Distinguishing relative aqueous alteration and heating among CM chondrites with IR spectroscopy

Abstract

Using infrared (IR) spectroscopy of thin sections, we characterize the relative degree of aqueous alteration and subsequent heating of a suite of CM chondrites to document spectral indicators of these processes that can contextualize observations of carbonaceous asteroids. We find that the progressive aqueous alteration of CMs manifests in two spectral regions. The low-wavenumber region (1200–400 cm−1; 8–25 μm) records the increasing proportion of Mg-Fe phyllosilicates relative to anhydrous silicates as aqueous alteration proceeds, with a highly correlated shift of the Christiansen feature (CF) to lower wavenumber and the Si-O bending band minimum to higher wavenumber, and an increase in depth of the Mg-OH band (~625 cm−1). The strongest correlation (R2 = 0.90) with petrologic subtype is the distance between the CF and Si-O stretching band minimum, which predicts the petrologic subtype of the sample to within 0.1. The high-wavenumber region (4000–2500 cm−1, ≤3.33 μm) probes the variation in abundance and composition of Mg-Fe serpentine and tochilinite among the altered CMs. All moderately to highly altered CMs (≤2.3) have an OH/H2O (‘3 μm’) band emission maximum of 3690 cm−1 (2.71 μm) indicative of Mg-bearing serpentine, and mildly aqueously altered CMs (≥ 2.5) have a wider band with a complex shape that results from contributions of Fe-bearing serpentine and tochilinite. Among weakly heated CMs (Stage II; 300–500 °C), the low-wavenumber region exhibits spectral features resulting from the dehydration and dehydroxylation of phyllosilicates that include broadening of the Si-O stretching band and a shift of its minimum to lower wavenumber, and the disappearance of the Mg-OH band. The location of the Si-O bending band minimum appears to be unaffected by mild heating. Extensively heated CMs (Stage III+; > 500 °C) have a low-wavenumber region dominated by the spectral features of secondary, Fe-bearing olivine and low-Ca pyroxene and thus are readily distinguished from unheated and mildly heated CMs. The OH/H2O band of all heated CMs is broad and rounded with an emission peak at lower wavenumbers (≤3636 cm−1; ≥2.75 μm) than in unheated CMs. However, spectral and petrographic evidence suggests that our heated CMs have been compromised by terrestrial rehydration. Our study confirms that thermal metamorphism effects are concentrated within the matrix and suggests that the matrix of the CM WIS 91600 had a CI-like mineralogy prior to heating.