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 < 30000; 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 μ 10 ppb U is needed at a spot size of 100 μm and rep rate of 10Hz; absolute uncertainty scales roughly with U concentration.