In 2008, October 6 at 5:46 A.M., asteroid 2008 TC3 fell to Earth in northern Sudan. See the Almahata Sitta webpage for the complete story of the discovery of this meteorite, results of the consortium analyses, and new models for the petrogenetic history of the ureilite parent body. The 2008 TC3 meteorite was sent to NASA's Johnson Space Center in Houston (Zolensky) and Carnegie Institution of Washington (Steele) for analysis and classification, and Alamahta Sitta was determined to be a polymict ureilite fragmental breccia composed of three main ureilite lithologies, along with a wide range of xenolithic clasts representing many different chondritic and achondritic lithologies in an assemblage similar to the polymict breccia Kaidun (Bischoff et al., 2010). Results of the analyses indicate that all of the clasts came from the Almahata Sitta fall; e.g., detection of short-lived cosmogenic nuclides, very low weathering grade (W0W0/1), multiple lithologies among fragments delimiting a strewn field, a high number of rare E-chondrite rock types found together, diffusion of PAHs among clasts (Sabbah et al., 2010), and the finding of new and unique meteorite fragments within a small area.
The heterogeneous composition of Almahata Sitta could reflect an assemblage derived from a catastrophic collision(s) between ureilte and chondrite objects (Kohout et al., 2010). In an alternative scenario, these diverse clasts could have become gravitationally bound within a common debris disk composed of a disrupted ureilite asteroid, and this disk subsequently re-accreted into one or more smaller second-generation asteroids. This second-generation asteroid was then lightly sintered together through multiple low-energy impacts resulting in a bulk porosity of ~50%. The highly porous ureilite material recovered from the Almahata Sitta fall, as represented by the recovered specimen MS-168, is consistent with the hypothesized lightly-sintered matrix of the second-generation asteroid 2008 TC3.
Among the wide variety of xenolithic clasts recovered from the Alamahta Sitta polymict ureilite fall is the 15.55 g inclusion MS-MU-012. This inclusion was analyzed at the Institut für Planetologie in Münster, Germany and classified as the first known plagioclase-bearing olivineaugite ureilite (Bischoff et al., 2015, #5092). In their analyses of MS-MU-012, Goodrich et al. (2015) determined a composition of ~52% olivine (with melt inclusions), 13% orthopyroxene, 21% augite, and 14% plagioclase-rich areas, the latter consisting of both pristine and shock-melted (with interstitial sulfide and metal) crystals. The unmelted pristine plagioclase has a composition of An68.4, which attests to an origin as a late-stage fractionate in which a significant amount of plagioclase remained, and cooling occurred prior to its complete extraction (Goodrich et al., 2016).
The MS-MU-012 inclusion is considered to be a paracumulate representing a mixture of residual olivine and cumulus pyroxene. A plausible formation scenario was presented in which an augite-saturated melt invades a region composed of a residual olivinepigeonite assemblage; following melt extraction, the lithology appears texturally similar to a cumulate (Berkley and Goodrich, 2001; D.W. Mittlefehldt, 2005). It is considered that the parental melts from which olivineaugite ureilites were formed originated at greater depths than melts parental to the ferroan olivinepigeonite ureilites (Goodrich et al., 2004). On the other hand, crystallization of the olivineaugite ureilites occurred after the parental melt had ascended to shallower depths, and after the degree of fractional melting had reached ~15%; at this stage most of the plagioclase had been removed and the magma had undergone reduction to higher olivine Fo compositions. Although the presence of plagioclase in MS-MU-012 is unique, the meteorite is otherwise indistinguishable from typical olivineaugite ureilites with respect to mineralogy, O-isotopic composition, and petrographic characteristics (Goodrich et al., 2016). See the HaH 064 page for further information about the olivineaugite ureilite subgroup.
Exclusive of the primary ureilite components, there was a broad diversity of lithologic types present in 2008 TC3, constituting <30% of all material recovered. However, with the vast bulk of 2008 TC3 thought to have been lost as fine dust (≥99.9% of the estimated 4283 ton pre-atmospheric mass), the bulk asteroid was likely composed of fine-grained, highly porous, and weakly consolidated ureilitic matrix material, consistent with the reflectance spectra obtained for the asteroid (Goodrich et al., 2015). Examples of some of the diverse samples that have been recovered are listed below (Bischoff et al., 2010, 2015, 2016; Horstmann and Bischoff, 2010; Hoffmann et al., 2016):
unique chondrite: MS-CH, type 3.8 [± 0.1], has petrographic and isotopic affinities to R-chondrites, but is mineralogically anomalous
Bencubbin-like carbonaceous chondrite: MS-181, a 58.6 g chondrule-like clast containing metal globules and silicates in a 60:40 ratio, having an O-isotopic composition consistent with bencubbinites
C2 carbonaceous chondrite: AhS 202 (photo; Fioretti et al., 2017)
niningerite-bearing, fine-grained ureilitic fragment (linking E chondrites)
sulfide-metal assemblage in a fine-grained ureilitic fragment: MS-158, -166
ungrouped enstatite- and metal-rich achondrite fragments: MS-MU-019 (characteristics similar to NWA 8173/10271; MS-MU-036 (similar to MS-MU-019 and Itqiy (Bischoff et al., 2016)
the first known plagioclase-bearing olivineaugite ureilite lithology: MS-MU-012
trachyandesitic clasts: 1) MS-MU-011 (view 1), MS-MU-011 (view 2), sample ALM-A; plagioclase-enriched (~70 vol%) with pockets of gemmy olivine (photo courtesy of Stephan Decker), likely sampling the UPB crust (or possibly an alkali- and water-rich localized melt pocket); calculated ArAr age of ~4.556 b.y. and PbPb age of ~4.562 b.y. (Bischoff et al., 2013, 2014; Delaney et al., 2015; Turrin et al., 2015; Amelin et al., 2015); 2) MS-MU-035; anorthoclase and/or plagioclase-enriched (~65 vol%) (Bischoff et al., 2016)
Special thanks to Siegfried Haberer and Stephan Decker for providing specimens of this special meteorite and many of its xenolithic inclusions to the scientific and collector communities. The photo of MS-MU-012 shown above is a 0.14 g partial slice, while the photo below shows the main mass.