Meteorite from Mars
DAG 476
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
Classification and mineralogy: was analyzed and classified at Germany's
Max-Planck-Institut für Chemie in Mainz. This is very appropriate since this
institute also developed the APXS instrument used during the Mars Pathfinder
Mission aboard the Sojourner Rover to analyze surface rock compositions.
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.
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