Dirty calcites 1. Uranium-series dating of contaminated calcite using leachates alone

Chemically precipitated calcite (travertine, speleothem, calcrete) generally contains admixed detritus which will contribute some Th and U during dissolution in dilute acid. This will introduce errors during230Th-23U dating of the calcite, unless corrections are made for the detrital components of230Th and234U. Isotopic data from analyses of leachates (L) of several coeval sub-samples of such rock can be used to obtain the age of the chemically precipitated calcite component, and thus the true age of the deposit; this is the leachate-only (“L/L”) method. Alternatively, one can use analyses of leachate and residue (R), the Ku & Liang “L/R” method. The two procedures give the same age only if there is no differential isotopic fractionation (DIF) of U or Th during the partial dissolution of the residue; if DIF occurs but is uniform for all leachates, then the L/L method gives the true age of the deposit, but the L/R age will generally be in error. The L/L method is also simpler than methods requiring total dissolution of the detrital component.