Behavior of the Mo, Tl, and U isotope systems during differentiation in the Kilauea Iki lava lake

Stable molybdenum (Mo), thallium (Tl), and uranium (U) isotope ratios were determined in a suite of samples from the 1959 Kilauea eruption and from Kilauea Iki lava lake with the aim of understanding the effects of igneous differentiation on these isotope systems. The samples range from olivine cumulate with MgO up to 27% to internal differentiates with MgO less than 3%, representing a tholeiitic differentiation series. Molybdenum, Tl, and U behave incompatibly during differentiation, and Mo and U isotope ratios do not systematically vary amongst the different samples. δ⁹⁸Mo values range from −0.17 to −0.31‰ and δ²³⁸U values range from −0.20 to −0.38‰. Most individual analyses for both isotope systems overlap within measurement uncertainty (± ~0.7 and ~ 0.6, respectively). Mean δ⁹⁸Mo and δ²³⁸U values are −0.22 ± 0.08‰ (2σ) and − 0.29 ± 0.09‰ (2σ), respectively, which overlap with Pacific mid ocean ridge basalt (MORB). In contrast, Tl isotopes show small but resolvable variations, with ε²⁰⁵Tl ranging from +1.20 to −1.38. The most negative ε²⁰⁵Tl values are confined to some of the lowest [Tl] samples, but the ε²⁰⁵Tl values do not otherwise vary smoothly with MgO or [Tl]. Possible mechanisms for thallium isotope fractionation are considered (e.g., degassing, water leaching, sulfide fractionation) but none are found to be satisfactory. Overall, the lack of resolvable variation in the Mo and U isotope systems and the small magnitude of heterogeneity in the Tl isotope system indicate that differentiation in tholeiitic systems is unlikely to be a major contributor to global variation in these isotope systems.