Effects of contact metamorphism on the lithium content and isotopic composition of kerogen in coal

Lithium isotopes (δ⁷Li) in coals have been shown to increase with thermal maturity, suggesting preferential release of ⁶Li from kerogen to porefluids. This has important implications for paleoclimate studies based on δ⁷Li of buried marine carbonates, which may incorporate Li from porefluids during recrystallization. Here, the Li content and isotopic composition of macerals from two coal seams intruded by dikes, were studied as a function of temperature across a thermal gradient into the unmetamorphosed coal. Samples were collected in Colorado (USA) from a Vermejo Fm. coal seam intruded by a mafic-lamprophyre dike and compared to a Dutch Creek No.2 coal seam intruded by felsic-porphyry dike; a potential source of Li-rich fluids. The Li-content and Li-isotope compositions of coal macerals were measured in situ by Secondary Ion Mass Spectrometry (SIMS). The macerals of the Vermejo coal samples, buried to VR_o 0.68% (T_max = 104 °C), contained <1.5 μg/g Li with an average vitrinite δ⁷Li of −28.4 ± 1.6‰, while liptinite and inertinite were heavier, averaging −15.4 ± 3.6‰ and − 10.5 ± 3.7‰, respectively. The contact metamorphosed vitrinite/coke showed the greatest change with temperature with δ⁷Li 18 to 37‰ heavier than the unmetamorphosed vitrinite. The Dutch Creek coal, buried to VRo 1.15% (T_max = 147 °C), prior to dike emplacement, may have released Li during burial, as less isotopic change was observed between contact metamorphosed and unmetamorphosed macerals. Overall, Li contents were < 1 μg/g, and the vitrinite in metamorphosed coal had δ⁷Li values 8 to 21‰ heavier than the unmetamorphosed coal. SIMS measurements on macerals near the dike did not show an increase in Li-content indicative of Li derived from dike fluids, however previous bulk measurements that included silicates showed slightly higher (2-3 μg/g) Li-contents near the dike, suggesting possible Li incorporation from dike fluid into metamorphic silicates. A negative correlation was observed between Li-content and ¹²C⁺/³⁰Si⁺ count ratios, indicating that at metamorphic temperatures Li becomes concentrated in silicates.