Hafnium, neodymium, and strontium isotope and parent‐daughter element systematics in basalts from the plume‐ridge interaction system of the Salas y Gomez Seamount Chain and Easter Microplate

Abstract

We present a comprehensive data set with Hf, Nd, and Sr isotope ratios and parent‐daughter trace element concentrations in 111 basalts and glasses from seamounts of the western Easter–Salas y Gomez Seamount Chain (ESC), the Easter Microplate (EMP) spreading centers, and the East Pacific Rise (EPR). Sr and Pb radiogenic isotope ratios and related ratios of highly incompatible parent to lesser incompatible daughter elements grade from high values near the Salas y Gomez (SyG) hot spot location to low values 1000 km west. Here the west rift of the EMP is dominated by typical depleted mid‐ocean ridge basalt (MORB). Hf and Nd radiogenic isotope ratios show the opposite gradients, which also reflect the long‐term enriched nature of the hot spot source and mixing of the hot spot with the depleted upper mantle. Gradients of these parameters occur north and south along the EMP boundaries. These observations confirm the plume‐ridge interaction model proposed for this region by Schilling et al. and further characterize the SyG hot spot. The binary mixing relationship evident in the isotope variations of the basalts is somewhat compromised when the trace elements are considered. This complexity can be explained by modification of trace element abundances during the process of partial melting of the two end‐member components (enriched hot spot and depleted upper mantle). In addition, the melting variability is evident in the basalt bulk compositions, which range from tholeiitic (EMP to 108°W) to alkali basalts (108°W to SyG). A single highly correlated data array in Pb‐isotope space and a linear Pb‐Sr isotope relation indicate that the SyG hot spot is homogeneous. The similarity of the Pb isotope ratios of the SyG hot spot to other long‐term high‐U/Pb mantle domains suggests an origin in subduction‐modified altered oceanic crust. The SyG end‐member isotope composition appears to be pervasive in the south central Pacific mantle, evoking a widespread mantle contamination event by the SyG hot spot in the past.