粒間流体の実態(<特集>地殻流体)

概要

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Investigation of intergranular regions of fine-grained metamorphic rocks using various electron microscopy reveals the presence of intergranular pores previously filled with an aqueous fluid. Such intergranular fluid inclusions exhibit a characteristic shape which minimizes interfacial energy at intergranular regions. The distribution of the inclusions indicates that they were formed from fluid filled intergranular microcracks that ovulated into inclusions due to the initially unstable form of a fluid film at grain boundaries. A simple calculation of interfacial stresses produced by anisotropic thermal contraction of quartz grains during cooling of the quartz aggregate demonstrates that most grain boundaries in crustal rocks experience intense intergranular cracking accompanied by infiltration of fluids. Presence of the fluid filled cracks might explain the observations of low electrical resistivity and seismic wave speeds at middle to shallow crustal depths. The inhibition of the crack formation due to the interfacial stress relaxation in ductile crustal regions results in higher electrical resistivity and seismic wave speeds at greater deep. Intergranular chemical components in mantle rocks are ubiquitously found in mantle xenoliths. We can attribute these components to intergranular melts which were present in the mantle.
2012-12-31