Thermal modeling of shock melts in four Martian meteorites; implications for preservation of Martian atmosphere and crystallization of high-pressure minerals from shock melts
achondrites, argon, atmosphere, chemical fractionation
The distribution of shock melts in four shergottites, having both vein and pocket geometry, has been defined and the conductive cooling time over the range 2500 degrees C to 900 degrees C calculated. Isolated 1 mm (super 2) pockets cool in 1.17 s and cooling times increase with pocket area. An isolated vein 1X7 mm in Northwest Africa (NWA) 4797 cools to 900 degrees C in 4.5 s. Interference between thermal haloes of closely spaced shock melts decreases the thermal gradient, extending cooling times by a factor of 1.4 to 100. This is long enough to allow differential diffusion of Ar and Xe from the melt. Small pockets (1 mm (super 2) ) lose 2.2% Ar and 5.2% Xe during cooling, resulting in a small change in the Ar/Xe ratio of the dissolved gas over that originally trapped. With longer cooling times there is significant fractionation of Xe from Ar and the Ar/Xe ratio increases rapidly. The largest pockets show less variation of Ar/Xe and likely preserve the original trapped gas composition. Considering all of the model calculations, even the smallest isolated pockets have cooling times greater than the duration of the pressure pulse, i.e., >0.01 s. The crystallization products of these shock melts will be unrelated to the peak shock pressure experienced by the meteorite.
Shaw, C. J., & Walton, E. (2013). Thermal modeling of shock melts in Martian meteorites; implications for preserving Martian atmospheric signatures and crystallization of high-pressure minerals from shock melts. Meteoritics & Planetary Science, 48(5), 758-770. doi:10.1111/maps.12100
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