Thallium isotopes as tracers of recycled materials in subduction zones: Review and new data for lavas from Tonga-Kermadec and Central America

Abstract

-Sediment is actively being subducted in every convergent margin worldwide. 18Yet, geochemical data for arc lavas from several subduction zones, such as Northern Tonga and 19 Costa Rica have revealed either attenuated or limited evidence for sediment in their mantle 20 source regions. Here we use thallium (Tl) isotopes to trace slab components in lavas from the 21 Tonga-Kermadec and Central American arcs. In general, both arcs display Tl isotope data that 22 are most compatible with addition of sediment to the sub-arc mantle from the subducting slab. and Central American arcs demonstrate that pelagic sediments are detectable in most arcs, 38 whereas altered oceanic crust components only become appreciable when sediment Tl 39 concentrations are very low (e.g. carbonate) or if sediments are no longer a significant 40 3 component of the subducting slab (e.g. slab melting in Western Aleutians). As such, Tl isotopes 41 is a promising tool to trace sediment subduction although this requires at least some pelagic 42 sediment is present in the subducted sediment package. 43We suggest that thallium partitioning between the slab and mantle wedge is most likely 44 controlled by retention in phengite or by partitioning into fluids. Residual phengite likely 45 produces high Cs/Tl ratios because Tl should be more compatible in phengite than is Cs, 46 however, this conclusion needs experimental verification. The stability of phengite is lower at 47 higher fluid contents, which results in hyperbolic relationships between Cs/Tl and possible 48indicators of fluids such as Sr/Nd and Ba/Th. Thus, combined Tl isotopic and elemental 49 systematics not only provide detailed information about the specific slab components that 50 contribute to arc lavas, but also potentially shed light on the mineralogy and physical conditions 51 of subducting slabs. 52 53

Thallium concentration and thallium isotope composition of lateritic terrains

“…Figure 1: Literature compilation of Tl concentrations and Tl stable isotopes (ɛ 205 Tl) for terrestrial and major marine sediments and reservoirs. (a -d) FeMn deposits (Rehkämper et al, 2002), (e) mantle (Nielsen et al, 2006b), (f) low temperature altered ocean crust (Nielsen et al, 2006a), (g -j) volcaniclastics/radiolarian and claystone/pelagic clays (Prytulak et al, 2013 andNielsen et al, 2017b), (j) diatom-bearing sediments (Nielsen et al, 2016), (k -l) river water and loess averages (Nielsen et al, 2005), (m) continental crust average (Nielsen et al, 2017a)., (n) soils (Kersten et al, 2014), and laterite values measured in this study. Where average ranges specified ɛ 205 Tl ± 1SD are reported.…”

Thallium (Tl) sorption onto illite and smectite: Implications for Tl mobility in the environment

“…In various environments, thallium is assumed to substitute potassium in secondary minerals (e.g. Nielsen et al, 2006Nielsen et al, , 2017Prytulak et al, 2017). For instance, in illite, K + takes place as counter ion on the negative surface charges and is strongly bounded to the structure in the interlayer space (Meunier, 2003).…”

Elemental Systematics in MORB Glasses From the Mid‐Atlantic Ridge

“…Prytulak et al () did not find evidence of fluid mobile behavior of Tl in the Mariana arc, while Jenner et al () found positive Tl enrichments (relative to Ce and Pr) from some Lau back‐arc lavas and Nielsen et al () found Tl strongly partitions into the fluid phase at Mariana forearc. Recent Tl isotopic work indicates that the Tl budget of arc lavas are dominated by Tl from pelagic clays from a sediment mélange (Nielsen et al, , ; Shu et al, ). Jenner et al () also found correlated enrichments of As, Tl, Pb, and Bi in backarc lavas from the Manus Basin, which were interpreted by Jenner () as evidence of fluid enrichment of As, Tl, Pb, Sb, and Bi to arc crust.…”