Developments in the Understanding and Application of Lithium Isotopes in the Earth and Planetary Sciences

Abstract

INTRODUCTIONThe signifi cant relative mass difference (c. 16%) between the two stable isotopes of Li (approximately 6 Li 7.5%, 7 Li 92.5%), coupled with broad elemental dispersion in Earth and planetary materials, makes this a system of considerable interest in fi ngerprinting geochemical processes, determining mass balances, and in thermometry. Natural mass fractionation in this system is responsible for c. 6% variation among materials examined to date ( Fig. 1). Although the "modern era" of Li isotope quantifi cation has begun, there are still many questions about the Li isotopic compositions of fundamental materials and the nature of fractionation by important mechanisms that are unanswered (e.g., Hoefs 1997). Figure 1. Summary of lithium isotopic compositions of Earth and planetary materials. Filled bars are solid samples, open bars are liquids. See text for references and details. Tomascak 154The purpose of this chapter is to summarize the current understanding of Li isotopes in geo-and cosmochemical systems and to indicate (1) where Li isotopes have a high probability of adding new understanding of these systems; (2) where some of the more signifi cant defi cits in knowledge exist. The small but burgeoning Li isotope community has not yet compiled the volume of peer-reviewed literature needed to adequately assess even that which has been studied to date. As a result, signifi cant portions of this chapter are based on data reported in abstracts, and as such are more than normally subject to revisions over time. This chapter is anticipated to serve as a starting point for those interested in research incorporating Li isotope geochemistry, or in understanding the state of extant research. BACKGROUND: LITHIUM ISOTOPES IN THE SCIENCES

Lithium isotopes as tracers of groundwater circulation in a peat land

“…The accuracy and the reproducibility of the procedure for solid samples (dissolution + 172 purification procedure + mass analysis) were tested by repeated measurement of the JB-2 173 basalt standard (Geological Survey of Japan), giving a mean value of δ 7 Li = +4.9‰ ± 0.6 (2σ, 174 n=17) which is in good agreement with published values (see Carignan et al, 2007, and 175 Tomascak, 2004, for data compilation). Consequently, based on long-term measurements of 176 both seawater and JB-2 basalt standards, we estimate the external reproducibility of our 177 method to be around ± 0.5‰ (2 σ).…”

“…The relative mass difference between the two isotopes is 56 considerable at 17%, generating significant mass-dependant fractionation during geochemical 57 processes. The range of variation in lithium-isotope compositions is more than 50‰ in 58 geological materials (see Coplen et al, 2002;Tomascak, 2004 for data compilation). As Li is 59 3 a recently developed isotopic tracer, not all the processes that could induce and control 60 isotopic fractionation are as yet well constrained.…”

“…The relative mass difference between the two isotopes is 56 considerable at 17%, generating significant mass-dependant fractionation during geochemical 57 processes. The range of variation in lithium-isotope compositions is more than 50‰ in 58 geological materials (see Coplen et al, 2002;Tomascak, 2004 …”

A stable (Li, O) and radiogenic (Sr, Nd) isotope perspective on metasomatic processes in a subducting slab

“…Lithium is mobile in hydrous fluids and there is a strong isotopic fractionation between its two stable isotopes at low temperatures due to the large relative mass difference (~16%) (Tomascak, 2004). Recent studies demonstrated that the Li isotope system is very sensitive to metasomatic enrichment processes (Marschall et al, 2009;Penniston-Dorland et al, 2010).…”

Lithium isotopes in island arc geothermal systems: Guadeloupe, Martinique (French West Indies) and experimental approach

“…The relative mass difference between the two isotopes is considerable (17%) and is likely to generate large mass dependent fractionation during geochemical processes, even at high temperature. The range of variation in Li isotopic compositions is more than 50‰ in geological materials (Coplen et al, 2002;Tomascak, 2004).…”