The Earth’s continental crust is thought to originate from melting of the mantle, but it is too felsic and depleted in Fe relative to a primary mantle melt. This depletion in Fe is also commonly found in continental arc magmas and is often attributed to magnetite crystallization. However, Fe depletion in arc magmas coincides with an enrichment in ferric Fe relative to ferrous Fe, which cannot be explained by removal of a ferric Fe-rich mineral like magnetite. Deep-seated garnet pyroxenite arc cumulates (arclogites) have Fe-rich compositions that complement the Fe-depleted nature of the continental crust and continental arc magmas, and are likely candidates for the “missing link” between basaltic mantle magmas and the felsic continental crust. To test this suggestion, we present high precision in-situ Fe valence data for garnets in arclogites and reconstruct whole rock Fe valence states. We show that arclogites have low bulk Fe3+/ΣFe due to the low Fe3+/ΣFe of garnets and the lack of magnetite. At high pressures, garnet crystallizes, but magnetite does not, the former causing preferential accumulation of ferrous Fe at the base of magmatic orogens. Arclogite fractionation thus leads to the formation of oxidized felsic residual liquids (Fe3+/ΣFe of 0.2-0.4). Such oxidation may profoundly influence the speciation of magmatic volatiles as well as the oxidative weathering capacity of the crust.
Tang, M., Lee, C. A., Costin, G., Höfer, H. E., (2019). Recycling reduced iron at the base of magmatic orogens. Earth and Planetary Science Letters. Elsevier B.V. 528, 115827. doi.org/10.1016/j.epsl.2019.115827