AGE AND PETROGENESIS OF PICRITIC SHERGOTTITE NWA 1068: Sm-Nd AND Rb-Sr ISOTOPIC

Introduction: NWA 1068 is a 577g picritic shergottite found in the Moroccan Sahara in 2001[1]. The meteorite resembles several other picritic shergottites, e.g. EETA79001B, DaG476, SaU005 and Dho019, in major-element chemistry and mineralogy, but it differs significantly from these meteorites in REE distribution pattern. It has a slightly LREE-depleted pattern commonly shared by some olivine-free basaltic shergottites, e.g. Shergotty, Zagami and Los Angeles, but not QUE94201. Detailed geochemical and mineralpetrological studies were given in [1]. We performed Rb-Sr and Sm-Nd isotopic analyses on this rock to determine its crystallization age and to study the petrogenetic relationship between this meteorite and other basaltic and pricritic shergottites. Samples: An aliquant sample of 1.62 g was allocated for isotopic studies at JSC. For this study, we processed 1.0 g of the interior of the meteorite. The whole rock samples were taken after the sample was coarsely crushed to <149μm. Then the sample was further crushed and sieved into finer 74-149μm and <74μm splits. The mineral separates pyroxene, olivine and plagioclase were obtained from these finer size fractions by magnetic and heavy-liquid density separations. So far, we have not obtained enough pure plagioclase for Sm-Nd analyses. Since the meteorite has been exposed to the severe desert environment, acid-washing was used to get rid of terrestrial alteration products for all the samples analysed. For cleaning whole rock and olivine samples, we used 2N HCl with sonication. For pyroxene samples, an additional step of 3N HF etching was used. Both acid-residue (r) and acid-leachate (l) were analyzed. The whole rock isotopic results were presented earlier [2]. Sm-Nd results: Sm-Nd isotopic results for bulk rock and pyroxene samples are shown in Fig. 1. The acid-washed olivine sample had too little Nd for good isotopic measurements. The unwashed whole rock and four acid-leachates of whole rock and mineral samples are clustered together. These five samples and three HF-washed pyroxenes form a linear array corresponding to an age of 185±11 Ma using the Williamson fit program [3]. The errors were ±14 Ma and ±16 Ma for the York [4] and Isoplot [5] programs, respectively. The datum for the gently HCl-washed WR(r) sample plots significantly above the isochron and may still contain some alteration products because it lies on a probable terrestrial alteration line (broken line) defined by the acid-washed samples, the desert caliche from Los Angeles shergottite [6] (square) and river aeolian particulates [7,8] (diamonds). If the isochron defined by leachates and pyroxenes represents the age of the meteorite, it certainly is consistent with the ages reported for several other shergottites [9] that have not been exposed to severe terrestrial weathering, e.g. Shergotty, Zagami, EETA79001A&B, ALHA 77005, LEW 88516. The corresponding initial εNd value for this meteorite is –7.0±0.2, also in good agreement with values for some of these less altered shergottites.