Effect of deformation on helium storage and diffusion in polycrystalline forsterite
Effect of deformation on helium storage and diffusion in polycrystalline forsterite
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DOI：
10.1016/j.gca.2020.01.018
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发表时间：
2020-03
影响因子：
5
通讯作者：
Rémi Delon;S. Demouchy;Y. Marrocchi;M. Bouhifd;J. Gasc;P. Cordier;S. Koizumi;P. Burnard
中科院分区：
文献类型：
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作者：
Rémi Delon;S. Demouchy;Y. Marrocchi;M. Bouhifd;J. Gasc;P. Cordier;S. Koizumi;P. Burnard
虽然最近的研究已经研究了未变形的地幔矿物中的氦行为，但塑性变形产生的缺陷对氦的储存和运输的影响仍然不受限制。为此，利用百特森压力机，在300 的压力下，在950、1050和1200 °C的压力下，对细粒无铁镁橄榄岩的合成致密集合体进行了变形。3个变形样品和1个未变形样品在静高压(1.00 ± 0.02 Gpa)和高温(1120 ± 20 °C)条件下，在活塞缸中掺入He 24 h。铀矿被用作惰性气体的来源。随后采用循环阶梯加热和惰性气体质谱仪相结合的方法对样品进行分析，以研究氦在变形的多晶镁橄榄岩聚集体中的储存和扩散。结果显示了复杂的扩散行为，不能用单一的线性回归来拟合。然而，单个阶梯加热循环可以由AF检验确定的几个线性回归来拟合，这表明在给定的温度范围内，扩散系数遵循阿雷尼乌斯定律。我们的结果突出了He在变形镁橄榄岩聚集体中的复杂扩散行为，这是由于与不同的He存储位置(镁空位、间隙位置、位错和晶界)相关的几种扩散机制之间的竞争所致。He在晶界扩散的参数(Ea= 36 ± 9 kJ·ol−1和D0= 10−10.57±0.58 m~2·S−1)和在间隙中扩散的参数(Ea= 89 ± 7 kJ·m ol−1和D0= 10−8.95±1.16 m~2·S−1)和镁空位扩散参数(E_a= 173 ± 14 kJ·m ol−1和D_0= 10−5.07±1.25 m~2·S−1)。此外，我们还测定了He沿位错扩散的Ea= 56 ± 1 kJ·−1和D0= 10−9.97±0.37 m2·S−1。这些结果表明，最大只有1.2%的He可以沿着位错储存在地幔矿物中，与以前研究报告的22%的晶界相比，这是微不足道的。这意味着晶格扩散系数几乎不受位错存在的影响，而储存在晶界的氦的比例可以显著提高地幔岩石的扩散系数。因此，变形过程可以通过减小晶粒尺寸(即通过动态再结晶)来显著增加He的存储容量，但不会充分增加位错密度以引起晶格中He的存储和迁移率的变化。此外，He在矿物晶格和晶界之间的快速重新分配可以提高变形橄榄岩与附近未变形(或较小变形)橄榄岩平衡时的整体He浓度。
Although recent studies have investigated He behavior in undeformed mantle minerals, the effect of defects generated by plastic deformation on He storage and transport remains unconstrained. For this purpose, synthetic dense aggregates of fine-grained iron-free forsterite were deformed under 300 MPa confining pressure at 950, 1050, and 1200 °C using a Paterson press. Three deformed samples and one undeformed sample were then doped with He under static high-pressure (1.00 ± 0.02 GPa) and high-temperature (1120 ± 20 °C) conditions for 24 h in a piston cylinder. Uraninite was used as a source of noble gases. The samples were subsequently analyzed using a cycled step heating protocol coupled with noble gas mass spectrometry to investigate He storage and diffusion in the deformed polycrystalline forsterite aggregates. Results show complex diffusive behaviors that cannot be fitted by a single linear regression. Nevertheless, individual step heating cycles can be fitted by several linear regressions determined by aF-test, suggesting that diffusivities follow Arrhenius law within the given temperature ranges. Our results highlight the complex diffusive behavior of He in deformed forsterite aggregates, which is due to the competition between several diffusion mechanisms related to different He storage sites (Mg vacancies, interstitial sites, dislocations, and grain boundaries). Diffusion parameters (activation energyEaand pre-exponential factorD0) for He diffusion in grain boundaries were refined from literature data (Ea= 36 ± 9 kJ·mol−1andD0= 10−10.57 ± 0.58m2·s−1), and those of He diffusion in interstitials (Ea= 89 ± 7 kJ·mol−1andD0= 10−8.95 ± 1.16m2·s−1) and Mg vacancies (Ea= 173 ± 14 kJ·mol−1andD0= 10−5.07 ± 1.25m2·s−1) were defined from our results and literature data. Furthermore, we determinedEa= 56 ± 1 kJ·mol−1andD0= 10−9.97 ± 0.37m2·s−1for He diffusion along dislocations. These results suggest that a maximum He fraction of only 1.2% can be stored along dislocations in mantle minerals, which is negligible compared to 22% in grain boundaries as reported by previous studies. This implies that bulk lattice diffusivities are barely affected by the presence of dislocations, whereas the proportion of He stored in grain boundaries can significantly enhance the bulk diffusivities of mantle rocks. Thus, deformation processes can significantly increase He storage capacity by decreasing grain size (i.e., via dynamic recrystallization), but will not sufficiently increase the dislocation density to induce a change in He storage and mobility within the crystallographic lattice. Furthermore, rapid redistribution of He between the mineral lattice and grain boundaries could enhance the bulk He concentrations of deformed peridotites upon equilibration with nearby undeformed (or less-deformed) peridotites.