Laser ablation (U-Th-Sm)/He dating of detrital apatite

Abstract

Laser ablation (U-Th-Sm)/He dating of apatite (LA-aHe) is an emerging technique that presents a number of advantages over conventional whole-grain methods. Increased analytical efficiency, through the elimination of He re-extractions and acid digestion, increases sample throughput and facilitates larger datasets. The spatially resolved selection of helium extraction locations eliminates the need for alpha-ejection corrections and allows for imperfect grains with inclusions, fractures, unusual morphologies, rounding, and surface frosting or coatings to be measured. These advantages allow the effective analysis of detrital samples, which opens new avenues for geologic applications. Time-resolved parent isotope concentration data, collected by LA-ICP-MS, enables the screening of grains for chemical zonation, a commonly cited cause of over-dispersion in (U-Th-Sm)/He dates. Simultaneous collection of other chemical (e.g., anion or trace-element compositions) or isotopic (UPb age) information for the grain may enhance provenance interpretations. We demonstrate the accuracy and uncertainty of the measurement method using two measurement sessions. We determine the age of the Fish Canyon Tuff apatite reference material to be 27.9 ± 0.6 Ma (n = 45; MSWD 1.5), which is in agreement with previous determinations. Our data reveal that zonation is a common feature in Fish Canyon Tuff apatite and can be resolved by LA-aHe methods to reduce dispersion. LA-aHe methods are ideally suited to detrital studies where a large number of measurements and a reduction in grain-selection bias compared to whole-grain methods should result in the sample better reproducing the source population. The utility of the method is demonstrated using a modern sand from the Wenatchee River, Washington, USA, previously characterized by conventional whole-grain aHe methods. Our LA-aHe dataset (n = 74) better reproduces the aHe ages of bedrock samples within the drainage in comparison to the age distribution derived from conventional aHe dating.