Degassing and associated crystallization of basalt magma

为了了解岛弧玄武岩的脱气过程，采取了三种方法：(L)估算岛弧玄武岩中的水分含量，分析斑晶中的玻璃包裹体；(2)确定含水岛弧玄武岩在高压下的熔融相关系；(3)研制适用于岛弧玄武岩成分的斜长石湿度计。熔融实验是在与地球物理观测估计的岩浆库深度相对应的压力下进行的。通过高压实验，将玄武岩岩浆中斜长石的组成与含水率之间的关系表示为湿度计。根据这一结果，推测岛弧火山前缘玄武岩的典型富钙斜长斑晶是由水含量大于3wt%的含水岩浆结晶而成。而富钙斜长石中熔融包裹体的水含量小于1.5wt%。这可能表明熔体包裹体中的部分水通过宿主矿物的扩散过程而丢失，也可能表明斜长石是在脱气过程中从过冷的岩浆中结晶出来的。对熔体包裹体中SO2的分析表明，SO2随水含量的降低而减少，表明熔体包裹体在脱气过程中被捕获在斜长石中。由此推测，富钙斜长石斑晶是在相对较浅的深度内从脱气含水玄武岩岩浆中结晶出来的。

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.