Experimental Studies on Phase Equilibria, Density and Sound Velocities of Martian Mantle Assemblages

Kurzfassung

The NASA InSight mission on Mars, equipped with the seismometer SEIS, is currently monitoring the seismic activity of the planet, and several recorded marsquakes have been exploited to gain information about Mars’ internal structure. Interpretation of these seismological observations in terms of composition and mineralogy requires the knowledge of phase equilibria as well as density and thermo-elastic properties of constituent materials at pertinent conditions. In order to relate bulk chemical compositions derived from analyses of Martian meteorites, to mineral assemblages as a function of depth, we have performed ex-situ phase equilibria experiments in piston cylinder and multi-anvil presses at pressures of 3, 8 and 10 GPa and temperatures between 1200 and 1600 °C, covering a range representative of Mars’ upper and mid-mantle conditions. Our results indicate the stability of a small fraction of magnetite next to olivine and pyroxene in Martian upper mantle phase assemblages as well as in most of the mid-mantle runs, together with Fe3+-rich garnets. Stability of Fe3+-bearing phases, and magnetite in particular, has not been reported before but is indicative of the oxidized synthesis environment. Selected aggregates were then used to carry out in-situ sound velocity and density measurements using ultrasonic interferometry combined with X-ray radiography and energy-dispersive X-ray diffraction over pressure and temperature conditions directly relevant to the Martian mantle. Both compressional and shear wave velocities for the upper and mid-mantle assemblages were generally slower than seismic wave velocities estimated by pre-landing models. This offset can be explained by the differences in phase proportions between experiments and models and the fact that current thermodynamic models are not taking into account the effect of Fe3+-incorporation in the constituent minerals. Our experimental data showed a substantial decrease of compressional and shear sound velocities upon increasing temperature, which is dominant over pressure-induced effects at shallow mantle conditions, and, independently from the here-considered mineralogy, suggest the presence of low-shear-wave-velocity zones between 100 and 400 km depth in the Martian upper mantle, which could explain InSight seismic observations.