Origin of the ungrouped achondrite NWA 4518: mineralogy and geochemistry of FeNi-metal

Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyright owners. For more information on Open Research Online's data policy on reuse of materials please consult the policies page. Introduction: Ungrouped achondrite NWA 4518 is an ultramafic polymict breccia with abundant FeNi-metal. The silicate chemistry of NWA 4518 is similar to that of winonaites, HEDs and silicate inclusions from IIE irons. However, its oxygen isotopic composition is very different from all known types of meteorites. Two thin polished sections of NWA 4518 were studied with optical microscopy and EMPA. Petro-graphy suggests that breccia probably was formed by impact mixing of molten metal with solid silicates and suffered a high temperature metamorphism and reducing metasomatism. In situ elemental compositions of 13 metal particles were determined by LA-ICP-MS in one thin section of NWA 4518. The metal is enriched in refractory siderophile elements (RSE) and W by ~100хCI and does not correspond to any other groups of iron meteorites. Results: The NWA 4518 is a recrystallized equigranular polymict breccia, consisting of olivine Fo 68 (Fe/Mn=36) and minor pyroxenes En 45 Wo 42 (Fe/Mn=29) and En 72 Wo 2 (Fe/Mn=42) [1]. FeNi-metal (2 vol%) occurs among silicates as polygonal grains of 50-800 µm size. These metal grains are interconnected by a network of thin metal-sulfide veins, which locally contain metal-silicate melt pockets. The majority of metal grains has a mosaic internal texture and consist of several polygonal areas, only slightly different in Ni content (6.5-7.2 wt%), but obviously visible in BSE images. Tiny Ni-rich inclusions are widespread in the metal grains. Taenite lamellae, up to 7 µm in width, occur on the borders of polygonal areas inside the metal grains and along the contacts of the metal with surrounding silicates. The lamellae have M-shaped profiles of Ni concentration from 22.5 wt% on the borders to 15 wt% in the center. The Ni content in kamacite varies in different grains in the range 4-7 wt% (average 6.24 wt%, Ni/Co=8.7). The metal in the veins has similar composition. All of metal phases lack Si and have constant P concentrations ~ 0.2 wt%. Sulfides in NWA 4518 occur among silicates as polygonal grains of Ni-poor troilite, up to 500 µm in size, interconnected by thin troilite veins with higher Ni content (0.2-0.3 wt% Ni). The large grains of troil-ite locally have straight sharp contacts with metal grains. …