Closed System Heating of Murchison and “Fake” Murchison CM2 Carbonaceous Chondrite: Spectral Refletance Properties

English Abstract:

A number of carbonaceous chondrites exist which underwent aqueous alteration followed by thermal metamorphism (termed ATCCs). They exhibit diverse spectral reflectance properties as well as different levels of aqueous alteration and thermal metamorphism. To better understand how the parent bodies of these meteorites can be recognized from spectroscopic data, and to relate spectral properties to changes in mineralogy, we subjected a powdered sample of the Murchison CM2 carbonaceous chondrite and a “fake” powdered Murchison (85% serpentine + 5% nanophase magnetite + 5% troilite + 5% shungite) to closed system heating under both dry and wet (1:1 water:rock ratio) using pressure vessels with the samples packed in air and heated to 200°C for 111 days. The samples were characterized before and after heating by reflectance spectroscopy (0.35-2.5 µm) and X-ray diffractometry. The dry run Murchison sample showed a small (<1.5% absolute) decrease in reflectance and loss of Fe²⁺ and Fe²⁺-Fe³⁺ serpentine-associated absorption bands. The wet run Murchison sample showed major spectral differences between the top crust of the sample and the lower part. The top crust spectrum was brighter than the starting material and resulting absorption bands are fully consistent with ferric oxyhydroxides, with no evidence of serpentine bands. The lower part of the sample was darker than the starting material with a shallow Fe³⁺-associated absorption “edge” near 0.6 µm and no serpentine absorption bands. The “fake” Murchison sample showed similar spectral changes. These results suggest that even moderate levels of thermal metamorphism can result in significant spectral changes, resulting in a loss of diagnostic Fe phyllosilicate absorption bands, likely accompanied by production of ferric oxyhydroxides in spectrally significant amounts.