A new martian meteorite from the Sahara: The shergottite Dar al Gani 489

Authors


folco@unisi.it

Abstract

Abstract— Dar al Gani 489 (DaG 489) is a meteorite fragment of 2146 g found in the Libyan Sahara by a meteorite finder during one of his search campaigns in 1997–98. It is a porphyritic rock with millimetersized olivine crystals (Fo79–59) set in a fine-grained groundmass (average grain size 0.1 mm) consisting of pigeonite (En75–57 Wo5–15) crystals and interstitial feldspathic glass (An67–56 Or0–1). Minor phases include enstatite (En82–71 Wo2–4), augite (En48–52 Wo29–32), chromite, Ti-chromite, ilmenite, pyrrhotite, merrillite, and secondary calcite and iron oxides. On the basis of mineralogical, petrographic, bulk chemical, O-isotopic, and noble gas data, DaG 489 can be classified as a highly shocked martian meteorite (e.g., Fe/Mn(bulk) = 42.1, Ni/Mg(bulk) = 0.002; δ17O = 2.89, δ18O = 4.98, and Δ17O = 0.305), belonging to the basaltic shergottite subgroup.

The texture and modal composition of DaG 489 are indeed those of basalts; nonetheless, the bulk chemistry, the abundance of large olivine and chromite crystals, and enstatitic pyroxene suggest some relationship with lherzolitic shergottites. As such, DaG 489 is similar to the hybrid shergottite Elephant Moraine (EET) A79001 lithology A; however, there are some relevant differences including a higher olivine content (20 vol%), the lack of orthopyroxene megacrysts, a higher molar Mg/(Mg + Fe)(molar) = 0.68, and a lower rare earth element content in the bulk sample. Therefore, DaG 489 has the potential of providing us with a further petrogenetic link between the basaltic and lherzolitic shergottites.

Noble gases data show that DaG 489 has an ejection age of ∼1.3 Ma. This young age lends support to the requirement of several ejection events to produce the current population of shergottites, nakhlites, and chassignites (SNC) meteorites.

In terms of texture, mineral and bulk compositions, shock level, and weathering features, DaG 489 is essentially identical to DaG 476, another basaltic shergottite independently found ∼25 km due northnortheast of DaG 489. Because DaG 489 also has the same exposure history as DaG 476, it is very likely that both meteorites are fragments of the same fall.

In addition to the existing hypotheses on the petrogenesis of the similar EETA79001 lithology A and the identical DaG 476, we propose that DaG 489 could have formed through high-degree partial melting of a lherzolite-like material.

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