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: 1424932
• 负责人: Elizabeth Cottrell
• 金额: $ 0.71万
• 依托单位: Smithsonian Institution
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-07-15 至 2019-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1424932&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.

地球上岩浆系统中最具影响力的化学成分之一是H2O(水)。它通过改变岩浆的密度、粘度以及在冷却过程中从岩浆中结晶出的矿物来影响岩浆的物理和化学行为。由于其体积随着岩浆喷发而变化很大，H2O也强烈地影响着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、氯和F相互作用，在与岩浆有关的矿床的形成中发挥重要作用。尽管它起着关键作用，但在低于饱和浓度的岩浆中(即水在岩浆的硅酸盐熔体部分溶解，但不存在流体/汽相)，或者在存在流体/气相但不包含纯水(即与另一种挥发性成分如二氧化碳混合)的岩浆中，水的化学行为相对不受实验室实验的限制。这种知识的缺失在目前对岩浆行为的理解上造成了很大的差距，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在这些条件下岩浆中水的化学活度来满足这一需求。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国家岩石和矿石收藏中心分享作为该项目一部分开发的校准眼镜的碎片。从那里，它们将被提供给国际研究界借阅。非饱和条件下岩浆中H2O化学活度的实验测量将由天然岩石成分在高压和高温下合成水合熔体来完成。这些熔体将在P-T-XH2O条件下在其液相线上方使用双胶囊方法与外囊中已知的氧逸度缓冲剂进行平衡。达到平衡后，熔体会迅速冷却成玻璃。通过使用X射线近边吸收结构光谱(XANES)和湿化学方法测量得到的玻璃中铁的氧化状态，就可以知道熔体的氧逸度，并可以根据熔体和实验氧缓冲液之间的氧逸度差来计算熔体中H2O的活度。通过改变添加到熔体中的H2O的初始浓度(以及压力和温度)，可以确定特定岩浆成分的活度-浓度关系。然后，这些数据将被用来开发描述性热力学方程，这些方程又将被用来改进现有的在H2O不饱和条件下的综合相平衡模型(即熔体)。