MnO/MgO ratios of arc basalts highlight the role of early garnet fractionation

Abstract

Multiple lines of evidence suggest that garnet may play an important role in the generation of arc magmas, either as a residual phase during mantle melting or subsequently during crystallisation differentiation. Moreover, garnet stability is strongly pressure sensitive, and therefore garnet fractionation can serve as an indirect indicator of fractionation pressure. Here, we introduce MnO/MgO ratios as a compositional proxy uniquely sensitive to garnet fractionation. We show that primary mantle melts have nearly invariant MnO/MgO ratios that are in equilibrium with mantle olivine. Therefore, the subsequent evolution of this ratio is only a function of magmatic differentiation. Further, based on compiled experimental studies, garnet is the only phase that crystallises from basaltic magmas and preferentially partitions MnO relative to MgO. Thus, limited increases in MnO/MgO ratios during magmatic differentiation, as we observe in most continental arcs, are strong evidence for early garnet fractionation and require that crystallisation differentiation begins at or below the Moho of many arcs.