Petrography and Geochemistry of Metals in Almahata Sitta Ureilites (original) (raw)

Introduction: Ureilites are ultramafic achondrites, predominantly composed of olivine and pyroxenes with accessory carbon, metal and sulfide. The majority of ureilites are believed to represent the mantle of the ureilite parent body (UPB) [1]. Although ureilites have lost much of their original metal [2], the metal that remains retains a record of the formative processes. Almahata Sitta is predominantly composed of unbrecciated ureilites with a wide range of silicate compositions [3,4]. As a fall it presents a rare opportunity to examine fresh ureilite metal in-situ, and analyzing their highly siderophile element (HSE) ratios gives clues to their formation. Bulk siderophile element analyses of Almahata Sitta fall within the range observed in other ureilites [5]. We have examined the metals in seven ureilitic samples of Almahata Sitta (AS) and one associated chondrite fragment (AS#25). Methods: Electron-beam techniques: Imaging (using a LEO 1455VP SEM with Oxford Instruments INCA software) and major element composition analyses (using a WDS Cameca SX100 EMPA) were conducted at the NHM. EBSD analysis was conducted at the University of Glasgow using an EDAX-TSL system attached to a Quanta 200F fieldemission SEM that was operated in low vacuum mode. LA-ICP-MS: Analyses to obtain trace element compositions were performed at NASA JSC using a New Wave UP-193 SS laser ablation system attached to a Thermo Element2-XR ICP-MS. Laser power was 1-2 GW/cm 2 , with a repetition rate of 10 Hz. Spot sizes of 20 to 100 µm were used, depending on metal grain size. Some grains were too small to analyze. The Filomena and Hoba iron meteorites, NIST 610 glass, and NBS 1168 and NBS 1178 steels were used as standards. Ni values from EPMA were used for internal calibration for all samples except AS#15, #25 and #33 where Fe+Co+Ni were summed to 100 wt%. The following isotopes were analyzed: