The redox state of arc mantle using Zn/Fe systematics

The redox state of arc mantle The redox state of the upper mantle beneath arcs is relevant to understanding mantle melting and melt differentiation. Because we have no access to samples from the convective mantle, this question has to be addressed indirectly using information derived from the chemistry of melts. Cin-Ty Lee et al . show that the ratio of zinc to total iron content constrains the valence state of iron in primary arc basalts and their mantle sources. They find that primitive arc magmas have Zn/Fe ratios identical to those of mid-ocean ridge basalts, hinting at a similar iron oxidation state of primary mantle melts in arcs and ridges. The results suggest that the subduction of oxidized crustal material may not significantly alter the redox state of the mantle wedge. They conclude that the observed higher oxidation states of arc lavas must therefore be, in part, a consequence of shallow-level differentiation processes. Here it is shown that the ratio of zinc to total iron content constrains the valence state of iron in primary arc basalts and their mantle sources. Primitive arc magmas have identical Zn/Fe T ratios (Fe T = Fe 2+ + Fe 3+ ) as mid-ocean-ridge basalts, indicating a similar iron oxidation state of primary mantle melts in arcs and ridges and that the subduction of oxidized crustal material may not significantly alter the redox state of the mantle wedge. It is concluded that the observed higher oxidation states of arc lavas must therefore be, in part, a consequence of shallow-level differentiation processes. Many arc lavas are more oxidized than mid-ocean-ridge basalts and subduction introduces oxidized components into the mantle 1 , 2 , 3 , 4 . As a consequence, the sub-arc mantle wedge is widely believed to be oxidized 3 , 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 source 6 , 7 . Here we show that Zn/Fe T (where Fe T = Fe 2+ + Fe 3+ ) 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, Fe 2+ and Zn behave similarly, but because Fe 3+ is more incompatible than Fe 2+ , melts generated in oxidized environments have low Zn/Fe T . Primitive arc magmas have identical Zn/Fe T to mid-ocean-ridge basalts, suggesting that primary mantle melts in arcs and ridges have similar Fe oxidation states. The constancy of Zn/Fe T d