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

在地球岩浆系统中发现的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来的矿物质来影响岩浆的物理和化学行为。由于岩浆喷发时其体积变化很大，H2O也强烈影响火山的爆炸性和危险性。也起到很大作用magma-related矿床的形成与其他重要的交互等成矿元素年代,Cl, f .虽然有着至关重要的作用,在岩浆水的化学行为低于饱和浓度(即水溶解在岩浆的硅酸盐熔体部分,但没有液/气相存在),或液/气相组成的存在但不是纯粹的水(即混合挥发性与另一个如二氧化碳),相对来说不受实验室实验的约束。这种知识缺陷造成了目前对岩浆行为理解的重大空白，并阻碍了对重要物理特征（如岩浆密度和粘度）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的化学活性来解决这一需求。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分享作为该项目一部分开发的校准眼镜的分割。从那里，它们将被提供给国际研究界借阅。对欠饱和条件下岩浆中H2O化学活性的实验测量将通过在高压和高温下由天然岩石成分合成含水熔体来完成。这些熔体将在P-T-XH2O条件下在其液相线以上使用双胶囊方法进行平衡，外胶囊中有已知的氧逸度缓冲液。在达到平衡后，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，可以知道熔体的氧逸度，并且可以通过熔体与实验氧缓冲液之间的氧逸度差来计算熔体中H2O的活性。通过改变加入熔体的水的初始浓度（以及压力和温度），可以确定给定岩浆成分的活度-浓度关系。这些数据将用于开发描述性热力学方程，进而用于改进现有的水欠饱和条件下的综合相平衡模型（即熔体）。

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.