He diffusion in olivine

Helium diffusion in olivine (dunite xenolith) has been measured in the temperature range 1180–1460°C; a linear Arrhenius function was obtained with an activation energy of 120₋₂₇⁺³² kcal/mole, and a pre-exponential factor (D₀) = 2.2 × 10⁸ cm²/s.

Diffusion mechanisms are not a viable means of degassing He from the mantle. Olivine phenocrysts can be expected to retain previously trapped He, during cooling in extrusive basalts, provided the flow units are thinner than 50 m; xenoliths will retain mantle He signatures only if magma transport times are less than 50 years, or if the He fugacity in the magma is high enough to prevent xenolith degassing. The lower oceanic crust is probably substantially degassed of He. Trapped He will be qualitatively retained in quenched submarine basalt glass only if the cooling rate is faster than 5 × 10¹⁴°C/m.y.; glass at several centimeters depth in a basalt flow (near the spherulite zone) will have cooling rates lower than this, so He loss may be significant in many basalt glass samples.