Petrology and Geochemistry of New Paired Martian Meteorites LAR 12240 and LAR 12095



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This project describes two Martian meteorites, paired olivine-phyric shergottites LAR 12240 and LAR 12095, in order to decipher their petrological context, and place constraints on the geological history of Mars. This project identifies all phases found in LAR 12240 and 12095 and analyzes them for major and trace elements. The textural relationships among these phases are examined in order to develop a crystallization history of the magma(s) that formed these basalts. Two polished, 100 µm thick sections of the paired Martian meteorites LAR 12240 and LAR 12095 were obtained for the purpose of this study. A petrographic microscope, a JEOL JXA 7600F scanning electron microscope (SEM), a Cameca SX100 Electron microprobe (EMP) and a laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) were used to complete this study. LAR 12095 and LAR 12240 are typical olivine-phyric shergottites with Mg-rich olivine megacrysts in a groundmass comprised of pyroxene, smaller Fe-rich olivines and interstitial plagioclase. The olivine megacrysts are likely antecrysts and contain numerous melt inclusions that are more evolved than the bulk-rock compositions of LAR 12095 and LAR 12240 or compositions of shergottite parent magmas from the literature. The crystallization of LAR 12240 and LAR 12095 likely began with the crystallization of Cr-rich spinels (T1) and the Mg-rich olivine megacryst cores followed by progressively less Cr-spinels (T2 and T3) crystallizing along with the outer edges of the olivine megacrysts and pigeonite. The olivine megacrysts were then entrained in a magma that crystallized at or near the Martian surface. The magma would have begun crystallizing more T2 and T3 along with 0.5-1 mm and 0.1 mm sized Fo60 olivines and the last of the pigeonite followed by augite, plagioclase feldspar, ulvöspinel (T4 and T5) sulfides and merrillite. These meteorites have calculated fo2 values range from ΔFMQ of -3.7 to -1.7 and depleted (light rare earth element) LREE patterns which place them into the reduced, depleted shergottite group. LAR 12095 and LAR 12240 most closely resemble SaU 005 for their major and trace element compositions. These meteorites also closely resemble DaG 489 for trace element compositions.