MINERALOGICAL CONTROL ON MOLECULAR STRUCTURE AND APPARENT THERMAL ALTERATION OF ANCIENT ORGANIC MATTER AS MEASURED BY RAMAN SPECTROSCOPY

Organic matter preserved in ancient rocks (kerogen) is one of the primary sources of information about the earliest life on Earth. Raman spectroscopy is typically used to assess the level of thermal alteration experienced by geologic units as indicated by the kerogen molecular structure. These previous studies, however, have each focused on a single lithology. This study provides evidence for variable molecular structures of kerogen on micron to millimeter scales within primary lithologies composed of different minerals.

Two Precambrian units were studied. The first is a Neoproterozoic black chert from the Black River Dolomite of Tasmania, Australia, which contains putative chert-apatite scales of vase-shaped microfossils embedded within fine-grained primary chert. These objects and the matrix chert both contain kerogen. The second unit is the Neoarchean Gamohaan Formation of South Africa, which contains kerogen-bearing carbonate stromatolites and layered black cherts.

The molecular structure of the kerogen from these two units was studied via Raman spectroscopy and the apparent level of thermal maturity was determined via standard spectral parameters such as D/G band intensity ratios, D-band FWHM, and Raman Index of Preservation. The Neoproterozoic chert-apatite objects contain kerogen that appears to be significantly more thermally altered than that hosted by the chert matrix despite the intimate association of the minerals. Similarly, kerogen preserved in the Neoarchean carbonates and cherts also shows apparent differences in thermal maturity. The measured molecular-structural variations are likely the result of bonding of the kerogen with the various minerals rather than actual fine-scale differences in thermal alteration of the samples.

Conventional wisdom states that a difference in perceived thermal alteration between a kerogenous object and the rest of the host rock is an indicator that they are not syngenetic. The results reported here, however, imply that this is not necessarily true and any scale of Raman thermal alteration for kerogen must be calibrated to the host mineral composition. Thus, these findings have implications for positively identifying early life on Earth and also for the study of potential organic matter on Mars via the Mars2020 SHERLOC (Raman) instrument.