Plagioclase as a recorder of magmatic H2O in explosive eruptions

Pre-eruptive magmatic H2O content influences whether eruptions are effusive or explosive, and therefore the hazard posed to nearby populations. Many arc volcanoes proximal to population centers (e.g., Pinatubo, Mt St Helens) have been active during historic times, elucidating the possible style of future eruptions. Others have been dormant, and knowledge of eruptive style (and its cyclicity) is indirect, highlighting the need for reliable proxies on pre-eruptive H2O content.

My research develops such a proxy for SiO2-rich magmas by experimentally determining the partitioning of H2O between and dacitic melts. Even though plagioclase is a nominally anhydrous mineral, it incorporates trace amounts of H2O, which obey thermodynamic equilibrium.

I measure H2O contents of plagioclase and coexisting dacitic glass by secondary ion mass spectrometry in experimental products synthesized on a Mount St Helens bulk composition. These experiments are performed in a piston-cylinder apparatus at conditions relevant for crustal arc magmatism. The first results show consistent behaviour between H2O in plagioclase and water fugacity, and important relationships between H2O partitioning and plagioclase composition.

To test this approach, I analyze H2O contents in plagioclase crystals and melt inclusions from the 1980-1982 Mount St Helens eruption. The temporal trends in plagioclase H2O contents from 1980 to 1982 parallel those recorded by melt, supporting the use of this new technique as a quantitative proxy for pre-eruptive melt H2O in explosive events. At the same time, the results highlight limitations for slowly cooled dome-forming eruptions, where H2O diffusive loss can obscure original signals.

These findings pave the way for reconstructing long-term variations in pre-eruptive H2O concentrations by analyzing H2O contents in plagioclase in volcanic deposits. This approach complements, and partially overcomes, the shortcomings of traditional proxies such as melt inclusions, mineral-melt equilibrium, and H2O contents in faster diffusing phases.

Speaker: Manuel Pimenta Silva, Woods Hole Oceanographic Institution

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