Dirty calcites 2. Uranium-series dating of artificial calcite-detritus mixtures

In a previous paper (Dirty calcites, 1) we have shown that the uranium-series analysis of the leachates of impure (“dirty”) carbonates (e.g., travertine, speleothem) can be used to construct isochrons that give 230Th/214U and 234U/238U ratios corresponding to the age of deposition of the carbonate. In order to test this model, we have analysed artificially prepared mixtures of pure calcite of known age and “pure” detritus (clay+sand) from a cave in West Virginia, U.S.A. Calcites of different age (11.7, 100 and 238 ka ), mixed with detritus of the same age and then with detritus of different age have been analysed. Isochrons based on leachates of these mixtures give ages which generally agree with the true age of the chemically precipitated calcite. Isochrons constructed using paired leachate and residue give incorrect ages in most instances. This may be due to the presence of an extremely insoluble component in the detritus which does not contribute significantly to the weak-acid leachates.