Characterization of Hydrogen Diffusion in Volcanic Feldspars

基本上所有的无水矿物，包括菱镁矿，都含有少量到痕量的氢。 这是至关重要的，以了解名义上无水矿物中的氢的掺入和保留的机制，如果这些矿物中的含水物种是用来了解地球化学和地质过程的性质。 这项工作通过两条研究路线限制了氢在晶石中的掺入和保留机制。 第一个项目通过模拟在干燥条件下和在固定氧逸度下获得的实验扩散数据，描述氢在含OH的方解石中在岩浆温度（800-1000摄氏度）下的扩散速率和机制。 氢在岩浆温度下的扩散速率决定了在一个典型的喷发事件中保存在长石斑晶中的OH的量，以及在何种程度上保存在具有复杂岩浆历史的长石斑晶中的OH的分区。 综合损失和扩散剖面模型将确定是否扩散取决于含水物质的浓度。 第二个项目是确定1980-1982年圣海伦火山喷发序列中斜长石斑晶及其熔融包裹体中的氢（OH和H2O）浓度。 喷发温度，氧逸度，和散装化学成分都是相当恒定的，通过这段时间，而连续喷发的水含量下降了几个重量百分比。 斜长石是这些岩石中最丰富的斑晶。 假设初步确定火山长石中氢的扩散系数，则湿喷发的斜长石应比干喷发的斜长石含有更高的OH浓度。氢在斜长石及其熔融包裹体之间的分配系数可以通过测定斜长石及其熔融包裹体的含水量来确定。 具有较长结晶历史的斑晶应保留OH分区的特征长度尺度，这可能有助于我们了解火山喷发引起的岩浆挥发分含量增加，由于在岩浆储层中进行结晶。 这项工作的结果应该适用于许多高温地质系统，其中水起着重要的作用。

Essentially all anhydrous minerals, including feldspars, contain minor to trace amounts of hydrogen incorporated into their structures. It is crucial to understand the mechanisms for incorporation and retention of hydrogen in nominally anhydrous minerals if hydrous species in these minerals are to be used to understand geochemical and geological processes in nature. This work constrains the mechanisms for incorporation and retention of hydrogen in feldspars through two lines of research. The first project characterizes the rate and mechanism of diffusion of hydrogen in OH-bearing feldspars at magmatic temperatures (800-1000 degrees C) through modeling of experimental diffusion data obtained under dry conditions and at a fixed oxygen fugacity. The diffusion rate of hydrogen at magmatic temperatures determines the amount of OH preserved in a feldspar phenocryst during a typical eruptive event, and the extent to which zoning of OH in feldspar phenocrysts with complex magmatic histories should be preserved. Models of integrated loss and diffusion profiles across heated slabs of feldspar will determine if diffusion is dependent on the concentration of hydrous species. The second project is to determine hydrogen (OH and H2O) concentrations in plagioclase phenocrysts and their melt inclusions from the 1980-1982 eruption sequence of Mount St. Helens. The eruption temperature, oxygen fugacity, and bulk chemical composition were all fairly constant through this time period while the water content of successive eruptions decreased by several weight percent. Plagioclase feldspar is the most abundant phenocryst in these rocks. Assuming a preliminary determination of a diffusion coefficient of hydrogen in volcanic feldspar, the plagioclase from wet eruption should contain higher OH concentrations than the plagioclase from the dryer eruptions. The partition coefficient of hydrogen between the plagioclase and their melt inclusions can be evaluated by measuring the water content of the feldspar and its melt inclusions. Phenocrysts with longer crystallization histories should preserve OH zoning on characteristic length scales which may aid our understanding of eruptions induced by increased magmatic volatile content due to progressive crystallization in the magma reservoir. The results from this work should be applicable to the many high-temperature geologic systems in which water plays an important role.