Phase Relations in Aleutian Basaltic Andesites at 1 atm, 1, and 2kbar: The Effects of Changing XH2O

智力上的功绩。了解不同C-O-H流体组成条件下弧状玄武岩安山岩的结晶和分异，是了解许多弧形火山的岩浆储存系统的关键。岩浆流体内容物对它们在储存过程中的分异以及它们后来是爆发性喷发还是喷发喷发起着根本作用。要理解后者，重要的是要限制在存储区域中发生的过程。以前的实验研究提供了关于流体中H2O摩尔分数(XH2O)与相关系的重要结果，但相对较少的研究集中在地壳压力(2kbar)下的H2O-不饱和体系。以前没有实验研究比较初始饱和H2O熔体和未饱和H2O熔体在减压过程中的结晶，这适用于了解岩浆上升过程中的微晶结构和结晶过程。这项研究将通过对来自Westdahl和Okmok火山的拉斑玄武岩进行熔融和逆转实验来帮助填补数据空白。这项工作的一个新方面将涉及探索斜长石结晶动力学在初始为H2O的不饱和岩浆与最初为H2O饱和的岩浆的减压过程中可能发生的变化。所有的物相组成和织构将使用电子探针、背散射电子图像、岩相光学显微镜和图像分析进行测量。高压实验还将使用反射傅里叶变换红外光谱进行分析，以确认熔体中溶解的H2O比例，并与之前实验研究计算的XH2O进行比较。这些实验将有助于更好地理解喷发前岩浆在阿留申群岛最近两次喷发之前的储存和上升。它们还将提供可普遍应用的数据，以了解在弧状玄武岩安山岩岩浆储存和上升过程中，岩浆流体成分变化对结晶过程的影响。实验的一个子集将由一名本科生进行，作为2007年春季高级岩石学课程的一部分。阿拉斯加大学费尔班克斯分校目前没有气体混合炉；购买一个将加强地质和地球物理系的基础设施。这一结果将为大地测量学研究生提供基于岩石学的数据，用于对奥克莫克火山的通货膨胀和通货紧缩信号进行建模。结果将通过会议发言、同行评议出版物以及与阿拉斯加火山观测站(AVO)联合开发的新开发的网络实验岩石学数据库传播。与AVO外联协调员合作，将向居住在火山附近的感兴趣的人传播有关Okmok和Westdahl未来喷发的岩浆动力学信息。

Intellectual Merit. Understanding crystallization and differentiation in arc basaltic andesite magmas under changing C-O-H fluid compositions is key to understanding the magma storage systems of many arc volcanoes. Magmatic fluid contents play a fundamental role in their differentiation during storage, and whether they erupt explosively or effusively later on. To understand the latter, it is important to constrain the processes occurring in the storage region. Prior experimental studies provide important results on phase relations as a function of the mole fraction of H2O (XH2O) in the fluid phase, but relatively few studies exist that focus on H2O-undersaturated systems at crustal pressures (2 kbar). No prior experimental studies have compared crystallization during decompression of initially H2O-saturated vs. H2O-undersaturated melts, applicable to understanding microlite textures and crystallization processes during magma rise. The study will help fill the data gap through melting and reversal experiments on tholeiitic basaltic andesites from Westdahl and Okmok volcanoes. A novel aspect of this work will involve exploring how plagioclase crystallization kinetics may change during decompression of an initially H2O-undersaturated magma, as compared with one that was initially H2O-saturated. All phase compositions and textures will be measured using electron microprobe, back scattered electron images, and petrographic light microscope and image analyses. The high-pressure experiments will also be analyzed using reflectance Fourier Transform Infrared spectroscopy to confirm the fraction of H2O dissolved in the melt, compared with the XH2O calculated following previous experimental studies. The experiments will result in improved understanding of preeruptive magma storage and ascent prior to two recent eruptions in the Aleutians. They will also provide data that can be generally applied to understanding the effects of changing magmatic fluid compositions on crystallization processes during storage and ascent of arc basaltic andesite magmas.Broader Impacts. A subset of the experiments will be conducted by an undergraduate student and as part of an Advanced Petrology class, Spring 2007. The University of Alaska Fairbanks currently does not own a gas-mixing furnace; acquiring one will enhance the infrastructure in the Department of Geology and Geophysics. The results will provide a geodesy graduate student with petrology-based data for modeling inflation and deflation signals at Okmok volcano. The results will be disseminated through conference presentations, peer-reviewed publications, and through a newly developed web experimental petrology database, in conjunction with the Alaska Volcano Observatory (AVO). In collaboration with the AVO outreach coordinator, information about magma dynamics relevant to future eruptions at Okmok and Westdahl will be disseminated to interested people living near the volcanoes.