Ages and Geologic Histories of Martian Meteorites

Abstract

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...

The Tissint Martian meteorite as evidence for the largest impact excavation

“…Although the constraints necessary for detailed shock modelling are all present in this meteorite, such a modelling venture is outside the scope of the present study. Notwithstanding, the stronger-than-expected shock metamorphism in Martian meteorites may explain the difficulty in the age dating of shergottites, especially for methods using isotopes of noble gases 39,40 . Furthermore, shock metamorphism can easily affect 'softer materials' (for example, plagioclase).…”

Geological Processes and Evolution

“…Two independent systems for dating Martian units complement each other, as discussed earlier in this volume by Nyquist et al (2001), Neukum et al (2001), and Hartmann and Neukum (2001). Dating of rock samples is believed to be relatively exact (barring any unknown systematic errors), but in the absence of sample return missions, we have samples only from unknown locations on Mars; thus, their provenance, context, and relation to geological units and history are unknown.…”

“…Nevertheless, the observation of young igneous crystallization ages, down to ∼165 Myr, among the meteorites shows that Martian volcanism continues essentially until the present day. The observation of a high proportion of young ages moreover suggests that Mars has been volcanically relatively active at recent times: 10 of 15 meteorites for which radiometric ages are summarized in Nyquist et al (2001) Because ∼40% of the Martian surface is highlands belonging to the oldest stratigraphic unit, the Noachian, whereas only 1 of 15 (∼7%) of the meteorites has a correspondingly old radiometric age, the answer to the first question appears to be negative. This conclusion invites comparison to the lunar case.…”

“…If instead, the cosmic ray exposure ages are used, there are seven apparent ejection events, also statistically acceptable. The two approaches are combined by Nyquist et al (2001). Better understanding will be facilitated via acquisition of new data and better statistics.…”

Untitled

“…A description of the different SNC silicate petrographies is given in the paper by Nyquist et al (2001). The main secondary mineral phases are carbonates, sulphates, halite and clay minerals (the latter particularly in the 3 nakhlites) although associated sulphides and ferric oxides are also sometimes present.…”