Determination of Uranium and Thorium in Zircon, Apatite, and Fluorite: Application to Laser (U-Th)/He Thermochronology

Abstract

We have developed a methodology for (U-Th)/He thermochronology on a variety of mineral species. With many laboratories initiating research in the area of (U-Th)/He thermochronology, we recognize that there may be interest in a review of analytical procedures for uranium and thorium determination in single crystals of apatite, zircon, rutile, and fluorite. Uranium and thorium are both determined by inductively coupled plasma mass-spectrometry using an isotope dilution method. While standard and spike solutions can be purchased, their isotopic composition and the concentration of the standard solution need to be verified. Digestion procedures for apatite and fluorite are relatively straightforward, but zircon decomposition requires the use of pressure vessels or fusion. Matrix effects are shown to have an insignificant effect on isotope ratios, although isobaric interferences, particularly of PtAr⁺ on U isotopes, can be a problem. We include complete thermochronology datasets for replicate analysis of Durango apatite, Yucca Mountain fluorite, and an Australian megacryst zircon.