Ages and geologic histories of Martian meteorites

We review the radiometric ages of the 16 currently known Martian meteorites, classified as 11 shergottites (8 basaltic and 3 lherzolitic), 3 nakhlites (clinopyroxenites), Chassigny (a dunite), and the orthopyroxenite ALH84001. The basaltic shergottites represent surface lava flows, the others magmas that solidified at depth. Shock effects correlate with these compositional types, and, in each case, they can be attributed to a single shock event, most likely the meteorite's ejection from Mars. Peak pressures in the range 15 - 45 GPa appear to be a launch window: shergottites experienced similar to 30 - 45 GPa, nakhlites similar to 20 +/- 5 GPa, Chassigny similar to 35 GPa, and ALH84001 similar to 35 - 40 GPa. Two meteorites, lherzolitic shergottite Y-793605 and orthopyroxenite ALH84001, are monomict breccias, indicating a two-phase shock history in toto: monomict brecciation at depth in a first impact and later shock metamorphism in a second impact, probably the ejection event. Crystallization ages of shergottites show only two pronounced groups designated S-1 (similar to 175 Myr), including 4 of 6 dated basalts and all 3 lherzolites, and S-2 (330 - 475 Myr), including two basaltic shergottites and probably a third according to preliminary data. Ejection ages of shergottites, defined as the sum of their cosmic ray exposure ages and their terrestrial residence ages, range from the oldest (similar to 20 Myr) to the youngest (similar to0.7 Myr) values for Martian meteorites. Five groups are distinguished and designated S-Dho (one basalt, similar to 20 Myr), S-L (two lherzolites of overlapping ejection ages, 3.94 +/- 0.40 Myr and 4.70 +/- 0.50 Myr), S (four basalts and one lherzolite, similar to2.7 - 3.1 Myr), S-DaG (two basalts, similar to1.25 Myr), and S-E (the youngest basalt, 0.73 +/- 0.15 Myr). Consequently, crystallization age group S-1 includes ejection age groups S-L, S-E and 4 of the 5 members of S, whereas S-2 includes the remaining member of S and one of the two members of S-DaG. Shock effects are different for basalts and lherzolites in group S/S-1. Similarities to the dated meteorite DaG476 suggest that the two shergottites that are not dated yet belong to group S-2. Whether or not S-2 is a single group is unclear at present. If crystallization age group S-1 represents a single ejection event, pre-exposure on the Martian surface is required to account for ejection ages of S-L that are greater than ejection ages of S, whereas secondary breakup in space is required to account for ejection ages of S-E less than those of S. Because one member of crystallization age group S-2 belongs to ejection group S, the maximum number of shergottite ejection events is 6, whereas the minimum number is 2. Crystallization ages of nakhlites and Chassigny are concordant at similar to1.3 Gyr. These meteorites also have concordant ejection ages, i.e., they were ejected together in a single event (NC). Shock effects vary within group NC between the nakhlites and Chassigny. The orthopyroxenite ALH84001 is characterized by the oldest crystallization age of similar to4.5 Gyr. Its secondary carbonates are similar to3.9 Gyr old, an age corresponding to the time of Ar-outgassing from silicates. Carbonate formation appears to have coincided with impact metamorphism, either directly, or indirectly, perhaps via precipitation from a transient impact crater lake. The crystallization age and the ejection age of ALH84001, the second oldest ejection age at 15.0 +/- 0.8 Myr, give evidence for another ejection event (O). Consequently, the total number of ejection events for the 16 Martian meteorites lies in the range 4 - 8. The Martian meteorites indicate that Martian magmatism has been active over most of Martian geologic history, in agreement with the inferred very young ages of flood basalt flows observed in Elysium and Amazonis Planitia with the Mars Orbital Camera (MOC) on the Mars Global Surveyor (MGS). The provenance of the youngest meteorites must be found among the youngest volcanic surfaces on Mars, i.e., in the Tharsis, Amazonis, and Elysium regions.