Abstract.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 ∼30 − 45 GPa, nakhlites ∼20 ± 5 GPa, Chassigny ∼35 GPa, and ALH84001 ∼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 (∼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 (∼20 Myr) to the youngest (∼0.7 Myr) values for Martian meteorites. Five groups are distinguished and designated S Dho (one basalt, ∼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, ∼2.7 − 3.1 Myr), S DaG (two basalts, ∼1.25 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 ∼1.3 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 ∼4.5 Gyr. Its secondary carbonates are ∼3.9 Gyr old, an age corresponding t...

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Crystals of halite and sylvite within the Monahans (1998) H5 chondrite contain aqueous fluid inclusions. The fluids are dominantly sodium chloride-potassium chloride brines, but they also contain divalent cations such as iron, magnesium, or calcium. Two possible origins for the brines are indigenous fluids flowing within the asteroid and exogenous fluids delivered into the asteroid surface from a salt-containing icy object.

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Small differences in the ratio of neodymium-142 to neodymium-144 in early formed mantle reservoirs in planetary bodies are the result of in situ decay of the extinct radionuclide samarium-146 and can be used to constrain early planetary differentiation and therefore the time scale of planetary accretion. The martian meteorite Nakhla (approximately 1.3 billion years old), the type sample of the nakhlite subgroup of the Shergottite-Nakhlite-Chassigny (SNC) meteorites, exhibits a 59 +/- 13 parts per million excess in the ratio of neodymium-142 to neodymium-144 relative to normal neodymium. This anomaly records differentiation in the martian mantle before 4539 million years ago and implies that Mars experienced no giant impacts at any time later than 27 million years after the origin of the solar system.