ASSESSING THE SUITABILITY OF MAGNETITE FOR (U-TH)/HE AND COSMOGENIC 3HE EXPOSURE DATING: A SURVEY OF INCLUSIONS AND LI CONCENTRATIONS

Magnetite is a ubiquitous mineral phase that is highly retentive to helium. Therefore, magnetite is a potential target phase for (U-Th)/He and cosmogenic ³He exposure dating in many geological environments and can be a useful tool to assess fluid-rock interactions, exhumation, weathering, and erosion. However, magnetite grains can contain mineral and fluid inclusions that contribute non-radiogenic ⁴He and non-cosmogenic ³He, leading to incorrect measured ages. Micro-computed X-ray tomography (microCT) is a useful tool to detect the presence and relative density of inclusions within individual magnetite grains. We test the suitability of magnetite separates from different lithologies and environments through a systematic investigation of inclusions in magnetite grains, including characterizing the number of inclusions and their respective mineral phases through microCT scanning and SEM/EDX. Studied samples include igneous (including volcanic and ultramafic) magnetite, porphyry Cu-related hydrothermal magnetite, and detrital magnetite grains derived from multiple sources. We also characterize the U, Th, and Li concentrations of grains with and without inclusions to assess the impact of the presence of inclusions on measured bulk (U-Th)/He and cosmogenic ³He exposure ages. We statistically analyze the mineralogy and frequency of inclusions, inclusion volumes, and elemental concentrations according to their respective lithologies. We find that inclusions vary by sample and only samples with sufficient inclusion-free grains are suitable for dating studies. This study establishes a pre-screening protocol for magnetite grains and provides insight into which lithologies might be preferable for selecting magnetite for (U-Th)/He and cosmogenic ³He exposure dating.