Thallium isotope systematics in volcanic rocks from St. Helena – Constraints on the origin of the HIMU reservoir

It is generally accepted that subduction of the oceanic crust plays an important role in creation of mantle heterogeneity. HIMU (stands for high μ = (²³⁸ U/²⁰⁴Pb)) is a rare and probably not volumetrically large mantle component, which is sampled at St. Helena in the Atlantic and in the Cook-Austral Islands in the South Pacific. Previous studies that used radiogenic isotopes have specifically invoked subducted hydrothermally altered oceanic crust (AOC) as a significant component in the source of HIMU. However, because radiogenic isotopes are affected by both parent-daughter fractionation and time that cannot be independently determined, it is difficult to unambiguously use these tracers to determine the source material of the HIMU reservoir. Thallium isotopes can be used to identify the involvement of sediments rich in Mn oxides or AOC, because these two components exhibit high Tl concentrations and highly fractionated Tl isotope compositons, whereas the upper mantle is depleted in Tl and homogenous with respect to Tl isotopes. Here we report Tl isotope data for a suite of St. Helena lavas previously characterized to investigate if the HIMU mantle reservoir can be linked to AOC. Samples show large Tl isotopic variation, from ε²⁰⁵Tl = − 9.8 to ε²⁰⁵Tl = + 3.9 and do not correlate with Sr, Nd, Hf or Pb isotopes. Eight samples have ε²⁰⁵Tl lighter than the mantle value (ε²⁰⁵T = − 2), with the lightest values similar to those observed for modern AOC. These features are consistent with the presence of different proportions of recycled oceanic crust in the source of St. Helena mantle source. Our results, therefore, support previous studies that inferred AOC as a significant component in the HIMU mantle reservoir and show that the Tl isotopic system can be used as a powerful tracer to detect contributions of AOC in the creation of mantle heterogeneities.