Shock Devolatilization of Hydrated Minerals

Richard Kraus, Harvard University

Hydrated minerals are generally associated with the oldest terrain on Mars and may have formed in the hydrothermal systems produced below impact craters. The shock wave from impact events can also modify or devolatilize hydrated minerals through mechanical deformation or thermal processes. Carbonaceous chondrites contain the hydrated mineral serpentine, and may be up to 20% water by weight. During accretion, devolatilization of hydrated minerals was a major source of the water on Earth and Mars. Therefore, in order to understand the origin and redistribution of water on Earth and Mars, the effects of shock waves on hydrated minerals needs to be understood. The current data on shock modification of hydrated minerals is difficult to interpret and can be contradictory. Early work on shock devolatilization of hydrated minerals considered the entropy gain during shock compression to be the most important factor in losing bound water. As recovery systems are often made of stainless steel, a clay mineral enclosed in steel will undergo a quasi-isentropic loading path to reach the peak stress. The final entropy in the clay will be significantly less than one shocked to the principal Hugoniot, and hence data on shock devolatilization was often plotted against the first step shock pressure in the ring-up. However, in a natural impact event the hydrated mineral would be shocked to a state on the principal Hugoniot. Much of the debate regarding shock modification of hydrated minerals stems from an uncertainty in whether to present the data as a function of entropy or peak shock stress. Here we present results from new shock recovery experiments on nontronite (a smectite clay observed on Mars) and identify the major issues in interpretation of previous work on devolatilization of hydrated minerals.

Abstract Author(s): Richard G. Kraus, Sarah T. Stewart, Ralph E. Milliken, Nicholas J. Tosca