Fluids in the Deep Earth: Raman Spectroscopy at High Pressures and Temperatures

Fluids play a huge role in the geochemistry of the Earth's deep interior, even though they are present at relatively low abundance relative to the solid, rocky mantle. These fluids are comprised primarily of the volatile components carbon, oxygen, hydrogen and sulfur (COHS). They are important in magma generation as even a small amount of fluid can drastically alter the melting behavior of a rock. They are also important in mass transfer by altering the rocks they pass through in a process called metasomatism. It is from such fluids that diamonds precipitate in the deep mantle. Perhaps their greatest influence is in the way they can mediate the oxidation state, or the so-called oxygen fugacity, of the mantle. For all their importance to mantle geochemistry, very little is known experimentally about what compounds, or species, are actually stable at mantle temperatures and pressures. Currently we must rely on thermodynamic calculations that are based on very little experimental data. Yet knowledge of the speciation of the COHS compounds that comprise mantle fluids is fundamental for understanding many mantle processes. This lack of information comes from an inherent difficulty in freezing in, or 'quenching', the fluid species once a typical sample held at high pressure and temperature is cooled and decompressed to ambient conditions - nearly all the information is lost during the quenching process. This necessitates a technique that allows one to probe fluid samples while they are at high pressures and temperatures. Micro-Raman spectroscopy is a technique that has been used with great success to identify, and in some cases to quantify, the species in fluid inclusions trapped in minerals, and this powerful in situ technique is beginning to be applied to fluids at high P and T in the diamond anvil cell. The purpose of this proposal is to develop the techniques in our lab required for in situ Raman spectroscopic measurements of COHS fluids held at high pressures (e.g. 1 - 50 GPa) and high temperatures (e.g. 1000 - 2000 K), and at a fixed oxygen fugacity. Experiments are done in a diamond anvil cell (DAC) which, because of the transparency of the diamonds, allows optical access to the fluid sample for spectroscopic measurements at high P and T. In this proposal we are requesting seed funding for a two-year program to modify our current experimental capabilities in order to carry out high P-T micro-Raman experiments in the DAC, develop and refine sample fabrication techniques, and carry out proof-of-concept and calibration experiments. This project will pave the way for a new and exciting experimental initiative in our laboratory.