High-resolution mapping of uranium and other trace elements in recrystallized aragonite-calcite speleothems from caves in the Pyrenees (France):: Implication for U-series dating

Aragonite and calcite speleothems were sampled in the cave of Pierre Saint-Martin (Pyrenees-Atlantiques, France). These speleothems contain significant amounts of uranium and strontium, which were measured spatially and chemically by nuclear microprobe analysis. Sr and U distributions are highly varied, with a marked contrast between aragonite and calcite because the aragonite concentrates Sr and U. On the other hand, Zn is preferentially located in calcite and in layers of organic matter trapped in CaCO3. Sr contents ranged from 440 to 1100 mu g/g in the calcite and 4900 to 18,500 mu g/g in the aragonite. U concentrations show remarkably high values from 11 to 19 mu g/g in the calcite and 89 to 350 mu g/g in the aragonite. These U and Sr concentrations are amongst the highest measured in supergene speleothems deposited by infiltrating meteoric waters. X-ray absorption experiments with synchrotron radiation suggested that uranium was incorporated in its hexavalent oxidation state in both aragonite and calcite. Recrystallization from aragonite into calcite was indicated by micromorphological observations for some of the samples. This recrystallization process may explain the uranium loss in neogenic calcite deposits and consequently the opening of the geochemical system, which can lead to errors in age dating with the U-series dating method. Uranium loss in recrystallized calcite is one of the important aspects to be considered in U-series dating and more generally for U geochemical stability in CaCO3. (c) 2005 Elsevier B.V. All rights reserved.