Towards a comprehensive use of garnet as an indicator of geodynamic-geochemical processes

Metamorphic garnet can be the ultimate source of information about geodynamic and geochemical processes in the Earth’s crust. Compositionally zoned garnet porphyroblasts preserve records of the host rock’s reaction path, its element transport properties, the fluid-rock interaction during metamorphism as well as absolute ages and rates of these processes. Especially variations of rare earth element (REE) concentrations in garnet are reflecting many of these geodynamic and geochemical processes. In order to extract this information, the thermodynamic equilibrium and kinetic contributions of the REE uptake in garnet must be distinguished and quantified, for which high resolution major- and trace element mappings together with numerical growth simulations are an indispensable tool.   

Utilizing high resolution trace element and µ-Raman mapping together with combined thermodynamic-geochemical-diffusion models we discriminate the equilibrium and kinetic aspects of the heavy (H) REE uptake in a garnet porphyroblast from a high-pressure/low temperature blueschist from the Dominican Republic. Like many metamorphic garnets from different rock types and tectonic settings, the HREE zoning in the investigated garnet comprises an inner, bell-shaped part with a pronounced central peak and an overall decrease of all HREE from core to inner rim. The central peak in the garnet core decreases in intensity with decreasing atomic number of the REE. This trend is followed by a concentric zone of HREE enrichment and a drastic HREE decrease towards the outermost rim. Superimposed on this common trend is a concentric pattern of minor recurring fluctuations in the HREE concentrations with regularly spaced sets of peaks and troughs along the entire garnet radius.

Comparison of the trace element mappings and thermodynamic-geochemical models show that the inner, bell-shaped part results from fractional garnet growth in an unchanged mineral assemblage. The model results further show that the width of the central peak is controlled by the bulk permeability of the interconnected transport matrix and the fraction of matrix minerals that the garnet equilibrates with. The correlation of high resolution µ-Raman mappings of the inclusion suite and the trace element mappings indicate that the REE enrichment zone is caused by HREE redistribution during the titanite-rutile transition.

The superimposed REE fluctuations result from changing element transport properties of the host rock and mark recurring changes from equilibrium REE uptake to transport-limited REE uptake in garnet. Such fluctuating element transport properties can be best explained by pulse-like fluid fluxes that rhythmically change the interconnectivity of the intercrystalline transport matrix. Increasing numbers of published spatially highly resolved REE analyses show that such trace element fluctuations are common in metamorphic garnet indicating that recurring changes in rock permeabilities due to pulsed fluid fluxes are a common phenomenon during metamorphism.