Volatile Elements in the Tissint Meteorite: Evidence for a Geochemical Signature for Martian Near-surface Alteration Preserved in Shock-generated Melt Pockets.

English Abstract:

Tissint contains an abundance of shock-generated melt glass formed by a variety of mechanisms including grain-boundary frictional melting, concentration of shockwaves along boundaries of minerals with contrasting shock impedance, and void collapse. Four shock melt pockets containing glass were analyzed by EPMA and SIMS. Melt pockets analyzed are basaltic in composition, consistent with the melting of local igneous phases; no excesses of P, Cl, or F are observed that could indicate a melted regolith component. H₂O content within glass ranges from <100 ppm to several thousand ppm. Cl exhibits a strong correlation with H₂O in shock melt glass, suggesting that Cl followed H₂­O when introduced into the rock. This strong correlation is not observed for H₂O and F or P; H₂O and Cl concentrations within Tissint glass cannot be explained by melting igneous apatite and are most likely a remnant geochemical fingerprint of aqueous processes affecting the rock near the Martian surface after the rock’s igneous crystallization. δD for Tissint melt glass exhibits a negatively-sloping trend against 1/H₂O, indicating a mixing line between two reservoirs. High δD values for these reservoirs are consistent with Martian sources of water such as the crust and atmosphere and suggest an absence of terrestrial contamination. In one melt pocket, H₂O concentration decreases and δD increases when approaching a vesicle; this may be a function of prolonged cooling that allowed H₂O time to devolatilize to the vesicle before quenching completed after the peak shock wave passed. Conditions of melt pocket formation may contribute to local-scale variations in shock melt composition: void collapse is the most likely mechanism to trap alteration products as these materials would be concentrated in voids within the pre-shocked rock. Volatiles in these voids also contribute to preferential melting by suppressing the local solidus. It appears that some shock melt pockets in Tissint contain a geochemical signature characteristic of Martian alteration products, preserved primarily in H₂O and Cl concentrations. The distribution of such melt pockets is likely heterogeneous as the precursors to these melt pockets (voids and cracks hosting alteration products) were likely heterogeneously distributed in the rock prior to shock melting induced by impact.