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Geochemical Journal
Geochemical Journal An open access journal for geochemistry
Published for geochemistry community from Geochemical Society of Japan.

Radium depletion and 210Pb/226Ra disequilibrium of Marítaro hydrothermal deposits, Los Azufres geothermal field, Mexico

CAROLYNE PICKLER, DANIELE L. PINTI, BASSAM GHALEB, VICTOR-HUGO GARDUÑO, ALAIN TREMBLAY
Geochemical Journal, Vol. 46, No. , P. 493-504, 2012

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/grock 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/gwater). 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.

KEYWORDS

hydrothermal deposits, 210Pb, 226Ra, radium adsorption, Los Azufres

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