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 Almahata 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 ureilite 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%. This fine-grained, highly porous, weakly consolidated matrix material is possibly represented by the recovered specimen MS-168 and/or the C1+URE+OC+EH regolith breccia clasts AhS 91/91A and 671; this would be consistent with the reflectance spectra obtained for the asteroid (Goodrich et al., 2015, 2019).
Clast 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 been identified, including LAP 031220 (EH4), QUE 94204 (EH7), Y-793225 (E-an), LEW 87223 [EL3-an; abs], 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.
The inner main belt Athor asteroid family (Xc-type in the Bus-DeMeo taxonomy), in which the largest member is ~42 km-diameter (161) Athor, has been identified by Avdellidou et al. (2022) as the unique parental source of the EL chondrite meteorites. Utilizing spectrographic (e.g., reflectance spectra, geometric albedo) and isotopic data, as well as thermochronometry and CRE age data, the research team determined that the predecessor of the Athor asteroid family was an EL-type chondritic planetesimal measuring 240420 km in diameter (Trieloff et al., 2022) that accreted within the terrestrial planet region about 4.5 b.y. ago, and which experienced a complex collisional history (see chronological illustration below). An initial severe collisional disruption occurred ~3 b.y. ago which led to the creation of an inferred 64 km-diameter daughter body composed predominantly of type 6 lithologies. This EL-chondrite daughter body ultimately migrated into a stable parking orbit in the inner main asteroid belt. Subsequent collisional fragmentation of this EL asteroid produced a gravitationally-bound association of various sized fragments recognized today as the Athor asteroid family. The identification by Trieloff et al. (2022) of a common CRE age of 33 m.y. for many EL6 chondrites attests to a major impact involving at least one of the Athor family fragments at this time. The location of this impact event is most likely near a dynamical resonance such as the Jupiter 3:1 mean motion resonance at 2.50 AU, which provides ejecta an efficient transfer mechanism into an Earth-crossing trajectory. For example, the EL6 Neuschwanstein meteorite was given a probability of 63 (±13) % of escaping via the Jupiter 3:1 mean motion resonance (Granvik and Brown, 2018). It is noteworthy that one of the three common CRE ages (i.e., major collisional events) among H-type chondrites is also 33 m.y. (Marti and Graf, 1992; Eugster et al., 2006, 2007), and that the H chondrite group is also located near the 3:1 mean motion resonance at 2.50 AU.
Collisional History of the EL Planetesimal
click on image for a magnified view
Schematic illustration credit: Avdellidou et al., Astronomy & Astrophysics, vol. 665, #L9, fig. 2 (2022 open accesslink)
'Athor asteroid family as the source of the EL enstatite meteorites'
The broad diversity of lithologic types present in 2008 TC3 constituted <30% of all material recovered. However, given that the vast bulk of 2008 TC3 is thought to have been lost as fine dust (≥99.9% of the estimated 4283 ton pre-atmospheric mass), the asteroid was likely composed predominantly of very fine-grained, highly-porous, weakly-consolidated matrix material, possibly represented by the recovered specimen MS-168 and/or the C1+URE+OC+EH regolith breccia clasts AhS 91/91A and 671; this would be consistent with the reflectance spectra and other data obtained for the asteroid (Goodrich et al., 2015, 2019; Bischoff et al., 2022). Examples of some of the diverse samples that have been recovered are listed below (Bischoff et al., 2010, 2015, 2016, 2018, 2019; Horstmann and Bischoff, 2010, 2014; Hoffmann et al., 2016; Fioretti et al., 2017; Goodrich et al., 2018, 2019):
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 (complete mass; cut section photo credit: Bischoff et al.2022; characteristics similar to NWA 8173/10271); MS-MU-036 (similar to MS-MU-019, Itqiy, and NWA 2526 [Bischoff et al., 2016; Zhu et al., 2021]); 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 olivineorthopyroxeneaugite ureilite lithology: MS-MU-012 (Goodrich et al., 2022)
trachyandesitic clasts: (1) MS-MU-011 (view 1), MS-MU-011 (view 2), MS-MU-011 (aka 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); (3) MS-277, 11.03 g; (4) MS-MU-065, 54.7 g
andesitic clast: AhS 3005, 16.84 g, composed of two different sectors: 1) "labradoriteopx" (plag cores = An5053); 2) "oligoclaseaugite" (plag cores = An3035) (Goodrich et al., 2022 #1065)
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.