The Chemistry of Aqueous Carbonic Fluids in Subduction

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At high pressures and temperatures in parts of the Earth's interior, water and carbon dioxide are both fluids. These fluids can dissolve into each other, either fully or partially. They can also undergo chemical reactions, both with each other and with other substances (minerals) in their surroundings. The extent to which they dissolve, and the nature and extent of reactions they undergo, effect phenomena as diverse as the crystallization of different minerals and the formation of magma that leads to volcanic eruptions. This project seeks to determine how much water can be dissolved into carbon dioxide, and how much carbon dioxide into water, at various pressures and temperatures, and the chemical changes that occur when they do. Beyond the geological perspective, both water and carbon dioxide are common, industrial solvents and thus an understanding of their interactions is generally desirable; it may, for instance, lead to the design of binary, chemically tun-able "green" solvents for use at the pressures and temperatures in which they are mutually soluble. In addition, water and carbon dioxide are primary constituents of modern high explosives, and their modelling requires a knowledge of these combined fluids at suitably high pressures and temperatures.The nature of high-pressure, high-temperature mixtures of water and carbon dioxide will be investigated, across conditions ranging up to 100,000 atmospheres and 1000C. The degree of miscibility of the two components will be acquired over the stated range, leading to the simple and basic knowledge of where (in pressure, temperature and composition) a single, homogeneous fluid can exist and where, conversely, it must segregate into two, co-existing fluids. A more fundamental perspective on these findings will be sought through investigation of the chemical species which actually exist in solution. In particular, the degree to which carbon dioxide will react with water and dissociate first to bicarbonate ion, and thence to carbonate will be quantified. The work will also encompass the addition of various solutes (e.g., sodium chloride) to the water/carbon dioxide mix and a determination of how large proportions of carbon dioxide affect the solubility of minerals such as calcium carbonate and quartz. Water and carbon dioxide will be loaded into a high-pressure diamond-anvil cell, which will then be placed in an oven. Temperature will be raised until the contents of the cell have formed a homogeneous solution, following which temperature will be lowered and limits of solubility determined visually as the temperature at which a second (different and co-existing) fluid appears. Solubility of minerals loaded with the fluids will similarly be determined visually. The species present, and their concentrations, will be determined by use of Raman spectroscopy, an optical technique well-suited to the diamond-anvil cell.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.