Thallium elemental and isotopic systematics in ocean island lavas

The Earth’s mantle exhibits marked chemical heterogeneity. We provide an examination of thallium systematics in ocean island basalts (OIB): new high-precision trace element analyses, including Tl, and Tl isotopic compositions for 48 OIB spanning the entire range observed Sr-Nd-Hf-Pb isotope ratios. All investigated are characterised by ubiquitous depletion requiring sources to have concentrations as low 0.2 ng/g, which is order magnitude lower than estimates primitive similar inferred depleted mantle. interpreted reflect near quantitative removal during subduction inefficient recycling into deeper If true, composition surface materials may not be readily translated OIB. dataset shows a >10 ?-unit primary variation, from ?205Tl = ?6.4 +6.6. However, majority samples (32 48) within uncertainty values (?205Tl ?2 ± 1), show no co-variation with radiogenic composition. Notably, only minor (11 samples) outside range. concentration contrast between inputs such sediments altered basalt so great that additions (<1% mass) high-Tl material will dominate budget lava, decoupling expected result. Thallium therefore difficult link directly variations components they reflect. Indeed, if isotopically distinct oceanic crust and/or were efficiently recycled sampled via OIB, more variation would observed. markedly unsystematic likely residual subducted material, acquired en route shallow-level crustal assimilation.