島弧における初生マグマの含水量の推定：実験岩石学とメルト包有物分析からの制約

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The slab-derived fluids and/or hydrous slab melts released from a subducted slab ascend into the mantle wedge, lower its melting temperature and thus induce generation of hydrous arc magmas. The estimation of H2O concentration in primary arc magmas provides an important constraint on pressure and temperature conditions of magma generation at subduction zones. This paper gives an overview of the estimation of H2O concentration in primary arc magmas by combining two petrological methods: experimental petrological studies and analyses of melt inclusions. Melting experiments of hydrous primary arc magmas have clarified that the P-T condition of magma generation shifts toward lower temperature and higher pressure with increasing H2O concentration. Another experimental constraint is that only primary magmas with low H2O (≤ 2 wt%) can erupt without modification of their primary composition by crystallization differentiation due to comparable dT/dP between olivine liquidus and basalt adiabat. However, this does not exclude presence of hidden H2O-rich primary magmas at depths. Indeed, the H2O concentrations in primary melt estimated from the analyses of primitive melt inclusions suggest wide variation (e.g., ~ 2 wt% at Kamchatka arc and ~ 4 wt% at Central American arc). H2O-rich primary magmas may ascend and erupt after differentiation and/or supply volatiles to magmas at shallower level and cause so-called “excess degassing”. Analyses of melt inclusions also clarified that the H2O concentration in primitive melt inclusions is almost constant or decrease from volcanic front to rear arc. This observation is opposite to a previous understanding that H2O concentration in primary melt increases as well as incompatible K2O across the arc.

島弧における初生マグマの含水量の推定：実験岩石学とメルト包有物分析からの制約

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