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

摘要

A natural quartz sample free of mineral and fluid inclusions was irradiated with a 200 MeV proton beam to produce spallogenic Ne-21, He-3 and He-4. 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 degrees 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 Ne-21 and He-3, 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 similar to 70 kJ/mol higher activation energy for neon diffusion compared to helium diffusion likely represents an energy barrier related to its similar to 13% greater diameter and provides a fundamental constraint with which to test theories of solid state diffusion. The diffusion parameters for proton-induced He-4 are indistinguishable from those for He-3, 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. Copyright (c) 2005 Elsevier Ltd.