Hydration of Mafic Crust – Analytical and Numerical Insights

Abstract

The grain-scale mechanisms of hydration of mafic lower crustal rocks are investigated through partially amphibolitized samples from the Hustad Igneous Complex in the Western Gneiss Region, Norway. Chemical maps were obtained on continuous rock sections of 20 cm by 2 cm and supplemented with EMP point measurements. Numerical models are employed to reproduce reactive fluid flow driven by a fluid pressure gradient. The Proterozoic pyroxenite body and dolerite dike show different responses to the exposure to hydrous fluids along fractures formed during late Caledonian extension and exhumation. While the dolerite reaches full amphibolitization in a cm-scale reaction halo with a dm-scale transition zone, the pyroxenite has experienced previous metamorphism and is less affected by this event. Dissolution precipitation reactions and slightly faster grain boundary assisted flow are identified as the main mechanisms of fluid flow through the rock and limited element mobility is documented by grain scale chemical gradients in forming amphibole. Phase diagram calculations yield a P/T-window between 650 – 730°C and 0.4 – 0.6 GPa for amphibolite formation. 1D numerical models of reactive fluid flow driven by a fluid pressure gradient show the reaction and reproduce differences in front propagation speed between the two lithologies as observed in the samples. A simple 2D model is employed to demonstrate that the gradual transition from dolerite to amphibolite can be achieved by implementing higher permeability along grain boundaries, supported by the observation that flow along boundaries continues before individual grains are fully replaced.