Magma degassing processes in view of hydrogen isotopic variations

火山喷出水的氢同位素变化是了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℃）下，熔体中溶解的水蒸气和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的脱气过程，从而导致不同的喷发方式。(1)测定了1991年云仙火山喷发熔岩和火山弹的整体岩石和矿物分离物（角闪石和黑云母）的含水量和氢同位素比值。喷发为典型的英安岩穹丘构造。结果表明，深部岩浆的初始含水量（5 ~ 6 wt.%）和氢同位素比（δ tad =-50）较高，在上升过程中失去了大部分水分，形成了低H_2O熔岩（0.5 wt%或更低）。观测到的h_2o -delta关系既不符合封闭系统脱气模型，也不符合开放系统脱气模型，表明在高粘性岩浆脱气过程中，动力学同位素效应起了重要作用。这些结果将很快发表在《火山学与地热研究杂志》上。(2)测定了1707年富士山喷发玄武质岩屑样品的水含量和氢同位素比值。deltaD值与H_2O含量呈负相关。在夏威夷基拉韦厄火山的玄武岩碎屑中也发现了同样的趋势。这说明玄武岩熔体中溶解的部分水与熔体中丰富的Fe^<2+>反应还原为氢，导致δ tad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.