Expanding the limits of laser-ablation U–Pb calcite geochronology

Abstract

U–Pb geochronology of calcite by laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS) is an emerging field with potential to solve a vast array of geologic problems. Because of low levels of U and Pb, measurement by more sensitive instruments, such as those with multiple collectors (MCs), is advantageous. However, whereas measurement of traditional geochronometers (e.g., zircon) by MC-ICPMS has been limited by detection of the daughter isotope, U–Pb dating of calcite can be limited by detection of the parent isotope if measured on a Faraday detector. The Nu P3D MC-ICPMS employs a new detector array to measure all isotopes of interest on Daly detectors. A new method, described herein, utilizes the low detection limit and high dynamic range of the Nu P3D for calcite U–Pb geochronology and compares it with traditional methods. Data from natural samples indicate that measurement of 238U by Daly is advantageous at count rates < 30 000; this includes samples low in U or those necessitating smaller spots. Age precision for samples run in this mode are limited by 207Pb counts and the maximum U ∕ Pbc. To explore these limits – i.e., the minimum U, Pb, and U ∕ Pb ratios that can be measured by LA-ICPMS – a model is created and discussed; these models are meant to serve as a guide to evaluate potential candidate materials for geochronology. As an example, for samples necessitating a < 1 Ma uncertainty, a minimum of ∼ 10 ppb U is needed at a spot size of 100 µm and rep rate of 10 Hz; absolute uncertainty scales roughly with U concentration.