RAMAN spectroscopy for the evaluation of thermal maturity in Jurassic?s Spinicaudatan

JIMENEZ, VICTORIA CECILIA; COPELLO GUILLERMO; GALLEGO, OSCAR FLORENCIO; MONFERRAN, MATEO DANIEL; NORA CABALERI; PELLERANO, ROBERTO GERARDO

Congreso; XI Congreso Argentino de Química Analítica; 2021

In paleontology, the use of Raman spectroscopy is a relatively new field (1, 2). This direct and non-invasive technique allows a determination of the chemical phases and specific mineralogy at the molecular level, as well as the thermal maturity of fossils. Investigations in the remains of spinicaudados fossils have focused mainly on the study of the mineralogical and elemental constitution of the carapaces while the study of preservation of carbonaceous material (possibly organic) has been neglected. In this work, Jurassic spinicaudatan carapace of the La Matilde Formation were analyzed by RAMAN spectroscopy to identify presence of organic matter and determine the thermal maturity of the sample. Raman spectroscopic analysis revealed the presence of amorphous carbon in the carapace, as evidenced by graphite ?G? and disordered ?D? bands. In addition, bands ?D? and ?G? deconvolved into Gaussian-Lorentzian component bands representing vibrational modes that become active Raman due to structural disorder (3). These bands include band D2, band G, band D3, band D1, and band D4 (3).The deconvolution of amorphous carbon peak yields full width at half maximum (FWHM), area (peak intensity), and height of each band. It has been shown that the ratio (R) of the band area of the D1 band to the sum of the areas of the D1, the G, and the D2 bands acts as a proxy for the total amount of thermal alteration experienced by the sample (4), so it can be used as a geothermometer at a higher metamorphic level degree (paleotemperature range 330?650 C).In this study, the presence of amorphous carbon was revealed in fossilized spinicaudate shells. In addition, the relative peak intensity between the bands and the resulting parameters allowed us to roughly estimate the paleotemperature inscribed in the carapace. Based on the geothermometer for high temperature conditions (4), the estimated paleotemperature ranged between 380-400 °C.However, we must considerer that although this detected carbonaceous material may be the rest of the organic matter of the invertebrate conserved in situ, but it could also derive from the fossyldiagenetic microbial activity or belong to inorganic carbonaceous material. RAMAN spectroscopy by itself does not provide all the necessary details to know the origin of the carbonaceous material and in case any of the last two origin proposals were true, the thermal estimation should not be used as a paleotemperatural state. Nevertheless, we it would allow to determine the taphonomic history experienced by the fossil.