Mantle source heterogeneity and melting processes beneath seafloor spreading centers: The East Pacific Rise, 18°–19°S

We present new major and trace element and Nd‐Pb‐Sr isotopic data on samples from the East Pacific Rise (EPR) axis and nearby seamounts in the Hump area, 18°–19°S. Most samples studied are normal mid‐ocean ridge basalt (N‐MORB); four samples from the southern seamounts are enriched MORB (E‐MORB). Dredge 52 samples from a southern seamount are depleted in incompatible elements yet possess “enriched” isotopic signatures. Except for the dredge 52 samples, all the samples show significant correlations between isotopic ratios and ratios of incompatible elements; that is, incompatible elements and isotopes are coupled. Sr and Nd isotopic ratios correlate significantly better with ratios of moderately incompatible elements than with ratios involving highly incompatible ones (e.g., Rb, Nb, and K) which appear to be “overenriched”. Both isotopic and incompatible element ratios also correlate with the extent of melting calculated from major elements. We interpret these correlations as mixing trends resulting from melting of a heterogeneous source containing enriched (“plume‐like”) domains of variable sizes. Overenrichment of highly incompatible elements in E‐MORB appears to be recent and is best explained by low‐degree‐melt infiltration in the source region prior to major melting events. The low‐degree melts are primarily derived from isotopically N‐MORB mantle. This low‐degree melt process also explains the incompatible element‐isotope decoupling throughout the EPR between 13° and 23°S. The dredge 52 samples too are consistent with such a process, but their immediate source is a site of low‐degree melt generation.