Thallium isotope systematics in volcanic rocks from St. Helena - Constraints on the origin 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.