Diffusion kinetics of 3He and 21Ne in quartz and implications for cosmogenic noble gas paleothermometry

For both He and Ne, modeled Arrhenius diffusion parameters (activation energy, Ea, and pre-exponential factor, D0) display a range of values in the quartz samples analyzed. For 3He, Ea ranges from 73.0 to 99.8 kJ/mol and D0 from 5.9 × 100 to 1.0 × 104 cm2 s−1 for the initial, low-temperature linear Arrhenius arrays; when observed, a second array at higher temperatures corresponds to Ea ranging from 85.2 to 106.4 kJ/mol and D0 from 1.7 × 10−1 to 3.5 × 100 cm2 s−1. For 21Ne, Ea ranges from 95.7 to 153.8 kJ/mol and D0 from 6.6 × 10−1 to 3.2 × 103 cm2 s−1 for the initial, low-temperature array; linearity at high temperatures is not well constrained, likely because the α- to β-quartz transition occurs during the relevant temperature range. When extrapolated to Earth surface temperatures and geologically relevant timescales, these results suggest that 1 mm-radius quartz grains lose significant amounts of cosmogenic 3He by diffusion at sub-zero temperatures from the low-retentivity domain over >103 yr timescales and from the high-retentivity domain over >104 yr, whereas quantitative retention of cosmogenic 21Ne occurs over >106 yr at temperatures ⩽40 °C in most cases. While these results are generally consistent with previously reported studies, they also reveal that sample-specific diffusion parameters are required for quantitative application of cosmogenic noble gas paleothermometry. The cosmogenic 3He abundance in one quartz sample with a simple Holocene exposure history and the stepwise degassing pattern of cosmogenic 3He and 21Ne from another quartz sample with a ∼1.2 Ma exposure history agree well with diffusion experiments on proton-irradiated aliquots of the same samples. For the sample with a simple Holocene exposure history, a production and diffusion model incorporating sample-specific diffusion parameters and the measured 3He abundance predicts an effective diffusion temperature consistent with the effective modern temperature at the sample location. This internal consistency demonstrates that the empirically determined, sample-specific diffusion kinetics apply to cosmogenic 3He and 21Ne in quartz in natural settings over geologic timescales.