Spectral evidence for amorphous silicates in least-processed CO meteorites and their parent bodies

Abstract

Least-processed carbonaceous chondrites (carbonaceous chondrites that have experienced minimal aqueous alteration and thermal metamorphism) are characterized by their predominately amorphous iron-rich silicate interchondrule matrices and chondrule rims. This material is highly susceptible to destruction by the parent body processes of thermal metamorphism or aqueous alteration. The presence of abundant amorphous material in a meteorite indicates that the parent body, or at least a region of the parent body, experienced minimal processing since the time of accretion. The CO chemical group of carbonaceous chondrites has a significant number of these least-processed samples. We present visible/nearinfrared and mid-infrared spectra of eight least-processed CO meteorites (petrologic type 3.0-3.1). In the visible/near-infrared, these COs are characterized by a broad weak feature that was first observed by Cloutis et al. (2012) to be at 1.3-µm and attributed to iron-rich amorphous silicate matrix materials. This feature is observed to be centered at 1.4-µm for terrestrially unweathered, least-processed CO meteorites. At mid-infrared wavelengths, a 21-µm feature, consistent with Si-O vibrations of amorphous materials and glasses, is also present. The spectral 2 features of iron-rich amorphous silicate matrix are absent in both the near-and mid-infrared spectra of higher metamorphic grade COs because this material has recrystallized as crystalline olivine. Furthermore, spectra of least-processed primitive meteorites from other chemical groups (CRs, MET 00426 and QUE 99177, and C2-ungrouped Acfer 094), also exhibit a 21-µm feature.Thus, we conclude that the 1.4-and 21-µm features are characteristic of primitive least-processed meteorites from all chemical groups of carbonaceous chondrites. Finally, we present an IRTF+SPeX observation of asteroid (93) Minerva that has spectral similarities in the visible/near-infrared to the least-processed CO carbonaceous chondrites. While Minerva is not the only CO-like asteroid (e.g., Burbine et al., 2001), Minerva is likely the least-processed COlike asteroid observed to date.

Miller Range 07687 and its place within the CM‐CO clan

“…This spectrum is similar to the one previously published by McAdam et al. (2018). Poorly defined aliphatic bands are also detectable at 3.4 and 3.5 μm (Fig.…”

“…The smooth and broad nature of the band is also indicative of silicates of low crystallographic order (Fu et al., 2017), hence consistent with amorphous phases detected by Vaccaro (2017) and McAdam et al. (2018).…”

“…McAdam et al. (2018) also suggested that the red NIR slope observed in MIL 07687 was due to the influence of terrestrial weathering products; however, Eschrig et al. (2020) found that weathered COs tend to have bluer NIR slopes (with poorly weathered COs and falls being red‐sloped).…”

“…In addition to its peculiar petrography, past research suggests MIL 07687 to be a highly primitive meteorite, notably by its apparently high concentration of amorphous material, 15 N‐enrichment, and elevated abundance of presolar grains (Haenecour et al., 2020; McAdam et al., 2018; Vaccaro, 2017). Indeed, amorphous silicates tend to be abundant in primitive chondrite matrices (Abreu and Brearley, 2010; Brearley, 1993; Dobrică and Brearley, 2020), and high bulk 15 N content probably reflects material of strong interstellar heritage (Nakamura‐Messenger et al., 2006).…”

NORTHWEST AFRICA (NWA) 12563 and ungrouped C2 chondrites: Alteration styles and relationships to asteroids

“…From Acfer 094 (Bischoff and Geiger, 1994;Newton et al, 1995;Greshake, 1997), Asuka 12169 (M. Kimura et al, 2019;Noguchi et al, 2020), DOM 08006 (Alexander et al, 2018;Nittler et al, 2018;Davidson et al, 2019) and MET 00426 and QUE 99177 (Abreu and Brearley, 2010;Harju et al, 2014;McAdam et al, 2018), among the most pristine carbonaceous chondrites, we can infer that the main primary phases in chondrules and matrix in 3.0 carbonaceous chondrites are olivine, pyroxene, glass, kamacite, sulfide (including nanosulfide <100 nm) and amorphous silicate. The transformation of these phases, along with ice and organic material, has led to different suites of secondary phases in C2 chondrites.…”