Vesuvianite: a new mineral species of (U–Th)/He geochronology†

Vesuvianite is a silicate mineral that forms during the early stages of contact metamorphism near marble in skarn deposits. In terms of geochronological information for vesuvianite, there are currently isotope dilution thermal ionization mass spectrometry (ID-TIMS) U–Pb dating, laser ablation sector field inductively coupled plasma mass spectrometry (LA-SF-ICP-MS) U–Pb dating, and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb dating techniques available. However, (U–Th)/He dating in vesuvianite has not yet been carried out. This research aims to use (U–Th)/He dating on previously studied samples of vesuvianite. Upon comparing the dating results with prior U–Pb ages, most samples yielded (U–Th)/He ages consistent with those obtained by LA-SF-ICP-MS U–Pb dating. Among them, Bufa, M1439, and Wilui have slightly older ages than those determined by the U–Pb method. The concentrations of U, Th, and Sm were found to be largely uniformly distributed throughout the fragments after analysing the LA-ICP-MS profiles of the fragments. Fragments of M784 and M6635 were selected to investigate the helium diffusion behavior in vesuvianite using high-precision laboratory step heating experiments based on the volume diffusion process. The activation energy of vesuvianite varied between 119.1 and 130.7 kJ mol⁻¹. The closure temperatures ranged from 82.5 to 139.8 °C (a cooling rate of 10 °C/Ma and an effective grain radius of 73–82 μm). All samples underwent Raman spectroscopic analysis, and the results showed that the peak at 410 cm⁻¹ probably reflects the extent of radiation damage. Since the 410 cm⁻¹ peak of the three samples with age differences had a larger full width at half maximum, we suspect that the age variation of the samples was either due to radiation damage inside the fragments or inadequate data size for statistics. Given that the experimental results suggested that this procedure is reliable and practicable, the (U–Th)/He of vesuvianite may prove to be an important tool for future research into skarn deposits.