GEOCHEMICAL CONSEQUENCES OF FLUID FLOW IN POROUS BASALTIC CRUST CONTAINING PERMEABILITY CONTRASTS

Abstract

Changes in mineral stability and solubility due to temperature or pressure variations along fluid flowlines are potential causes of metasomatism in monolithologic hydrothermal systems. The conditions under which significant changes in rock composition can occur have been evaluated by combining steady state models of fluid flow and heat transport in a section of porous basaltic crust with reaction path calculations. A region of enhanced permeability, which may result from a combination of faulting and dyke emplacement, was represented by a vertically aligned 100 m wide layer located within the zone of fluid upflow. In basaltic systems at temperatures between 200 and 350°C and pressures below 1 kb, the reactions most likely to be observed involve Ca and Na mass transfer between pore fluids and greenschist facies mineral assemblages (e.g., albite, epidote, prehnite, chlorite, quartz, actinolite). For models where permeability (k) is homogeneous with k = 10-15 m2 or less, and for fluids of approximately seawater salinity, very little metasomatic reaction is expected over timescales less than 105 years. Addition of a high permeability layer within the upwelling portion of the hydrothermal system results in formation of regions of intense metasomatism, characterised by Ca enrichment and alkali removal, located near the boundary between the two permeability domains. These metasomatic rocks form within timescales from 105 to 104 years in systems where the high permeability layer has k = 5 x 10-15 to 2 x 10-13 m2. These reactions occur in response to temperature variations, and pressure effects are expected to be negligible. Within the permeable layer, one can expect to see a zone of feldspar dissolution and epidote and quartz precipitation advancing across a zone of earlier albitisation. Similar mineralogical changes are seen in epidosite rocks from ophiolites, which formed under conditions similar to those modelled here.