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

Abstract

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.

Introduction

The field of non-traditional stable isotopes has greatly expanded in the past decade and a half. One common attribute of nearly all of these emergent tracers is the occurrence of large measurable isotope fractionation at low temperatures, often in oceanographic settings (see Teng et al., 2017 and references therein), and one of the major applications for these systems has been in paleoceanography, particularly research pertaining to the rise of oxygen (e.g., Arnold et al., 2004; Nielsen et al., 2005; Anbar and Rouxel, 2007; Wille et al., 2007; Czaja et al., 2012; Planavsky et al., 2014; Andersen et al., 2016; Wang et al., 2016; Ostrander et al., 2017, Ostrander et al., 2019; Them et al., 2018). It is inevitable that fractionated isotope signatures diagnostic of Earth-surface conditions will be transferred to solid Earth reservoirs via subduction, and variations in non-traditional isotope ratios in igneous rocks and their minerals perhaps tied to such processes are increasingly being reported (e.g., Nielsen et al., 2006; Wille et al., 2007; Czaja et al., 2012; Andersen et al., 2015; Freymuth et al., 2015; Nielsen et al., 2017a; Rader et al., 2018). The Mo, Tl, and U isotope systems are of particular interest as numerous studies in recent years have documented major isotopic variations in igneous systems, especially in arc volcanic rocks (e.g., Willbold and Elliott, 2017; Nielsen et al., 2017a; Andersen et al., 2015; Freymuth et al., 2019). In order to determine if measured isotopic variations can be traced to subduction of isotopically fractionated material, we must first establish if these isotope systems can experience fractionation by igneous processes, such as fractional crystallization.

This contribution explores the behavior of the Mo, Tl, and U isotope systems during the crystallization of Kilauea Iki lava lake, a tholeiitic picrite body. The lava lake has been an ideal locality for testing whether closed-system igneous differentiation leads to isotope fractionation and has been targeted in studies of several other stable isotope systems that include Li, Mg, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, and Sn (Tomascak et al., 1999; Teng et al., 2007, Teng et al., 2008; Chen et al., 2013; Savage et al., 2015; Badullovich et al., 2017; Kato et al., 2017; Zhang et al., 2018; Johnson et al., 2019; Ding et al., 2020; Shen et al., 2020; Zhao et al., 2020). We show here that negligible isotope fractionation occurs for the Mo and U systems during crystallization. We also report minor heterogeneity in Tl isotope composition that does not correlate straightforwardly with degree of differentiation and discuss potential explanations for this heterogeneity.