[S-VC45] [EE] Wet volcanology

[SVC45-05] Investigating submarine eruptions using H₂O contents of volcanic glasses: application of a new FTIR spectroscopy method

*Iona McIntosh¹、Alex Nichols²、Kenichiro Tani³、Ed Llewellin⁴ (1.Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan、2.University of Canterbury, Christchurch, New Zealand、3.National Museum of Nature and Science, Tsukuba, Japan、4.Durham University, Durham, United Kingdom)

キーワード：FTIR spectroscopy, hydration, submarine volcanology

Volatile degassing during magma ascent plays a fundamental role in determining eruption style and pyroclast formation, and the analysis of the dissolved volatile contents preserved in volcanic glasses (both melt inclusions and matrix glasses) is therefore an important tool for investigating eruption processes. Given its strong control on parameters such as melt viscosity, H₂O is one of the most important magmatic volatiles, especially for felsic magmas where it is often the most volumetrically abundant volatile. In addition, the well-constrained pressure-dependence of H₂O solubility means that the H₂O contents of pyroclast glasses can provide a record of the pressure at which pyroclasts quenched. For submarine eruptions, where vent depth and pyroclast formation are rarely observed directly, this is particularly valuable information. Unfortunately, silicate glasses are vulnerable to secondary hydration (i.e. the addition of H₂O from the surrounding environment at low temperature in the time following deposition), and thus volcanic glasses deposited in the submarine environment are often hydrated. To obtain meaningful H₂O data from these hydrated glasses it is therefore necessary to distinguish between the original final dissolved H₂O content and the H₂O added subsequently during hydration. Since H₂O added during hydration is added as molecular H₂O (H₂O_m), and the species interconversion between H₂O_m and hydroxyl (OH) species is negligible at ambient temperature, the final OH content of the glass remains unaltered during hydration. By using H₂O speciation models to find the original H₂O_m content that would correspond to the measured OH content of the glass, the original total H₂O (H₂O_t) content of the glass prior to hydration can be reconstructed. These H₂O speciation data are obtained using Fourier Transform Infra-red (FTIR) spectroscopy. In many cases OH cannot be measured directly and instead is calculated indirectly as OH = H₂O_t - H₂O_m. Here we demonstrate the importance of using a species-dependent H₂O_t molar absorptivity coefficient to obtain accurate H₂O_t and H₂O speciation data and outline a methodology for calculating such a coefficient for both hydrated and unhydrated rhyolite and andesite glasses. Using this method we present reconstructed final H₂O_t contents of hydrated felsic pyroclasts from submarine volcanoes in the Japanese Izu-Bonin Arc and use these data to investigate submarine felsic eruptions and the processes that produce submarine pyroclasts, in order to understand their associated hazards.