Widespread contamination of the Pacific upper mantle during the mid-Cretaceous

Abstract

This study presents new ¹⁷⁷Hf/¹⁷⁶Hf measurements on variably altered samples from the Early Cretaceous Pacific oceanic crust subducting into the Izu-Bonin trench. The new results are then combined with existing major and trace element and NdPb isotopic data to provide further constraints on existing models for the evolution of the Mesozoic Pacific upper mantle. The samples, from an across flow line transect of the oceanic crust accreted along the Pacific-Izanagi ridge system, can be separated into a relatively older (140–127 Ma), mostly normal mid-ocean ridge basalt (N-MORB) group and a younger (<127 Ma) group that includes many enriched (E-)MORBs. The HfNd (±Pb) isotope and alteration-resistant incompatible trace element composition of the older N-MORBs is similar to those of the geochemically depleted Jurassic-Early Cretaceous MORBs and Shatsky-Hess Rise platform lavas. On the other hand, the younger E-MORBs are compositionally similar to those of coeval lavas from Ontong Java, Manihiki and Hikurangi plateaus, which were suggested to be previously con-joined as Ontong Java Nui super oceanic plateau, and Ojin Rise Seamounts, which were suggested to be the younger manifestation of the Shatsky-Hess Rise plume. We propose that the older Pacific-Izanagi N-MORBs originated from a depleted MORB mantle (DMM):FOZO (focal zone) mixed source whereas the younger E-MORBs originated from a DMM:FOZO:EMI (enriched mantle 1) ± high μ (HIMU) mixed source. Our results support the proposed widespread contamination of the Pacific upper mantle by geochemically enriched materials that formed the ancestral Ontong Java Nui plateau starting at ca. 125 Ma.