Mineral reactions and associated fractionation processes, like dehydration reactions, trigger arc-related volcanism, mantle metasomatism and control the trace element transport into the mantle wedge. Fractional crystallisation in the subducted plate recycles elements in refractory phases into the deeper mantle. Quantifying these fractionation processes is an important task in petrology. Major element zonations in UHP minerals, such as garnet, often preserve detailed information about the temporal physico-chemical evolution of the host rock. With the aid of thermodynamic forward models that consider element fractionation along distinct P-T paths detailed information about the shape of the P-T path, mineral reactions and changing phase relations during subduction can be extracted from such growth zonations.
Apart from major elements metamorphic minerals often preserve complex trace element zonations that contain information about mineral reactions and associated kinetic processes, like transport- or interface-controlled mineral growth. Knowledge of such disequilibrium processes is crucial for the understanding of mineral growth and yields insight into element transport within and between reacting bodies.
In this work we combine thermodynamic forward models that bear detailed information on mode and major element composition of stable phases with a mass balance distribution of rare earth elements (REE) among the modelled stable assemblage during high pressure metamorphism. The aim is to interpret a combination of major- and trace element zonation patterns in complexely zoned UHP garnets from the Western Gneiss Region (Norway), in order to study trace element distribution and thermodynamic equilibrium among fluid and solid phases in a subducted slab.
All investigated garnets show multiple growth zones and complex growth zonation patterns with respect to major and rare earth elements. Despite differences in the major element zonation the REE patterns are almost identical in all garnets. The REE core-to-rim variations can be divided into four zones with characteristic patterns. Due to fractionation effects and contrasting REE patterns in the reactants, core-to-rim REE zonation in garnet develops distinct enrichment zones characteristic for the minerals involved in the garnet-forming reaction.
Our thermodynamic models can reproduce the complex major element zonations and growth zones in the natural garnets and predict garnet growth during four different reactions: (1) chlorite-breakdown, (2) epidote-breakdown, (3) amphibole-breakdown and (4) garnet growth from cpx at UHP conditions. Mass-balanced REE distribution among the stable phases yielded REE zonations in garnet that closely resemble those observed in the natural samples. Our results indicate that garnet fractionation largely controls the REE budget in the subducted slab and explain the partition of light and heavy REEs between garnet cores and rims which has important implications for geochronology.
