Physics
Scientific paper
Oct 1984
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984e%26psl..70..297h&link_type=abstract
Earth and Planetary Science Letters (ISSN 0012-821X), vol. 70, no. 2, Oct. 1984, p. 297-302.
Physics
22
Gaseous Diffusion, Helium, Mineralogy, Olivine, Dunite, Earth Crust, Geochemistry, Outgassing
Scientific paper
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 +32, -27 kcal/mole, and a preexponential factor of 2.2 x 10 to the 8th sq cm/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 about 50 m; xenoliths will retain mantle He signatures only if magma transport times are less than about 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 about 5 x 10 to the 14th C/Myr.; 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.
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