Enstatite Achondrite, ungrouped
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Fell October 7, 2008
20° 43.04' N., 32° 30.58' E.
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).
Among the wide variety of xenolithic clasts recovered from the Almahata Sitta polymict ureilite fall is the 177.1 g inclusion MS-MU-036. This inclusion was analyzed at the
Institut für Planetologie in Münster, Germany and classified as a unique metal-rich enstatite achondrite similar to the 14.0 g inclusion MS-MU-019 (Bischoff et al., 2016, #6319; Harries and Bischoff, 2020). It is composed of three different enstatite populationsclinoenstatite (~En98.5Wo1.3), orthoenstatite (~En96.5Wo3.2), and high-Ca clinopyroxene (augite; ~En60Wo40)hosted in a Si-bearing (~2.3 wt%) FeNi-metal phase with minor sulfides in the form of alabandite, oldhamite, and daubréelite. Rare forsterite and plagioclase grains are both present in abundances of <1 vol%. Inclusion MS-MU-019 is similarly composed of the same two enstatite populations of clinoenstatite and orthoenstatite along with augite, also hosted in a Si-bearing FeNi-metal phase (Bischoff et al., 2015, #5092; Harries and Bischoff, 2020). Another sample from the fall, MS-245, is mentioned as being compositionally similar to these two meteorites. In addition, it is considered by Hoffmann et al. (2016, #1874) that MS-MU-019 might have similarities to the metal-rich enstatite achondrites NWA 8173 (photo courtesy of Gary Fujihara) and NWA 10271.
It is noteworthy that these two enstatite achondrite inclusions have also been compared to Itqiy (Bischoff et al., 2016), which itself has been compared to a number of other anomalous metal-rich, enstatite achondrite-related meteorites. In particular, metal in Mount Egerton and in the anomalous iron meteorite Horse Creek (as well as the anomalous irons LEW 85369, LEW 88055, and LEW 88631) has been described as being compositionally similar (i.e., having complementary HSE patterns in metal) to metal in the anomalous enstatite achondrite NWA 2526, which like Itqiy is a partial melt residue after ~20% partial melt extraction (Keil and Bischoff, 2008; Humayun et al., 2009; M. Humayun, 2010). These meteorites might have a common origin on an enstatite parent body unique from the Shallowater, EH, EL, and main-group aubrite parent bodies (Keil and Bischoff, 2008). Continued studies of these meteorites could help resolve potential genetic links among them.
Based on mineralogical and petrological analyses, along with some assumptions about the formation conditions for the MS-MU-019/036 restite, including a coremantle boundary location under ~0.1 GPa pressure on a parent body composed of 20 wt% metallic iron, Harries and Bischoff (2020) estimated that the minimum diameter of the meteorite parent body was ~500 km. Furthermore, their data indicate that as temperatures reached ~1260°C, this enstatite chondrite body experienced a catastrophic disruption accompanied by rapid cooling at a rate larger than 1 K/hr. This thermal history is similar to that which is proposed to have occurred on the Shallowater parent body, and may have been a common scenario during the period of terrestrial body accretion in the inner Solar System.
Exclusive of the primary ureilite components, there was a broad diversity of lithologic types present in 2008 TC3, which constituted <30% of all material recovered. However, since 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 primarily of fine-grained, highly-porous, weakly-consolidated ureilitic matrix material consistent with the reflectance spectra obtained for the asteroid (Goodrich et al., 2015, 2019). 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 (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)
Thanks to Stephan Decker's Meteorite Shop and Museum for providing specimens of this special meteorite and many of its xenolithic inclusions to the scientific and collector communities. The photo of MS-MU-036 shown above is a 0.24 g partial slice. The photo below highlights the high metal content in this enstatite achondrite, exhibiting a textural similarity to the EH7-an Itqiy (compare to Itqiy photo here).