Many arc lavas are more oxidized than mid-ocean-ridge basalts and subduction introduces oxidized components into the mantle1,2,3,4. As a consequence, the sub-arc mantle wedge is widely believed to be oxidized3,5. The Fe oxidation state of sub-arc mantle is, however, difficult to determine directly, and debate persists as to whether this oxidation is intrinsic to the mantle source6,7. Here we show that Zn/FeT (where FeT = Fe2+ + Fe3+) is redox-sensitive and retains a memory of the valence state of Fe in primary arc basalts and their mantle sources. During melting of mantle peridotite, Fe2+ and Zn behave similarly, but because Fe3+ is more incompatible than Fe2+, melts generated in oxidized environments have low Zn/FeT. Primitive arc magmas have identical Zn/FeT to mid-ocean-ridge basalts, suggesting that primary mantle melts in arcs and ridges have similar Fe oxidation states. The constancy of Zn/FeT during early differentiation involving olivine requires that Fe3+/FeT remains low in the magma. Only after progressive fractionation does Fe3+/FeT increase and stabilize magnetite as a fractionating phase. These results suggest that subduction of oxidized crustal material may not significantly alter the redox state of the mantle wedge. Thus, the higher oxidation states of arc lavas must be in part a consequence of shallow-level differentiation processes, though such processes remain poorly understood.
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Discussions and debates with D. Canil, R. Lange, E. Cottrell and K. Kelley are appreciated. We especially thank H. O’Neill for insights. This work was facilitated by a Geological Society of America award (to C.-T.A.L.) F.A. was supported by the Keith-Weiss Visiting Professorship at Rice University.
The authors declare no competing financial interests.
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Lee, CT., Luffi, P., Le Roux, V. et al. The redox state of arc mantle using Zn/Fe systematics. Nature 468, 681–685 (2010). https://doi.org/10.1038/nature09617
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