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, t he lherzolite LEW88516, and orthopyroxenite Allan Hills 84001. The lat ter one was separated into orthopyroxene, chromite, and maskelynite fr actions. Ejection times were calculated for all Martian meteorites usi ng cosmic ray exposure ages based on He-3, Ne-21, and Ar-38 concentrat ions and literature data on the terrestrial ages. We discuss the argum ents for a scenario in which they were ejected from Mars by asteroidal or cometary impact as small meteoroids and were delivered to Earth wi thin and up to 15 million years. The basaltic shergottites QUE94201, S hergotty, and Zagami show an ejection age cluster at 2.76 Ma, the four th basaltic shergottite EET79001 being younger (0.82 Ma). Lherzolites LEW88516 and ALH77005 were ejected by a later event 3.84 Ma ago. All n akhlites, Nakhla, Governador Valadares, and Lafayette, originate from an event 11.0 Ma ago. Although Chassigny is chemically distinct from t he nakhlites, its ejection age is 11.6 Ma. Whether this is a separate event cannot be decided from the available data. Finally, orthopyroxen ite ALH84001 was launched from Mars 14.3 Ma ago. The distribution of t he delivery times for Martian meteorites to Earth reflect individual i mpact events onto five or six different source terrains. This result i s significant in the context of a comprehensive knowledge of the Marti an crust composition. Finally, we derived the K-Ar-40 gas retention ag e of ALH84001 mineral fractions adopting two different models: Assumin g a component of Martian atmospheric Ar with Ar-40/Ar-36 = 1790 trappe d 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 rep orted Sm-Nd crystallization age. Copyright (C) 1997 Elsevier Science L td.