Centre for Planetary Sciences

Modal abundances of Ca,Al-rich inclusions (CAIs) are poorly known and reported data scatter across large ranges. We combine reported CAI modal abundances and our own set, and present a complete list of CAI modal abundances in carbonaceous chondrites. This includes (in area%): CV: 2.98, CM: 1.21, Acfer 094: 1.12, CO: 0.99, CK/CV (Ningqiang & DaG 055): 0.77, CK: 0.2, CR: 0.12 and CB: 0.1. CAIs are Poisson distributed and if only small areas (<1000 mm2) are studied, the data are probably not representative of the true CAI modal abundances, explaining their reported large scatter in a single chondrite group. Carbonaceous chondrites have excess bulk Al concentrations when compared to the CI-chondritic value. We find a correlation between this excess and CAI modal abundances and conclude that the excess Al was delivered by CAIs. The excess Al is only a minor fraction (usually ~10 rel%, but 25 rel% in case of CVs) of the bulk chondrite Al and cannot have contributed much 26Al to heat the chondrite parent body. Ordinary, enstatite, R- and K-chondrites have an Al deficit relative to CI chondrites and only very low CAI modal abundances, if any are present at all. Carbonaceous chondrites also had an initial Al deficit if the contribution of Al delivered by CAIs is subtracted. Therefore all chondrites probably lost a refractory rich high-T component. Only minor amounts of CAIs are present in the matrix or have been present in the chondrule precursor aggregates. Most CAI size distributions contain more than one size population, indicating that CAIs from within a single meteorite group had different origins.

The Almahata Sitta fall event provides a unique opportunity to gain insight into the nature of ureilitic objects in space and the delivery of ureilite meteorites to Earth. From thermal events recorded in the mineralogy, petrology, and chemistry of ureilites recovered from the fall area, we reconstruct a timeline of events that led to their genesis. This history is similar to that of other known ureilites and supportive of a disrupted ureilite parent body hypothesis. Temperatures of final mantle equilibrium were 1200-1300 °C, but this high-temperature history was abruptly terminated by rapid cooling and reduction associated with pressure loss. The onset of late reduction reactions and onset of rapid cooling must have been essentially simultaneous, most likely engendered by the same event. Cooling rates of 0.05-2 °C h-1 determined from reversely zoned olivines and pyroxenes in Almahata Sitta imply rapid disassembly into fragments tens meters in size or smaller. This phenomenon seems to have affected all known portions of the ureilite parent body mantle, implying an event of global significance rather than localized unroofing. Reaccretion of one or more daughter asteroids occurred only after significant heat loss at minimum time scales of weeks to months, during which time the debris cloud surrounding the disrupted parent was inefficient at retaining heat. Fragments initially dislodged from the ureilite parent body mantle underwent subsequent size reduction and mixed with various chondritic bodies, giving rise to polylithologic aggregate objects such as asteroid 2008 TC3.

Modal abundances of Ca,Al-rich inclusions (CAIs) are poorly known and reported data scatter across large ranges. We combine reported CAI modal abundances and our own set, and present a complete list of CAI modal abundances in carbonaceous chondrites. This includes (in area%): CV: 2.98, CM: 1.21, Acfer 094: 1.12, CO: 0.99, CK/CV (Ningqiang & DaG 055): 0.77, CK: 0.2, CR: 0.12 and CB: 0.1. CAIs are Poisson distributed and if only small areas (<1000 mm 2 ) are studied, the data are probably not representative of the true CAI modal abundances, explaining their reported large scatter in a single chondrite group. Al concentrations when compared to the CIchondritic value. We find a correlation between this excess and CAI modal abundances and conclude that the excess Al was delivered by CAIs. The excess Al is only a minor fraction (usually ~10 rel%, but 25 rel% in case of CVs) of the bulk chondrite Al and cannot have contributed much 26 Al to heat the chondrite parent body. Ordinary, enstatite, R-and Kchondrites have an Al deficit relative to CI chondrites and only very low CAI modal abundances, if any are present at all. Carbonaceous chondrites also had an initial Al deficit if the contribution of Al delivered by CAIs is subtracted. Therefore all chondrites probably lost a refractory rich high-T component. Only minor amounts of CAIs are present in the matrix or have been present in the chondrule precursor aggregates. Most CAI size distributions contain more than one size population, indicating that CAIs from within a single meteorite group had different origins.

Most unbrecciated ureilites are believed to represent intact samples of the ureilite parent body (UPB) mantle, consisting predominantly of olivine and pyroxenes with accessory carbon, metal and sulfide [1]. Although much of the original metal has been lost [2], what remains records partial melting and differentiation processes on the UPB. Knowing the modal abundances of metals within ureilite samples helps with the interpretation of bulk siderophile elements [2,3] and of magnetic susceptibility [4]. X-ray computed tomography (CT) scanning allows non-destructive 3D imaging of samples, visualising 3D spatial relationships which are not apparent in traditional petrographic studies of individual 2D thin sections. We have analysed a range of ureilite samples in 2D and 3D, including several samples of the anomalous ureilite Almahata Sitta (AS) [5]. As a quickly recovered fall, AS samples do not show oxidation of metal as seen in most other ureilites. Methods: Samples were scanned using the Metris X-Tek HMX ST 225 CT system at the NHM and reconstructed using CT-PRO 2.0. Analysis of the scans was performed using VG Studio Max 2.0, using greyscale thresholding to separate phases. Thin sections or probe blocks were examined using the LEO 1455 VP SEM at the NHM. Whole section EDS X-ray chemical maps were acquired using Oxford Instruments INCA software and combined to differentiate minerals. These were processed by pixel counting in Photoshop to obtain modal mineralogies. Results and discussion: Metal in Almahata Sitta shows a large variation in modal percent and appearance between samples. For example, from SEM data AS#22 has ~1.8% metal with grain boundary metal not exceeding ~35μm in width, whereas AS#44 shares many features with the anomalous ureilite LAR 04315 and is particularly metal-rich, with ~3.4% metal and some unusually large grains >250μm in size. These percentages include siderophile-bearing metal in veins and primary inclusions as well as siderophile-free secondary reduction metal. All metal affects magnetic susceptibility, whereas bulk siderophile concentrations are not affected by reduction metal. Reduction metal makes up a greater percentage of the metal content of AS#22 than AS#44. Preliminary data analysis suggests that whilst it is easier to differentiate metal and sulfide using the SEM, carbon is harder to quantify in 2D as SEM samples are carbon-coated. The main sources of error in extrapolating the data are likely to be small sample size and sample heterogeneity. Acknowledgements: We would like to thank R. Abel, A. T. Kearsley and A. Ball for equipment assistance. We thank ANSMET and the NHM for comparative ureilite samples.

Ureilite meteorites contain iron silicide minerals including suessite (Fe,Ni) 3 Si, hapkeite (Fe 2 Si) and xifengite (Fe 5 Si 3 ). Despite occurring mostly in brecciated varieties presumed to be derived from the regolith of the ureilite parent asteroid, suessite has also been confirmed in one lithology of a dimict ureilite (NWA 1241). In contrast, Si-bearing Fe-metals occur in both brecciated and unbrecciated ureilites, implying that they were formed throughout the ureilite parent asteroid. We examined major, minor and trace element data of Fe-metals in seven brecciated ureilites (DaG 319, DaG 999, DaG 1000, DaG 1023, DaG 1047, EET 83309, and EET 87720) in addition to the dimict ureilite NWA 1241. In this study we show that the silicides and Si-bearing metals in ureilites have similar siderophile trace element patterns; therefore, the precursors to the silicides were indigenous to the ureilite parent body. Sifree kamacite grains in brecciated ureilites show flatter, more chondritic siderophile element patterns. They may also be derived from the interior of the ureilite parent body, but some may be of exogenous origin (impactor debris), as are rare taenite grains. On Earth, iron silicides are often formed under high-temperature and strongly reducing conditions (e.g. blast furnaces, lightning strikes). On the Moon, hapkeite (Fe 2 Si) and other silicides have been found in the regolith where they were formed by impact-induced space weathering. In the Stardust aerogel, iron silicides derived from comet Wild2 were also formed by an impact-related reduction process. Silicides in ureilite regolith breccias may have formed by similar processes but ureilites additionally contain abundant elemental carbon which probably acted as a reducing agent, thus larger and more abundant silicide grains were formed than in the lunar regolith or cometary material. The origin of suessite in NWA 1241 may be analogous to that of reduced lithologies in the terrestrial mantle, although a regolith origin may also be possible since this sample is shown here to be a dimict breccia.

Introduction: Ureilites are ultramafic achondrites made up of olivine (ol), pyroxenes (px) and minor carbon, metal and sulfide accessory phases. Although olivine-pigeonite (ol-pig) assemblage is the most common, specimens featuring low-calcium pyroxene of nominally orthopyroxene (opx) composition (i.e., Wo ≤5) and/or augite (aug) are also found occurring with or without pigeonite e.g., [1, 2]. In the majority of ureilites exsolution of pyroxene is not evident. Silicate grains are generally chemically homogenous; other than the reverse zoning normally affecting the outermost grain rims caused by reduction, a ubiquitous and characteristic feature of ureilites, or mosaicized regions [1, 2]. Ol-pig and ol-opx samples are believed to represent restites of silicate partial melting and are believed to have the same origin despite different pyroxene compositions based on chemical similarities, such as falling along the same Fe/Mg-Fe/Mn trend [2]. The Almahata Sitta (AhS) fall of 2008 is com...