Thallium Isotope Fractionation During Magma Degassing: Evidence From Experiments and Kamchatka Arc Lavas

Abstract

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 give 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 tran...

Thallium Isotopic Compositions in Hawaiian Lavas: Evidence for Recycled Materials on the Kea Side of the Hawaiian Mantle Plume

“…If higher Ce/Tl values are indicative of degassing, this is either decoupled from Tl isotope systematics in Hawaiian shield lavas, or the samples with highest Ce/Tl are additionally altered with respect to Ce and/or Tl. The heavy ε 205 Tl in two samples from a single eruption on Terceira, Azores are interpreted to result from degassing, although the authors stress that in general the majority of subaerial OIB lavas are likely unaffected by degassing (Nielsen et al, 2021).…”

“…However, recent work by Nielsen et al (2021) shows that degassing can change the ε 205 Tl of subaerial basalts: affected rocks show a positive correlation between ε 205 Tl and Ce/Tl due to the preferential loss of the lighter 203 Tl isotope. Three of the four Hawaiian samples with isotopically heavy Tl have higher Ce/Tl than the other Hawaiian samples (Figure 3a): the highest Hawaiian values of Ce/Tl (2,000-4,500) are higher than upper mantle values (∼1,280; Nielsen et al, 2014), although are not as high as the values measured in one potentially degassed lava from Terceira, Azores (up to 17,000; Nielsen et al, 2021). Importantly, of the seven samples with Ce/Tl > 2,000, only three have heavy ε 205 Tl: the other four have ε 205 Tl < −0.7, including the samples with the highest Ce/Tl values measured (West Maui; 02-WA-10 and 00-LP-01; Figure 3a).…”

“…For example, Tl has a similar ionic radius and charge to K, Rb, and Cs and so these elements should behave similarly during mantle melting and crystallization (e.g., Nielsen et al, 2014Nielsen et al, , 2016. Decoupling of these elements might indicate secondary processes: for example, the loss of volatile Tl through volcanic degassing or the addition/loss of mobile alkalic metals through post-eruptive alteration and fluid mobilization (e.g., Nielsen et al, 2021;Schiano et al, 1993). Below, we assess the possibility that degassing or alteration may have caused ε 205 Tl > 0 in the four samples from the Kea geochemical group (Figure 2b).…”

“…Isotopically heavy values of ε 205 Tl in OIB have previously been attributed to the presence of either ancient ferromanganese or pelagic sediment recycled through the mantle (e.g., Brett et al, 2021;Nielsen, Rehkämper, Norman, et al, 2006), modern ferromanganese or pelagic sediment assimilated during magma ascent (e.g., Brett et al, 2021;Nielsen et al, 2007), or fractionation from volcanic degassing (Nielsen et al, 2021). The extreme Tl concentration contrast between the mantle (<1 ng/g) and potential surface inputs (>>100 ng/g) can disturb Tl isotopic systematics without resolvable effect on other geochemical signatures (radiogenic isotopes, trace elements) (e.g., Blusztajn et al, 2018 2a.…”

“…The analysis of multiple samples is required from a single volcano to determine whether degassing has affected their ε 205 Tl values because this interpretation relies on the presence of a progressive increase in Ce/Tl with ε 205 Tl in a related sample suite as the lighter 203 Tl is removed from the system (Nielsen et al, 2021). The samples from West Maui, which include two samples heavy in ε 205 Tl, form the opposite trend, where Ce/Tl decreases as ε 205 Tl increases (Figure 3a).…”