High-Precision Th-Pb Dating by Isotope-Dilution TIMS+MC-ICPMS – Preliminary Results

Abstract:

Relative to the U-Pb decay series, the Th-Pb decay series has received relatively little attention for geochronology although it offers promising applications in both high-resolution geochronology as well as thermochronology. The limitations are partly because Th measurements by thermal ionization have proven notoriously difficult, and for many geochronological applications, the additional information provided by Th-Pb has not been worth the effort required. However, the current generation of MC-ICPMS instruments provides nearly two orders of magnitude better sensitivity of Th than TIMS, eliminating this barrier to measurement and opening up this chronometer for new use. We have developed a method to measure relative abundances of Th, U and Pb by isotope dilution for high-precision geochronology. This method leverages the strengths of two instruments, the high sensitivity and stable mass fractionation of MC-ICPMS instruments for Th and U, and the low background, lack of isobaric interference, and signal stability of TIMS instruments for Pb. To make these measurements, we have calibrated a new synthetic isotope tracer that comprises a mixture of ²²⁹Th-²³³U-²³⁶U-²⁰²Pb-²⁰⁵Pb. The analytical strategy developed for accessory minerals is a hybrid of techniques previously developed for U-Pb ID-TIMS and U/²³⁰Th ID-MC-ICPMS, with a two-column HCl-HNO₃ anion exchange procedure (one to separate U and Pb from matrix+Th, and the other to separate Th from matrix), followed by a recombination of the U and Th fractions for MC-ICPMS analysis while the Pb fraction is measured by TIMS. Th-Pb measurements are complementary to, and potentially as precise as the U-Pb gold standard, and the method will find application in certain geological problems, including (1) precisely determining the relative decay rates of ²³²Th and ²³⁸U, (2) dating materials that are young enough to be substantially affected by ²³⁰Th or ²³¹Pa disequilibria, and (3) dating small quantities of high-Th minerals like monazite, allanite, and perovskite. We present here a description of the analytical strategy and preliminary measurements of calibration and reference materials.