Fe-Zn isotopes of hydrous oxidized mantle-derived magmas and implications for the control of mantle fertility

Subduction delivers volatile-rich recycled materials into the mantle, producing hydrous and oxidized magmas that are theoretically expected to show heavy Fe and Zn isotopes, due to their sensitivity to oxidized components. However, many oxidized magmas derived from subduction-metasomatized mantle such as arc basalts, generally display lighter Fe-Zn isotopes than mid-oceanic ridge basalts (MORBs). Whether this discrepancy reflects the isotopically light Fe-Zn carried by recycled components or intrinsic mantle depletion remains unclear. To address this issue, we present combined Fe-Zn isotopic data for primitive (Mg# = 65-75), oxidized, volatile-rich lamprophyres and basalts from the eastern North China Craton (NCC). These rocks were derived from the subcontinental lithospheric mantle (SCLM) that was metasomatized by diverse recycled components following prior depletion. They show strong slab fluid/melt fingerprints (e.g., enriched Sr-Nd isotopes, high Ba/Nb) and elevated Fe³⁺/ΣFe (0.35-0.48). The lamprophyres and basalts exhibit MORB-like δ⁵⁶Fe (0.07‰ ± 0.03‰ and 0.08‰ ± 0.02‰, 2SD, respectively) and δ⁶⁶Zn (0.25‰ ± 0.10‰ and 0.27‰ ± 0.02‰, 2SD, respectively), accompanied by relatively low Fe-Zn contents (FeO_t = 8.0-8.4 wt.%; Zn = 93-100 μg/g). Crucially, Fe-Zn isotopes show no correlation with those subduction or redox indicators. In particular, the lamprophyres as low-degree melts generated by preferential melting of fusible and metasomatized portions of the SCLM, still show no significant metasomatic effect on Fe-Zn isotopes. These features, consistent with observations from arc magmas, suggest that metasomatism enriches the depleted mantle in certain incompatible elements (e.g., Sr, Ba) without noticeably replenishing the Fe-Zn budget. Instead, mantle fertility proxies such as Nb/Yb and Zr/Nb in these samples are comparable to MORBs and significantly depleted relative to ocean island basalts (OIBs). Our samples, together with magmas from distinct tectonic settings, define a positive Fe-Zn isotopic trend and also show broad covariations of these isotopes with Nb/Yb and Zr/Nb. We propose that the resultant mantle fertility, regardless of the derivation from prior melt depletion or mantle metasomatism, exerts first-order control on Fe-Zn isotope compositions in mantle-derived magmas.