Spectral reflectance properties of carbonaceous chondrites: 8. “Other” carbonaceous chondrites: CH, ungrouped, polymict, xenolithic inclusions, and R chondrites

Abstract

We have analyzed reflectance spectra (0.3–2.5μm) of a number of ungrouped or tentatively grouped carbonaceous chondrites (CCs), possible CC-type xenoliths in an aubrite (Cumberland Falls) and a howardite (PRA 04401), a CH chondrite (PCA 91467), a CC polymict breccia (Kaidun), and some R chondrites. The best approach to analysis relies largely on characterizing spectrally active phases – i.e., those phases that contribute diagnostic absorption features, involving absorption band wavelength position, band depth, shape of absorption features, combined with albedo and spectral slope. Mafic silicate (hydrous and/or anhydrous) absorption features are ubiquitous in the CCs and R chondrites we have examined. Combining information on these features along with albedo and spectral slopes allows reasonable inferences to be made concerning their uniqueness. Reflectance spectra of Coolidge show contributions from both olivine and Fe oxyhydroxides (from terrestrial weathering), and its high reflectance and mafic silicate band depths are consistent with a petrologic grade >3 and inconsistent with CVs. The CC nature of the Cumberland Falls inclusions from spectral analysis is inconclusive, but they do exhibit spectral features consistent with their overall mineralogy. DaG 430, which has petrologic characteristics of both CV and CK chondrites, has a spectrum that is not fully consistent with either group. The spectrum of EET 96029 is consistent with some, but not all CM2 chondrites. GRO 95566, a meteorite with some affinities to CM2s, most resembles the Renazzo CR2 chondrite, consistent with their similar mineralogies, and its spectral properties can be related to its major mineralogic characteristics. Spectra of Kaidun are most consistent with CR chondrites, which form the bulk of this meteorite. The reflectance spectrum of MCY 92005 is consistent with its recent classification as a CM2 chondrite. The R3 chondrite MET 01149 shares many characteristics with CKs, but differs in terms of its slightly red slope and 2μm region absorption feature. The combination of high reflectance, deep 1μm band and, to a lesser extent, slightly red slope and weak 2μm region absorption band, distinguishes PRE 95404 from CV3s, to which it was initially assigned. The LAP 04840 R6 spectrum is dominated by olivine, consistent with a petrologic grade >3. Its reflectance is somewhat lower than for the R3 chondrites, and falls within the range of many CCs. Its most characteristic feature is the metal–OH absorption bands in the 2.3μm region. Analysis and assignment of PCA 91467 (CH3) is complicated by the presence of terrestrial weathering products. Its red spectral slope is consistent with its high metal content. Reflectance spectra of the howardite PRA 04401, which contains ∼40% CM2-like inclusions, is dominated by the howardite’s pyroxene absorption bands, and expected CM2-type absorption bands near 0.7 and 1.1μm are not seen. The CC xenoliths do reduce overall reflectance and pyroxene absorption band depths significantly, and probably add an overall red slope, when compared to inclusion-free howardites. QUE 99038, which has been linked to CM2, CO, or CR chondrites is not spectrally consistent with any of these groups. The 2μm band, high overall reflectance, and dominant olivine absorption band are all generally inconsistent with CM2 and CR2–3 chondrites. It resembles the CO3 chondrite ALH 77003 in the 1μm region, but differs in the 2μm region. The red-sloped, nearly featureless spectrum of Tagish Lake is unique among carbonaceous chondrites. It probably arises from the highly aromatic nature of the organic component and its intimate association with the phyllosilicate-rich matrix, which makes up a high proportion of this meteorite. Our results suggest that the meteorites included in this study can usually be determined to be either unique or to be placed with a reasonable degree of confidence into established CC groups. Our analysis has also provided insights into the degree to which spectral analysis can be used for characterization, the spectral features that can be used for characterization, and their limitations.