Spectral reflectance properties of carbonaceous chondrites 4: Aqueously altered and thermally metamorphosed meteorites

Abstract

We examined the spectral reflectance properties of 26 carbonaceous chondrites (CCs) that show evidence of aqueous alteration and subsequent thermal metamorphism (termed ATCCs). We also reviewed the thermal and aqueous alteration history of these meteorites and searched for trends between spectral parameters and temperature histories in order to uncover spectral–compositional relationships. Aqueous alteration results in the production of phyllosilicates from anhydrous silicate precursors – largely serpentine group phyllosilicates, and increasing amounts of saponite group phyllosilicates with increasing aqueous alteration. Thermal metamorphism results in dehydration of these phyllosilicates and production of abundant amorphous material except at the highest temperatures (≳900°C), as well as alteration of carbonaceous components. ATCCs are a spectrally diverse group in almost all respects. Spectral slopes, as measured by the ratio of reflectance at 2.4μm to the local peak or inflection in the 0.5–0.8μm region and 2.4/1.5μm ratios range from 0.78 to 1.48, and 0.93 to 1.24, respectively (blue-sloped spectra have ratio values of <1). ATCC powder spectra (<75, <100, or <125μm) are generally dark, with maximum reflectance at the local peak or inflection in the 0.5–0.8μm region, or maximum reflectance at any wavelength ranging from 2.6% to 8.9%, and 3.5% to 10.3%, respectively. All ATCC spectra exhibit an absorption feature in the ∼0.8–1.3μm region, with band depths ranging from ∼1% to 8%. This feature is diverse in terms of number of apparent absorption bands. The presence of mixed valence Fe2+–Fe3+ phyllosilicates, as evidenced by an absorption band near 0.7μm with a depth of up to 5%, and Mg-bearing phyllosilicates, as evidenced by an MgOH combination band in the 2.3–2.4μm region, are seen in many of the least thermally metamorphosed ATCC spectra. The depth of the 0.7μm band generally decreases with increasing temperature. Olivine-associated absorption bands in the 0.8–1.3μm region seem to be more prevalent in the more metamorphosed ATCC spectra. However clearly-resolvable olivine absorption bands are not present in ATCC spectra, suggesting that thermal metamorphism did not lead to the production of widespread crystalline Fe2+-bearing olivine. The reddest ATCC powder spectra are generally the darkest, and C content is correlated with decreasing overall reflectance and weakly correlated with spectral slope. When the degree of thermal metamorphism was compared to various spectral measures of slope, band depth, and overall reflectance, no strong correlations emerged. However, it does appear that the most thermally metamorphosed ATCCs have generally flatter spectral slopes. ATCC chip spectra are brighter and less red-sloped than powder spectra, but band depths are generally comparable. Laboratory-heated CIs and CMs generally exhibit the same types of spectral changes seen in naturally thermally metamorphosed ATCCs. For laboratory-heated CM and CI chondrites, and ATCCs for which temperature estimates are available, reflectance generally decreases with increasing temperature to ∼500°C, and then increases to higher temperatures. Silicate absorption band depths are generally least for temperatures of ∼600–800°C. Below this temperature interval, ATCC spectra show more phyllosilicate-like absorption bands. ATCC spectra generally become flatter with increasing temperature above ∼400°C. Temperatures in excess of those experienced by the ATCCs (∼900°C) are required for the appearance of well-resolved olivine absorption bands.