The Bulk Composition of Coarse-grained Meteorites from Laser Ablation Analysis of Their Fusion Crusts

Introduction: Many new meteorites representing rare achondrite groups are being recovered from both hot and cold deserts. A significant fraction of these meteorites are both coarse-grained in texture and small in size, making accurate determinations of bulk rock compositions rather challenging. We successfully obtained elemental analyses of several angrites using ~1 mm 2 rasters of polished sections by laser ablation ICP-MS [1, 2]. This technique yielded inconsistent answers for two coarse-grained angrites, NWA 4590 and NWA 4801, leading us to try an alternative method for sampling one of these coarse-grained meteorites by means of analysis of its fusion crust. The fusion crust covering a meteorite is formed by the melting of the surface minerals during passage through the Earth's atmosphere. Due to the dynamic environment under which it forms, the elemental abundances of fusion crust may be sufficiently representative of the bulk composition of the meteorite [3]. The degree to which fusion crusts are representative of a meteorite's bulk-rock composition must be interpreted cautiously as mineral point-sources may contribute heterogeneity [4]. Meteorites recovered from the hot deserts of Oman, Libya and Northwest Africa pose an additional challenge , since it is well known that elemental abundances of such meteorites can be significantly influenced by terrestrial weathering [e.g, 5]. In this study, we report new results on the composition of the fusion crust of a coarse-grained angrite (NWA 4590) and an olivine basaltic shegrottite (NWA 4468), and on the cleaning protocol that removed most of the terrestrial contaminants from their fusion crusts. Samples and Analytical method: NWA 4590 is a fresh, coarse-grained (0.6-12 mm) plutonic igneous cumulate angrite covered with fusion crust [6]. NWA 4468 is a superbly fresh, olivine basaltic shergottite [7]. We analyzed two chips of fusion crust from each meteorite by directly ablating the exterior surface of the fusion crust without epoxy mounting or polishing. The fusion crust fragments of NWA 4590 were analyzed both before and after application of a surface cleaning procedure. To remove terrestrial contaminants, the fusion crust surfaces of each meteorite were brushed with a nylon toothbrush and DI water, then polished using 1 µm alumina powder. The polished samples were washed in deionized water in an ultrasonic bath.