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Diffusion kinetics of proton-induced ^(21)Ne, ^3He, and ^4He in quartz

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A natural quartz sample free of mineral and fluid inclusions was irradiated with a 200 MeV proton beam to produce spallogenic ^(21)Ne, ^3He and ^4He. Temperature-dependent diffusivities of these three nuclides were then determined simultaneously by high precision stepped-heating and noble gas mass spectrometry. The outward mobility of proton-induced nuclides reflects diffusion through the quartz lattice. In the studied range of 70 to 400°C the helium diffusion coefficients exceed those of neon by 5–7 orders of magnitude. The implied diffusion parameters E_a = 153.7 ± 1.5 (kJ/mol) and ln(D_o/a^2) = 15.9 ± 0.3 (ln(s^(−1))) and E_a = 84.5 ± 1.2 (kJ/mol) and ln(D_o/a^2) = 11.1 ± 0.3 (ln(s^(−1))) for proton-induced ^(21)Ne and ^3He, respectively, indicate that cosmogenic neon will be quantitatively retained in inclusion-free quartz at typical Earth surface temperatures whereas cosmogenic helium will not. However, the neon diffusion parameters also indicate that diffusive loss needs to be considered for small (<1 mm) quartz grains that have experienced elevated temperatures. Since natural quartz often contains fluid inclusions which may enhance noble gas retentivity, these parameters likely represent an end-member case of purely solid-state diffusion. The ∼70 kJ/mol higher activation energy for neon diffusion compared to helium diffusion likely represents an energy barrier related to its ∼13% greater diameter and provides a fundamental constraint with which to test theories of solid state diffusion. The diffusion parameters for proton-induced ^4He are indistinguishable from those for ^3He, providing no evidence for the commonly expected inverse square root of the mass diffusion relationship between isotopes. We also find preliminary indication that increased exposure to radiation may enhance neon and helium retentivity in quartz at low temperatures.

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