Development of thallium as a tracer of igneous process in contrasting tectonic settings

Abstract

Some volcanoes erupt relatively undifferentiated melts, which can be used to investigate the otherwise inaccessible mantle. Tectonic setting influences which portions of the mantle are sampled by volcanic activity. At mid-ocean ridges, passive upwelling causes decompression melting of the upper mantle. In subduction settings, arc lavas provide chemical evidence for the incorporation of slab materials and fluids from the downgoing plate. The geochemistry of ocean island basalts (OIB) indicate that at least some of this surface material is recycled into the deep mantle and then re-sampled. However, identifying geochemical signatures that can be linked to a specific type of subduction input is often challenging. This thesis investigates the potential of thallium stable isotope and elemental systematics to further constrain recycling of surface materials through the mantle via subduction. Thallium (Tl) is a heavy (mass ∼204), volatile, highly incompatible trace metal, which is both depleted and isotopically homogeneous in mid-ocean ridge basalts (MORB). In contrast, much higher concentrations of isotopically diverse Tl are found in potential subduction inputs, meaning that addition of exotic Tl to the mantle should be readily identifiable. I provide new analyses of global OIB and the first analyses of thallium stable isotopes in samples from a continental arc volcano, alongside trace element compilations for MORB, OIB, and arcs. The accuracy and precision of these datasets are supported by characterisation and evalu- ation of igneous reference materials. I show that the mantle is likely to be even more depleted in Tl than previously recognised, and that recycling of Tl into the deep mantle and OIB source regions is likely limited. Variations in the Tl isotopic composition of OIB reflect some combination of heterogeneous residual slab Tl and shallow-level assimilation during magma ascent. Instead, the majority of subducted Tl is reprocessed through subduction-related volcanism, eventually accumulating in the continental crust.