The age of SNC meteorites and the antiquity of the Martian surface
Résumé
We report new Sm–Nd, Lu–Hf, and Pb–Pb mineral and whole-rock isotope data for the basaltic shergottite Zagami, as well as Pb–Pb whole-rock isotope data for the basaltic shergottite Los Angeles, the lherzolitic shergottite Dar-al-Gani 476 (DaG 476), and the clinopyroxenite Nakhla. In agreement with previous findings, our new Sm–Nd and Lu–Hf mineral ages on the Martian meteorite Zagami are young (155 and 185 Ma, respectively). The 207Pb/206Pb–204Pb/206Pb compositions of the insoluble fractions of shergottites (Zagami, Los Angeles, and literature data for Shergotty and EETA79001) form an excellent alignment indicative of a 4.0 Ga crystallization age. The range of Pb isotope compositions observed in the leachates of these samples attests to negligible contamination of the shergottites by terrestrial Pb and argues against mixing relationships. The age of 4.048 ± 0.017 Ga (MSWD = 1.5) provided by the Pb isotope compositions of the Zagami whole-rock and residues is therefore taken to date the crystallization of this rock, which, so far, was believed to be only not, vert, similar 180 Ma old. Based on this result, we argue that the lithosphere of Mars is extremely old and that most mineral ages were reset recently by acidic aqueous solutions percolating through the Martian surface. This interpretation is consistent with photographic interpretations of erosional features on Mars. It also relieves the constraint imposed by the presence of anomalies of 142Nd and 182W (both products of extinct radioactive nuclides) that the Martian mantle should have preserved primordial isotopic heterogeneities, thus allowing for the planet interior to be actively convecting.