Lithium in kerogen: A source of isotopically light Li in the global geochemical cycle

Abstract

Lithium is a critical resource for batteries, therefore understanding its sources is key to identifying economic reserves. Recently, Li-isotopes of marine sediments have been used for interpreting paleo-weathering as an indicator of global climate change. The contribution of Li from organic sources to porefluids is often neglected, therefore this study evaluated the changes in Li-content and isotopic composition of organic maceral groups (vitrinite, liptinite, inertinite) in coal as a function of thermal maturation across two coal seams intruded by dikes. Samples were collected from a single depositional layer in the Vermejo Fm. (Colorado USA) where high-volatile bituminous coal was intruded by a mafic-lamprophyre dike, and from the Dutch Creek No.2 mine where a medium volatile bituminous coal was intruded by a felsic-porphyry dike. Samples were collected across the contact zone and into unmetamorphosed country rock.The Li-content and isotopic composition (δ 7 Li ‰) of coal macerals were measured in situ by secondary ion mass spectrometry (SIMS). The organic macerals of the unmetamorphosed Vermejo coal (VRo 0.68%) contained <1μg/g Li, however the δ 7 Li of the vitrinite, which makes up the bulk of these coals, was 18 to 37‰ lighter than the metamorphosed vitrinite and coke, indicating that isotopically light Li is released during metamorphism. Unmetamorphosed coal had vitrinite with average δ 7 Li of -28.4±1.6‰ (2σ), liptinite -15.4±3.6‰, and inertinite -10.5±3.7‰. Organic macerals may release their isotopically light Li at different temperatures.The Dutch Creek coal (VRo 1.15‰) had been heated by burial to ~150 ºC before felsic dike emplacement and had likely released organo-Li during diagenesis, so there was a smaller range in δ 7 Li. Nonetheless, vitrinite in the unmetamorphosed country rock had δ 7 Li values 8 to 20‰ lighter than vitrinite and coke in the contact zone, again showing that 6 Li is preferentially released from organics as temperature increases. A negative correlation was observed between Li-contents (<10μg/g) and C/Si ratios, indicating that at elevated temperatures Li becomes concentrated in authigenic silicates. Thus, isotopically light Li from organic sources must be considered when using δ 7 Li of authigenic minerals to interpret past global weathering due to climate change.