Laser microprobe (U-Th)/He geochronology

A new analytical method had been developed to enable high-spatial-resolution (U-Th)/He dating of accessory minerals. It involves the use of a focused ArF excimer to ablate pits in a polished grain surface, with the evolved gases spiked for isotope-dilution measurement of radiogenic ⁴He. These data are converted to concentrations by precise measurement of each pit using an optical interferometric microscope. U, Th, and Sm concentration measurements are made using one of several alternative microanalytical techniques (e.g., wavelength-dispersive electron microprobe analysis or laser-ablation, inductively coupled plasma mass spectrometry). By way of illustration, we present both conventional and laser microprobe (U-Th)/He dating results for a Brazilian monazite sample. Laser microprobe data (28 measurements on two crystal fragments) yield a weighted mean (U-Th)/He date of 455.3 ± 3.7 Ma (2SE). This result is statistically indistinguishable from the mean conventional (U-Th)/He date for three separate grain fragments: 449.6 ± 9.8 Ma (2SE). The agreement of conventional and laser ablation dates should encourage a wide variety of applications of the technique, including: (1) detrital mineral dating for provenance and unroofing studies; (2) the dating of broken, included, highly zoned, or irregular grains which are not easily corrected for α-ejection; and (3) measuring ⁴He loss profiles that can be inverted to determine cooling histories.