Mid-infrared reflectance spectroscopy of carbonaceous chondrites and Calcium-Aluminum-rich inclusions

Kurzfassung

We obtained mid-infrared reflectance spectra of powdered size fractions (0–25 ​μm, 25–63 ​μm, 63–125 ​μm and 125–250 ​μm) of 5 carbonaceous chondrites HaH 280 (CK4), Allende (CV3), NWA 10574 (CM2) and Murchison (CM2), and C2 (ungrouped) Tagish Lake. In addition, in-situ micro-FTIR reflectance spectra of CAIs in Allende, Ornans and Vigarano have been made. These spectra are part of a database for the comparison with remote sensing data of asteroids and planetary bodies, but also for laboratory studies of meteorites. Carbonaceous chondrites HaH 280 (CK4), Allende (CV3) and NWA 10574 (CM2) show mid-IR spectra with Reststrahlen bands (RB) dominated by forsterite and pyroxene features. The transparency features (TF) are at 12.1 ​μm–12.9 ​μm and the Christiansen features (CF) at 8.5 ​μm–9.3 ​μm. Murchison (CM2) and ungrouped Tagish Lake (C2) show mainly phyllosilicate features in their spectra. The CF is between 8.6 ​μm and 9.0 ​μm. Micro-FTIR spectra of CAIs from Allende, Vigarano, and Ornans have spinel features between 14.1 ​μm and 14.3 ​μm. In addition, melilite was identified in the Vigarano and Ornans samples with characteristic features at 11.7 ​μm and 12.4 ​μm. Nepheline and sodalite features have been identified around 14 ​μm in the Allende CAI as alteration phases resulting from thermal metamorphism. A band shift in spinel with increasing Fe content from thermal metamorphism could be spectral indicator for parent body metamorphism. A comparison with other bulk samples from the same meteorites/type shows high similarity. Comparison of the finest size fractions modelled in emissivity with remote sensing data of asteroids shows some similarity between a linear mixture of Allende and Murchison and the spectra of asteroid 253 Mathilde. However, the main 10 ​μm plateau is clearly shifted by 0.5 ​μm between asteroid and meteorite spectra. Also, the general band shapes are different. Differences between asteroidal environment (e.g. fairy castle structure) and laboratory make direct comparisons difficult.