Water in volcanic glass: From volcanic degassing to secondary hydration

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• 作者：Angela N. Seligman ; ^{seligman@uoregon.edu" class="auth_mail" title="E-mail the corresponding author} ; Ilya N. Bindeman ^{bindeman@uoregon.edu" class="auth_mail" title="E-mail the corresponding author} ; James M. Watkins ^{watkins4@uoregon.edu" class="auth_mail" title="E-mail the corresponding author} ; Abigail M. Ross ^{aross4@uoregon.edu" class="auth_mail" title="E-mail the corresponding author}
• 关键词：Secondary hydration ; Hydrogen isotopes ; Diffusion ; Water ; Paleoenvironments ; Kamchatka
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2016
• 出版时间：15 October 2016
• 年：2016
• 卷：191
• 期：Complete
• 页码：216-238
• 全文大小：5327 K

文摘

Volcanic glass is deposited with trace amounts (0.1–0.6 wt.%) of undegassed magmatic water dissolved in the glass. After deposition, meteoric water penetrates into the glass structure mostly as molecular H₂O. Due to the lower δD (‰) values of non-tropical meteoric waters and the ∼30‰ offset between volcanic glass and environmental water during hydration, secondary water imparts lighter hydrogen isotopic values during secondary hydration up to a saturation concentration of 3–4 wt.% H₂O. We analyzed compositionally and globally diverse volcanic glass from 0 to 10 ka for their δD and H₂O_t across different climatic zones, and thus different δD of precipitation, on a thermal conversion elemental analyzer (TCEA) furnace attached to a mass spectrometer. We find that tephrachronologically coeval rhyolite glass is hydrated faster than basaltic glass, and in the majority of glasses an increase in age and total water content leads to a decrease in δD (‰), while a few equatorial glasses have little change in δD (‰). We compute a magmatic water correction based on our non-hydrated glasses, and calculate an average 10³lnα_glass-water for our hydrated felsic glasses of −33‰, which is similar to the 10³lnα_glass-water determined by Friedman et al. (1993a) of −34‰. We also determine a smaller average 10³lnα_glass-water for all our mafic glasses of −23‰. We compare the δD values of water extracted from our glasses to local meteoric waters following the inclusion of a −33‰ 10³lnα_glass-water. We find that, following a correction for residual magmatic water based on an average δD and wt.% H₂O_t of recently erupted ashes from our study, the δD value of water extracted from hydrated volcanic glass is, on average, within 4‰ of local meteoric water. To better understand the difference in hydration rates of mafic and felsic glasses, we imaged 6 tephra clasts ranging in age and chemical composition with BSE (by FEI SEM) down to a submicron resolution. Mafic tephra have more bubbles per unit area (25–77 mm⁻²) than felsic tephra (736 mm⁻²) and thicker average bubble walls (0.07 mm) than felsic tephra (0.02 mm). We use a simplified diffusion model to quantify the hydration rate of vesicular glass as a function of the diffusivity of water and the average bubble wall thickness. Based on fits to our hydration rate data, we estimate the initial low-temperature diffusivity at 0.1 wt.% H₂O_t in volcanic glass (mafic and felsic) to be on the order of 10⁻³ to 10⁻⁴ μm²/year and find that differences in hydration rates between mafic and felsic tephra can be attributed primarily to differences in vesicularity, although slightly slower hydration of basalt cannot be precluded. We also observe no consistent temporal difference in secondary meteoric water uptake in wet versus dry and hot versus cold climates.