ASSESSING INCLUSIONS AND Li CONCENTRATIONS IN MAGNETITE FOR COSMOGENIC 3He DATING

Magnetite is a ubiquitous mineral that is highly retentive to helium and, therefore, a potential target phase for cosmogenic ³He exposure studies. However, magnetite grains frequently contain mineral and fluid inclusions, which can increase ³He concentrations and lead to erroneous results. Previous studies have shown that using micro computed tomography (microCT) pre-screening to select only grains without inclusions for geochronologic applications can significantly improve the resulting data. This study evaluates the influence of lithology and geologic environments on the number of inclusions as well as Li concentrations to inform sampling strategies. We surveyed >3000 magnetite grains from a range of lithologies, including felsic to ultramafic volcanic and intrusive igneous rocks, as well as hydrothermal and detrital magnetite. Grains were characterized by type, number, and volume fraction of inclusions using 3D microCT and this data was compared to 2D SEM pre-screening. MicroCT pre-screening detected more inclusions than SEM and is thus a more reliable method. We also measured Li concentrations in a number of magnetite grains because nucleogenic ³He can be produced from ⁶Li. This process increases the overall ³He concentration and can lead to erroneous results if not taken into account. Magnetite grains from (ultra-)mafic rocks are generally more suitable for ³He studies since they contain few inclusions and have low Li concentrations. Magnetite grains from felsic rocks, however, have higher Li concentrations and more inclusions than those from mafic lithologies, making them potentially more problematic for cosmogenic ³He applications. We discuss study design and sampling strategies to optimize the selection of magnetite grains for cosmogenic ³He studies, including in-situ exposure ages and catchment-averaged erosion rates.