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Isotopic fractionation accompanying helium diffusion in basaltic glass

Citation

Trull, T and Kurz, MD, Isotopic fractionation accompanying helium diffusion in basaltic glass, Journal of Molecular Structure, 485 pp. 555-567. ISSN 0022-2860 (1999) [Refereed Article]

DOI: doi:10.1016/S0022-2860(99)00057-5

Abstract

The relative rates of 3He and 4He diffusion in basaltic glass set limits on the extent to which diffusive loss can alter initial magmatic 3He/4He ratios. Typically a value of 1.15 for the isotopic diffusivity ratio, D3He/D4He, has been assumed, because this corresponds to the inverse square-root of the masses ratio, (m4/m3)(1/2). Measurements of the isotopic compositions of sequential releases of He from mid-ocean-ridge and seamount basalt glasses heated in-vacuo reveal this to be an overestimate. The observed isotopic diffusivity ratio ranges from 1.06 at room temperature to 1.10 at 500°C. Assuming Arrhenius temperature dependence and extrapolating suggests that insignificant He isotopic fractionation will occur at seafloor temperatures (D3He/D4He = 1.02 ± 0.03), with more pronounced fractionation at magmatic temperatures (e.g. 1.12 ± 0.02 at 1100°C). These low values mean that significantly larger He losses are required to alter initial 3He/4He ratios than was previously assumed, and these larger losses require much longer loss times. For example, lowering an initial 3He/4He ratio by 10% requires 65% gas loss for D3He/D4He equal to 1.15, but 80% loss for D3He/D4He of 1.08, which requires twice as long to occur. Consideration of solid-state diffusion theory, and comparison to cation and other noble gas diffusivities, suggests that the low values and positive temperature dependence of the D3He/D4He ratio result from the relatively widely spaced vibrational energy levels of the He atoms in the silicate glass structure. Temperature dependent diffusive He isotopic fractionation is likely in other geologic materials.

Item Details

Item Type:Refereed Article
Research Division:Earth Sciences
Research Group:Geochemistry
Research Field:Geochemistry not elsewhere classified
Objective Division:Expanding Knowledge
Objective Group:Expanding Knowledge
Objective Field:Expanding Knowledge in the Earth Sciences
Author:Trull, T (Professor Thomas Trull)
ID Code:16934
Year Published:1999
Web of Science® Times Cited:24
Deposited By:CRC-Antarctic and Southern Ocean Environm
Deposited On:1999-08-01
Last Modified:2011-08-05
Downloads:0

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