Spectral reflectance properties of carbonaceous chondrites – 5: CO chondrites

Abstract

We examined the spectral reflectance properties of 16 CO-type carbonaceous chondrites (CCs) in order to better understand their range of spectral properties, develop spectral–compositional correlations, and provide information that may aid in the search for CO parent bodies. As a group, our CO powder spectra have some similarities and differences. COs have experienced varying degree of thermal metamorphism, with petrologic subgrades ranging from ∼CO3.0 to ∼CO3.8. Their reflectance spectra are characterized by a ubiquitous absorption feature in the 1μm region, and a nearly ubiquitous feature in the 2μm region that appears in CO >3.1 spectra. The 1μm region feature is attributable to abundant Fe-bearing amorphous phases (and Fe-poor olivine) in the lower petrologic subtypes, which gradually transforms to more abundant and Fe-rich olivine with increasing metamorphism. The increase in depth and decrease in wavelength position of this feature are consistent with this transformation.All but the least-altered COs also exhibit an absorption feature in the 2μm region whose depth also generally increases with increasing metamorphic grade, resulting in increasingly blue-sloped spectra and larger band area ratios. The wavelength position and change in depth of this feature (ranging from 0% to 12.2%) is consistent with increasing Fe2+ in spinel, which is present in calcium–aluminum and ameboid olivine inclusions. Reflectance of a local reflectance maximum near 0.8μm increases with increasing thermal metamorphism and this is likely due to the loss and aggregation of carbonaceous phases. The increasing reflectance is negatively correlated with various measures of spectral slope (i.e., brighter=bluer), and while this cannot be uniquely attributed to any one cause, it is consistent with increasing spinel Fe2+ content and decreasing carbonaceous material abundance or aggregation. With decreasing grain size, CO spectra normally become brighter and more red-sloped. The 0.6/0.5μm ratios of CO falls are consistently higher than CO finds, suggesting that terrestrial weathering has affected the visible wavelength region spectral properties of finds.Unmetamorphosed CO spectra may be difficult to distinguish from the least altered CM chondrites. However above petrologic grade ∼3.1, COs can be uniquely discriminated from CI, CM, metamorphosed CI and CM, and CR chondrites, by the presence of both olivine and spinel absorption bands. Some K-class asteroids exhibit olivine and spinel absorption bands, consistent with CO chondrites, although modeled olivine:spinel ratios are generally lower in these asteroids than in CO chondrites.