Precursor and metamorphic condition effects on Raman spectra of poorly ordered carbonaceous matter in chondrites and coals

Geothermometers based on Raman spectrometry of carbonaceous matter and covering a wide range of temperatures (100–650 °C) have been developed over recent years. While Raman data have been largely interpreted in terms of temperature, they are also the fingerprint of certain metamorphic conditions, especially in the low temperature range relevant to poorly ordered carbonaceous matter. This study investigates the Raman spectra of two series of chondritic carbonaceous matter and coal samples formed from different precursors and under different metamorphic conditions.The Raman spectra of Polyaromatic Carbonaceous Matter (PCM) from 42 chondrites and 27 coal samples, measured with visible (514 nm) and ultra-violet (244 nm) excitation wavelengths, are analyzed. The Raman spectra of low rank coals and chondrites of petrologic types 1 and 2, which contain the more disordered PCM, reflect the distinct carbon structures of their precursors. The 514 nm Raman spectra of high rank coals and chondrites of petrologic type 3 exhibit continuous and systematic spectral differences reflecting different carbon structures present during the metamorphism event. They result from differences in the chemical structures of the precursors concerning for instance the reticulation of polyaromatic units or an abundance of ether functional groups, or possibly from a lack of carbonization processes to efficiently expel oxygen heteroatoms, due to weak lithostatic pressure and confinement. These results suggest that the use of low temperature carbon thermometers should be restricted to a given geological context. At the same time, the sensitivity of Raman spectra to precursors and certain metamorphic conditions could be used to obtain information other than temperature.The analysis also provides evidence of the accretion of relatively homogeneous PCM precursors among ordinary CO and CV carbonaceous chondrite parent bodies, given that the 514 nm Raman spectra of PCM efficiently trace the metamorphism grades. Looking closer, however, the 514 nm Raman data are more scattered in chondrites than in coals and the maturity tracers are less sensitive and miscorrelate with the atomic H/C ratio, suggesting slight compositional and structural differences among the PCM precursors accreted.