Quartz grain boundaries as fluid pathways in metamorphic rocks

Abstract

[1] 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.

Pressure solution in rocks: focused ion beam/transmission electron microscopy study on orthogneiss from South Armorican Shear Zone, France

“…Recently, Kruhl et al (2013) has shown that partially opened grain and phase boundaries are commonly observed in different types of rocks, such as quartzites, granitoids and marbles. Our observations support this study and document a number of partially opened interfaces which serve as fluid pathways especially at the late stage of the rock evolution at temperature < 300°C, which is the temperature for the brittle-ductile transition in quartz (Voll 1976).…”

“…For the operation of such mechanism, the space for fluid transport has to be available, either in the form of porosity, partially-opened grain boundaries or fractures. In natural rocks such as granites and quartzites, the grains and phase boundaries are usually not perfectly straight, and many grain and phase boundaries might be partially opened (Kruhl et al 2013) thus forming an interconnected network. If a fluid phase is present at the grain/phase boundary and the interface is simultaneously subject to high differential stress, dissolution occurs at the surface where solubility of a phase is enhanced and higher density of crystal lattice defects occurs.…”

Reconstruction of 3D grain boundaries from rock thin sections, using an advanced polarised‐light microscopy method

“…Assuming a linear superposition, the retardation of light passing through the boundary between two grains 1 and 2 can be approximated by the linear relationship: where d 1, d 2 are the distances traversed in the respective grains and Δ n 1, Δ n 2 the corresponding birefringence. However, thin amorphous films may occur along grain boundaries (Wirth, ), or they may be open on the nanometre scale (Kruhl et al ., ), which can lead to light scattering. Therefore, Eq.…”

“…where d1, d2 are the distances traversed in the respective grains and n1, n2 the corresponding birefringence. However, thin amorphous films may occur along grain boundaries (Wirth, 1996), or they may be open on the nanometre scale (Kruhl et al, 2013), which can lead to light scattering. Therefore, Eq.…”

On the survival of intergranular coesite in UHP eclogite

“…Alternatively, the ability of the host to delay infiltration of fluid during exhumation could be critical. At lower pressures, open grain boundaries or fracturing may create fluid pathways (Kruhl, Wirth, & Morales, 2013), but at the associated lower temperatures reaction kinetics are slower inhibiting inversion (Mosenfelder & Bohlen, 1997;Mosenfelder et al, 2005). Other processes once considered important include rapid exhumation and continuous cooling during exhumation, but these have been discounted as primary factors for the reasons discussed by Mosenfelder et al (2005).…”