¹R.D.Hanna,²V.E.Hamilton,³C.W.Haberle,^4,5A.J.King,⁶N.M.Abreu,^7,8J.M.Friedrich
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113760]
¹Jackson School of Geosciences, University of Texas at Austin, Austin, TX 78712, USA
²Department of Space Studies, Southwest Research Institute, Boulder, CO 80302, USA
³School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA
⁴School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
⁵Planetary Materials Group, Department of Earth Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, UK
⁶Earth Science Program, Pennsylvania State University, Du Bois, PA 15801, USA
⁷Department of Chemistry, Fordham University, Bronx, NY 10458, USA
⁸Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024, USA
Copyright Elsevier

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 MgFe phyllosilicates relative to anhydrous silicates as aqueous alteration proceeds, with a highly correlated shift of the Christiansen feature (CF) to lower wavenumber and the SiO 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 SiO 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 MgFe 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 SiO stretching band and a shift of its minimum to lower wavenumber, and the disappearance of the Mg-OH band. The location of the SiO 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 higher 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.

¹Jaques S. Schmidt,²Ruth Hinrichs
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13467]
¹Division of Geochemistry, PETROBRAS R&D Center, Rua Horácio Macedo 950, Rio de Janeiro, RJ, Brazil
²Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500, Porto Alegre, RS, Brazil
Published by arrangement with John Wiley & Sons

A Raman method to estimate the aromatization degree of carbonaceous matter (CM) was established, and calibrated by literature data respecting differences in acquisition procedures. The generated equations allowed a comparison of the G‐band width to vitrinite reflectance, and to additional structural organization parameter extracted from X‐ray powder diffraction. The relations between aromatization degree and peak temperatures from literature were then propagated to G‐band width parameter. The applicability of these geothermometers is demonstrated by the consistency of data obtained for carbonaceous chondrites with previous estimations. Since the CM aromatization is controlled by peak temperature and time span, it is possible to estimate the duration of chondrite’s heating events. The G‐band width relation to elemental ratio trends of organic matter in chondrites, interplanetary dust particles, glucose, and saccharose‐based semi‐coke favors synthesis by formose route. These findings provide new approaches for the future development of chemical kinetic models for organic matter of chondrites. A reliable chemical model may allow the external calibration of numerical models for accurately evaluating the peak temperatures and cooling durations of chondrites.