Raman spectromicroscopy allows fast identification and localization of minerals and organic matter. Raman (vibrational) spectroscopy can identify most minerals and allows automated comparison with reference databases. During map acquisitions, one spectrum is recorded at each pixel, forming a hyperspectral mapping dataset. Statistical analysis of hyperspectral allows the distinction of the different phases contained in a sample and the imaging of their distribution with 0.6 to 1 µm resolution. Organic matter that has been deeply buried displays strong resonant Raman backscattering which facilitates its localization in most mineral matrices. Below is shown an example of Raman mapping of organic matter distribution in association with various micrometric to sub-micrometric mineral grains.
The Raman spectra of organic matter display a series of partly overlapping bands D1 to D4 and G, as shown in the spectrum below. Deconvolution of this spectrum in Gaussian or semi-Gaussian bands can be used to derive an estimate of the peak burial temperature in the range 160-650°C, a temperature domain that encompasses the upper boundary of the oil window to high-grade metamorphism.
We use Renishaw inVia (at UMET, Lille) and Jobin Yvon Labram HR (at LOG and LASIR, Lille) microspectrometers.
Example of Raman study of microfossils and organic matter thermometry:
Schopf, J.W., Kudryavtsev, A.B., Agresti, D.G., Czaja, A.D. and Wdowiak, T.J., 2005. Raman Imagery: A New Approach to Assess the Geochemical Maturity and Biogenicity of Permineralized Precambrian Fossils. Astrobiology, 5, 333-371.
Lepot, K., Benzerara, K. and Philippot, P., 2011. Biogenic versus metamorphic origins of diverse microtubes in 2.7 Gyr old volcanic ashes: Multi-scale investigations. Earth and Planetary Science Letters, 312, 37–47.
Beyssac, O., Goffe, B., Chopin, C. and Rouzaud, J.N., 2002. Raman spectra of carbonaceous material in metasediments: a new geothermometer. Journal of Metamorphic Geology, 20, 859-871.
10.1046/j.1525-1314.2002.00408.x