Fluid-rock interactions and amphibolitisation of the lower continental crust (The Kråkenes Gabbro, Western Gneiss Region, Norway)

To understand numerous geological processes, like element recycling or plate dynamics, the quantification of fluid-induced reactions in the Earth’s crust and mantle is an important but challenging subject, especially for short-lived events including substantial mass exchange. Lithium can serve as a powerful tool to quantify timing and fluid-flow mechanisms that happen on short geological timescales, because it is a very fast diffusing element and usually appears as a trace element in both fluid and rock.

The Kråkenes Gabbro is part of a fossil continent-continent collision zone, located in the Western Gneiss Region in Norway, and shows the effects of fluid-rock interaction perfectly.  The low permeability gabbro is cross-cut by strictly N-S-trending fractures, which opened during exhumation, serving as a pathway for an aqueous fluid to infiltrate the rock. Metasomatism occurred under amphibolite-facies conditions, resulting in a sharp amphibolite-generating reaction front propagating on dm-scale into the magmatic gabbro. This reaction is driven by strong chemical gradients between the reactive fluid and the dry, metastable gabbro. Samples were taken as continuous profiles (~ 30 cm length) perpendicular to the vein and analyzed using a) SEM automated quantitative mineralogy mapping to quantify evolving mineral assemblages during amphibolite-facies metamorphism and b) MC ICP-MS to determine variations in bulk rock lithium concentrations and isotope compositions along the profile.

To understand fluid-flow mechanisms, reactive flow-based diffusion models were created, and model accuracy was checked by integrating measured mineral and lithium data. Mass balance calculations and recalculations of the gabbro and amphibolite mineral assemblages give information on the fluid composition and its transported elements, showing that the fluid-induced reaction is not diffusion-limited only. Furthermore, these models portray the evolving reaction front and the evolution of physical parameters such as mineral assemblage, density or porosity within it. Our investigations into lithium concentrations and δ⁷Li values show that lithium is transported by the fluid into the formerly almost dry system and thus propagated into the gabbro. Reaction-induced variations in e.g. porosity and partition coefficients are included into lithium-diffusion models to find the minimum misfit between measured and modelled lithium data to estimate the duration of the fluid-induced reaction.