Reevaluating the oxidation effect of garnet crystallization

Garnet has been proposed to be an important fractionating phase during magmatic differentiation in thick volcanic arcs because garnet fractionation can reproduce the hallmarks of continental crust by driving residual magmas to higher silica, lower total iron, and higher iron oxidation states. Recently, Holycross and Cottrell (2023b) have measured the partition coefficients for Fe2+ and Fe3+ between garnet and melt, which allow for more precise modeling of the oxidation effect of garnet fractionation. Using their new experimental results combined with fractional crystallization modeling, Holycross and Cottrell (2023a, 2023b) concluded that garnet crystallization has little influence on arc magma oxidation. However, these conclusions are based on conceptual errors in modeling the effects of garnet. Firstly, they ignored garnet fractionation beyond 50% crystallization which would cause substantial Fe3+ enrichment. Limiting garnet fractionation to the initial 50% crystallization of arc basalts is inconsistent with existing experiments including their own experiments. Secondly, Fe2+ partitioning was indirectly and incorrectly parameterized using FeMg exchange coefficients, inconsistent with experimental partition coefficients. Application of these recent models leads to unrealistic Fe2+ partitioning behavior with progressive fractionation. When these models are corrected with proper mass balance and partitioning relationships, garnet as a driver of oxidation remains a viable and testable hypothesis.