Chemistry of grain boundaries in mantle rocks
Chemistry of grain boundaries in mantle rocks
复制标题
地幔岩石晶界的化学
DOI：
10.2138/am-2003-0709
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发表时间：
2003
影响因子：
3.1
通讯作者：
D. Kohlstedt
中科院分区：
文献类型：
--
作者：
T. Hiraga;I. Anderson;D. Kohlstedt
摘要用扫描透射电子显微镜（STEM）通过能量色散X射线（EDX）谱分析了橄榄岩超糜棱岩、玄武岩中的橄榄石斑晶和橄榄石+透辉石合成压块中橄榄石晶界的成分。在天然和合成样品的晶界上的组合物分布表现出5 nm的特征宽度和镁的耗尽和伴随的钙，铝，钛和铬的富集。已知化学偏析会影响晶界过程，例如晶界扩散、滑动、断裂和迁移，所有这些都会影响多晶聚集体的流变性质。此外，由于晶界富含微量元素，因此这些晶界可能是地幔岩石中这些元素的重要储存场所。幔源熔体与不寻常的组合物，如那些富含钙和/或钛，可能被解释为富集在这些元素中的橄榄石晶界的优先熔融。所有样品中晶界处的共同化学特征表明，化学偏析是一种能量有利的现象，因此应该发生在地球地幔的其他地方。微量元素向晶界的偏聚在地幔动力学和地球化学过程中起着重要作用。
Abstract The compositions of olivine grain boundaries have been analyzed with scanning transmission electron microscopy (STEM) via energy dispersive X-ray (EDX) spectrum profiling in three specimens: a peridotite ultramylonite, olivine phenocrysts in a basaltic rock, and synthesized compacts of olivine + diopside. Composition profiles across grain boundaries in both natural and synthetic samples exhibit a characteristic width of 5 nm and a depletion of Mg and concomitant enrichments of Ca, Al, Ti, and Cr. Chemical segregation is known to affect grain boundary processes such as grain boundary diffusion, sliding, fracture, and migration, all of which influence the rheological properties of polycrystalline aggregates. Also, because grain boundaries are enriched in trace elements, the boundaries can be important storage sites for such elements in mantle rocks. Mantlederived melts with unusual compositions, such as those rich in Ca and/or Ti, might be explained by preferential melting of olivine grain boundaries enriched in these elements. The common chemical signatures at grain boundaries in all samples indicate that chemical segregation is an energetically favorable phenomenon and thus should occur elsewhere in Earth’s mantle. Segregation of trace elements to grain boundaries may play an important role in dynamical and geochemical processes in Earth’s mantle.