EVALUATING THERMAL EFFECTS ON LITHIUM ISOTOPES IN KEROGEN: IMPLICATIONS FOR ENERGY RESOURCES

Lithium is a valuable trace element due to its mobility during weathering and under hydrothermal conditions (Pistiner and Henderson, 2003). There is limited understanding of what determines the Li-content and isotopic compositions of carbonaceous rocks. The Li content of carbonaceous rocks is high (10s – 100s ppm) compared to seawater (0.2 ppm), and its content varies with depositional environment and thermal grade. The global average Li content in coal is 12 ppm (Ketris and Yudovich, 2009), with values reported >500 ppm (Dai et al., 2012). Most Li is thought to be hosted by silicates in coal (Finkelman et al., 2018), but Swaine (1990) reports an association with kerogen as well.

Secondary ion mass spectrometry (SIMS) was used for Li-isotope measurements on bulk coal powders from the PSU Coal Repository and coal samples from mines in the Rocky Mountain region, as well as coal/shale heated by dikes. The samples range from lignite to anthracite in rank, come from different depositional environments, have varying ages and amounts of vitrinite, inertinite and liptinite. Nano-SIMS was used to evaluate Li-isotope ratios on polished samples of black shale of varying thermal maturity. In situ analyses of C-rich vs Si-rich areas at nano-scale resolution avoided the need to isolate kerogen chemically.

Results show that low rank coals are ⁶Li enriched (as light as δ⁷Li –30.8 ± 3.5‰) compared to high rank coals (δ⁷Li up to –4.6 ± 1.5‰) showing a linear trend with increasing temperature. Dike cut coals show ⁶Li enriched country rock with increasing δ⁷Li up to ~400˚C. Near the contact, δ⁷Li values decrease again, possibly due to igneous fluids. Nano-SIMS measurements on C-rich areas vs Si-rich areas of Bakken shales (0.5% & 1.0 % VRo) show that C-rich regions have lighter δ⁷Li than Si-rich regions.

Observed trends suggest that organically bound Li, enriched in ⁶Li, is released with increasing thermal maturity and is taken up by authigenic clays. Elucidating the temperature of Li redistribution and associated isotopic fractionation in coals will lead to a better understanding of the input of organic-Li to fluids during diagenesis, and its role in the global geochemical cycle.

References:

Dai, S. (2012) Int. J. Coal Geol., 98, 10–40.

Finkelman, R.; Palmer, C.; Wang, P., (2018) Int. J. Coal Geol., 185, 138-160.

Ketris, M.; Yudovich, Y., (2009) Int. J. Coal Geol., 78, 135–148.

Pistiner, J.; Henderson G., (2003) EPSL, 214, 327-339.

Swaine, D., (1990) , Trace Elements in Coal, Elsevier, 292 pp.