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

1 atm、1 和 2kbar 下阿留申玄武安山岩的相关系：改变 XH2O 的影响

• 批准号: 0636118
• 负责人: Jessica Larsen
• 金额: $ 28.4万
• 依托单位: University of Alaska Fairbanks Campus
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-01-01 至 2011-03-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0636118&HistoricalAwards=false
• 关键词: Phase; Relations; Aleutian; Basaltic; Andesites

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.

智力优势。 了解弧玄武安山岩岩浆在变化的C-O-H流体成分下的结晶和分异作用是了解许多弧火山岩浆储存系统的关键。岩浆流体的含量对它们在储存过程中的分异起着根本性的作用，以及它们以后是爆发式喷发还是喷溢式喷发。要了解喷溢式喷发，重要的是要限制储存区发生的过程。先前的实验研究提供了重要的结果，作为在流体相中的H2O（XH 2 O）的摩尔分数的函数的相位关系，但相对较少的研究存在，专注于在地壳压力（2千巴）的H2O-欠饱和系统。没有以前的实验研究比较初始H2O饱和与H2O不饱和熔体减压过程中的结晶，适用于了解微晶纹理和岩浆上升过程中的结晶过程。这项研究将有助于通过对来自Westdahl和Okmok火山的拉斑玄武岩安山岩进行熔化和逆转实验来填补数据空白。这项工作的一个新的方面将涉及探索斜长石结晶动力学如何可能会改变减压过程中的最初H2O-不饱和的岩浆，相比，最初的H2O-饱和。所有的相组成和纹理将使用电子探针，背散射电子图像，岩相光学显微镜和图像分析测量。高压实验也将使用反射傅里叶变换红外光谱进行分析，以确认溶解在熔体中的H2O的分数，与先前的实验研究计算的XH 2 O相比。这些实验将导致对阿留申群岛最近两次喷发前岩浆储存和上升的更好理解。他们还将提供的数据，可以普遍适用于了解在弧玄武安山岩岩浆的存储和上升过程中，岩浆流体成分的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，并作为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。研究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Okmok火山的通货膨胀和通货紧缩信号进行建模。研究结果将通过会议报告、同行评议出版物以及与阿拉斯加火山观测站（AVO）合作新开发的网络实验岩石学数据库进行传播。与AVO外联协调员合作，将向生活在火山附近的有关人员传播与Okmok和Westdahl火山今后喷发有关的岩浆动力学信息。