Neon diffusion kinetics in olivine, pyroxene and feldspar: Retentivity of cosmogenic and nucleogenic neon

We performed stepwise degassing experiments by heating single crystals of neutron- or proton-irradiated olivine, pyroxene and feldspar to study diffusion kinetics of neon. This is important in evaluating the utility of these minerals for cosmogenic 21Ne measurements and, potentially, for Ne thermochronometry. Degassing patterns are only partially explained by simple Arrhenius relationships; most samples do not exhibit a precisely-determined activation energy in an individual diffusion domain. Regardless, we find clear differences in diffusion kinetics among these minerals. Based on sub-selected data, our estimates for neon diffusion kinetics (activation energy Ea and pre-exponential factor Do, assuming the analyzed fragments approximate the diffusion domain) in each mineral are as follows: for the feldspars, Ea ranges from ∼65 to 115 kJ/mol and Do from 3.9 × 10−3 to 7.1 × 102 cm2s−1; for the pyroxenes, Ea ranges from ∼292 to 480 kJ/mol and Do from 1.6 × 102 to 2.9 × 1011 cm2s−1; for the olivines, Ea ranges from ∼360 to 370 kJ/mol and Do from 1.5 × 106 to 5.0 × 106 cm2s−1. Differences in these parameters are broadly consistent with the expected effect of structural differences between feldspar, and olivine and pyroxene. These results indicate that cosmogenic 21Ne will be quantitatively retained within olivine and pyroxene at Earth surface temperatures over geological timescales. The diffusion kinetics for feldspars, on the other hand, predicts that 21Ne retention at Earth surface temperatures will vary significantly with domain size, crystal microtexture, surface temperature, and exposure duration. Quantitative retention is expected only in favorable conditions. This conclusion is reinforced by additional measurements of cosmogenic 21Ne in coexisting quartz and feldspar from naturally irradiated surface samples; sanidine from a variety of rhyolitic ignimbrites exhibits quantitative retention, whereas alkali-feldspar from several granites does not.