Concordant 230Th and 231Pa ages of marine phosphorites

Abstract

The reliability of uranium-series age determinations of marine phosphorites has been tested by simultaneous determination of 230 Th/ 234 U and 231 Pa/ 235 U. The measured 230 Th/ 234 U and 231 Pa/ 235 U activity ratios in representative phosphorite samples from the continental margins of Eastern Australia and Peru are consistent with the theoretically expected development with time of these isotope ratios on a concordia diagram and hence support the assumption that the initial 230 Th and 231 Pa contents are insignificant in comparison with the uranium contents, and that the phosphorites have remained a chemically closed system with respect to uranium, thorium and protactinium. Alpha-recoil-induced leakage of 234 U from the phosphorites appears to be too slow to cause any serious errors in the 230 Th ages, and hence can be neglected. These results demonstrate that reliable uranium-series age determinations can be obtained for marine phosphorites on the basis of a simple, closed system model.