlocking the H2O archive in plagioclase: experimental plagioclase -- dacite melt H2O partitioning at crustal conditions

Manuel Pimenta Silva 1, Brian D. Monteleone2, Glenn A. Gaetani2, Dawnika L. Blatter3, Thomas W. Sisson3

  • Affiliations: 1Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, United States; 2Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, United States; 3U.S. Geological Survey, California Volcano Observatory, Moffett Field, CA 94035, USA

  • Presentation type: Talk

  • Presentation time: Thursday 09:45 - 10:00, Room S150

  • Programme No: 1.1.6

  • Theme 1 > Session 1

Abstract

H2O has a fundamental impact on volcanic petrology and hazards, particularly in arc volcanoes. The proximity of many arc volcanoes to population centers (e.g., Pinatubo, Mount St. Helens) thus highlights the need for reliable proxies on pre-eruptive H2O concentrations. Our goal is to establish a robust hygrometric proxy for dacitic magmas by experimentally determining the partitioning of H2O between plagioclase and dacite melt, which has so far been unreported. Plagioclase is well suited for this purpose due to its ubiquity in arc magmas, extended thermal stability, and lower H diffusivity relative to other silicate phases1. We measured H2O concentrations of plagioclase and coexisting dacitic glass by SIMS in vapor-saturated experiments conducted on a Mount St. Helens dacitic composition. Experiments were performed in piston-cylinder devices2,3 at 900-1000 ºC, 400-1310 MPa, and fO2 conditions near the Re-ReO2 buffer. Preliminary results from six experiments (An43-60) show 3.7-5.6 wt.% H2OTglass and 94-137 µg/g H2Oplag, resulting Dplag-glass~H2O ~ in  between 2-3*10-3. Key trends include the positive correlation between H2Oplag and pressure, and a negative An#-Dplag-glassH2O correlation. These findings pave the way for reconstructing long-term variations in pre-eruptive H2O concentrations by analyzing H2O contents in plagioclase in volcanic deposits, partially overcoming shortcomings of other proxies, such as melt inclusions, H contents in faster diffusing phases or plagioclase-melt hygrometry.   1Johnson & Rossman (2013), Am Mineral, 98, 1779-1787 2Blatter, Sisson & Hankins (2017), Contrib Mineral Petrol, 172, 27 2Blatter, Sisson & Hankins (2023), Contrib Mineral Petrol, 178, 33