1. Hydrogen concentration in plagioclase as a hygrometer of magmas: Approaches from melt inclusion analyses and hydrous melting experiments M. Hamada 1 *, M. Ushioda 1, T. Fujii 2,3 and E. Takahashi 1 1 Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan 2 Earthquake Research Institute, University of Tokyo, Tokyo, Japan 3 Crisis & Environmental Management Policy Institute, Tokyo, Japan (*E-mail: hamada@geo.titech.ac.jp) V11C-2776 Plagioclase is one of the nominally anhydrous minerals (NAMs) which accommodates hydrogen, and plagioclase in volcanic rocks essentially contains structural OH in it. Hydrogen concentration in NAMs can be a useful proxy for dissolved H 2 O in silicate melts if the partitioning of hydrogen between plagioclase and melts is known. Here, we performed two parts of studies in order to determine hydrogen partitioning between plagioclase and hydrous basaltic melt: (i) analyses of low-H 2 O melt inclusions (H 2 O ≈ 0.3 wt.%) hosted by plagioclase in mid-ocean ridge basalt (MORB) and (ii) hydrogen partitioning experiments between Ca-rich plagioclase (An 95 ) and hydrous arc basaltic melt (0.8 ~ 5.5 wt.% H 2 O) at 0.35 GPa. Obtained formulation is Hydrogen in plagioclase (wt. ppm water) ≈ 94.3×√(H 2 O in melt, wt.%). This empirical formulation can be used as a practical hygrometer of magmas. In this poster, we apply this formulation to understand the 1986- 1987 summit eruption of Izu-Oshima volcano, a frontal-arc volcano in Izu arc, and discuss eruption process of H 2 O-saturated magma. Purpose and quick summary Part 1) Analyses of plagioclase-melt inclusion pairs from MORB Figure is from Nakamura et al. (2007 Marine Geol.) KH93-3 DR9 50 μm 500 μm Whole-rock composition SiO 2 50.5 TiO 2 1.52 Al 2 O 3 15.1 FeO* 11.1 MnO 0.19 MgO 7.44 CaO 11.1 Na 2 O 2.80 K 2 O 0.10 P 2 O 5 0.15 Total 100 (wt.%) Studied sample: Sample# KH93-3 DR9 from Rodriguez Triple Junction in the Indian Ocean Part 2) Hydrous melting experiments to determine H partitioning between plagioclase and melt Experimental procedures volcanic front 35ºN Izu-Oshima volcano 30ºN 45ºN 145ºE140ºE135ºE130ºE Miyakejima volcano SiO 2 TiO 2 Al 2 O 3 FeO* MnO MgO CaO Na 2 O K 2 O Total 51.2 0.96 17.5 11.2 0.21 4.9 11.5 2.2 0.25 100 (wt.%)) Starting material of melting experiments (MTL rock, collected from Miyakejima volcano in Izu arc) H 2 O in melt (wt.%) Temperature ( ℃ ) plagioclase-in augite-in magnetite-in plagioclase-inaugite-in magnetite-in Partitioning experiment of hydrogen between An 95 plagioclase and melt Au 80 Pd 20 capsule 200 μm Crushing volcanic rock Separation of An 95 plagioclase Hydrous melting of crushed rock 10 mg of hydrous glass 1 mg of An 95 plagioclase Melting at 0.35 GPa for 24-48 hours using internally-heated pressure vessel Backscattered electron image of the recovered sample (1130℃, 2.6 wt.% in melt) Obtained phase diagram of MTL rock at 0.35 GPa An 95 plagioclase f O 2 ~NNO+3 300 H 2 O in glass (wt.%) 0 50 100 150 200 250 0 0.005 0.010 0.015 0123456 H in plagioclase (wt.ppm water) D H (plag/melt) C H 2 O (wt. ppm) ≈94.3× C H 2 O (wt. %) meltplag Hydrous melting experiments Melt inclusions 0.01±0.005 0.008±0.002 0123 OH in glass (wt.%) ☞ Linear correlation between H concentration in plagioclase and OH in melt suggests that H is accommodated in plagioclase as OH. Wavenumber (cm -1 ) Absorption per cm #RTJ-pl20 (11 wt. ppm water, 0.3 wt.% H 2 O in melt inclusion) #RTJ-pl24 (38 wt. ppm water, 0.3 wt.% H 2 O in melt inclusion) #MTL17 (68 wt. ppm water, 0.8 wt.% H 2 O in melt) #MTL05 (153 wt. ppm water, 2.3 wt.% H 2 O in melt) #MTL39 (157 wt. ppm water, 3.5 wt.% H 2 O in melt) #MTL37 (210 wt. ppm water, 4.5 wt.% H 2 O in melt) #MTL41 (225 wt. ppm water, 5.6 wt.% H 2 O in melt) Natural plagioclase in MTL lava (5 wt. ppm water) residual resin? Longer hydrogen bond length d(O ⋯ O) Shorter hydrogen bond length d(O ⋯ O) ☞ Peak of infrared absorption spectra of plagioclase shifts from lower wavenumbers (peak position: 3200-3400 cm -1 ) under H 2 O-poor conditions to higher wavenumbers (peak position: 3600 cm -1 ) under H 2 O-rich conditions, meaning expansion of O- H ⋯ O bond length with increasing H 2 O. These observations suggest that hydrogen site in plagioclase slightly changes with increasing H 2 O, which also changes hydrogen partitioning between plagioclase and melt as shown above. － Representative absorption spectra of plagioclase － ☞ High hydrogen concentration in plagioclase from Izu-Oshima volcano, a frontal-arc volcano in Izu arc ( ≥ 200 wt. ppm water, Hamada et al., 2011 EPSL), suggests crystallization of plagioclase from H 2 O-rich ( ≥ 4 wt.%) melt. Hamada et al. (2011, EPSL) An80An85An90An95 Anorthite content of plagioclase ☞ Hydrogen partition coefficient D H slightly decreases with increasing H 2 O in coexisting melt. Internally-heated pressure vessel installed at Magma Factory, Tokyo Tech. 40ºN 1160 ℃ 1130 ℃ 1050 ℃ 1000 ℃ 1050 ℃ ☞ H concentration in plagioclase can be approximated as a square root of H 2 O in melt. Decreasing temperature lowers H concentration in plagioclase. However, the effect of temperature is minor. Plagioclase (An 95 ) Olivine (Fo 78-81 ) Clinopyroxene Orthopyroxene Magnetite 16 2 0 0 (vol.%)) Modal composition Plagioclase (An 74-89 ) Olivine (Fo 90 ) 5 1 (vol.%)) Modal composition Cross-Nicol image of studied MORB. Quenched glass contains ~0.3 wt.% H 2 O. Backscattered-electron image of plagioclase and melt inclusion. Analytical result using FT-IR An content of plagioclase Hydrogen concentration in plagioclase (wt. ppm water) H 2 O in plagioclase-hosted melt inclusions (wt.%) Hydrogen concentration in plagioclase (wt. ppm water) ≈ 0.01 Integration of Part 1 and Part 2 Application ☞ Hydrogen concentration in plagioclase and H 2 O concentration in glass was measured using FT-IR.