Rb-Sr AND Sm-Nd STUDIES OF OLIVINE-PHYRIC SHERGOTTITES RBT 04262 AND LAR 06319: ISOTOPIC EVIDENCE FOR RELATIONSHIP TO ENRICHED BASALTIC SHERGOTTITES

Introduction: RBT 04262 and LAR 06319 are two Martian meteorites recently discovered in Antarctica. Both contain abundant olivines, and were classified as olivinephyric shergottites [1]. A detailed petrographic study of RBT 04262 by [2] suggested it should be reclassified as a lherzolitic shergottite. However, the moderately LREE-depleted REE distribution pattern [3] indicated that it is closely related to enriched basaltic shergottites like Shergotty, Zagami, Los Angeles, etc. In earlier studies of a similarly olivinephyric shergottite NWA 1068 which contains 21% modal olivine, it was shown that it probably was produced from an enriched basaltic shergottite magma by olivine accumulation [4, 5]. As for LAR 06319, recent petrographic studies suggested that it is different from either lherzolitic shergottites or the highly LREE-depleted olivine-phyric shergottites [6]. We performed Rb-Sr and Sm-Nd isotopic analyses on RBT 04262 and LAR 06319 to determine their crystallization ages and Sr and Nd isotopic signatures, and to better understand the petrogenetic relationships between them and other basaltic, lherzolitic and depleted olivine-phyric shergottites. Samples: Aliquant samples of RBT 04262 weighing 3.1 g and of LAR 06319 weighing 1.4 g were allocated to us for isotopic studies. The bulk rock samples (WR) were taken after the samples were coarsely crushed to <149μm. Then, the sample was further crushed and sieved into finer 74149μm and <74μm splits. The mineral separates – pyroxene (Px), olivine (Ol) and plagioclase (Plag) were obtained from these finer size fractions by magnetic and heavy-liquid density separations. Most samples were washed with weak HCl (1-2N) to remove possible surface contaminants. This procedure was especially important for the Rb-Sr isochron study of RBT 04262 (see Fig.1). Both acid-residue (r) and combined mineral leachates (Leach) were analyzed. Rb-Sr results: The Rb-Sr isotopic system for RBT 04262 is highly disturbed (Fig. 1). Five acid-washed samples and one unwashed plagioclase sample (solid circles) form a linear array corresponding to an age of 167±6 Ma for λ(Rb) =0.01402 Ga and the initial Sr/Sr =0.722745±19. However, two acid leachates and three unwashed bulk rock and pyroxene and olivine samples (open circles) lie considerably below the isochron, and were probably contaminated. In contrast, the Rb-Sr isotopic system for LAR 06319 is not significantly disturbed (Fig. 2). All nine bulk rock and mineral samples including two acid leachates form an isochron with an age of 207±14 Ma and an initial Sr/Sr =0.722509±69. The RBT 04262 age is within the range of Rb-Sr ages of olivine-free enriched basaltic shergottites (161-177 Ma) as well as previously reported ages for lherzolitic shergottites (148-185 Ma). It is also in agreement with the Rb-Sr age of olivine-bearing basaltic shergottite NWA 1068 (166±37 Ma). However, LAR 06319 is apparently older by 40±15 Ma. Olivine-phyric Shergottite RBT 04262