14:45 〜 15:00
[SGC37-05] FTIR analysis of H2O in hydrated volcanic glasses: insights into submarine eruption processes
キーワード：FTIR volatile analysis, secondary hydration, submarine eruption
Magmatic volatiles (such as H2O, CO2, Cl, S) drive volcanic eruptions through bubble nucleation and growth as magma ascends. Their solubilities vary in response to changes in pressure, temperature and co-existing volatile concentrations. Accordingly, volcanic processes can be reconstructed based on dissolved volatile concentrations recorded in volcanic glasses. H2O is often the most volumetrically abundant volatile in silicic magmas such as rhyolite, and therefore is a key volatile record of eruption processes. However, the strong susceptibility of high silica glasses to secondary hydration (i.e. slow diffusive addition of non-magmatic water in the time after eruption and deposition that alters eruptive H2O concentrations) has limited the use of matrix glass H2O concentrations in high surface area material such as tephra and pumice, particularly in submarine deposits.
H2O is dissolved in melts and glasses as two species, molecular H2O (H2Om) and OH, which can be measured using Fourier Transform Infra Red spectroscopy (FTIR). Low temperature secondary hydration increases H2Om but does not alter OH. Using imaging FTIR analyses and a species-dependent H2Ot molar absorptivity coefficient (1) to overcome analytical issues relating to thin and water-rich glasses, it is now possible to accurately measure OH concentrations and thus to investigate volcanic processes even in glasses affected by secondary hydration.
We present here the example of how FTIR H2O analyses of hydrated glasses have been used to investigate past eruptions of Oomurodashi, a shallow silicic submarine volcano ~60 km from Tokyo Bay (2, 3):
i) Ages of submarine lavas are estimated from the difference between their matrix glass OH contents (a record of eruption pressure, hence depth below sea level) and their current day depth below sea level, which reveal that they were erupted at a time when sea level was lower than the present day.
ii) Matrix glass OH contents of submarine pumice reveal the minimum water depths reached during eruption, with high porosity pumice reaching shallower water depths than low porosity pumice.
iii) Matrix glass OH contents of three different tephra layers (O58 on Izu-Oshima island, O3T on Toshima island, and Od-1 in marine sediment core C9010E) are consistent with their explosive fragmentation within the upper 100-200 m of the shallow submarine volcanic edifice in a shallow phreatomagmatic eruption.
iv) Matrix glass H2Om contents of marine tephra Od-1 have been modified by secondary hydration, and numerical modelling of this low temperature hydration can be a viable method for estimating ages of marine tephra.
(1) McIntosh et al (2017) American Mineralogist 102(8), 1677-1689 doi: 10.2138/am-2017-5952CCBY
(2) McIntosh et al (2022) Geology 50(10), 1111-1115 doi: 10.1130/G50148.1
(3) McIntosh et al (2022) Frontiers in Earth Science doi: 10.3389/feart.2022.963392
H2O is dissolved in melts and glasses as two species, molecular H2O (H2Om) and OH, which can be measured using Fourier Transform Infra Red spectroscopy (FTIR). Low temperature secondary hydration increases H2Om but does not alter OH. Using imaging FTIR analyses and a species-dependent H2Ot molar absorptivity coefficient (1) to overcome analytical issues relating to thin and water-rich glasses, it is now possible to accurately measure OH concentrations and thus to investigate volcanic processes even in glasses affected by secondary hydration.
We present here the example of how FTIR H2O analyses of hydrated glasses have been used to investigate past eruptions of Oomurodashi, a shallow silicic submarine volcano ~60 km from Tokyo Bay (2, 3):
i) Ages of submarine lavas are estimated from the difference between their matrix glass OH contents (a record of eruption pressure, hence depth below sea level) and their current day depth below sea level, which reveal that they were erupted at a time when sea level was lower than the present day.
ii) Matrix glass OH contents of submarine pumice reveal the minimum water depths reached during eruption, with high porosity pumice reaching shallower water depths than low porosity pumice.
iii) Matrix glass OH contents of three different tephra layers (O58 on Izu-Oshima island, O3T on Toshima island, and Od-1 in marine sediment core C9010E) are consistent with their explosive fragmentation within the upper 100-200 m of the shallow submarine volcanic edifice in a shallow phreatomagmatic eruption.
iv) Matrix glass H2Om contents of marine tephra Od-1 have been modified by secondary hydration, and numerical modelling of this low temperature hydration can be a viable method for estimating ages of marine tephra.
(1) McIntosh et al (2017) American Mineralogist 102(8), 1677-1689 doi: 10.2138/am-2017-5952CCBY
(2) McIntosh et al (2022) Geology 50(10), 1111-1115 doi: 10.1130/G50148.1
(3) McIntosh et al (2022) Frontiers in Earth Science doi: 10.3389/feart.2022.963392