R8: Metamorphic rocks and tectonics

[R8-11] Precipitation of graphite in continental crust from CO₂-rich fluids: Evidence from fluid inclusions, Raman spectroscopy and carbon stable isotopes

*Madhusoodhan SATISH-KUMAR¹, Sasidharan KIRAN¹, Kana UMETSU¹, Sanjeewa MALAVIARACHCHI² (1. Niigata University, 2. University of Peradeniya)

Keywords:Graphite, Fluid inclusions, Carbon isotopes, Raman spectra

Carbon, the fourth most abundant element in the solar system, is a key element that controls life, environment and resources. Carbon isotope geochemistry is a widely applied tool for understanding the movement of carbon through different reservoirs in the Earth. Here we present the results of a detailed study on fluid inclusions and Raman spectroscopy of graphite-bearing fluid inclusions and combine them with carbon isotopic compostion in a vein-type graphite occurrence at Kahatagaha mine in Sri Lanka. Graphite occurs in a variety of lithological units in the basement rocks of Sri Lanka (Binu-Lal et al., 2003), however the most spectacular occurrence in Sri Lanka is the vein-type graphite (Touret et al., 2018; Touzain et al., 2010), which is well known for its high purity, high crystallinity, and large reserves. Graphite vein and associated rocks from the Kahatagaha mine were reexamined. Irrespective of various morphological varieties of graphite crystals, the carbon isotope composition of graphite showed only small variation, with an average value of δ¹³C = -6.79 +/- 0.42‰ (n=28). All fluid inclusions preserved in quartz pods embedded in graphite vein are pure CO₂, H₂O-CO₂ and H₂O-graphite inclusions. Raman spectroscopy of primary fluid inclusions suggested that the fluid from which the vein-type graphite was precipitated is enriched in CO₂, and the absence of CH₄ indicates no mixing of fluids has occurred. The temperature of graphite precipitation was estimated to be around 500 ^oC. A comprehensive analysis of carbon isotopic composition and fluid inclusion studies helped to understand the precipitation mechanism of graphite from fluids (e.g. Satish-Kumar, 2005; Touzain et al., 2010) and vein graphite formation in the continental crust.
References: Binu-Lal, S.S., et al. 2003. Chemical Geology 197, 253 – 270; Satish-Kumar, M., 2005, Geochimical et Cosmochimica Acta, 69, 3841-3856; Touret, J.L.R., et al. Chemical Geology, 508, 167-181; Touzain, P., et al., 2010. Canadian Mineralogist 48, 1373-1384.