Ureilite
Trachyandesite
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 (W0–W0/1), 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). 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%.
Very small feldspathic mineral clasts previously found in polymict ureilites (regolith breccias) have been determined by mineralogy and oxygen isotopes to be indigenous to the ureilite body likely sampling the UPB crust. These clasts generally fall into two distinct populations: an albitic lithology (An~1–25) and a labradoritic lithology (An~35–60) (Goodrich and Wilson, 2014). Two individual feldspathic clasts (trachytic/syenitic) designated MS-MU-011 (aka ALM-A; 24.2 g) and MS-MU-035 (26.9 g) were recovered from the Almahata Sitta strewn field and have been the subjects of in-depth studies. Both clasts are from the more common albitic lithology and consist of 70 vol% oligoclase (An10–30, ave. An15), 20 vol% augite, and 5 vol% pigeonite, along with trace Ni-poor metal and K-rich glass (Collinet and Grove, 2020). These feldspathic clasts could represent a low-degree melt (10–15 wt%) of an alkali-undepleted ([Na+K]/[Ca+Na+K] = 0.5) chondritic composition (Collinet and Grove, 2020). This early buoyant melt phase was the first to migrate from the shallowest source region to the surface through veins and dykes.
Results of a trace element study of a suite of unbrecciated ureilites led Barrat et al. (2016) to propose a revised melting history for the UPB. They concluded that following the segregation of a S-rich metallic core, ureilite precursor material experienced two successive steps of melting and melt extraction. The initial extracted melts were low-density feldspathic lavas containing high abundances of Al, alkalis, and silica, comparable to some clasts identified in polymict ureilites and to the two trachyandesitic lithologies MS-MU-011 and MS-MU-035. A presumed scenario was presented in which this relatively buoyant silicic liquid was extracted, leaving behind an olivine and pigeonite residue, and ultimately reached the surface to form a crust of some extent. A representative of the next sequential melt to be extracted has not yet been identified among known meteorites, but it is inferred to have been an Al-poor, alkali-depleted liquid possibly more dense than the resulting ureilitic residues. Consequently, this melt material may have been located deeper in the mantle of the UPB where sampling would be a less likely event. Regardless of the actual density of this second-stage melt, the model scenario infers a partial melting degree of ~17–28% that was sustained throughout the entire melting event, and therefore the mass of this second-stage melt would be proportionately less than that represented by the rare trachyandesitic lithologies.
The Al–Mg age determined for the ALM-A trachyandesite sample indicates that the disruption of the ureilite parent body occurred no earlier than ~6.5. m.y. after CAIs (Bischoff et al., 2014). An Ar–Ar isochron age of 4.556 (±0.015) b.y. was obtained for pyroxene in MS-MU-011 by Turrin et al. (2015), while a Pb–Pb isochron age of 4.5620 (±0.0034) b.y. was obtained by Amelin et al. (2015) based on an assumed 238U/235U ratio of 137.79. Whether this Pb–Pb age represents the slow crystallization of an extruded lava or the rapid quenching of an intrusive magma, it attests to a basaltic crustal formation at least by 5.3 (±3.4) m.y. after CAIs; this pre-dates the catastrophic breakup of the UPB (Barnes et al., 2019). Notably, the classification of the ungrouped achondrite NWA 6698 was revised in Vaci et al. (2021 #2378 [O-plot]). This meteorite is now recognized as a ureilite-related dioritic cumulus phase closely related to the extrusive, rapidly-cooled ALM-A/MS-MU-011/035 trachyandesite lithologies (see diagram below).
Total Alkali vs. Silica (TAS) Plot
click on diagram for a magnified view
Diagram credit: Vaci et al., 51st LPSC, #1697 (2020)
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 42–83 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):
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, 20.057 g, w/ tremolite amphibole (Fioretti et al., 2017 #1846 [photo]; Goodrich et al., 2019 #1551, 2020 #1223 [O plot]; Hamilton et al., 2020 #1122; Hamilton et al., 2021; Miller et al., 2021 #2360 [O–Cr plot]; Dodds et al., 2022 #2158)
C1+URE+OC+EH regolith breccia: AhS 91/91A and 671 (Goodrich et al., 2018 #1321 [photo], 2019 #1551, #1312, article, 2021 #1331, article); MS-266 (polymict breccia with C1 clast; Bischoff et al., 2022)
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 olivine–orthopyroxene–augite 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 Ar–Ar age of ~4.556 b.y. and Pb–Pb 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) "labradorite–opx" (plag cores = An50–53); 2) "oligoclase–augite" (plag cores = An30–35) (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-035 shown above is a 0.094 g (5 × 7 mm) crusted partial slice; the sample is courtesy of Stephan Decker. The top photo below is the complete mass of MS-MU-035 with a glassy fusion crust, while the bottom photos show gemmy olivines in the interior of the similar MS-MU-011. Excellent high-resolution photos of a 10.23 g specimen of AhS trachyandesite were presented by Vincent Haberer on the MPOD website for 29 July 2020.
MS-MU-011—trachyandesite clast with gemmy olivines
click on image for a magnified view
Photos above shown courtesy of Stephan Decker—Meteorite Shop and Museum
