Thallium isotopes

Abstract

High precision measurements of Tl isotope ratios have only been possible since 1999, but this isotope system has already found a number of novel applications. To date, studies have revealed that Tl isotopes fractionate substantially in the marine realm with ferromanganese sediments and low temperature hydrothermal alteration displaying a total isotope variation of about 35 ε205Tl-units. The isotopic variation can most likely be accounted for by invoking a combination of conventional mass dependent equilibrium isotope fractionation and nuclear field shift isotope effects (Bigeleisen, 1996; Schauble, 2007). It has been suggested that preferential oxidative sorption of205Tl onto the Mn oxide hexagonal birnesite is one of the primary mechanisms causing Tl isotope variation in the ocean and sediments (Peacock and Moon, 2012; Nielsen et al., 2013). Thallium isotopes have thus far mainly been applied to investigate past and present marine environments where Tl isotopes may be utilized as a monitor of the Mn supply rate or Mn oxide deposition rate over million year time scales (Rehkämper et al., 2004; Baker et al., 2009; Nielsen et al., 2009). Thallium isotopes can also be used to calculate the magnitude of hydrothermal fluid circulation through ocean crust. Such calculations can be performed both for high and low temperature fluids (Nielsen et al., 2006c). Lastly, it has been shown that marine ferromanganese sediments can be detected in mantle-derived basalts (Nielsen et al., 2006b), which confirms that marine sediments subducted at convergent plate margins can be recycled to the surface via mantle plumes (e.g., Hofmann and White, 1982).