NEBULA SCALE MIXING BETWEEN NON-CARBONACEOUS AND CARBONACEOUS CHONDRITE RESERVOIRS: TESTING THE GRAND TACK MODEL WITH ALMAHATA SITTA STONES

Introduction: There is an increasing number of Cr-O-Ti isotope studies [1-6] that show that solar system materials are divided into two main populations, one carbonaceous chondrite (CC)-like and the other is non-carbonaceous (NCC)-like, with minimal mixing between them attributed to a gap opened in the propoplanetary disk due to Jupiter's formation [5,6,7,8]. The Grand Tack model [9] suggests that there should be a particular time in the disk history when this gap is breached and ensuring a subsequent large-scale mixing between S- and C-type asteroids (inner solar system and outer solar system materials), an idea supported by our recent work on chondrule ?17O-?54Cr isotope systematics [10]. The Almahata Sitta (AhS) meteorite provides a unique opportunity to test the Grand Tack model. The meteorite fell to Earth in October 2008 from the impact of asteroid 2008 TC3 which was discovered just prior to the fall of the AhS stones [11-13]. The AhS meteorite is composed of up to 700 individual pieces with ~190 of those pieces having some geochemical and/or petrologic studies [14,23]. Almahata Sitta is an anomalous polymict ureilite with other meteorite components, including enstatite, ordinary, and carbonaceous chondrites with an approximate abundance of 70-80% ureilites and 20-30% chondrites [14,15]. This observation has lead to the suggestion that TC3 2008 was a loosely aggregated rubble pile-like asteroid with the non-ureilite sample clasts within the rubble-pile (e.g., [14,15] and references therein). Due to the looselyaggregated nature of AhS, the object disintegrated during atmospheric entry resulting in the weakly held clasts falling predominantly as individual stones in the AhS collection area. These stones are regarded as clasts from a highly heterogeneous breccia that probably represents ureilitic regolith [15]. Recent work [16,23] has identified one sample of AhS, sample 91A, which shows both ureilitic and chondritic lithologies co-existing within a single stone. The predominate lithology type in 91A is a C1 chondrite based on mineralogy, but the C1 material contains clasts of olivine, pyroxene, and plagioclase that have ureilite-like compositions, as well as chondrules derived from OC and primitive CC, and metal blebs from OC and EC [26,23]. It has been suggested that similar material comprised the bulk of asteroid 2008 TC3 [16,23]. A second AhS stone, 671, has now been discovered to be very similar to 91A [23]. Previous Cr isotope investigations into AhS stones are few and limited to wholly ureilitic samples. The data show nearly uniform isotopic composition similar to that of typical ureilites with negative ?54Cr values [17,18].