9-3 Raman geothermometry of oolitic hematite

Session: Early Career Investigators in Mineralogy and Crystallography

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Abstract:

Despite the extensive distribution of oolitic hematite deposits, the origin and conditions of their formation are still under considerable debate. Currently, it is postulated that these enigmatic deposits are mineralogically composed of hematite, chamosite, or iron oxyhydroxides. Likewise, little is known about the temperature of the fluids, and whether biological or non-biological processes occur to induce the precipitation and hence formation of these extensive deposits. Therefore, to shed light on the thermal history of these deposits, we have undertaken Raman spectroscopic analysis of hematitic oolites from the Lower Silurian Red Mountain Formation, which contains abundant, spatially widespread kerogen. Raman spectra of kerogen in sedimentary and metamorphic rocks exhibit a systematic change with progressively increasing thermal stress. It has been well established that spectral features such as intensity (i.e., height) ratio, area (i.e., integrated intensity) ratio, and width (i.e., full width at half maximum; FWHM) between several different peaks can be used to calculate thermal maturity. The kerogen is spatially associated with fine-grained oolitic hematite, hematite cement, oolitic apatite, fine-grained dolomite, and diagenetic pyrite. High-resolution Raman microscopy enabled in situ analysis without chemical pretreatment by targeting kerogen in mineral phases other than hematite, which exhibits a strong 2LO mode (~1320 cm⁻¹) that overlaps the D band. We employed the multi-band fitting deconvolution methods of Kouketsu et al. (2014), including pseudo-Voigtian and mixed pseudo-Voigtian and Lorentzian functions, and we calculated peak temperatures based on 4-band deconvolution D1-FWHM, yielding a mean of 194 ± 30 °C, while 5-band deconvolution results yield a mean of 186 ± 30 °C. Significantly, our results indicate a greater thermal maturity than previously inferred. Furthermore, the calculations are supported by D2-FWHM-based estimates derived from the same study and by R2 parameter trends described in Rahl et al. (2005). These thermal constraints support broader interpretations of the diagenetic history and biosignature preservation in iron-rich oolitic deposits, with significant implications for terrestrial paleoenvironmental reconstruction and astrobiological exploration.

Kouketsu, Y., Mizukami, T., Mori, H., Endo, S., Aoya, M., Hara, H., Nakamura, D., Wallis, S. (2014). A new approach to develop the Raman carbonaceous material geothermometer for low-grade metamorphism using peak width. Island Arc, 23, 33–50.

Rahl, J., Anderson, K., Brandon, M., Fassoulas, C. (2005). Raman spectroscopic carbonaceous material thermometry of low-grade metamorphic rocks: Calibration and application to tectonic exhumation in Crete, Greece. Earth and Planetary Science Letters, 240, 339–354.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8780

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