Apatite fission-track chronometry using laser ablation ICP-MS

Abstract

Comparison is made of apatite fission-track (FT) ages measured by using conventional thermal neutron irradiation and Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) to determine sample uranium content. Spontaneous track densities were determined on specified apatite grains from the Durango, Fish Canyon Tuff, and Mount Dromedary Complex age standards. Neutron-induced track densities and ICP-MS uranium values were determined for the same specified grains. Ages determined using the two approaches show reasonable consistency, indicating the possibility of routine fission-track analysis using LA-ICP-MS. Uncertainties and assumptions in the LA-ICP-MS approach include the laser ablation of chemically etched mineral surfaces, the suitability of reference materials for calibration, and the derivation of FT ages using the alternatives of selection of a λ f value or a revised zeta comparative approach. The use of LA-ICP-MS in FT chronometry would dramatically increase the speed of analysis and hence sample throughput, as well as avoiding the need for neutron irradiation and the production, transport and handling of radioactive samples. With the acknowledged dependency of FT annealing in apatite on composition, such grain-by-grain, multi-element analysis could provide a more robust definition of the specific track annealing characteristics of each apatite crystal.