Thallium elemental and isotopic systematics in ocean island lavas

Abstract

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 of observed Sr-Nd-Hf-Pb isotope ratios. All investigated OIB are characterised by ubiquitous Tl depletion requiring OIB mantle sources to have Tl concentrations as low as 0.2 ng/g, which is an order of magnitude lower than estimates for the primitive mantle and similar to Tl concentrations inferred for the depleted mantle. The low Tl concentrations inferred for OIB mantle sources are interpreted to reflect near quantitative removal of Tl during subduction and inefficient Tl recycling into the deeper mantle. If true, the Tl isotopic composition of surface materials may not be readily translated to the mantle sources of OIB. The new OIB dataset shows a >10 ε-unit range in primary isotopic variation, from ε205Tl = −6.4 to +6.6. However, the majority of samples (32 of 48) are within uncertainty of mantle values (ε205Tl = −2 ± 1), and show no co-variation with radiogenic isotopic composition. Notably, OIB with only minor Tl depletion (11 samples) have Tl isotopic compositions outside the mantle range. The Tl concentration contrast between the mantle and inputs such as sediments and altered basalt is so great that minor additions (<1% by mass) of high-Tl material will dominate the isotopic budget of a lava, with decoupling of Tl and radiogenic isotopic compositions as an expected result. Thallium isotopic compositions of OIB are therefore difficult to link directly to radiogenic isotope variations and the mantle components they may reflect. Indeed, if isotopically distinct Tl from altered oceanic crust and/or sediments were efficiently recycled into the mantle and sampled via OIB, more variation in the Tl isotopic composition of OIB would be expected than is observed. The markedly unsystematic primary Tl isotopic variations in OIB therefore likely reflect the residual Tl isotopic composition of subducted material, and/or Tl acquired en route to the surface via shallow-level crustal assimilation.