This contribution summarizes the current state of understanding and recent advances made in the field of stable thallium (Tl) isotope geochemistry. High precision measurements of Tl isotope compositions were developed in the late 1990s with the advent of multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) and subsequent studies revealed that Tl, despite the small relative mass difference of the two isotopes, exhibits substantial stable isotope fractionation, especially in the marine environment. The most fractionated reservoirs identified are ferromanganese sediments with ɛ205Tl ≈ +15 and low temperature altered oceanic crust with ɛ205Tl ≈ −20. The total isotopic variability of more than 35 ɛ205Tl-units hence exceeds the current analytical reproducibility of the measurement technique by more than a factor of 70. This isotopic variation can be explained by invoking a combination of conventional mass dependent equilibrium isotope effects and nuclear field shift isotope fractionation, but the specific mechanisms are still largely unaccounted for.
Thallium isotopes have been applied to investigate paleoceanographic processes in the Cenozoic and there is evidence to suggest that Tl isotopes may be utilized as a monitor of the marine manganese oxide burial flux over million year time scales. In addition, Tl isotopes can be used to calculate the magnitude of hydrothermal fluid circulation through ocean crust. It has also been shown that the subduction of marine ferromanganese sediments can be detected with Tl isotopes in lavas erupted in subduction zone settings as well as in ocean island basalts.
Meteorite samples display Tl isotope variations that exceed the terrestrial range with a total variability of about 50 ɛ205Tl. The large isotopic diversity, however, is generated by both stable Tl isotope fractionations, which reflect the highly volatile and labile cosmochemical nature of the element, and radiogenic decay of extinct 205Pb to 205Tl with a half-life of about 15 Ma. The difficulty of deconvolving these two sources of isotopic variability restricts the utility of both the 205Pb–205Tl chronometer and the Tl stable isotope system to inform on early solar system processes.