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

Margaret M. McAdam, Jessica M. Sunshine, Kieren T. Howard, Conel M. Alexander, Timothy J. McCoy, Schelte J. Bus

Icarus, Volume 306, 15 May 2018, Pages 32-49


Update (6 Feb 2018): PDF (OPEN ACCESS)

• Least-processed meteorites have experienced minimal parent body processing (including thermal metamorphism and aqueous alteration). They are characterized by up to 30 vol.% amorphous matrix silicates.
• We present near and mid-infrared spectroscopy of 8 least-processed CO meteorites, two CR3 meteorites and one ungrouped least-processed meteorite.
• Least processed meteorites from all chemical groups exhibit 1.4-µm and 21-µm features produced by the abundant amorphous silicates present in these meteorites.
• This is the first mid-infrared detection of amorphous material in the spectra of least-processed meteorites.
• One asteroid, (93) Minerva, appears to have a similar near-infrared spectrum as the least-processed CO meteorites, indicating Minerva has a large abundance of amorphous materials on its surface. Minerva, therefore, preserves these primitive materials and can be considered as the least-processed, most primitive asteroids observed.”

“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/near-infrared 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 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 CO-like asteroid observed to date.”