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摘要
Lattice diffusion of neon in single, gem-quality crystals of quartz, forsterite, and natural iron-bearing olivine was determined from experiments using two different methods to introduce Ne. In the first set of experiments, Ne-22 was implanted into polished samples, which were then annealed for various times and temperatures in 1-atm furnaces. In the second group of experiments, referred to as soaking experiments, polished slabs of crystals were placed in open containers and inserted into cold-seal type pressure vessels where they were exposed to Ne gas at 340-1340 bars Ne pressure and elevated temperatures. Nuclear reaction analysis, using the reactions Ne-22(p,gamma)Na-23 and Ne-20(alpha,gamma)Mg-24 for the implantation and soaking experiments, respectively, was used to directly measure Ne distributions in the samples following diffusion anneals. The following Arrhenius relations were obtained for Ne diffusion parallel to c: %26lt;br%26gt;Forsterite : D-Fo = 1.4x10(-13) exp(-131 +/- 6 kJ mol(-1)/RT) m(2)s(-1) %26lt;br%26gt;Quartz : D-Qtz = 1.6x10(-14) exp(-115 +/- 4 kJ mol(-1)/RT) m(2)s(-1) %26lt;br%26gt;There are no significant differences in Ne diffusion along the investigated crystallographic axes in either quartz or olivine, and Ne diffusivities for synthetic forsterite and natural Fe-bearing olivine are similar. In addition, diffusivities determined from the soaking experiments and the implantation experiments are similar, indicating little effect on diffusion of any lattice damage that might be induced by the 22Ne ion implantation. The lack of damage is also confirmed by imaging with high-resolution electron microscopy. %26lt;br%26gt;Ne diffusivities in quartz measured in this study are about 10 orders of magnitude slower than those determined from the bulk-release experiments of Shuster and Farley (2005). In the case of olivine, Ne diffusivities in olivine measured in the present study are 1-3 orders of magnitude slower than diffusivities determined by Futagami et al. (1993) and 2-5 orders of magnitude slower than Ne diffusivities determined by Gourbet et al. (2012) over the temperature range of overlap of the respective studies. In addition, activation energies for diffusion differ widely, with the value from of Futagami et al. (1993) significantly lower (87 kJ/mol) than that measured in the present work, and the preferred values of Gourbet et al. (2012) (360-370 kJ/mol) significantly higher than our findings. These differences may reflect the influence of factors that may disproportionately affect measurements of Ne diffusion determined through bulk-outgassing methods compared with direct profiling techniques measuring lattice diffusion such as those employed in this study. %26lt;br%26gt;Our data indicate that both quartz and olivine may be very retentive of Ne at temperature conditions typical near the surface of the earth, with 100 mu m radius quartz and olivine grains experiencing less than 1% Ne loss over times on order of the age of the earth at 100 and 150 C, respectively. Under mantle conditions, effective diffusion distances for Ne in olivine would be about 1 m and 0.2 m over 1 billion years at 1550 and 1100 degrees C, respectively. In contrast to Ne, helium is much more mobile, with effective diffusion distances of 1500 m over 1 billion years at 1500 degrees C, compared with 0.8 m for Ne under the same time-temperature conditions.
- 出版日期2014-4-15
