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

在地球上的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、粘度和冷却过程中结晶出来的矿物质来影响岩浆的物理和化学行为。由于其体积的巨大变化，岩浆喷发，H2O也强烈影响火山的爆炸性和危险性。它还通过与其他重要成矿元素如S、Cl和F的相互作用，在岩浆型矿床的形成中起着重要作用。尽管H2O的化学行为具有关键作用，但在低于饱和浓度的岩浆中（即H2O溶解在岩浆的硅酸盐熔体部分中，但不存在流体/蒸气相），或存在流体/蒸气相但不包含纯H2O（即与另一种挥发性组分如CO2混合）的化学行为相对不受实验室实验的约束。这种知识的缺乏造成了一个显着的差距，在目前的理解岩浆的行为，并阻碍了重要的物理特性，如岩浆密度和粘度的建模，以及了解整体的化学演化的岩浆体。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。这项研究包括支持一名本科生。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玻璃的碎片。从那里，它们将被借给国际研究界。在欠饱和条件下，岩浆中H2O化学活性的实验测量将通过在高压和高温下从天然岩石成分合成含水熔体来完成。这些熔体将在高于其液相线的P-T-XH 2 O条件下使用双胶囊方法平衡，在外胶囊中具有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，将知道熔体的氧逸度，并且熔体中H2O的活度可以从熔体和实验氧缓冲剂之间的氧逸度差计算。通过改变加入到熔体中的H2O的初始浓度（以及压力和温度），将确定给定岩浆成分的活度-浓度关系。然后，这些数据将被用来开发描述性的热力学方程，这反过来又将被用来改善现有的综合相平衡模型（即MELTS）在H2O不饱和条件。

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.