AR-AR CHRONOLOGY OF THE MARTIAN METEORITE ALH84001 - EVIDENCE FOR THETIMING OF THE EARLY BOMBARDMENT OF MARS

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 o n assumptions made about Ar-39 recoil effects and on whether significa nt quantities of Ar-40 from the Martian atmosphere are trapped in the meteorite. If, as suggested by xenon and nitrogen isotope studies, Mar tian atmospheric argon is present, then it must reside predominantly i n the K-rich phase maskelynite. Independently determined Xe-129 abunda nces in the maskelynite can be used to place limits on the concentrati on of the atmospheric Ar-40. These indicate a reduction of around 80 M a to ages calculated on the assumption that no Martian atmosphere is p resent. 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 sigma statistical errors), giving a weighted mean value of 3,920 Ma. Ambiguities in the interpretation of Ar-39 re coil effects and in the contribution of Martian atmospheric Ar-40 lead to uncertainties in the Ar-Ar age which are difficult to quantify, bu t we suggest that the true value lies somewhere between 4,050 and 3,80 0 Ma. This age probably dates a period of annealing of the meteorite s ubsequent to the shock event which gave it its cataclastic texture. Th e experiments provide the first evidence of an event occurring on Mars coincident with the time of the late heavy bombardment of the Moon an d may reflect a similar period of bombardment in the Southern Highland s of Mars. Whether the age determined bears any relationship to the ti me of carbonate deposition in ALH84001 is not known. Such a link depen ds on whether the temperature associated with the metasomatic activity was sufficient to cause argon loss from the maskelynite and/or whethe r the metasomatism and metamorphism were linked in time through a comm on heat source. Copyright (C) 1997 Elsevier Science Ltd.