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

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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.