Formation of saline brines and salt in the lower crust by hydration reactions in partially retrogressed granulites from the Lofoten Islands, Norway

Abstract

Anorthosites, gneisses and mangeritic rocks from the Lofoten Islands in northern Norway contain Cl-rich amphibole (up to 3.5 wt percent Cl) and biotite (up to 1.5 wt percent Cl) that developed mostly during regional metamorphism at ~600°C and ~9 kb by replacement of older anhydrous granulite facies mineral assemblages. (Na,K)Cl halide solid solutions are closely associated spatially with amphiboles that have the highest measured Cl-contents. Rocks with Cl-rich amphiboles and biotites always have minerals that exhibit a wide range in OH/Cl; one sample from a shear zone contains virtually Cl-free amphibole within a few millimeters of Cl-rich amphibole coexisting with halide crystals. Textural evidence indicates that amphibole and biotite evolved from Cl-poor to Cl-rich compositions within individual samples; OH-rich, Cl-poor compositions are always much more abundant than Cl-rich ones. Cl-rich amphiboles and biotites exclusively occur in rocks that contain relict granulite facies minerals (pyroxene and/or olivine); samples in which no granulite facies minerals remain contain amphiboles and biotites without elevated Cl-contents. In one particular case, the restricted development of Clrich amphiboles and biotites within a shear zone and the absence of these minerals from undeformed lithologic equivalents outside the shear zone demomstrate that the amphibole and biotite developed by reaction between a preexisting granulite facies mineral assemblage and an aqueous fluid rather than by fluid-ansent metamorphism or by magmatic processes. We infer from this example that Cl-rich amphibole nad biotite with the same grani size and texture as in the shear zone developed throughout the Lofoten Islands by fluid-rock reaction. Mineralfluid equilibria indicate that the aqueous fluid was variable in composition ranging from XH2O (=[H2O/(H2O+Cl)] of 0.99 for equilibrium with the most Cl-poor amphiboles and biotites to 0.4-0.6 for equilibrium with the most Cl-rich minerals that coexist with salt. The most saline fluids correspond to NaCl equivalent concentrations of over 25 mol/l. The range in OH/Cl of amphiboles and biotite in individual samples and the restricted occurrence of Cl-rich minerals in rocks with relict pyroxene and/or olivine can be explained by a desiccation model in which small amounts of normal low-Cl crustal fluid infiltrates rocks with granulite facies minerals and is subsequently enriched in Cl by preferential consumption of H2O during the resulting hydration reaction. Saline fluids developed at the extremes of reaction progress and then produced the Cl-rich amphibole and biotite compositions coexisting with halide crystals. The desiccation model further explains variations in the whole-rock concentration of Cl in rocks from the Lofoten Islands, the absence of Cl-rich minerals from completely retrogressed granulites that lack olivine or pyroxene, the measured frequency distribution of amphibole and biotite compositions in individual samples, and the calculated evolution of fluid composition with reaction progress. Our results have implications for interpreting measurements of elevated electrical conductivity in the lower crust and for models of granulite genesis that appeal to fluids with low H2O activity. Amer. J. Science (1998), 298, 705-757