Degassing and associated crystallization of basalt magma

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In order to understand the degassing process from island-arc basalt, the following three approaches have been taken : (l) Estimation of water content in island arc basalt analyzing glass inclusions in phenocryst ; (2) Determination of melting phase relations of hydrous island-arc basalt at high pressure ; (3) Development of plagioclase hygrometer applicable to the island-arc basalt composition. Melting experiments were performed at pressure corresponding to the depth of magma reservoir which was estimated by geophysical observation. From the high pressure experiments, the relationship between composition of plagioclase and water content in basaltic magma was formulated as a hygrometer. Base on this result, it is estimated that the Ca-rich plagioclase phenocrysts typical for basalt from the volcanic front of island arc are crystallized from hydrous magmas which contain water more than 3 wt %. While it was found that the melt inclusions in Ca-rich plagioclase contain water less than 1.5 wt %. This may indicate part of water in melt inclusion was lost by diffusion process through host mineral, or alternatively indicate that plagioclase was crystallized from supercooled magma during degassing. Analysis of SO2 in melt inclusions indicates that SO2 decreases with decreasing water content, suggesting that melt inclusions were trapped in plagioclase during the process of degassing. From this observation, it could be proposed that Ca-rich plagioclase phenocrysts are crystallized from degassing hydrous basalt magma at relatively shallow depth.