Mass bias corrections for U-Pb isotopic analysis by secondary ion mass spectrometry: Implications for U-Pb dating of uraninite.

Abstract

Uranium (U)-lead (Pb) geochronology of uraninite is critical to the study of the genesis of U deposits throughout the world. Previous attempts at developing a technique to date uraninite using secondary ion mass spectrometry (SIMS) have, however, had limited success. Improper correction of mass bias results in incorrect reported U-Pb ratios. When these results are plotted on Concordia diagrams they produce erroneous ages, complicating the study of U deposits. Uranium and Pb isotope ratios were measured in three uraninite reference materials (RMs) with varying Pb content and three samples with unknown U-Pb isotope compositions using a CAMECA 7f SIMS instrument. Measurements were made using a primary beam of O(-) accelerated at 12.5 kV. A mass resolving power of 1300 and a 50-V offset were used to minimize interferences. The study demonstrates that the mass bias for U-Pb isotope ratio measurements in uraninite by SIMS varies as a function of Pb content. A three-point calibration curve was developed using uraninite RMs with low-, intermediate- and high-Pb contents. Corrected ratios for both concordant and discordant uraninite were plotted on a Concordia diagram to demonstrate the effect that different correction techniques have on the resulting age. Accurate determination of U-Pb ratios in uraninite using a SIMS instrument requires a suite of RMs with varying Pb content and the construction of a calibration curve, or a uraninite RM with a Pb content similar to that of the unknowns. Failure to standardize correctly will result in erroneous ages being calculated using Concordia diagrams. Copyright © 2016 John Wiley & Sons, Ltd.