Manganese partitioning between minerals and melts: Implications for tracing garnet fractionation during differentiation of continental arc magmas

Garnet fractionation is proposed to play a crucial role in driving calc-alkaline differentiation of continental arc magmas and the genesis of andesitic continental crust, yet identifying clear geochemical signatures of garnet fractionation remains challenging. Because of its high compatibility in garnet, manganese (Mn) offers a promising proxy for tracing garnet fractionation. Here, we experimentally determine Mn partition coefficients (DMn) between mafic minerals (olivine, orthopyroxene, clinopyroxene, garnet, and amphibole), magnetite and silicate melts at 0.5–3.5 GPa and 850–1400 °C with fO2 varying from FMQ−2.0 to FMQ+8.2 (FMQ: fayalite-magnetite-quartz fO2 buffer). Our results confirm that Mn is more compatible in garnet (DMn=2.8–19.5) than in other mafic minerals (0.6–7.7) and magnetite (1.7–5.6), and reveal that melt composition, particularly MgO content, exerts the dominant control on mineral-melt Mn partitioning. Using these data, we model MnO-MgO trends produced by fractionation of various mineral assemblages. The modeling demonstrates that only substantial fractionation (> 20–30 wt.%) of garnet and/or magnetite can produce a pronounced decrease in magma MnO with decreasing MgO. Given that magnetite rarely exceeds 10 wt.% of the fractionating assemblage due to its high Fe content, and assuming that fractional crystallization dominates the compositional variation of evolved arc magmas, we propose that a rapid decline in MnO content with decreasing MgO provides a robust geochemical indicator of significant garnet fractionation. Application of this Mn-based index to continental arc magmas reveals that significant garnet fractionation occurs only during the late stage of magma differentiation (MgO ≤ 5 ± 1.5 wt.%) in the Northern Andean, Central Andean, Cascades, and Mexican arcs. These findings provide new constraints on the role of garnet in the evolution of continental arc magmas and the formation of andesitic continental crust.