Diffusion of helium in SiC and implications for retention of cosmogenic He

Diffusion of helium has been characterized in silicon carbide of cubic and hexagonal (4H and 6H) forms. Polished sections of SiC were implanted with ³He at 100 keV at a dose of 1 × 10¹⁵/cm². The implanted SiC samples were sealed under vacuum in silica glass ampoules, and annealed in 1-atm furnaces. ³He distributions following all experiments were measured with Nuclear Reaction Analysis using the reaction ³He(d,p)⁴He. For He diffusion in cubic SiC and 4H hexagonal SiC we obtain the following Arrhenius relations: D_cubic=1.83×10^-6exp(-254±10kJmol^-1/RT)m²s^-1. D_4H=4.78×10^-7exp(-255±29kJmol^-1/RT)m²s^-1. While He diffusion is considerably slower in SiC than in many silicate phases, He retentivity may be limited under some conditions. For example, helium will be lost from SiC grains over much shorter timescales than potential survival times of SiC presolar grains in the solar nebula. When exposed to impact heating followed by slow cooling, nearly complete loss of He from SiC grains near the site of impact will occur within several hours to a few days. For SiC grains at greater distance from impact sites, He would be better retained, depending on the rapidity of cooling. At tens of km away from a large impactor, where peak T would be ∼800 K, SiC grains would lose about 50% of their He if the grains cooled within a few thousand years, and 5% if they cooled within a few tens of years. At greater distances where heating is more modest (500 K and lower), SiC grains would be quite retentive of He even for cases of very slow cooling. Helium would also be retained in cases of impact heating followed by very rapid cooling. For these short heating pulses, 10 μm diameter SiC grains would retain more than 50% of their He for peak heating temperatures of 2173, 1973 and 1773 K for durations of 3, 10 and 60 s, respectively.