Single-crystal elasticity of martian mantle minerals and a flexible CO2 laser heating system

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Observations of the seismic wave velocity structure of the Martian interior are becoming increasingly available from the SEIS seismometer on the NASA InSight lander. The interpretation of such data relies crucially on the ability to model the mineralogy and seismic velocities of the Martian interior in order to test plausible compositions and temperature gradients. To date, however, such models for the Martian mantle are constructed using thermodynamic parameters that are either estimated, not determined from the most recent phase equilibria and elasticity data or are not suitable for determining Martian compositions. Very few elasticity measurements exist at simultaneous high pressure and temperature conditions, requiring data for most minerals to be extrapolated to some extent, which introduces significant uncertainties.In the first period of this project a new system was developed to measure acoustic wave velocities at pressures and temperatures corresponding to the entire Martian mantle. The system, where Brillouin spectroscopy measurements are performed simultaneously with CO2-laser heating in a diamond anvil cell, has been successfully benchmarked by performing measurements on single crystals of pyrope. By combining these data with further measurements on Fe-rich ringwoodite and recent data from the literature, an updated mineral-physics model for the base of the Martian mantle has been obtained. Significant differences exist with previous models based on properties of terrestrial materials. Using the new model to interpret a proposed Martian mantle discontinuity at 1140 km, implies a temperature at this depth in the range 1870-1970 K. In the renewal phase, simultaneous single crystal X-ray diffraction measurements will be also implemented, to obtain a truly unique system capable of determining the full elastic tensor of any mineral throughout the conditions of any terrestrial planet. Using this system, the determination of the full elastic tensors of the main Martian minerals will be completed by examining Fe-rich single crystals of majoritic garnet, olivine and even the low symmetry mineral clinopyroxene, at pressures and temperatures of their stability in the Martian mantle. These data will be used to develop a an internally consistent thermodynamic model to predict the mineralogy and seismic wave velocities of the Martian mantle with vastly reduced uncertainties. This model will not only be used to interpret the emerging observations of Martian seismic structure and assess the uncertainties in these interpretations, but will also provide a first assessment of how seismic anisotropy has the potential to influence observations of the Martian interior. Moreover, by studying minerals comprised of different solid solution components, we will address a central issue in mineral physics as to whether the properties of intermediate compositions can be effectively described using linear combinations of end member properties.