M. Hamada 1 *, M. Ushioda 1 , T. Fujii 2,3 and E. Takahashi 1

1. V11C-2776 (*E-mail: hamada@geo.titech.ac.jp) Part 2) Hydrous melting experiments to determine H partitioning between plagioclase and melt Integration of Part 1 and Part 2 300 Purpose and quick summary ☞H concentration in plagioclase can be approximated as a square root of H2O in melt. Decreasing temperature lowers H concentration in plagioclase. However, the effect of temperature is minor. Hydrous melting experiments 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 H2O 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-H2O melt inclusions (H2O ≈ 0.3 wt.%) hosted by plagioclase in mid-ocean ridge basalt (MORB) and (ii) hydrogen partitioning experiments between Ca-rich plagioclase (An95) and hydrous arc basaltic melt (0.8 ~ 5.5 wt.% H2O) at 0.35 GPa. Obtained formulation is Hydrogen in plagioclase (wt. ppm water) ≈ 94.3×√(H2O 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 H2O-saturated magma. 250 Melt inclusions 1050 ℃ 200 Experimental procedures 1000 ℃ 1130 ℃ Crushing volcanic rock H in plagioclase (wt.ppm water) 150 130ºE 135ºE 140ºE 145ºE 1160 ℃ 1050 ℃ 45ºN 100 Separation of An95 plagioclase Hydrogen concentration in plagioclase as a hygrometer of magmas: Approaches from melt inclusion analyses and hydrous melting experiments ☞Linear correlation between H concentration in plagioclase and OH in melt suggests that H is accommodated in plagioclase as OH. 1 mg of An95 plagioclase 50 40ºN Hydrous melting of crushed rock plag melt CH2O (wt. %) CH2O (wt. ppm) ≈94.3× 0 10 mg of hydrous glass 0 1 2 3 Izu-Oshima volcano OH in glass (wt.%) 35ºN 0.015 0.01±0.005 Miyakejima volcano 0.010 DH(plag/melt) 0.008±0.002 Au80Pd20 capsule 30ºN 0.005 ☞Hydrogen partition coefficient DH slightly decreases with increasing H2O in coexisting melt. 0 volcanic front 0 1 2 3 4 5 6 Melting at 0.35 GPa for 24-48 hours using internally-heated pressure vessel H2O in glass (wt.%) Starting material of melting experiments (MTL rock, collected from Miyakejima volcano inIzu arc) － Representative absorption spectra of plagioclase － SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O Total 51.2 0.96 17.5 11.2 0.21 4.9 11.5 2.2 0.25 100 (wt.%)) residual resin? Part 1) Analyses of plagioclase-melt inclusion pairs from MORB #RTJ-pl20 (11 wt. ppm water, 0.3 wt.% H2O in melt inclusion) #RTJ-pl24 (38 wt. ppm water, 0.3 wt.% H2O in melt inclusion) Studied sample: Sample# KH93-3 DR9 from Rodriguez Triple Junction in the Indian Ocean #MTL17 (68 wt. ppm water, 0.8 wt.% H2O in melt) #MTL05 (153 wt. ppm water, 2.3 wt.% H2O in melt) M. Hamada1*, M. Ushioda1, T. Fujii2,3 and E. Takahashi1 1Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan 2Earthquake Research Institute, University of Tokyo, Tokyo, Japan 3Crisis & Environmental Management Policy Institute, Tokyo, Japan KH93-3 DR9 Absorption per cm Whole-rockcomposition SiO2 50.5 TiO2 1.52 Al2O3 15.1 FeO* 11.1 MnO 0.19 MgO 7.44 CaO 11.1 Na2O 2.80 K2O 0.10 P2O5 0.15 Total 100(wt.%) #MTL39 (157 wt. ppm water, 3.5 wt.% H2O in melt) #MTL37 (210 wt. ppm water, 4.5 wt.% H2O in melt) Modal composition 500 μm Plagioclase (An95) Olivine (Fo78-81) Clinopyroxene Orthopyroxene Magnetite #MTL41 (225 wt. ppm water, 5.6 wt.% H2O in melt) 16 2 0 0 0 (vol.%)) Cross-Nicol image of studied MORB. Quenched glass contains ~0.3 wt.% H2O. Internally-heated pressure vessel installed at Magma Factory, Tokyo Tech. Natural plagioclase in MTL lava (5 wt. ppm water) Figure is from Nakamura et al. (2007 Marine Geol.) Wavenumber (cm-1) 50 μm Backscattered-electron image of plagioclase and melt inclusion. Longer hydrogen bond length d(O⋯O) Shorter hydrogen bond length d(O⋯O) Modal composition Obtained phase diagram of MTL rock at 0.35 GPa Plagioclase (An74-89) Olivine (Fo90) 5 1 (vol.%)) ☞Peak of infrared absorption spectra of plagioclase shifts from lower wavenumbers (peak position: 3200-3400 cm-1) under H2O-poor conditions to higher wavenumbers (peak position: 3600 cm-1) under H2O-rich conditions, meaning expansion of O-H⋯O bond length with increasing H2O. These observations suggest that hydrogen site in plagioclase slightly changes with increasing H2O, which also changes hydrogen partitioning between plagioclase and melt as shown above. fO2~NNO+3 plagioclase-in augite-in magnetite-in Partitioning experiment of hydrogen between An95 plagioclase and melt Analytical result using FT-IR An95 plagioclase plagioclase-in Application Temperature (℃) 200 μm augite-in Hydrogen concentration in plagioclase (wt. ppm water) Hydrogen concentration in plagioclase (wt. ppm water) magnetite-in Backscattered electron image of the recovered sample (1130℃, 2.6 wt.% in melt) ☞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 H2O-rich (≥4 wt.%) melt. ☞Hydrogen concentration in plagioclase and H2O concentration in glass was measured using FT-IR. H2O in plagioclase-hosted melt inclusions (wt.%) An content of plagioclase An80 An85 An90 An95 Anorthite content of plagioclase ≈ 0.01 H2O in melt (wt.%) Hamada et al. (2011, EPSL)