| The rheological behaviour of crustal and upper mantle rock plays a key role in controlling virtually all geodynamic phenomena occurring in the outer Earth. In much of the crust, time dependent deformation (creep) of rock is dominated by stress-induced diffusive mass transfer through water-bearing mineral grain boundaries (pressure solution). However, the kinetics of the underlying inter-granular reaction and diffusion processes remain poorly understood and poorly quantified. The present project aims to determine the dissolution behaviour and diffusive properties of silicate grain contacts undergoing active pressure solution, by means of a new technique. This involves measuring the electrical impedance of pressure-dissolving bi-crystal contacts, using in-situ impedance spectroscopy carried out in a purpose-built, high pressure-temperature cell recently constructed at Utrecht. Grain contacts studied will include quartz-quartz, quartz-mica, quartz-feldspar and feldspar-feldspar boundaries. Dilute electrolyte solutions will be used as the aqueous fluid phase, enabling grain boundary diffusivities to be calculated from impedance data using the Nernst-Einstein relation. The results will help solve long-standing controversy regarding the diffusive properties and dissolution behaviour of wetted grain boundaries in silicate rocks, and will provide much-needed data for modelling fluid-assisted deformation phenomena in the Earth. Phenomena of special interest include the creep and healing behaviour of fault zones, the seismic cycle and porosity reduction in sedimentary basins. The implications of our results for these and other geodynamic processes will be assessed using constitutive models derived from our data plus selected tectonophysical modelling experiments. |
