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. delta Mo-98 values range from -0.17 to -0.31 parts per thousand and delta U-238 values range from -0.20 to -0.38 parts per thousand. Most individual analyses for both isotope systems overlap within measurement uncertainty (+/- similar to 0.7 and similar to 0.6, respectively). Mean delta Mo-98 and delta U-238 values are -0.22 +/- 0.08 parts per thousand (20) and - 0.29 +/- 0.09 parts per thousand (20), respectively, which overlap with Pacific mid ocean ridge basalt (MORB). In contrast, Tl isotopes show small but resolvable variations, with epsilon Tl-205 ranging from +1.20 to -1.38. The most negative epsilon Tl-205 values are confined to some of the lowest [Tl] samples, but the epsilon Tl-205 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.

Automated summary

Stable molybdenum, thallium, and uranium isotope ratios were analyzed in samples from the 1959 Kilauea eruption and Kilauea Iki lava lake to understand the effects of igneous differentiation. The study found no significant variation in molybdenum and uranium isotope systems, while thallium isotopes showed minor changes. Overall, differentiation in tholeiitic systems was determined to have a low impact on global variation in these isotope systems.