Magma degassing processes in view of hydrogen isotopic variations

火山喷发中水的氢同位素变化是理解岩浆脱气过程的一个很好的指标，因为即使在岩浆温度(-800ﾟC)下，水蒸气和熔体中溶解的水之间的氢同位素分馏也很大。本研究旨在研究具有不同化学性质的岩浆导致不同喷发方式的脱气过程。(1)测定了1991年温泉火山喷发的熔岩和火山弹中的全岩和矿物分离物(角闪石和黑云母)的水含量和氢同位素比值。喷发为典型的英安岩穹隆状构造。结果表明，深部岩浆具有较高的初始含水率(5~6wt.%)和氢同位素比值(增量D=-50)，在上升到地表过程中失去了大部分水，导致低H2O熔岩(0.5wt%或更低)。观测到的H_2O-DeltaD关系既不符合封闭体系脱气模型，也不符合开放体系脱气模型，表明动力学同位素效应在高粘性岩浆脱气过程中起着重要作用。这些结果将很快发表在《火山学和地热研究》杂志上。(2)测定了1707年富士山喷发的玄武岩渣岩样品的水含量和氢同位素比值。结果表明，D值与H2O含量呈负相关。夏威夷基拉韦厄火山的玄武岩渣也有同样的趋势。这表明玄武岩熔体中溶解的部分水通过与熔体中丰富的Fe~(2+)&Gt~(2+)反应还原为氢，导致D与H_2O呈负相关。

Hydrogen isotopic variation of water in volcanic eject is a good indicator to understand the degassing processes of magma, because large hydrogen isotopicfractionation between vapor and water dissolved in melt is significantly large even at magmatic temperatures (-800ﾟC). This research aimed at studying the degassing processes magmas with different chemical properties that lead to different eruption styles.(1) Water contents and hydrogen isotopic ratios were determined for whole rocks and mineral separates (hornblende and biotite) from lavas and bombs obtained from the 1991 eruption of Unzen volcano. The eruption was of typical dacite dome formation. The data indicated that the deep-seated magma characterized by a high initial water conten (5-6 wt.%) and hydrogen isotopic ratio (deltaD=-50) lost most of water during ascent to the Earth's surface, resulting in the low H_2O lavas (0.5 wt% or lower). The observed H_2O-deltaD relationship followed neither the closed- nor open-system degassing models, suggesting that a kinetic isotopic effect playd an important role during degassing of highly viscous magma. These results will be published shortly in Journal of Volcanology and Geothermal Research.(2) Water contents and hydrogen isotopic ratios were determined for basaltic scoria samples from the 1707 eruption of Mt.Fuji. It was found that the deltaD values negatively correlate with H_2O contents. The same trend was found for basaltic scoria from Kilauea volcano, Hawaii. This suggests that part of water dissolved in basaltic melt was reduced to hydrogen through reaction with abundant Fe^<2+> in the melt, resulting in the negative correlation between deltaD and H_2O.