New insights into the structure and formation of coals, terrestrial and extraterrestrial kerogens from resonant UV Raman spectroscopy

摘要

Resonant UV (244 nm-excitation) Raman spectroscopy was used to characterize a series of coals, type II kerogens, Insol- uble Organic Matter (IOM) extracted from primitive chondrites and two stratospheric Interplanetary Dust Particles. UV - Raman spectra of immature terrestrial kerogens and coals are distinct from those of extraterrestrial kerogens extracted from type 1 and 2 primitive chondrites, allowing unambiguous identification. The degree of disorder in the refractory carbonaceous matter in the considered IDPs is found to be higher than in primitive chondrites, confirming former interpretation of visible (514 nm-excitation) Raman spectra of IDPs. Spectral and structural features of laboratory analogues of chondritic organic matter formed in cold plasma reactors at 500-700 degrees C differ from those of natural samples, challenging the hypothesis that chondritic organic matter within the protosolar disk may form through plasma reactions. Finally, UV Raman data shed new light in the structural evolution of coals during maturation and support the presence of an sp 2 structural transition at vitrinite reflectance Ro similar to 0.4-0.5%. Comparison with former visible Raman and infrared spectroscopic data confirms the exis- tence of this structural transition, which is related to a dramatic drop of oxygenated species (carboxyl, hydroxyl, ketone and ether groups). Oxygen thus plays an indirect but significant role in the control of the Raman spectra of polyaromatic mate- rials. A potential application of this technique is the determination of the biotic versus abiotic origin of kerogens in ancient rocks on Earth, and of soils and rocks on Mars that is otherwise difficult with classical visible -excitation Raman spectroscopy. (C) 2020 Elsevier Ltd. All reserved.