期刊
JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH
卷 118, 期 5, 页码 1957-1967出版社
AMER GEOPHYSICAL UNION
DOI: 10.1002/jgrb.50099
关键词
quartz grain boundary; quartz cell reduction; grain boundary cavity; fluid pathway; transmission electron microscopy; focused ion beam technique
资金
- German Research Council [KR 691/33]
TEM and SEM/FIB sequential imaging of quartz grain boundaries from contact and regional metamorphic rocks show that most of the grain boundaries are open on the nanometer scale. Three types of voids occur. (i) Roughly 40-500nm wide open zones parallel to the grain boundaries. They are suggested to be caused by general volume reduction as a result of anisotropic cooling contraction at temperatures decreasing below ca. 300 degrees C, the threshold temperature of diffusion in quartz and of decompression expansion at pressures decreasing below several hundred MPa. (ii) Cavities of variable shape and up to micrometer size along the open grain boundaries and (iii) cone-shaped, nanometer-sized depressions at sites where dislocation lines meet the open grain boundaries. The latter two types are generated by dissolution-precipitation processes. Open grain boundaries, cavities, and depressions form a connected network of porosity, which allows fluid circulation and may affect physical properties of the rocks. The same process is suggested to occur along grain and phase boundaries in other rocks as exemplified in this study, and it should be expected along intracrystalline cracks or cleavage planes.
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