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 a F-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 energy Ea and pre-exponential factor D0) for He diffusion in grain boundaries were refined from literature data (Ea = 36 ± 9 kJ·mol−1 and D0 = 10−10.57 ± 0.58 m2·s−1), and those of He diffusion in interstitials (Ea = 89 ± 7 kJ·mol−1 and D0 = 10−8.95 ± 1.16 m2·s−1) and Mg vacancies (Ea = 173 ± 14 kJ·mol−1 and D0 = 10−5.07 ± 1.25 m2·s−1) were defined from our results and literature data. Furthermore, we determined Ea = 56 ± 1 kJ·mol−1 and D0 = 10−9.97 ± 0.37 m2·s−1 for 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.

中文翻译：

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变形对多晶镁橄榄石中氦储存和扩散的影响

尽管最近的研究调查了未变形地幔矿物中的氦行为，但塑性变形产生的缺陷对氦存储和运输的影响仍然不受限制。为此，使用帕特森压机在 950、1050 和 1200 °C 下，在 300 MPa 围压下使细粒无铁镁橄榄石的合成致密骨料变形。然后将三个变形样品和一个未变形样品在静态高压（1.00±0.02 GPa）和高温（1120±20°C）条件下在活塞缸中掺杂 24 小时。铀矿被用作稀有气体的来源。随后使用循环步骤加热协议与惰性气体质谱联用对样品进行分析，以研究变形多晶镁橄榄石聚集体中的 He 储存和扩散。结果显示了无法通过单个线性回归拟合的复杂扩散行为。尽管如此，单个步骤加热循环可以通过 F 检验确定的几个线性回归来拟合，这表明扩散率在给定的温度范围内遵循阿伦尼乌斯定律。我们的结果突出了变形镁橄榄石聚集体中 He 的复杂扩散行为，这是由于与不同 He 存储位点（镁空位、间隙位点、位错和晶界）相关的几种扩散机制之间的竞争。He 在晶界扩散的扩散参数（活化能 Ea 和指前因子 D0）从文献数据（Ea = 36 ± 9 kJ·mol−1 和 D0 = 10−10.57 ± 0.58 m2·s−1）中细化，以及间隙中的 He 扩散（Ea = 89 ± 7 kJ·mol−1 和 D0 = 10−8.95 ± 1。16 m2·s−1) 和 Mg 空位（Ea = 173 ± 14 kJ·mol−1 和 D0 = 10−5.07 ± 1.25 m2·s−1）是根据我们的结果和文献数据定义的。此外，我们确定了 Ea = 56 ± 1 kJ·mol−1 和 D0 = 10−9.97 ± 0.37 m2·s−1 沿位错的 He 扩散。这些结果表明，沿着地幔矿物中的位错存储的最大氦分数仅为 1.2%，与先前研究报告的晶界中的 22% 相比，这一比例可以忽略不计。这意味着块体晶格扩散率几乎不受位错的影响，而储存在晶界中的 He 的比例可以显着提高地幔岩石的块体扩散率。因此，变形过程可以通过减小晶粒尺寸（即通过动态再结晶）来显着增加 He 存储容量，但不会充分增加位错密度以引起晶格内 He 存储和迁移率的变化。此外，在与附近未变形（或变形较小）橄榄岩平衡后，He 在矿物晶格和晶界之间的快速重新分布可以提高变形橄榄岩的体积 He 浓度。