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). Alternatively, it is considered likely that these diverse clasts could have become gravitationally bound within a common debris disk composed of a disrupted ureilite asteroid, and that this disk then re-accreted into one or more smaller second-generation asteroids. This second-generation asteroid later became lightly sintered together through subsequent 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.
Inclusion MS-MU-002 was analyzed and classified at the University in Münster, Germany (A. Bischoff), and it was determined to be a very rare and uniquely pristine EL3 chondrite associated with the Almahata Sitta fall. The entire MS-MU-002 inclusion had dimensions of 1.28 × 1.15 × 1.07 cm, and weighed only 3.0 g before it was sectioned into several slices. Other EL3 clasts recovered from the Almahata Sitta strewn field include MS-17 and MS-177, initially characterized by Goresy et al. (2012).
Although classification of MS-MU-002 by magnetic susceptibility alone (5.26; Hoffmann et al., 2016) cannot discriminate between EH and EL chondrite groups, the observed values for a variety of parameters can help resolve the genetic relationship. For example, FeNi-metal in MS-MU-002 contains kamacite with ~1.4 wt% Si (EL: 0.32.1 wt%; EH: 1.93.8 wt%). Also, the sulfides that are present include troilite, oldhamite, and keilitic alabandite. Keilitic alabandite is representative of the Fe,Mn-rich end member of the solid solution series ([Mg,Mn,Fe]S) which occurs only in EL chondrites, in contrast to the Mg-rich end member (niningerite) found in EH chondrites. Inclusion MS-MU-002 contains chondrules with diameters of 0.20.5 mm (no average available), a size which overlaps the averages for both E-chondrite groups (EL3: ave. 0.55 mm; EH3: ave. 0.22 mm). A more complete list of EL/EH discriminators can be found on the NWA 3132 page.
Weyrauch et al. (2018) analyzed the mineral and chemical data from 80 enstatite chondrites representing both EH and EL groups and spanning the full range of petrologic types for each group. They found that a bimodality exists in each of these groups with respect to both the Cr content in troilite and the Fe concentration in niningerite and alabandite. In addition, both the presence or absence of daubréelite and the content of Ni in kamacite were demonstrated to be consistent factors for the resolution of four distinct E chondrite groups: EHa, EHb, ELa, and ELb (see table below).
ENSTATITE CHONDRITE SUBGROUPS
Weyrauch et al., 2018
Cr <2 wt%
Cr >2 wt%
Cr <2 wt%
Cr >2 wt%
Fe <20 wt%
Fe >20 wt%
Fe <20 wt%
Fe >20 wt%
Ni <6.5 wt%
Ni >6.5 wt%
Ni <6.5 wt%
Ni >6.5 wt%
A few other E chondrites with intermediate mineralogy have also been identified, including LAP 031220 (EH4), QUE 94204 (EH7), Y-793225 (E-an), LEW 87223 (E-an), and PCA 91020 (possibly related to LEW 87223). Studies have determined that these meteorites were not derived from the EH or EL source through any metamorphic processes, and some or all of them could represent separate E chondrite asteroids. The revised E chondrite classification scheme of Weyrauch et al. (2018) including selected examples from their 80-sample study can be found here. It was determined that MS-MU-002 is a member of the ELb subgroup.
Petrographic and isotopic evidence indicate that the EL parent body accreted hot silicate and metal components that were formed through repeated nebular condensation processes rather than by impact-heating events on a parent body (Weisberg et al., 2013). Observations consistent with a nebular condensation origin include the absence of shock-induced features, the lack of high-pressure polymorphs, the presence of metalsinoiteoldhamitegraphite assemblages composed of crystalline sinoite and poorly graphitized carbon, and the pristine texture of discrete chondrules and metal nodules, while the isotopic inventory is inconsistent with a large-scale impact-heating event. A high abundance of metal in the final agglomeration is also expected with the presumed nebular condensation sequence.
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, 2018; Horstmann and Bischoff, 2010, 2014; Hoffmann et al., 2016):
niningerite-bearing, fine-grained ureilitic fragment (linking E chondrites): MS-20
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]); AhS 38 (similar to MS-MU-019 and Itqiy but contains olivine [Goodrich et al., 2018]); AhS 60 (possible E IMR analogous to Portales Valley [Goodrich et al., 2018])
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-002 shown above is a 0.350 g full slice. The photo below shows the 3.00 g main mass.