URANIUM AND THORIUM ISOTOPE RATIO MEASUREMENTS USING MULTIPLE COLLECTOR INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY WITH MULTI-ION-COUNTING DETECTORS

摘要

Measurement of U and Th isotope compositions in carbonate samples for the purpose of Quaternary geochronology presents two main analytical challenges; the low ~(234)U/~(238)U ratio and the low absolute abundance of Th. We describe analytical strategies designed to meet these challenges using a multi-collector ICP-MS equipped with multiple channels of ion-counting detectors. For U, we present ~(234)U/~(238)U data obtained using a combination of Daly-photomultiplier and Faraday detectors for reference materials which span a wider range of ~(234)U/~(238)U than is expected for marine carbonates such as corals. For Th, we describe the performance characteristics of ion-counting channeltron detectors. Our data indicate that an array of such detectors can constitute a sensitive detector system with maximum duty cycle efficiency provided the detectors are well characterised for effective dead time and monitored for gain fluctuation between and during analytical sessions.