Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo Lithium isotope compositions of U.S. coals and source rocks: Potential tracer of hydrocarbons Zebadiah Teichert ⁎ , Maitrayee Bose, Lynda B. Williams School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, United States of America ARTICLEINFO Editor: Oleg Pokrovsky Keywords: Lithium isotopes Kerogen Coal Hydrocarbon source rocks Secondary Ion Mass Spectrometry (SIMS) NanoSIMS ABSTRACT Kerogen in organic-rich rocks contains trace amounts of lithium (Li) that has been overlooked as a contributor to the global Li geochemical cycle. This study examined a variety of coals where kerogen is concentrated (> 50% organic carbon) and hydrocarbon source rocks of diferent ages, depositional environments and thermal ma- turity to determine their range of Li isotopic compositions (δ 7 Li‰) and factors that infuence their compositions. Using Secondary Ion Mass Spectrometry (SIMS), we analyzed 22 coals and 4 hydrocarbon source rocks (Types I, II, III), to determine the δ 7 Li of kerogen in situ, without chemical isolation of phases that can alter their original isotopic compositions. The δ 7 Li values of the coals surveyed are distinctly isotopically light (< 0‰) compared to most natural minerals and fuids. In immature coals, with a vitrinite refectance in oil (VRo) of ≤0.5%, kerogen δ 7 Li values average –23.4 ± 1.1‰ and become heavier with increasing thermal grade to temperatures of gas generation (VRo ~1.3%). The linear correlation between δ 7 Li and VRo suggests that 6 Li may be preferentially released to pore fuid from kerogen during thermal maturation. Notably, authigenic clays forming at diagenetic temperatures substitute Li from pore fuids into silicate layers, therefore, the Li isotopic composition of the clays may record fuid isotopic compositions infuenced by organic-Li sources. NanoSIMS isotopic maps of the Lower Bakken Shale, and SIMS measurements of the Green River Shale show isotopically light Li associated with C-dominated areas, and heavier δ 7 Li with Si-dominated areas of the hy- drocarbon source rocks. We conclude that kerogen is a source of isotopically light Li that contributes to fuids during thermal maturation and hydrocarbon generation. Kerogen may be a signifcant contributor of Li to pore fuids and its distinctly light Li isotopic composition relative to other terrestrial waters and minerals could provide a tracer of organic inputs to the global geochemical cycle. 1. Introduction Lithium isotopes are being increasingly utilized to aid in under- standing a host of terrestrial and extraterrestrial processes. In terrestrial systems, there has been signifcant use of Li isotopes in low temperature crustal environments to investigate the geologic processes of weath- ering and fuid dynamics (Penniston-Dorland et al., 2017; Tomascak et al., 2016). However, there is limited information regarding how Li from organic compounds may play a role in these processes. If there is a signifcant amount of Li bound in organic compounds, it is important to know its isotopic composition and how Li from organics would con- tribute to the global geochemical cycle if the Li were released from the organics (e.g. with increasing burial temperature). In this study, we measured the Li isotope compositions on a suite of 22 coals from across the United States, representing diferent ages, depositional environ- ments and thermal maturities. In addition, 4 hydrocarbon source rocks containing Type I, II and/or III kerogen were analyzed using Secondary Ion Mass Spectrometry (SIMS) and NanoSIMS. The goal of this study is to evaluate the range of Li isotopic compositions in organic-rich rocks consisting of a variety of organic macerals which were exposed to a range of thermal alteration conditions. 1.1. Lithium resources Lithium is an important natural resource that has increased in de- mand due to the rising use of Li ion batteries. The primary economic sources for Li are brines, salt lakes and pegmatite deposits (Kesler et al., 2012). Seawater contains only ~0.17 mg/L Li but it is extracted eco- nomically using manganese oxide membranes to concentrate Li (Hong et al., 2018). Lithium is more concentrated in oilfeld brines with concentrations up to several 100 mg/L (Collins, 1975; Eccles and Berhane, 2011; Macpherson, 2015; Millot et al., 2011; Phan et al., https://doi.org/10.1016/j.chemgeo.2020.119694 Received 21 December 2019; Received in revised form 22 May 2020; Accepted 25 May 2020 ⁎ Corresponding author at: Physical Sciences F-686, 550 E. Tyler Mall, Arizona State University, Tempe, AZ, United States of America. E-mail address: zteicher@asu.edu (Z. Teichert). Chemical Geology 549 (2020) 119694 Available online 01 June 2020 0009-2541/ © 2020 Elsevier B.V. All rights reserved. T