Reconstructing the P-T-t evolution recorded in garnet through growth and multicomponent diffusion modelling

Ben Steven Knight , Christopher Clark, Ian Fitzsimons

Interpreting the P –T evolution of a region recorded in garnet requires understanding of how the major elements in garnet (Fe, Mg, Mn and Ca) are modified by diffusion. We present an integrated approach that combines thermodynamic modelling with MAGEMin and multicomponent diffusion modelling using UWDiffusion, built on underworld3, to constrain the P–T–t evolution of a garnet pyroxenite from the Western Gneiss Region, Norway. We use the differential evolution minimisation technique to first estimate prograde P–T conditions from intersecting garnet end-member isopleths, which indicate garnet growth occurred between 9.5 and 14 kbar at isothermal conditions (∼475 ◦C) and then records burial and heating up to 19 kbar and 580◦C. Peak conditions of above 20 kbar and ∼650◦C were attained based on the lack of amphibole present in the sample and amphibole stability from thermodynamic modelling. Although thermodynamic modelling predicts garnet stability up to peak conditions, local depletion of garnet-forming elements stops growth. The modelling reveals that the preservation of compositional zoning indicates that temperatures remained below ∼700◦C during exhumation, preventing substantial diffusional re-equilibration. Thermodynamic modelling reveals that the measured bulk stabilises epidote rather than the observed zoisite and a systematic investigation of the epidote–zoisite transition as a function of Fe2O3 content demonstrates that zoisite is stable only when the effective matrix Fe2O3 drops below ∼1.0 wt%, highlighting a prograde redox transition as the rock subducted, which coincides with garnet growth ceasing. This integrated approach demonstrates how coupling thermodynamic and diffusion modelling can be utilised to resolve the P–T–t record of a region through the observed mineral assemblage and garnet zoning.