Volcanic Processes in Volatile-Rich Magmas: High Pressure Experiments on Amphibole-Plagiclase-Melt Equilibria and C-O-S-Cl-H

该项目建立在以前的实验和分析研究的基础上，以限制富挥发分岩浆斑晶-熔体组合的演化。第一项任务将涉及校准斜长石-角闪石-熔体平衡，压力高达600兆帕，温度高达角闪石分解，具有代表性的镁铁质安山岩和英安岩岩石成分。角闪石-斜长石组合广泛存在于中成分岩浆中，在前人工作的基础上，其成分中含有重要的P-T信息。拟议的实验将在300-600兆帕的范围内进行，这是以前的研究中很少获得的，旨在获得与这些矿物的平衡组合非常接近的结果。这些结果将极大地改善从天然火山岩分析中获得的有关次火山岩浆系统的信息。这些新数据将专门用于解释海伦斯圣山岩浆系统的次火山岩浆历史。第二项任务将是确定富含二氧化碳的气相的组成，这种气相肯定是在镁铁质弧岩浆系统中压力超过400兆帕时产生的，要么是岩浆上升过程中减压的结果，要么是冷却的结果。对这种气相的H2O、S和氯含量的研究将为评价已提出的解释弧形环境中高度氧化的富S硅质岩浆成因的模型提供数据。它还将有助于评估在火山喷口测量的、经常用于喷发预测的相对挥发性丰度的重要性。还提议开展第三项一般性任务，包括继续对蒙特塞拉特圣海伦斯火山和苏弗里埃尔山正在喷发的物质进行岩石学和分析研究。

This project builds on previous experimental and analytical studies to constrain the evolution of of volatile-rich magma phenocryst-melt assemblages. The first task will involve the calibration of plagioclase-amphibole-melt equilibria at pressures up to 600 MPa and temperatures up to the amphibole breakdown for representative mafic andesite and dacite rock compositions. The amphibole-plagioclase assemblage is widespread in intermediate composition magmas, and their compositions contain significant P-T information based on previous work. The proposed experiments will be in the 300-600 MPa range, rarely obtained in previous studies, and are designed to obtain a close approximation to an equilibrium assemblage of these minerals. The results will greatly improve the information about subvolcanic magma systems that can be obtained from analyses of natural volcanic rocks. The new data will be specifically used to interpret the subvolcanic magma history of the Mount St, Helens magma system. The second task will be to determine the composition of the CO2-rich gas phase that is certainly generated at pressures in excess of 400 MPa in mafic arc magma systems either as a result of decompression during magma ascent or by cooling. Study of the H2O, S and Cl content of this gas phase will provide data to assess models that have been proposed to explain the origin of very oxidized and S-rich silicic magmas in arc environments. It will also be useful in assessing the importance of the relative volatile abundances measured at volcanic vents and often used in eruption prediction. A third general task is also proposed that involves the continued petrologic and analytical study of the materials being erupted in the ongoing eruptions of Mount St. Helens and Soufrierre Hills, Montserrat.