The Cosmogenic 3He Production Rate in Ilmenite and Redistribution of Spallation-Produced 3He in Fine-Grained Minerals

Cosmogenic nuclide surface exposure dating and erosion rate measurements in landscapes with basalt bedrock rely primarily on measurement of ³He in olivine or pyroxene. However, geochemical investigations using ³He have been impossible in the substantial fraction of basalts that lack separable olivine or pyroxene crystals, or where such crystals were present, but have been chemically weathered. Fine-textured basalts often contain small grains of ilmenite, a weathering-resistant mineral that is a target for cosmogenic ³He production with good He retention and straightforward mineral separation, but with a poorly constrained production rate. We empirically calibrated the cosmogenic ³He production rate in ilmenite by measuring ³He concentrations in basalts with fine-grained (~20 μm cross-section) ilmenite and co-existing pyroxene or olivine from the Columbia River and Snake River Plain basalt provinces. The concentration ratio of ilmenite to pyroxene and olivine is 0.78±0.02, yielding an apparent cosmogenic ³He production rate of 93.6±7.7 atom g^-1 yr^-1 that is 20-30% greater than expected from prior theoretical and empirical estimates for compositionally similar minerals. The production rate discrepancy arises from the high energy with which cosmic ray spallation reactions emit tritium and ³He and the associated long stopping distances that cause them to redistribute within a rock. Fine-grained phases with low cosmogenic ³He production rates, like ilmenite, will have anomalously high production rates owing to net implantation of ³He from the surrounding, higher ³He production rate, matrix. Semi-quantitative modeling indicates implantation of spallation ³He increases with decreasing ilmenite grain size, leading to production rates that exceed those in a large grain by ~10% when grain radii are <150 μm. The modeling predicts that for the ilmenite grain size in our samples, implantation causes production rates to be ~20% greater than expected for a large grain, and within uncertainty resolves the discrepancy between our calibrated production rate, theory, and rates from previous work. These findings indicate grain size should be quantified to reduce uncertainly in exposure ages and erosion rates determined by measuring ³He in ilmenite.