Experimental Study of Plagioclase Textures and Amphibole Reaction Rims: Implications for Rates of Magmatic Processes

斜长石结构和角闪石反应边缘的实验研究：对岩浆过程速率的影响

• 批准号: 0106658
• 负责人: Jessica Larsen
• 金额: $ 17.2万
• 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: 2001
• 资助国家: 美国
• 起止时间: 2001-07-01 至 2004-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0106658&HistoricalAwards=false
• 关键词: Experimental; Study; Plagioclase; Textures; Amphibole

Larsen and GardnerEAR-0106658Mineral phases in crustal magma bodies record information about magma mixing, heating, and degassing in the form of reaction textures that occur as the mineral responds to the change in state of the magma. Two minerals that commonly show these features are plagioclase and amphibole, exhibiting sieve, dusty, and patchy zoning (dissolution and growth) and reaction rims (degassing and heating) respectively. These textures are often used to estimate the rates of magma mixing and magmatic ascent processes. We propose to experimentally constrain the rates at which these textures form at conditions applicable to shallow-level crustal reservoirs, using natural (rhyo)dacite and andesite starting material, and clean, unzoned, euhedral seed crystals. Our goal for this proposed study is to provide the vital calibrations needed to make observations of these reaction textures quantitatively useful for volcanologists when examining the products of crustal magma reservoirs. Previous work (refs) investigating the rates and causes of these textures have provided a good basis for our proposed study. However, limitations to this work exist, necessitating further study to make constraints useful to a wide range of mineral-melt compositions and P-T conditions applicable to crustal magma reservoirs. We will accomplish our goals in two tasks that are designed to experimentally constrain the rates of dissolution and regrowth textures in plagioclase resulting from heating and compositional disequilibrium, and amphibole reaction rims produced by melt degassing or heating. The first task will focus on plagioclase dissolution and rim growth in Aniakchak rhyodacite and andesite, using plagioclase seed phenocrysts of An20, An37, and An80 composition, at pressures between 0.1 and 300 MPa and temperatures between 825 and 1100 degrees C. The goal of this task is to constrain rates of plagioclase dissolution as a function of heating above the plagioclase solidus, and to examine dissolution and re-growth of plagioclase due to chemical disequilibrium in melts in which plagioclase of another composition is stable. The second task will investigate pargasite and hornblende amphibole breakdown in Redoubt dacite and andesite at P-T conditions appropriate for amphibole stability in these compositions. The goal of this task is to extend known calibrations of this reaction to different melt compositions and magmatic conditions. In addition, the thermal breakdown of amphibole will be investigated in order to help petrologists distinguish between decompression and heating rims. The results of these studies will be constraints on the rates of formation of important plagioclase and amphibole reaction textures in shallow-level crustal reservoirs.

Larsen 和 GardnerEAR-0106658 地壳岩浆体中的矿物相以反应结构的形式记录有关岩浆混合、加热和脱气的信息，这些信息是随着矿物对岩浆状态变化的响应而发生的。通常表现出这些特征的两种矿物是斜长石和角闪石，分别表现出筛状、尘状和斑状分区（溶解和生长）和反应边缘（脱气和加热）。这些纹理通常用于估计岩浆混合和岩浆上升过程的速率。我们建议通过实验限制这些纹理在适用于浅层地壳储层的条件下形成的速率，使用天然（流纹）英安岩和安山岩起始材料以及干净的、未分区的自形晶种。我们提出的这项研究的目标是提供必要的重要校准，以便在火山学家检查地壳岩浆库的产物时对这些反应纹理进行定量观察。之前研究这些纹理的发生率和原因的工作（参考文献）为我们提出的研究提供了良好的基础。 然而，这项工作存在局限性，需要进一步研究，以制定适用于地壳岩浆储层的广泛矿物熔体成分和 P-T 条件的约束条件。我们将通过两项任务来实现我们的目标，这两项任务旨在通过实验限制由于加热和成分不平衡而产生的斜长石中的溶解和再生纹理的速率，以及通过熔融脱气或加热产生的角闪石反应边缘。第一项任务将重点关注 Aniakchak 流纹英安岩和安山岩中的斜长石溶解和边缘生长，使用 An20、An37 和 An80 成分的斜长石晶种斑晶，在 0.1 至 300 MPa 的压力和 825 至 1100 摄氏度之间的温度下进行。这项任务的目标是限制斜长石溶解速率与加热温度以上的关系。 斜长石固相线，并检查由于熔体中的化学不平衡而导致斜长石的溶解和再生长，其中另一种成分的斜长石是稳定的。 第二项任务将研究在适合这些组合物中角闪石稳定性的 P-T 条件下，堡垒英安岩和安山岩中的斜角闪石和角闪石角闪石的分解。这项任务的目标是将这种反应的已知校准扩展到不同的熔体成分和岩浆条件。此外，还将研究角闪石的热分解，以帮助岩石学家区分减压和加热边缘。这些研究的结果将限制浅层地壳储层中重要的斜长石和角闪石反应结构的形成速率。