Ambient compositional heterogeneity of the Pacific upper mantle revealed by the Cretaceous Naifeh-Plumeria seamounts

The origin of compositional heterogeneities in the upper mantle remains a subject of intense debate. This work examines the geochemical and isotopic features of the off-axis Cretaceous Naifeh-Plumeria (N-P) seamount chains in the North Pacific (∼88–85 Ma), shedding light into the composition of the ambient upper mantle away from mid-ocean ridges. The N-P seamounts are composed of strongly differentiated alkalic lavas ranging from hawaiites to mugearites. The lavas display a strong enrichment in trace elements that are highly incompatible during partial melting of the mantle, with a positive Nb-Ta anomaly and relative depletion of heavy rare-earth elements. Radiogenic isotope ratios reveal a dominantly FOZO mantle component with ⁸⁷Sr/⁸⁶Sr_i of 0.7029 to 0.7032, εNd_i of +3.7 to +7.8, and ²⁰⁶Pb/²⁰⁴Pb_i of 18.78 to 19.65. These isotopic signatures are distinct from typical Pacific E-MORB. The lack of a west-to-east age progression and the compositional similarities with mugearites from the Pukapuka Ridge suggest the N-P seamounts originated from the mantle upwelling caused by small-scale convection underneath ∼20 Ma-old oceanic lithosphere. We propose that the small-scale convection cells were generated perpendicular to the ancient Pacific-Farallon Ridge, contaminating the upper mantle with plume-like material from one or more of the nearby active hotspots. Our results suggest the presence of enriched heterogeneities in the Pacific upper mantle beyond the common type of enriched source that is recorded by E-MORB from modern Pacific spreading centers.