Meteorite from Mars
Martian Meteorite, Shergottite, 1 piece of ~15 x 10 cm and weighing 2,015 g was found MAY, 1998 in the desert of the DaG Plateau, Central Sahara
DaG 476 contains an unusually high abundance of olivine (~20 vol%) in the form of xenocrysts, derived earlier from a lherzolitic rock, embedded in a fine-grained groundmass composed mostly of Ca-poor pigeonite and feldspathic glass with minor Ca-rich augite. Micron-sized chromite grains and other minor phases are present within the olivine, giving it a speckled appearance. Both olivine and orthopyroxene xenocrysts in this meteorite show close mineralogical, petrological, and trace element similarities to the lherzolitic shergottites, and in particular to EETA79001 lithology A. While the bulk chemistry of DaG 476 is closer to that of the lherzolitic shergottites, the REE pattern (LREE-depleted) and Sm-Nd systematics imply that a close association once existed with the basaltic shergottite QUE94201, as well as to Nakhla and Chassigny. The conditions under which DaG 476 crystallized were more reducing than those of other basaltic shergottites, and it is the most magnesian member of the basalt subgroup. Overall, its mineralogy and bulk chemistry indicate that it is a distinct shergottite intermediate between the basaltic and lherzolitic subgroups. DaG 476 has a young crystallization age of ~474 m.y. (Sm-Nd), with cooling rates that are consistent with a burial depth during crystallization of less than 1 m. It is thought to have formed through a high-degree of partial melting of a lherzolite-like source material, followed by segregation of a melt containing unmelted phases of olivine, enstatite, and chromite. Furthermore, a residue containing a fraction of the unmelted phases was removed from this "crystal mush", leaving behind the fraction that would eventually form DaG 476. The texture of olivine xenocrysts and pyroxene crystals are indicative of flow alignment within an extruded lava flow near the surface. High shock features including twinning of clinopyroxene, mosaicism of olivine, and plagioclase converted to feldspathic glass, as well as abundant impact melt pockets, correspond to a shock stage of at least S5.
Comparisons with Viking inert gas measurements as well as results from chemical, mineralogical, petrographic, and oxygen isotopic studies clearly identify DaG 476 as Martian. Combining the 21Ne-based CRE age of 1.05 (±0.1) m.y. and the calculated terrestrial age of 60 (±20) k.y., a Mars ejection age of 1.1 (±0.1) m.y. ago is derived. Exposure ages of all members of both the basaltic and lherzolitic subgroups represent only a few ejection events from Mars; shergottites correspond to ejections at ~1.1, ~2.8, and ~20 m.y., and lherzolites at ~3.8 m.y. As a result of the uncertain terrestrial age for the basaltic shergottite EETA79001, which has a CRE age of ~0.6 m.y., its ejection age may either be similar to that of DaG 476, or represent a unique ejection event