Ejection times of Martian meteorites

Noble gas isotopic analyses were made for four meteorites whose origin is Mars: basaltic shergottites Queen Alexandra Range 94201, Zagami, the lherzolite LEW88516, and orthopyroxenite Allan Hills 84001. The latter one was separated into orthopyroxene, chromite, and maskelynite fractions. Ejection times were calculated for all Martian meteorites using cosmic ray exposure ages based on He-3, Ne-21, and Ar-38 concentrations and literature data on the terrestrial ages. We discuss the arguments for a scenario in which they were ejected from Mars by asteroidal or cometary impact as small meteoroids and were delivered to Earth within and up to 15 million years. The basaltic shergottites QUE94201, Shergotty, and Zagami show an ejection age cluster at 2.76 Ma, the fourth basaltic shergottite EET79001 being younger (0.82 Ma). Lherzolites LEW88516 and ALH77005 were ejected by a later event 3.84 Ma ago. All nakhlites, Nakhla, Governador Valadares, and Lafayette, originate from an event 11.0 Ma ago. Although Chassigny is chemically distinct from the nakhlites, its ejection age is 11.6 Ma. Whether this is a separate event cannot be decided from the available data. Finally, orthopyroxenite ALH84001 was launched from Mars 14.3 Ma ago. The distribution of the delivery times for Martian meteorites to Earth reflect individual impact events onto five or six different source terrains. This result is significant in the context of a comprehensive knowledge of the Martian crust composition. Finally, we derived the K-Ar-40 gas retention age of ALH84001 mineral fractions adopting two different models: Assuming a component of Martian atmospheric Ar with Ar-40/Ar-36 = 1790 trapped by this meteorite we obtained an age of 3390 Ma. On the other hand, assuming no contribution of Martian atmospheric Ar, the resulting K-Ar-40 age is about 4100 Ma, that is, roughly 500 Ma younger than the reported Sm-Nd crystallization age. Copyright (C) 1997 Elsevier Science Ltd.