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Quartz grain boundaries as fluid pathways in metamorphic rocks
Author(s) -
Kruhl Jörn H.,
Wirth Richard,
Morales Luiz F. G.
Publication year - 2013
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1002/jgrb.50099
Subject(s) - grain boundary , quartz , metamorphic rock , grain size , grain boundary strengthening , geology , materials science , mineralogy , composite material , microstructure , petrology
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 40–500 nm 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°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.