Thallium Isotopes Reveal Brine Activity During Carbonatite Magmatism

Carbonatite volcanism remains poorly understood compared to silicic volcanism due to the scarcity of carbonatite volcanoes worldwide and because volcanic H 2 O and CO 2 —major components in carbonatite volcanic systems—are not well preserved in the rock record. To further our understanding of carbonatite genesis, we utilize the non‐traditional thallium (Tl) isotope system in Khanneshin carbonatites in Afghanistan. These carbonatites contain 250–30,000 ng/g Tl and have ε 205 Tl values (−4.6 to +4.6) that span much of the terrestrial igneous range. We observe that δ 18 O VSMOW (+8.6‰ to +23.5‰) correlates positively with δ 13 C VPDB (−4.6‰ to +3.5‰) and ε 205 Tl up to δ 18 O = 15‰. Rayleigh fractionation of calcite from an immiscible CO 2 ‐H 2 O fluid with a mantle‐like starting composition can explain the δ 18 O and δ 13 C—but not ε 205 Tl—trends. Biotite fractionates Tl isotopes in other magmatic settings, so we hypothesize that a Tl‐rich hydrous brine caused potassic metasomatism (i.e., biotite fenitization) of wall rock that increased the ε 205 Tl of the residual magma‐fluid reservoir. Our results imply that, in carbonatitic volcanic systems, simultaneous igneous differentiation and potassic metasomatism increase ε 205 Tl, δ 18 O, δ 13 C, and light rare earth element concentrations in residual fluids. Our fractionation models suggest that the Tl isotopic compositions of the primary magmas were among the isotopically lightest (less than or equal to ε 205 Tl = −4.6) material derived from the mantle for which Tl isotopic constraints exist. If so, the ultimate source of Tl in Khanneshin lavas—and perhaps carbonatites elsewhere—may be recycled ocean crust.