Multi-aliquot method for determining (U + Th)/He ages of hydrothermal hematite: Returning to Elba

We have used a multi-aliquot method to obtain precise (U + Th)/He ages of hydrothermal hematite and to assess the extent to which He loss from fine-grained hematite caused by diffusion and recoil. Hematite (n = 6) from the Rio Marina mine, Elba (Italy) yields (U + Th)/He ages that range from 5.36 ± 0.33 to 5.64 ± 0.11 Ma, giving a weighted mean age of 5.53 ± 0.14 Ma and an isochron age of 5.25 ± 0.20 Ma. 40Ar/39Ar data from cogenetic adularia yield flat age spectra with analytically indistinguishable plateau ages (5.575 ± 0.008 and 5.583 ± 0.013 Ma). An additional adularia has a more complex spectrum and yields an interpreted age of 5.64 ± 0.03 Ma. The hematite (U + Th)/He ages overlap the 40Ar/39Ar ages, albeit they are less precise (2–6% vs. 0.2–0.5%). This indicates that the loss of in situ radiogenic 4He from complex fine-grained hematite, either by diffusion and recoil, is insignificant. The study shows that multi-aliquot method has the potential to reliably deliver precise and accurate ages for iron oxide mineralisation that has not suffered significant post-crystallisation thermal perturbation.