Resolving rehydration in pumice: Methods for deriving original H2O content and quenching pressures using H2O speciation and H isotope analysis

Water content of pumiceous matrix glass has been utilized in numerous studies to interpret the pressure at which magma quenches, hence depth within the volcanic conduit or below an overlying ice or water mass. The diffusion and addition of external water into quenched glass after eruption, i.e. rehydration, complicates interpretations as total water content (H₂O_t) then overestimates quenching pressure. Rehydration presents a significant issue when interpreting volatile contents of pumice from submarine eruptions, as prolonged residence time in contact with seawater can add several wt. % H₂O_t to the matrix glass. We present two methods that identify and resolve these rehydration signatures and determine accurate pumice quenching pressures for a number of silicic submarine systems. 1) Following the method of McIntosh et al., (2017), we use Fourier Transform Infra-Red spectroscopy (FTIR) measurements of OH concentration (unaffected by low temperature hydration) in conjunction with an H₂O speciation model (Nowak and Behrens, 2001), and interpret speciation with reference to a H₂O_t-solubility model (Newman and Lowenstern, 2002) to determine pumice quenching pressures. 2) We use high Temperature Conversion Elemental Analyzer (TC/EA) measurements of H isotopes (δD_VSMOW) and H₂O_t together with a speciation-dependent degassing model (Taylor et al., 1991), and a mixing model with known seawater δD_VSMOW to determine the pre-rehydration H₂O_t and hence pumice quenching pressures via the H₂O_t-solubility model. Using both models, results provide valuable insight into submarine quenching and fragmentation mechanisms of pumice over a range of hydrostatic-equivalent vent depths. We will explore the uncertainties of both methods and offer best-practice recommendations for future studies of rehydrated pumice, in both the subaerial and subaqueous environment.

Newman and Lowenstern (2002) Computers & Geosciences 28(5): 597-604. Nowak and Behrens (2001) EPSL 184(2): 515-522. McIntosh et al (2017) American Mineralogist 102(8): 1677-1689. Taylor et al (1991) Geochem. Soc. Spec. Publ. 3: 339-353.