Laser-Driven Shock Compression Studies of Planetary Compositions.

摘要

The physical and chemical properties of materials are profoundly affected by high pressure and temperature and differ significantly from the behavior commonly observed at ambient conditions. Understanding the interior dynamics and evolution of planetary bodies thus requires measurement of the equations of state of materials at these extreme conditions. Experiments employing laser-driven shock compression now permit exploration of phase space spanning orders of magnitude in pressure and temperature.;Results are presented here from a suite of laser-driven shock experiments on three major mineral phases of significance to the terrestrial mantle: SiO 2, MgO and MgSiO3. New optical diagnostics, including an absolutely calibrated streaked optical pyrometry system were developed to measure shock temperatures from ∼ 4000-60,000 K. This system was applied to observe high-pressure phase transitions and melting at previously unexplored conditions.;Experiments on two polymorphs of SiO2 are used to validate experimental technique and pyrometry calibration and are compared to previous results. Data on MgO and MgSiO3 constrain controversial predications for the ultra-high pressure melt curves and support melting temperatures at the Earth's core-mantle boundary higher than most previous predictions. In the case of MgSiO3, the first observations of a distinct liquidliquid phase transformation in a silicate material are presented. Experiments on amorphous and crystalline MgSiO3 starting materials show evidence of a transition to a high-pressure liquid phase approximately 10% denser than the low-pressure counterpart. Isochemical liquid-liquid phase separation may represent a previously unrecognized means of geochemical partitioning in early planetary history. Finally, discussions of the transport properties of each material are given and it is found that all three transform to metallic liquids upon melting with high thermal and electrical conductivity, suggesting the possibility of enhanced electromagnetic coupling across the core-mantle boundary in the molten state.