科研之友
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摘要
Diffusion of helium has been characterized in four carbonates: calcite, dolomite, magnesite, and aragonite. Cleaved or oriented and polished slabs of carbonate minerals were implanted with 100 keV or 3 MeV He-3 at doses of 5 x 10(15) He-3/cm(2) and 1 x 10(16) He-3/cm(2), respectively, and annealed in 1-atm furnaces. He-3 distributions following diffusion experiments were measured with nuclear reaction analysis using the reaction He-3(d,p)He-4. Our results show that He diffusion in calcite is the fastest among the carbonates studied, with diffusivities progressively slower in magnesite, dolomite and aragonite. In the case of the isomorphic trigonal carbonates (calcite, dolomite, magnesite), these observations are broadly consistent with predictions based on lattice characteristics such as unit cell size and inter-atomic apertures, with diffusivities faster in more open carbonate structures. Dolomite is an exception to this trend, suggesting that its unique ordered R3 crystal structure may play a role in slowing helium diffusion. Diffusion is anisotropic in all of the trigonal carbonates, and is typically slowest for diffusion along the c direction, and faster for diffusion normal to c and in directions normal to cleavage surfaces. The patterns of diffusional anisotropy are predicted to first order by the size of limiting inter-atomic apertures along any given crystallographic direction, providing additional support to the concept of modeling crystal lattices as "molecular sieves" with regard to diffusion of helium. When the effects of anisotropy and diffusion domain size are considered, our results are in reasonable agreement with previous results from bulk degassing of natural samples. Modeling of helium diffusive loss shows that calcite and magnesite are unlikely to be retentive of helium on the Earth's surface for typical grain sizes and time/temperature conditions. Dolomite and aragonite may be retentive under cooler conditions, but because helium retention is strongly dependent on diffusion domain size, general predictions are difficult given the structural complexities of natural samples. Our axis-specific diffusion measurements across a range of carbonate compositions, evaluated through direct profiling, offer important constraints for modeling helium mobility in carbonates, and for understanding the influence of the complexities of carbonate structures on He outgassing patterns.
- 出版日期2015-9-15
