Asteroid 2008 TC3 (approximately 4 m diameter) was tracked and studied in space for approximately 19 h before it impacted Earth’s atmosphere, shattering at 44–36 km altitude. The recovered samples (>680 individual rocks) comprise the meteorite Almahata Sitta (AhS). Approximately 50–70% of these are ureilites (ultramafic achondrites). The rest are chondrites, mainly enstatite, ordinary, and Rumuruti types. The goal of this work is to understand how fragments of so many different types of parent bodies became mixed in the same asteroid. Almahata Sitta has been classified as a polymict ureilite with an anomalously high component of foreign clasts. However, we calculate that the mass of fallen material was ≤0.1% of the pre-atmospheric mass of the asteroid. Based on published data for the reflectance spectrum of the asteroid and laboratory spectra of the samples, we infer that the lost material was mostly ureilitic. Therefore, 2008 TC3 probably contained only a few percent nonureilitic materials, similar to other polymict ureilites except less well consolidated. From available data for the AhS meteorite fragments, we conclude that 2008 TC3 samples essentially the same range of types of ureilitic and nonureilitic materials as other polymict ureilites. We therefore suggest that the immediate parent of 2008 TC3 was the immediate parent of all ureilitic material sampled on Earth. We trace critical stages in the evolution of that material through solar system history. Based on various types of new modeling and re-evaluation of published data, we propose the following scenario. (1) The ureilite parent body (UPB) accreted 0.5–0.6 Ma after formation of calcium-aluminum-rich inclusions (CAI), beyond the ice line (outer asteroid belt). Differentiation began approximately 1 Ma after CAI. (2) The UPB was catastrophically disrupted by a major impact approximately 5 Ma after CAI, with selective subsets of the fragments reassembling into daughter bodies. (3) Either the UPB (before breakup), or one of its daughters (after breakup), migrated to the inner belt due to scattering by massive embryos. (4) One daughter (after forming in or migrating to the inner belt) became the parent of 2008 TC3. It developed a regolith, mostly ≥3.8 Ga ago. Clasts of enstatite, ordinary, and Rumuruti-type chondrites were implanted by low-velocity collisions. (5) Recently, the daughter was disrupted. Fragments were injected or drifted into Earth-crossing orbits. 2008 TC3 comes from outer layers of regolith, other polymict ureilites from deeper regolith, and main group ureilites from the interior of this body. In contrast to other models that have been proposed, this model invokes a stochastic history to explain the unique diversity of foreign materials in 2008 TC3 and other polymict ureilites.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/maps.12401

Origin and history of ureilitic material in the solar system:the view from asteroid 2008 TC3 and the Almahata Sitta meteorite

https://www.boulder.swri.edu/~bottke/Reprints/Reddy_2014_Icarus_237_116_Chelyabinsk_meteorite_spectra.pdf

Chelyabinsk meteorite explains unusual spectral properties of Baptistina Asteroid Family

Brecciation among 2280 ordinary chondrites – Constraints on the evolution of their parent bodies

Meteoritic breccias are valuable samples as they can contain rare materials from the early solar system as clasts. Volatile‐rich, CI‐ and CM‐like clasts may represent parent body lithologies, which cannot be found as individual meteorites in today's meteorite collections. In order to reveal a better knowledge about the presence and chemical characteristics and variability of volatile (water‐bearing) materials in the early solar system these clasts play an important role. Such materials may have been available as the volatile component during the accretion of terrestrial planets. To understand the distribution of volatile‐rich materials in the solar system, we studied CI‐ and CM‐like clasts in brecciated meteorites including polymict ureilites, HEDs, CR, CB, CH, and ordinary chondrites. CI‐like clasts occur throughout all of the mentioned meteorite groups, whereas the CM‐like clasts have only been identified in HEDs and ordinary chondrites. The abundance of volatile‐rich clasts in general decreases in the order CH > CR > ureilites > HEDs > CB > OC > R. The mineralogy of CI‐like clasts is similar to CI chondrites, but their compositions of phyllosilicates differ. The mineralogy of CM‐like clasts clearly links them to CM chondrites. They must have been delivered to the HED parent body by low‐velocity impacts after differentiation and volcanism, as there is no evidence for high shock and heating processes. Additionally, we propose that CI‐like clasts in the CR, CB, and CH chondrites are a primary component of the appropriate parent bodies (accretionary breccias). Conversely, the CI‐like clasts in polymict ureilites and HEDs represent an infall as (micro)meteorites or as low‐velocity impactors, which happened after the accretion and differentiation of the appropriate parent bodies.

Mineralogy of volatile‐rich clasts in brecciated meteorites

The brecciation and shock classification of 2280 ordinary chondrites of the meteorite thin section collection at the Institut für Planetologie (Münster) has been determined. The shock degree of S3 is the most abundant shock stage for the H and LL chondrites (44% and 41%, respectively), while the L chondrites are on average more heavily shocked having more than 40% of rocks of shock stage S4. Among the H and LL chondrites, 40–50% are “unshocked” or “very weakly shocked.” Considering the petrologic types, in general, the shock degree is increasing with petrologic type. This is the case for all meteorite groups. The main criteria to define a rock as an S6 chondrite are the solid‐state recrystallization and staining of olivine and the melting of plagioclase often accompanied by the formation of high‐pressure phases like ringwoodite. These characteristics are typically restricted to local regions of a bulk chondrite in or near melt zones. In the past, the identification of high‐pressure minerals (e.g., ringwoodite) was often taken as an automatic and practical criterion for a S6 classification during chondrite bulk rock studies. The shock stage classification of many significantly shocked chondrites (>S3) revealed that most ringwoodite‐bearing rocks still contain more than 25% plagioclase (74%). Thus, these bulk chondrites do not even fulfill the S5 criterion (e.g., 75% of plagioclase has to be transformed into maskelynite) and have to be classified as S4. Studying chondrites on typically large thin sections (several cm2) and/or using samples from different areas of the meteorites, bulk chondrites of shock stage S6 should be extremely rare. In this respect, the paper will discuss the probability of the existence of bulk rocks of S6.

Shock stage distribution of 2280 ordinary chondrites—Can bulk chondrites with a shock stage of S6 exist as individual rocks?

Chelyabinsk — Not only Another Ordinary LL5 Chondrite, but a Spectacular Chondrite Breccia

On October 7, 2008, the asteroid 2008 TC3 exploded as it entered the Earth’s atmosphere, producing significant dust (in the atmosphere) and delivering thousands of stones in a strewn field in Sudan, collectively known as the Almahata Sitta (AhS) stones. About 600 fragments were officially recovered in 2008 and 2009. Further rocks were collected since the fall event by local people. From these stones, 249 were classified at the Institut für Planetologie in Münster (MS) known as MS‐xxx or MS‐MU‐xxx AhS subsamples. Most of these rocks are ureilitic in origin (168; 67%): 87 coarse‐grained ureilites, 60 fine‐grained ureilites, 15 ureilites with variable texture/mineralogy, four trachyandesites, and two polymict breccias. We identified 81 non‐ureilitic fragments, corresponding to 33% of the recovered samples studied in Münster. These included chondrites, namely 65 enstatite chondrites (43 EL; 22 EH), 11 ordinary chondrites (OC), one carbonaceous chondrite, and one unique R‐like chondrite. Furthermore, three samples represent a unique type of enstatite achondrite. Since all AhS stones must be regarded as individual specimens independent from each other, the number of fresh ureilite and enstatite chondrite falls in our meteorite collections has been increased by several hundred percent. Overall, the samples weigh between <1 and 250 g and have a mean mass of ~15 g. If we consider—almost 15 years after the fall—the mass calculations, observations during and after the asteroid entered the atmosphere, the mineralogy of the C1 stones AhS 91A and AhS 671, and the experimental work on fitting the asteroid spectrum (e.g., Goodrich et al., 2019; Jenniskens et al., 2010; Shaddad et al., 2010), the main portion of the meteoroid was likely made of the fine‐grained (carbonaceous) dust and was mostly lost in the atmosphere. In particular, the fact that C1 materials were found has important implications for interpreting asteroid 2008 TC3's early spectroscopic results. Goodrich et al. (2019) correctly suggested that if scientists had not recovered the “water‐free” samples (e.g., ureilites, enstatites, and OC) from the AhS strewn field, 2008 TC3 would have been assumed to be a carbonaceous chondrite meteoroid. Considering that the dominating mass of the exploding meteoroid consisted of carbonaceous materials, asteroid 2008 TC3 cannot be classified as a polymict ureilite; consequently, we state that the asteroid was a polymict carbonaceous chondrite breccia, specifically a polymict C1 object that may have formed by late accretion at least 50–100 Ma after calcium–aluminum‐rich inclusions.

/pdf/asteroid-2008-tc3-not-a-polymict-ureilitic-but-a-polymict-c1-79imb28z.pdf

Asteroid 2008 TC3, not a polymict ureilitic but a polymict C1 chondrite parent body? Survey of 249 Almahata Sitta fragments

Introduction: After asteroid 2008 TC3 impacted Earth in 2008, a highly diverse range of meteorite types was identified among the numerous meteorite fragments collected in the Almahata Sitta strewn field, including various types of ureilitic rocks and chondrites (e.g. [1-10]). Here, mineralogical characteristics of 18 new samples (MS-MU021 – MS-MU-038) are presented, complemented with the oxygen isotope signature of MS-MU-021.. Results: The samples represent (like earlier studied fragments) one single lithology weighing between 7.4 (MSMU-028) and 221.5 g (MS-MU-034). Together with MS-MU-034 some other large samples of the strewn field are characterized: MS-MU-036 (177.1 g), -037 (149.2 g), and -038 (148.2 g). Among the samples are 5 E chondrites, 12 samples of ureilitic origin (including a trachyandesite (MS-MU-035)), and an enstatiteand metal-rich achondrite. E-chondrites: Based on the low Si-concentration in metal (0.8-1.3 wt%) all five enstatite chondrites are from the EL-subgroup (2 EL3, 1 EL4, 2 EL6 (MS-MU-024, -026)). The EL3 chondrites (MS-MU-023, -031) can be characterized by their well-preserved chondritic texture and the occurrence of olivine. Especially in MS-MU-031 abundant Fa-poor olivine was found within the porphyritic chondrules. Olivine is lacking in MS-MU-029 (EL4). Ureilites: MS-MU-022, -034, -037, and -038 are coarse-grained ureilites. Within MS-MU-022 olivine has cores of Fa11-13, whereas the Fa-contents of olivine cores from MS-MU-034 and -038 are much higher and very similar (23-24 mol%). Olivine within MS-MU-034 contains abundant tiny Cr-rich inclusions (exsolved chromites; <<1 μm). MS-MU-037 has abundant pyroxene (~Fs18Wo5) and broad reduction rims around the olivine (Fa19-21). Two ureilitic fragments have complex textures with highly variable grain sizes (MS-MU-021 (Fig. 1), -028). The composition of the olivine cores in MS-MU-028 is Fa15-17, whereas Fa-contents of up to 26 mol% were analyzed in olivines from MS-MU-021. In this sample a small number of large olivine grains are embedded in a fine-grained groundmass (Fig. 1). The bulk sample has δO = 3.98‰ and δO = 8.29‰, which is consistent with an ureilitic origin. MS-MU-025, -027, -030, -032, and -033 are heavily-shocked, fine-grained ureilites. MS-MU-027 shows a high abundance of C-phase(s) and typical olivine has very low Fa contents (<3 mol%). The samples MS-MU-030 and MS-MU-032 contain abundant metal and sulfide and their core olivines have about 10 and 9-12 mol% Fa, respectively. MS-MU-033 is a typical fine-grained ureilite with olivine cores of Fa19-21. Ureilitic trachyandesite: The thin section inspection revealed that this sample (MS-MU-035) is similar to MSMU-011 (ALM-A [9]) having abundant (~65 vol%) subhedral, zoned anorthoclase and/or plagioclase (typically ~An9-25) embedding Cr-bearing Ca-pyroxene (~Fs20Wo38) and Ca-poor pyroxene (~Fs35.5Wo8) (Fig. 2). Enstatiteand metal-rich achondrite: MS-MU-036 is an unusual enstatiteand metal-rich achondrite with three coexisting pyroxenes (~En98.5Wo1.3, ~En96.5Wo3.2, and~En60Wo40). The silicates are embedded within a Si-bearing metal matrix (Fig. 3). Additional phases include oldhamite, alabandite, and daubréelite. Based on texture and mineralogy, it might represent a unique type of meteorite, however, similarities exist to MS-MU-019 and Itqiy [11].

Almahata Sitta News: Well-Known Varieties and New Species in the Zoo

MS‐MU‐012, a 15.5 g clast from the Almahata Sitta polymict ureilite, is the first known plagioclase‐bearing main group ureilite. It is a coarse‐grained (up to 4 mm), equilibrated assemblage of 52% olivine (Fo 88), 13% orthopyroxene (Mg# 89.2, Wo 4.5), 11% augite (Mg# 90.2, Wo 37.3), and 14% plagioclase (An 68), plus minor metal and sulfide. The plagioclase grains have been secondarily remelted and internally recrystallized, but retain primary external morphologies. Melt inclusions occur in olivine. Rounded chadocrysts of olivine and orthopyroxene are enclosed in augite grains. In terms of texture, mineralogy, major and minor element mineral compositions, and oxygen isotopes, MS‐MU‐012 is virtually identical to the archetypal Hughes‐type main group ureilites, with the significant addition of primary plagioclase. We conclude that MS‐MU‐012 formed as a cumulate in a common lithologic unit with the Hughes‐type ureilites. Based on reconstructed compositions of melts trapped in olivine, orthopyroxene, and augite in the Hughes‐type samples, we infer that the parent magma of the Hughes unit originated as a late melt in the incremental melting of the ureilite parent body (UPB), near the end of the melting sequence, but was not completely extracted from the mantle like earlier melts and was emplaced in an intrusive body. MELTS calculations indicate that olivine began to crystallize at ~1260 °C, followed shortly thereafter by co‐crystallization of orthopyroxene and augite. Plagioclase began to crystallize at ~1170–1180 °C. Graphite was buoyant in the melt and became heterogeneously distributed in flotation cumulates. Residual silicate liquid was extracted from the cumulate pile and could have crystallized to form the “labradoritic melt lithology” (with plagioclase of An ~68‐35), which is partially preserved as clasts in polymict ureilites. The final equilibration temperature recorded by the Hughes unit was ~1140–1170 °C, just before catastrophic disruption of the UPB. MS‐MU‐012 provides a critical missing link in the differentiation history of this asteroid.

The first main group ureilite with primary plagioclase: A missing link in the differentiation of the ureilite parent body

STREWN FIELD: OLD FRIENDS AND BRAND-NEW FELLOWS. A. Bischoff, S. Ebert, M. Patzek, M. Horstmann, A. Pack, J.-A. Barrat, and S. Decker. Institut fur Planetologie, Wilhelm-Klemm-Str. 10, 48149 Munster, Germany. E-mail: bischoa@uni-muenster.de. Uni Gottingen, Geowissenschaftliches Zentrum, Goldschmidtstr. 1, 37077 Gottingen, Germany. Universite de Bretagne Occidentale, 29280 Plouzane, France. Meteorite-Museum, Oberstr. 10a, 55430 Oberwesel, Germany.

/pdf/new-individuals-from-the-almahata-sitta-strewn-field-old-4z7972hpwv.pdf

New Individuals from the Almahata Sitta Strewn Field: Old Friends and Brand-New Fellows

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