Radiogenic and Stable Isotope Evidence for Recycling of Ancient Oceanic Crust in High‐Nb Basalts From Baja California, Mexico

Abstract The genesis of high‐Nb basalts (HNB) in subduction zones remains controversial, attributed to melting products of metasomatized mantle linked to recent or ancient subduction events. Here we present radiogenic isotope and molybdenum compositions for post‐subduction HNBs and adakites from Baja California, Mexico. The HNBs show more radiogenic Sr‐Pb and less unradiogenic Nd‐Hf isotopes compared to the adakites. Their Mo isotopic enrichment (δ 98/95 Mo), up to −0.12‰ relative to the depleted mantle (−0.21‰), is linked to FOZO (Focal Zone)‐like Sr‐Nd‐Hf‐Pb isotopic signatures. These features indicate compositional contribution of recycled, ancient oceanic crust (>1 Gyr), carrying heavy Mo under reduced conditions through serpentinite‐related interactions and losing varying amounts of Rb, Nd, Hf, and Pb relative to bulk oceanic crust during subduction dehydration. Our new data reveal ancient contributions to HNB formation and underscore the role of subducted oceanic crust and serpentinite to the formation of FOZO mantle signatures.