IMPACT MELT POCKETS IN MARTIAN METEORITES AS RECORDERS OF IMPACT PROCESSES ON MARS

Impact melt pockets are ubiquitous in Martian meteorites, and their characteristics are closely tied to impact processes on Mars. The abundances and isotope compositions of volatiles in the pockets bear useful information about the Martian magmatic sources of the host igneous rocks and the surface and subsurface environments where the rocks resided. However, different Martian meteorites of similar ejection ages display significant differences in volatile characteristics. This study compares the impact melt pockets and their volatile signatures in two Martian meteorites, Tissint and EETA 79001, to discuss their implications in understanding possible effects of impact processes.

Tissint and EETA 79001 have similar cosmic-ray exposure ages (~0.7 million years), suggesting that they might be ejected in a single impact event. Impact glasses from both samples have volatile signatures indicative of contributions from Martian surface or sub-surface volatile sources. However, in contrast to the well-defined mixing trend of the Tissint data that suggest the presence of distinctive volatile sources near the Martian surface, water concentrations and hydrogen isotope compositions in the impact melt pockets in EETA 79001 fall within small ranges and do not show clear mixing trend. To resolve this issue, we investigated the volatiles in the silicate minerals in EETA 79001 and found large and correlated variations in water concentration and hydrogen isotope compositions, confirming that the water in EETA 79001 were from different sources.

The contrasting data from Tissint and EETA 79001 could be related to different geological history of the precursor rocks in the Martian crust or different impact processes. Here we hypothesize that different cooling history of the impact melt pockets in the two meteorites may be the main factor. The impact melts in Tissint might be cooled down sufficiently fast and thus preserved the initial stage of the mixing of two volatile sources. The impact melts in EETA 79001 investigated here are much larger than those in Tissint, likely resulted in longer cooling time, more extensive mixing, and more homogeneous volatile signatures. Quantitative modeling is being conducted to investigate other factors on preservation of volatile signatures in Martian meteorites.