Isotopic fractionation accompanying helium diffusion in basaltic glass
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The relative rates of (3)He and (4)He diffusion in basaltic glass set limits on the extent to which diffusive loss can alter initial magmatic (3)He/(4)He ratios. Typically a value of 1.15 for the isotopic diffusivity ratio, D(3)He/D(4)He, has been assumed, because this corresponds to the inverse square-root of the masses ratio, (m(4)/m(3))(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 degrees C. Assuming Arrhenius temperature dependence and extrapolating suggests that insignificant He isotopic fractionation will occur at Seafloor temperatures (D(3)He/D(4)He = 1.02 +/- 0.03), with more pronounced fractionation at magmatic temperatures (e.g. 1.12 +/- 0.02 at 1100 degrees 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 (3)He/(4)He ratio by 10% requires 65% gas loss for D(3)He/D(4)He equal to 1.15, but 80% loss for D(3)He/D(4)He 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 D(3)He/D(4)He 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. (C) 1999 Elsevier Science B.V. All rights reserved.