The first samples from Almahata Sitta showing contacts between ureilitic and chondritic lithologies: Implications for the structure and composition of asteroid 2008TC₃

Abstract

Almahata Sitta (AhS), an anomalous polymict ureilite, is the first meteorite observed to originate from a spectrally classified asteroid (2008 TC3). However, correlating properties of the meteorite with those of the asteroid is not straightforward because the AhS stones are diverse types. Of those studied prior to this work, 70–80% are ureilites (achondrites) and 20–30% are various types of chondrites. Asteroid 2008 TC3 was a heterogeneous breccia that disintegrated in the atmosphere, with its clasts landing on Earth as individual stones and most of its mass lost. We describe AhS 91A and AhS 671, which are the first AhS stones to show contacts between ureilitic and chondritic materials and provide direct information about the structure and composition of asteroid 2008 TC3. AhS 91A and AhS 671 are friable breccias, consisting of a C1 lithology that encloses rounded to angular clasts (<10 μm to 3 mm) of olivine, pyroxenes, plagioclase, graphite, and metal‐sulfide, as well as chondrules (~130–600 μm) and chondrule fragments. The C1 material consists of fine‐grained phyllosilicates (serpentine and saponite) and amorphous material, magnetite, breunnerite, dolomite, fayalitic olivine (Fo 28‐42), an unidentified Ca‐rich silicate phase, Fe,Ni sulfides, and minor Ca‐phosphate and ilmenite. It has similarities to CI1 but shows evidence of heterogeneous thermal metamorphism. Its bulk oxygen isotope composition (δ18O = 13.53‰, δ17O = 8.93‰) is unlike that of any known chondrite, but similar to compositions of several CC‐like clasts in typical polymict ureilites. Its Cr isotope composition is unlike that of any known meteorite. The enclosed clasts and chondrules do not belong to the C1 lithology. The olivine (Fo 75‐88), pyroxenes (pigeonite of Wo ~10 and orthopyroxene of Wo ~4.6), plagioclase, graphite, and some metal‐sulfide are ureilitic, based on mineral compositions, textures, and oxygen isotope compositions, and represent at least six distinct ureilitic lithologies. The chondrules are probably derived from type 3 OC and/or CC, based on mineral and oxygen isotope compositions. Some of the metal‐sulfide clasts are derived from EC. AhS 91A and AhS 671 are plausible representatives of the bulk of the asteroid that was lost. Reflectance spectra of AhS 91A are dark (reflectance ~0.04–0.05) and relatively featureless in VNIR, and have an ~2.7 μm absorption band due to OH− in phyllosilicates. Spectral modeling, using mixtures of laboratory VNIR reflectance spectra of AhS stones to fit the F‐type spectrum of the asteroid, suggests that 2008 TC3 consisted mainly of ureilitic and AhS 91A‐like materials, with as much as 40–70% of the latter, and <10% of OC, EC, and other meteorite types. The bulk density of AhS 91A (2.35 ± 0.05 g cm−3) is lower than bulk densities of other AhS stones, and closer to estimates for the asteroid (~1.7–2.2 g cm−3). Its porosity (36%) is near the low end of estimates for the asteroid (33–50%), suggesting significant macroporosity. The textures of AhS 91A and AhS 671 (finely comminuted clasts of d...

The polymict carbonaceous breccia Aguas Zarcas: A potential analog to samples being returned by the OSIRIS‐REx and Hayabusa2 missions

“…This composition is very similar to that of the phyllosilicates in Almahata Sitta 91A, a C1 chondrite lithology of very unusual isotopic composition (Goodrich et al. 2019). Analytical EPMA totals for the phyllosilicate are low (65–86 wt%), as expected for fully hydrated material.…”

Insights into the formation of silica‐rich achondrites from impact melts in Rumuruti‐type chondrites

“…In the decade since the first papers on GRA 06128/9, a handful of other >4.5 Gyr-old SiO 2 -rich achondrites have been discovered. These include the Almahata-Sitta clasts MS-MU-011 (ALM-A) and the paired sample MS-MU-035 that represent melts complementary to the ureilites (Bischoff et al 2014(Bischoff et al , 2016Goodrich et al 2019). ALM-A is the largest and most representative sample of albitic lithology of the feldspar-rich clasts recognized in ureilite polymict breccias, and substantiated the prior evidence for SiO 2 -rich magmatism on the ureilite parent body (Ikeda and Prinz 2001;Cohen et al 2004;Kita et al 2004;Goodrich et al 2007;Goodrich and Wilson 2014).…”

“…, ; Goodrich et al. ). ALM‐A is the largest and most representative sample of albitic lithology of the feldspar‐rich clasts recognized in ureilite polymict breccias, and substantiated the prior evidence for SiO 2 ‐rich magmatism on the ureilite parent body (Ikeda and Prinz ; Cohen et al.…”

Evidence for the presence of chondrule‐ and CAI‐derived material in an isotopically anomalous Antarctic micrometeorite

“…Second, the classical dichotomy (i.e., carbonaceous versus noncarbonaceous chondrites) observed in oxygen isotope space should be reconsidered as the oxygen isotope compositions of carbonaceous chondrites appear to be much more diverse (see also e.g., Goodrich et al. 2019; Kebukawa et al. 2019; King et al.…”

“…First, the diversity among solar system materials is much larger than our contemporary (micro)meteorite inventory suggests (see also, e.g., Yada et al 2005;Gounelle et al 2009;Suavet et al 2010;Cordier and Folco 2014;Van Ginneken et al 2017;Goderis et al 2020;Suttle et al 2020). Second, the classical dichotomy (i.e., carbonaceous versus noncarbonaceous chondrites) observed in oxygen isotope space should be reconsidered as the oxygen isotope compositions of carbonaceous chondrites appear to be much more diverse (see also e.g., Goodrich et al 2019;Kebukawa et al 2019;King et al 2019;Kerraouch et al 2020;Suttle et al 2020). Micrometeorite WF1202A-001 may thus possibly serve as an example for the (hidden) diversity present among the carbonaceous chondrite class.…”