Ar---Ar chronology of the Martian meteorite ALH84001: Evidence for the timing of the early bombardment of Mars

Details Ar---Ar chronology of the Martian meteorite ALH84001: Evidence for the timing of the early bombardment of Mars Ar---Ar chronology of the Martian meteorite ALH84001: Evidence for the timing of the early bombardment of Mars

Sep 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997gecoa..61.3835t&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 61, Issue 18, pp.3835-3850

Physics

30

Scientific paper

ALH84001, a cataclastic cumulate orthopyroxenite meteorite from Mars, has been dated by Ar---Ar stepped heating and laser probe methods. Both methods give ages close to 3,900 Ma. The age calculated is dependent on assumptions made about 39 Ar recoil effects and on whether significant quantities of 40 Ar from the Martian atmosphere are trapped in the meteorite. If, as suggested by xenon and nitrogen isotope studies, Martian atmospheric argon is present, then it must reside predominantly in the K-rich phase maskelynite. Independently determined 129 Xe abundances in the maskelynite can be used to place limits on the concentration of the atmospheric 40 Ar. These indicate a reduction of around 80 Ma to ages calculated on the assumption that no Martian atmosphere is present. After this correction, the nominal ages obtained are: 3940 ± 50, 3870 ± 80, and 3970 ± 100 Ma. by stepped heating, and 3900 ± 90 Ma by laser probe (1 statistical errors), giving a weighted mean value of 3,920 Ma. Ambiguities in the interpretation of 39 Ar recoil effects and in the contribution of Martian atmospheric 40 Ar lead to uncertainties in the Ar---Ar age which are difficult to quantify, but we suggest that the true value lies somewhere between 4,050 and 3,800 Ma. This age probably dates a period of annealing of the meteorite subsequent to the shock event which gave it its cataclastic texture. The experiments provide the first evidence of an event occurring on Mars coincident with the time of the late heavy bombardment of the Moon and may reflect a similar period of bombardment in the Southern Highlands of Mars. Whether the age determined bears any relationship to the time of carbonate deposition in ALH84001 is not known. Such a link depends on whether the temperature associated with the metasomatic activity was sufficient to cause argon loss from the maskelynite and/or whether the metasomatism and metamorphism were linked in time through a common heat source.

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