Applying phase equilibria modelling to igneous systems by coupling trace element partitioning and accessory phase saturation to compositionally variable thermodynamic modelling in Rcrust

Abstract

Phase equilibria modelling techniques can constrain the equilibrium conditions of major rock-forming minerals, however current thermodynamic databases and activity-composition models do not accommodate minor and trace elements as chemical components in modelled systems. This paper presents the integration of trace element partitioning routines between melt and solid phases in the thermodynamic modelling tool Rcrust in order to effectively estimate the trace element composition of stable phases when melt is present. Since trace elements occur in large abundances in accessory phases for which we do not currently have activity-composition models this paper further presents the integration of accessory phase saturation routines for apatite and monazite, two commonly occurring accessory phases in granitic rocks.This provides a novel methodology for approximating trace element distribution among thermodynamically constrained phases with thermodynamically unconstrained accessory phases, apatite and monazite. A case study of the Peninsula Pluton granodiorite, South Africa, is used to validate the use of accessory phase saturation and trace element partitioning with phase equilibria modelling. The results highlight the importance of major element components that also contribute to accessory phase formation such as calcium in apatite (which can accommodate a non-trivial quantity of the available calcium) and the resultant effect on phase equilibria of major phases. Saturating apatite can alter phase equilibria, mainly for calcium-bearing phases, that is equivalent to up to 20°C change in temperature. Modelling results for the Peninsula Pluton predicts LREE concentrations of apatite and monazite that match the natural samples at temperatures below 725°C, corroborating previous findings for emplacement P-T constraints. However, phase assemblages predicted through phase equilibria modelling show that the rock equilibrated to P-T below the emplacement estimates of previous studies. By modelling both major and accessory phases we provide constraints on the formation and crystallisation of crustal magmas which has important implications on geothermobarometry and thermochronology in the crust.