Deep open storage and shallow closed transport system for a continental flood basalt sequence revealed with Magma Chamber Simulator

Abstract

Abstract The Magma Chamber Simulator (MCS) quantitatively models the phase equilibria, mineral chemistry, major and trace elements, and radiogenic isotopes in a multicomponent–multiphase magma + wallrock + recharge system by minimization or maximization of the appropriate thermodynamic potential for the given process. In this study, we utilize MCS to decipher the differentiation history of a continental flood basalt sequence from the Antarctic portion of the ~ 180 Ma Karoo large igneous province. Typical of many flood basalts, this suite exhibits geochemical evidence (e.g., negative initial εNd) of interaction with crustal materials. We show that isobaric assimilation-fractional crystallization models fail to produce the observed lava compositions. Instead, we propose two main stages of differentiation: (1) the primitive magmas assimilated Archean crust at depths of ~ 10‒30 km (pressures of 300–700 MPa), while crystallizing olivine and orthopyroxene; (2) subsequent fractional crystallization of olivine, clinopyroxene, and plagioclase took place at lower pressures in upper crustal feeder systems without significant additional assimilation. Such a scenario is corroborated with additional thermophysical considerations of magma transport via a crack network. The proposed two-stage model may be widely applicable to flood basalt plumbing systems: assimilation is more probable in magmas pooled in hotter crust at depth where the formation of wallrock partial melts is more likely compared to rapid passage of magma through shallower fractures next to colder wallrock.