Thallium isotope fractionation during magma degassing: evidence from experiments and Kamchatka arc lavas
Recycling of oceanic crust and sediments into the mantle at subduction zones is one of the most important processes occurring in the solid Earth. It controls the vast majority of mass transfer between the surface and deep Earth including water and other volatiles as well as radiogenic heat-producing elements like K, Th, and U. Thus, crustal recycling is the primary determinant of the viscosity and heat budget of the Earth's mantle. In addition, these recycled crustal reservoirs are thought to
... ve rise to mantle plumes that are expressed at the Earth's surface as ocean island basalts (OIBs) or large igneous provinces (LIPs) (e.g., Hofmann & White, 1982) . Much research has focused on understanding the physical and chemical consequences of crustal recycling, both those occurring during subduction itself and those associated with Abstract Thallium (Tl) isotope ratios are an emerging tool that can be used to trace crustal recycling processes in arc lavas and ocean island basalts (OIBs). Thallium is a highly volatile metal that is enriched in volcanic fumaroles, but it is unknown whether degassing of Tl from subaerial lavas has a significant effect on their residual Tl isotope compositions. Here, we present Tl isotope and concentration data from degassing experiments that are best explained by Rayleigh kinetic isotope fractionation during Tl loss. Our data closely follow predicted isotope fractionation models in which TlCl is the primary degassed species and where Tl loss is controlled by diffusion and natural convection, consistent with the slow gas advection velocity utilized during our experiments. We calculate that degassing into air should be associated with a net Tl isotope fractionation factor of α net = 0.99969 for diffusion and natural gas convection (low gas velocities) and α net = 0.99955 for diffusion and forced gas convection (high gas velocities). We also show that lavas from three volcanoes in the Kamchatka arc exhibit Tl isotope and concentration patterns that plot in between the two different gas convection regimes, implying that degassing played an important role in controlling the observed Tl isotope compositions in these three volcanoes. Literature inspection of Tl isotope data for subaerial lavas reveals that the majority of these appear only minorly affected by degassing, although a few samples from both OIBs and arc volcanoes can be identified that likely experienced some Tl degassing. Plain Language Summary Volcanic degassing is an important process for understanding emissions to the atmosphere of greenhouse gasses and toxic metals. Thallium (Tl) is a toxic metal that is highly volatile, which in recent years has been developed as a stable isotope system that can be applied to understand the transfer of material from the Earth surface to the deep mantle via measurements of volcanic lavas. However, degassing of Tl from volcanic lavas could potentially alter the isotope ratio of the residual lava and, thus, render applications of Tl isotopes in lavas difficult to relate to deep Earth material transfer. Here, we perform the first study of Tl isotopes in degassing experiments. These reveal that Tl isotope ratios progressively become enriched in the heavy isotope with loss of Tl. We also show that natural lavas from Kamchatka follow similar Tl depletion and isotope enrichment patterns as our experiments, which for the first time demonstrates that degassing in nature is associated with diffusioncontrolled loss of the light Tl isotope. NIELSEN ET AL.