Micro-FTIR reflectance spectroscopy of Ryugu, CI chondrites and volatile-rich clasts – Comparing spectral features in the Mid-IR (2.5–16.5 μm) region

Abstract

Although C1 clasts in carbonaceous chondrites are usually mineralogically similar to CI chondrites, they often exhibit distinct chemical or isotope characteristics, indicating that the diversity of carbonaceous matter is larger than represented by currently known meteorites. Samples returned by the Hayabusa2 mission provide an excellent opportunity to directly compare remote sensing data with laboratory spectra and elaborate on meteorite-asteroid links. We obtained reflectance spectra from 10 carbonaceous samples of extraterrestrial origin to identify spectral differences in the wavelength region between 2.5 and 16.5 μm. We investigated seven volatile-rich clasts, two CI chondrites, and a fragment from the asteroid Ryugu, recently returned by the Hayabusa2 mission. To obtain representative spectra from a lithology, we performed multiple analysis with an aperture size of 100 μm × 100 μm. Subsequently, spectral features were correlated with petrographic and chemical data. The phyllosilicate composition of the investigated C1 and C2 clasts is on average more Fe-rich compared to bulk CI chondrites, which is spectrally reflected in lower Christiansen feature (CF)/Reststrahlenband (RB) ratios. Our results confirm previous studies that indicate that the band area of the OH absorption band at 2.7 μm is dependent on the phyllosilicate composition. A high Mg abundance in phyllosilicates leads to a stronger OH absorption band. Varying degrees of aqueous alteration cause mineralogic differences that are observable in the reflectance spectra. Either in form of a band center shift towards smaller or longer wavelengths, depending on the metal cation giving rise to the M-OH absorption band, and/or a generally weaker OH absorption band, and a broad Reststrahlen band (RB) at 10 μm, with two minor RBs emerging at 11.3 and 12 μm. In contrast, most C1 clasts show a single RB at ≈10 μm, and a constant OH band position at 2.70 μm. The abundance of minor constituents, such as sulfides and carbonates, can also affect the spectrum. Dolomite produces two diagnostic bands at 6.5 and 11.3 μm, whereas pyrrhotite, devoid of diagnostic bands in this wavelength region, increases the background while decreasing the RB intensity. Our findings indicate that within a laboratory framework, subtle mineralogic differences among hydrated carbonaceous materials can be spectroscopically detected. The spectra of Ryugu sample A0008 show a distinctive OH absorption band, as seen in the globally retrieved data by the NIRS3 instrument for Ryugu (Kitazato et al., 2019). Under specific circumstances, micro-FTIR reflectance spectra can be qualitatively compared to remote sensing spectra, and help to further elaborate on meteorite-asteroid links.