Metamorphic fluids and anomalous porosities in the lower crust

Abstract

Because of the plastic behavior of rocks at high temperature and confining pressure, the porosity, φ, of lower crustal rocks is a variable dependent on the amount of free fluid present. On the basis of hydrologic theory it is predicted that the permeability, k, of such rocks can be expressed by a relationship of the form k ≈ ωφ n , where ω and n are material specific constants. The exponent n is likely to vary between 3 and 6, whereas ω, which is ultimately related to grain size and shape, probably varies over several orders of magnitude. Consequently, substantial contrasts between the φ- k relationships of lower crustal rocks are to be expected. The case in which thin crustal layers, “aquitards”, are characterized by relatively low values of ω or n, with respect to the crust in general, is of particular interest. Such layers would acquire high porosities in the presence of miniscule steady state crustal fluid fluxes and are a possible explanation for anomalous seismic “brightspots” and high electrical conductivities in the lower crust. Modelling of collision belt metamorphism suggests that even long after the cessation of active metamorphism and isostatic reequilibration, relict metamorphic fluid fluxes could generate aquitard porosities, between 0.1 and 0.4%, adequate to explain such geophysical anomalies.