Collaborative Research: An Experimental Determination of the Activity of H2O in Natural Melts at Undersaturated Conditions

合作研究：不饱和条件下天然熔体中 H2O 活性的实验测定

• 批准号: 1755966
• 负责人: Gordon Moore
• 金额: $ 6.98万
• 依托单位: University of California-Davis
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2017
• 资助国家: 美国
• 起止时间: 2017-08-01 至 2018-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1755966&HistoricalAwards=false
• 关键词: Collaborative; Research; Experimental; Determination; Activity

One of the most influential chemical components found in magmatic systems on Earth is H2O (water). It affects both the physical and chemical behavior of magmas by changing their density, viscosity, and the minerals that crystallize from them during cooling. Due to its large change in volume as magmas erupt, H2O also strongly influences how explosive and hazardous a volcano may be. It also plays a large role in the formation of magma-related ore deposits by interacting with other important ore-forming elements such as S, Cl, and F. Despite its critical role, the chemical behavior of H2O in magmas at concentrations below saturation (i.e. where H2O is dissolved in the silicate melt portion of the magma, but no fluid/vapor phase is present), or where a fluid/vapor phase is present but not composed of pure H2O (i.e. is mixed with another volatile component such as CO2), is relatively unconstrained by laboratory experiments. This knowledge deficit creates a significant gap in the current understanding of magmatic behavior, and hampers the modeling of important physical characteristics such as magma density and viscosity, as well as an understanding of the overall chemical evolution of a magma body. The experimental work of this project will address this need by directly measuring the chemical activity of H2O in magmas at these conditions. This study includes support for one undergraduate student. The researchers also plan to share splits of the calibrationglasses developed as part of this project with the National Rock and Ore Collection at the Smithsonian. From there, they will be made available for loan to the international research community. The experimental measurement of the chemical activity of H2O in magmas at undersaturated conditions will be accomplished by synthesizing hydrous melts from natural rock compositions at high pressure and temperature. These melts will equilibrate at P-T-XH2O conditions above their liquidus using a double capsule method with a known oxygen fugacity buffer in the outer capsule. After coming to equilibrium, the melts will be rapidly quenched to a glass. By measuring the oxidation state of iron in the resulting glass using both X-ray Absorption Near Edge Structure spectroscopy (XANES) and wet chemistry, the oxygen fugacity of the melt will be known, and the activity of H2O in the melt can be calculated from the oxygen fugacity difference between melt and the experimental oxygen buffer. By varying the initial concentration of H2O added to the melt (and pressure and temperature), the activity-concentration relations for a given magma composition will be determined. This data will then be used to develop descriptive thermodynamic equations, which in turn will be used to improve existing comprehensive phase equilibria models (i.e. MELTS) at H2O-undersaturated conditions.

地球上岩浆系统中发现的最有影响力的化学成分之一是 H2O（水）。它通过改变岩浆的密度、粘度以及冷却过程中结晶的矿物质来影响岩浆的物理和化学行为。由于岩浆喷发时体积发生巨大变化，H2O 也会强烈影响火山的爆炸性和危险性。它还通过与其他重要的成矿元素（如 S、Cl 和 F）相互作用，在与岩浆相关的矿床的形成中发挥着重要作用。尽管它发挥着至关重要的作用，但 H2O 在浓度低于饱和度的岩浆中的化学行为（即，H2O 溶解在岩浆的硅酸盐熔体部分中，但不存在流体/蒸汽相），或者存在流体/蒸汽相但不由纯 H2O 组成（即混合）与另一种挥发性成分（如 CO2）相比，相对不受实验室实验的限制。这种知识缺陷在当前对岩浆行为的理解中造成了重大差距，并阻碍了对岩浆密度和粘度等重要物理特征的建模，以及对岩浆体整体化学演化的理解。该项目的实验工作将通过直接测量这些条件下岩浆中 H2O 的化学活性来满足这一需求。这项研究包括对一名本科生的支持。研究人员还计划与史密森尼国家岩石和矿石收藏馆共享作为该项目一部分开发的校准眼镜的碎片。从那里，它们将可供国际研究界借用。不饱和条件下岩浆中 H2O 化学活性的实验测量将通过在高压和高温下从天然岩石成分合成含水熔体来完成。这些熔体将使用双胶囊方法在其液相线以上的 P-T-XH2O 条件下达到平衡，并在外胶囊中使用已知的氧逸度缓冲液。达到平衡后，熔体将迅速淬火成玻璃。通过使用 X 射线吸收近边结构光谱 (XANES) 和湿化学测量所得玻璃中铁的氧化态，可以了解熔体的氧逸度，并且熔体中 H2O 的活度可以通过以下公式计算：熔体和实验氧气缓冲液之间的氧逸度差异。通过改变添加到熔体中的水的初始浓度（以及压力和温度），将确定给定岩浆成分的活动-浓度关系。然后，这些数据将用于开发描述性热力学方程，进而用于改进 H2O 不饱和条件下现有的综合相平衡模型（即 MELTS）。