Isotopic fractionation accompanying helium diffusion in basaltic glass

The relative rates of He-3 and He-4 diffusion in basaltic glass set limits on the extent to which diffusive loss can alter initial magmatic He-3/He-4 ratios. Typically a value of 1.15 for the isotopic diffusivity ratio, (DHe) -He-3/(DHe)-He-4, has been assumed, because this corresponds to the inverse square-root of the masses ratio, (m(4)/m(3))(1/2). Measurements of the iso topic compositions of sequential releases of He from mid-ocean-ridge and se amount basalt glasses heated in-vacuo reveal this to be an overestimate. Th e observed isotopic diffusivity ratio ranges from 1.06 at room temperature to 1.10 at 500 degrees C. Assuming Arrhenius temperature dependence and ext rapolating suggests that insignificant He isotopic fractionation will occur at Seafloor temperatures (D3He/(DHe)-He-4 = 1.02 +/- 0.03), with more pron ounced fractionation at magmatic temperatures (e.g. 1.12 +/- 0.02 at 1100 d egrees C). These low values mean that significantly larger He losses are re quired to alter initial 3He/4He ratios than was previously assumed, and the se larger losses require much longer loss times. For example, lowering an i nitial He-3/He-4 ratio by 10% requires 65% gas loss for (DHe)-He-3/(DHe)-He -4 equal to 1.15, but 80% loss for (DHe)-He-3/(DHe)-He-4 of 1.08, which req uires 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 (DHe)-He-3/(DHe) -He-4 ratio result from the relatively widely spaced vibrational energy lev els 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.