Abstract:

Lithium (Li) as a non-traditional stable isotope is a strategic and critical metal for the development of emerging industries. This review summarizes the geochemical properties of Li as well as its isotope distribution characteristics, analytical techniques, and fractionation mechanisms, and provides a comprehensive discussion on the latest research application of Li isotopes in plate subduction, crust-mantle material evolution, metallogenic mechanism, surface weathering, carbon cycle, and human activities. The relative mass difference between the two stable isotopes, ⁶Li and ⁷Li, can reach 17%. Significant Li isotopic fractionation occurs due to changes in environmental conditions (both physical and chemical) during tectonic evolution, and δ⁷Li can vary up to 60‰ between different reservoirs. Lithium stable isotopes have great potential for ore prospecting and geochemical tracing. Lithium as a lithophile element with strong fluid activity is widely distributed in the crust, where ⁷Li is more likely to enter the aqueous phase as tetravalent cations during fluid migration, which results in higher δ⁷Li in natural reservoirs. Lithium isotopic fractionation is significant at low temperature by forming secondary clay minerals, and it is less likely to occur at high temperature, where Li diffusivity and partition coefficient in minerals are the controlling factors. The rapid development of Li isotope detection techniques such as MC-ICP-MS and in-situ microanalysis greatly improves the accuracy of Li isotopic analysis (up to 0.2‰) and promotes use of Li isotopes in geoscience research. One example is in the study of dehydration and metasomatism during plate subduction. The preferential partitioning of ⁷Li in the aqueous phase affects Li isotopic composition of mantle wedge fluid and island arc lavas, where the absence of Li isotopic fractionation in the deep, high temperature environment causes low δ⁷Li values in the deep fluids, similar as in the subduction plate; whilst Li isotopic variations in mantle-derived xenoliths reflect different degrees of metasomatism. Li isotopes are also effectively used to study the genesis of ore deposits and ore prospecting. Lithium in salt brine are mainly sourced from weathering of Li-rich parent rocks and transported by bottom-up hydrothermal fluids, and the dissolution of sediments further promotes Li enrichment. The low δ⁷Li granopegmatite type lithium deposits mainly formed during late-stage magmatic differentiation. Rivers, rainwater, aerosols, and clay formation jointly affect Li isotopic fractionation via epigenetic effects. This review provides a reference for the geochemical application of Li stable isotopes. Lithium isotopic analysis can be more broadly applied in geological studies as the accuracy of isotopic measurements is further improved and the mechanism of Li isotopic fractionation under complex conditions is further clarified.

Key words: lithium isotope, fractionation mechanism, metallogenic mechanism, oceanic crust tracer, surface weathering, MC-ICP-MS