ESR dating of barite taking into account the contribution from extinct ²²⁸Ra to the equivalent doses

Barite (BaSO₄) is one of the most ubiquitous sulfate minerals in mineral deposits and is suitable for electron spin resonance (ESR) dating. In sea-floor hydrothermal deposits with barite, ²²⁶Ra and ²²⁸Ra, which substitute for Ba in barite, are the main sources of natural radiation together with their daughter nuclides. In ESR dating, the contribution of ²²⁸Ra is often ignored because it is unobservable in most samples due to its short half-life. However, this may not be appropriate for certain young samples in which ²²⁸Ra has already decayed out. In this study, we estimated the initial ²²⁸Ra activity in such barite samples, evaluated its contribution to natural radiation dose, and applied age corrections.

We assumed that the initial ²²⁸Ra/²²⁶Ra ratio incorporated into barite was identical to the present-day ratio in hydrothermal fluids. This ratio was determined by gamma-ray spectrometry of Mn fibers, which was deployed on the seafloor to absorb Ra from the hydrothermal fluid. Independently, the natural radiation doses accumulated in barite extracted from the sulfide- and sulfate-rich deposits were determined by ESR. Using the estimated initial ²²⁸Ra activity, the extinct contributions of ²²⁸Ra and its daughter nuclides to the total natural dose were calculated, and age corrections were applied to barite samples collected from the Higashi-Aogashima Knoll Caldera hydrothermal field.

The conventional ESR ages of two barite samples, in which ²²⁸Ra was not detected, were 973 and 357 years. After correction for the possible contribution of extinct ²²⁸Ra and its daughters, the age differences were only about 2 years. This indicates that the contribution of ²²⁸Ra is negligible in practice unless the initial ²²⁸Ra/²²⁶Ra ratio in particularly high. However, it is still necessary to investigate further if the fluid ratio may vary spatially or temporally.