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Isotopic signatures of magmatic fluids and seawater within silicic submarine volcanic deposits

Citation

Mitchell, SJ and Hudak, MR and Bindeman, YN and Carey, RJ and McIntosh, IM and Houghton, BF and Rubin, KE, Isotopic signatures of magmatic fluids and seawater within silicic submarine volcanic deposits, Geochimica Et Cosmochimica Acta, 326 pp. 214-233. ISSN 0016-7037 (2022) [Refereed Article]


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DOI: doi:10.1016/j.gca.2022.03.022

Abstract

Addressing questions of magma ascent, volcanic eruption dynamics, and volatile fluxes requires accurate measurement of magmatic water concentrations in volcanic glass. Glass in volcanic rocks in the deep-sea environment can experience rehydration (addition of external water) across a range of temperatures, and from different sources (e.g., seawater and hydrothermal fluids), which can lead to overestimation of magmatic-H2O. This study used H and O isotopes (δD and δ18O) to identify sources of rehydration in pumice and lava from the deep-sea 2012 volcanic eruption of Havre volcano, Kermadec Arc, and from a variety of older (tens to thousands of years) silicic submarine deposits across the Izu-Bonin Arc and Lau Basin. We find that old seafloor pumices were rehydrated up to 6 wt.% H2O by the diffusion of cold seawater over 100s to 1000s of years, and thus, enriched in δD, bulk-δ18O, and water-in-glass (wig) δ18O up to −30, +9 and −5 respectively. By contrast, the young Havre deposits exhibit a much wider range of both isotopic enrichment and depletion with δD = −50 to −120, δ18Obulk = +5.7 to +6.2, and δ18Owig = −10 to +4, depending on the eruptive units. Using magmatic degassing and vapor δD-H2O modeling, a volatile-melt δ18O-geothermometer, and previous textural studies of Havre 2012 deposits, we identify multiple high-temperature rehydration sources, timescales, and mechanisms. δD-depleted pumices were likely rehydrated by vapor co-existing in bubbles and vesicles at temperatures around and below the glass transition (320670 C) over timescales of a few minutes during clast cooling above the eruptive vent. Conversely, δD-enriched Havre pumice and lava from different deposits were most likely rehydrated by heated, δD-enriched seawater at glass temperatures > 100 C. These results provide a natural confirmation of recent experimental findings, which tackle the fundamentals of H and O diffusion and isotope exchange in silicate materials at temperatures of 100400 C. Addressing the effects of rehydration, and thus accuracy of H2O measurements, in volcanic glasses and crystals is important for improving our understanding of conduit processes during volcanic eruptions, and the kinetics of glass hydration and alteration.

Item Details

Item Type:Refereed Article
Keywords:submarine volcanism, rehydration, stable isotopes, seawater, diffusion, volcanic glass, volatiles, magma, eruption
Research Division:Earth Sciences
Research Group:Geology
Research Field:Geology not elsewhere classified
Objective Division:Expanding Knowledge
Objective Group:Expanding knowledge
Objective Field:Expanding knowledge in the earth sciences
UTAS Author:Carey, RJ (Associate Professor Rebecca Carey)
ID Code:149347
Year Published:2022
Funding Support:Australian Research Council (DE150101190)
Deposited By:Earth Sciences
Deposited On:2022-03-26
Last Modified:2022-05-16
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