ABSTRACT

In order to calculate the chronology of hydrothermal deposits precipitated from hot springs at Marítaro, Los Azufres geothermal field, Mexico, their ²²⁴Ra, ²²⁶Ra, ²²⁸Ra and ²¹⁰Pb activities were measured. Hot springs contain large amounts of ²²⁶Ra which should be trapped in the lattice of precipitated minerals or be adsorbed on authigenic minerals. Since the time of deposition, a precise chronology can be obtained by measuring the daughter product ²¹⁰Pb produced in situ, assuming a closed system. The final goal of the study was to test whether ²¹⁰Pb-²²⁶Ra chronology could record past changes in the geothermal regime of the area. A 50-cm long sediment core was sampled but measured ²²⁶Ra, ²²⁸Ra and ²¹⁰Pb activities were much lower than expected. The ²²⁶Ra activity ranges from 0.5 to 1.1 dpm/g_rock which contrast with the high ²²⁶Ra content measured in the geothermal wells and hot springs of the Los Azufres field (from 118 to 3098 dpm/g_water). This depleted content can be explained by the highly acidic conditions (pH < 3) of the Marítaro hot springs which have inhibited radium adsorption on secondary minerals. The absence of a clear trend in the ²¹⁰Pb/²²⁶Ra activity ratio with depth might indicate that the system was not completely closed in the past to the exportation of ²¹⁰Pb by acid leaching or ²²²Rn diffusion. Given these limitations, an average age of the bulk deposit was constrained at 20-50 yrs using the measured ²¹⁰Pb/²²⁶Ra activity ratios.