Structure and chemistry of grain boundaries in deformed, olivine + basalt and partially molten lherzolite aggregates: evidence of melt-free grain boundaries
Structure and chemistry of grain boundaries in deformed, olivine + basalt and partially molten lherzolite aggregates: evidence of melt-free grain boundaries
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变形橄榄石玄武岩和部分熔融二辉橄榄岩聚集体中晶界的结构和化学：无熔体晶界的证据
DOI：
10.1007/s00410-002-0394-1
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发表时间：
2002
影响因子：
3.5
通讯作者：
D. Kohlstedt
中科院分区：
文献类型：
--
作者：
T. Hiraga;I. Anderson;M. Zimmerman;S. Mei;D. Kohlstedt
抽象。橄榄石晶界变形聚集体的橄榄石+玄武岩和部分熔融二辉橄榄岩进行了分析与各种电子显微镜技术测试薄（0.5-10 nm）晶间熔体膜的存在下。高分辨率透射电子显微镜（HREM）观察显示，大多数边界不包含薄的非晶相，尽管一小部分晶粒被相对厚（约1 μm）的熔体层隔开。然而，由于橄榄石-熔体界面能的各向异性，熔体通常在1至2 μm的长度上从三重结逐渐变细到相邻的晶界，从而产生仅约2°的有效二面角。通过扫描透射电子显微镜（STEM）使用能量色散X射线（EDX）分析橄榄石-橄榄石晶界的化学，探针尺寸<1.5 nm。Ca、Al和Ti偏析到晶界，形成宽度小于7 nm的富集区。虽然这些元素集中在玻璃相中，但与大块玻璃相具有相同化学组成的玻璃膜的存在不能解释边界处的其他元素（例如Si和Al）的浓度。结合HREM结果，STEM/EDX分析表明固体晶粒之间存在化学偏析，但没有薄的晶界熔体膜。此外，如果熔体膜存在沿着所有的晶界，如报告的类似样品的其他团体，岩石应大大削弱。本研究中分析的部分熔融岩石的蠕变实验表明，在熔体含量较低的情况下，几乎没有减弱，这与我们对无熔体晶界的观察一致。
Abstract. Olivine grain boundaries in deformed aggregates of olivine + basalt and partially molten lherzolite were analyzed with various electron microscopy techniques to test for the presence of thin (0.5–10 nm) intergranular melt films. High-resolution transmission electron microscopy (HREM) observations reveal that most of the boundaries do not contain a thin amorphous phase, although a small fraction of grains are separated by relatively thick (~1 µm) layers of melt. However, due to the anisotropy of the olivine-melt interfacial energy, melt often tapers from a triple junction into an adjoining grain boundary over a length of 1 to 2 µm, giving an effective dihedral angle of only ~2°. The chemistry of olivine-olivine grain boundaries was analyzed using energy dispersive X-ray (EDX) profiling by scanning transmission electron microscopy (STEM) with a probe size of <1.5 nm. Ca, Al and Ti segregate to grain boundaries forming enriched regions of <7 nm width. Although these elements are concentrated in the glass phases, the presence of glass films with the same chemical composition as the bulk glass phases cannot explain concentrations of other elements such as Si and Al at the boundaries. Combined with the HREM results, the STEM/EDX profiling demonstrates the existence of chemical segregation between solid grains but the absence of thin, grain boundary melt films. Additionally, if melt films exist along all of the grain boundaries, as reported for similar samples by other groups, the rock should be substantially weakened. Creep experiments on the partially molten rocks analyzed in this study reveal little weakening at small melt contents, consistent with our observations of melt-free grain boundaries.