Mineralogy and petrology of the Dar al Gani 476 martian meteorite: Implications for its cooling history and relationship to other shergottites

Dar al Gani 476, the 13th martian meteorite, was recovered from the Sahara in 1998. It is a basaltic shergottitic rock composed of olivine megacrysts reaching 5 mm (24 vol%) set in a fine-grained groundmass of pyroxene (59 vo l%) and maskelynitized plagioclase (12 vol%) with minor amounts of accessor y phases (spinel, merrillite, ilmenite). Dar al Gani 476 is similar to lith ology A of Elephant Moraine A79001 (EETA79001) in petrography and mineralog y, but is distinct in several aspects. Low-Ca pyroxenes in the Dar al Gani 476 groundmass are more magnesian (En(76)Fs(21) Wo(3)similar to En(58)Fs(30 )Wo(12)) than those in lithology A of EETA79001 (En(73)Fs(22)Wo(5)similar t o En(45)Fs(43)Wo(12)), rather similar to pyroxenes in Iherzolitic martian m eteorites (En(76)Fs(21)Wo(3)similar to En(63)Fs(22)Wo(15)) Dar al Gani 476 olivine is less magnesian and shows a narrower compositional range (Fo(76-5 8)) than EETA79001 olivine (Fo(81-53)), and is also similar to olivines in Iherzolitic martian meteorites (Fo(74-65)). The orthopyroxene-olivine-chrom ite xenolith typical in the lithology A of EETA79001 is absent in Dar al Ga ni 476. It seems that Dar al Gani 476 crystallized from a slightly more pri mitive mafic magma than lithology A of EETA79001 and several phases (olivin e, pyroxene, chromite, and ilmenite) in Dar al Gani 476 may have petrogenet ic similarities to those of Iherzolitic martian meteorites. Olivine megacry sts in Dar al Gani 476 are in disequilibrium with the bulk composition. The presence of fractured olivine grains in which the most Mg-rich parts are i n contact with the groundmass suggests that little diffusive modification o f original olivine compositions occurred during cooling. This observation e nabled us to estimate the cooling rates of Dar al Gani 476 and EETA79001 ol ivines, giving similar cooling rates of 0.03-3 degreesC/h for Dar al Gani 4 76 and 0.05-5 degreesC/h for EETA79001. This suggests that they were cooled near the surface (burial depth shallower than about 3 m at most), probably in lava flows during crystallization of groundmass. As is proposed for lit hology A of EETA79001, it may be possible to consider that Dar al Gani 476 has an impact melt origin, a mixture of martian Iherzolite and other martia n rock (Queen Alexandra Range 94201, nakhlites?).