Anharmonicity and Phase Transitions of Mantle Phases Using Simultaneous High Pressure-High Temperature Raman Spectroscopy in the Laser Heated Diamond Cell

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ChopelasEAR-0106120Measurement of vibrational modes at simultaneous high pressures and temperatures offers many important opportunities to expand the state of the art in Raman spectroscopy in geophysics. First, phase boundaries can be quickly determined in situ. Second, new unquenchable phases can be easily detected, with probable structures or space groups identified based on spectral patterns. Third, these experiments directly measure the intrinsic anharmonicity, a value required for extrapolating important parameters such as thermal expansivity from 1 atm to deep earth conditions or calculating densities of a material in P-T space using the Mie-Gruneisen equation of state. Earth temperature, compositional, and dynamical models generally rely on extrapolation of these properties of candidate minerals to deep Earth conditions.Our current capabilities include a nearly identical experimental arrangement as those in Mainz: Raman double monochromator with CCD camera and photomultiplier tube, diamond cells, and excitation laser. Preliminary data from the Mainz laboratory using a CO2 laser for heating show that this method is feasible. A CO2 laser is requested in this proposal. We propose to extend the current Raman spectroscopic capabilities in our laboratory at UNLV to simultaneous high temperatures and pressures using CO2 laser heating in the diamond anvil cell. Once the laboratory is upgraded, the study will first focus on the two geophysically important minerals forsterite and magnesium silicate perovskite, both important mantle phases. Prior Raman studies of these at either elevated temperatures or elevated pressures are available for comparison. First results will focus on providing direct constraints on the Mie Gruneisen equation of state and on thermal expansivity at mantle conditions using a Maxwell relation.