The geochemistry of basalts from the Easter microplate boundaries and the western Easter-Salas y Gomez seamount chain: A comprehensive study of mantle plume-spreading center interaction.

摘要

A comprehensive data set with Hf, Nd, Sr, and Pb isotope ratios, 33 trace element concentrations in 118 basalts and basalt glasses from seamounts of the western Easter-Salas y Gomez seamount chain (ESC), and the Easter microplate (EMP) spreading centers and the East Pacific Rise (EPR) is presented. In addition, Deuterium/Hydrogen isotope ratios were measured in 22 of these glasses. Sr and Pb radiogenic isotope ratios, D/H, and ratios of highly incompatible elements to less incompatible elements grade from high values near Salas y Gomez (SyG) to low values 1000 km west of the SyG hotspot, where the west rift of the EMP is characterized by typical depleted mid-ocean ridge basalt (MORB) similar to MORBs from the EPR. Hf and Nd radiogenic isotope ratios show the opposite gradients from low values at SyG to high values at the EMP west rift, also reflecting the long-term enriched nature of the plume source and mixing of the plume material with the depleted upper mantle. These observations confirm the plume-ridge interaction model proposed for this region. The positive correlations of δD with Sr and Pb isotope ratios indicate that the Salas y Gomez mantle plume with high time-integrated U/Pb ratio (HIMU)-affinity is rich in deuterium and in H₂O. These observations are consistent with an ancient hydrothermal alteration of oceanic crust, which is then recycled as a mantle plume.; Binary mixing is the dominant process responsible for the variation in isotope ratios across the region based on the tight nearly linear data array in Pb-Pb isotope space. Other radiogenic isotope versus radiogenic isotope relationships are also nearly linear in confirmation of the binary mixing hypothesis. In representations of isotope ratio versus trace element ratios or trace element ratios versus trace element ratios, the binary mixing relationship is compromised by increasing scatter in proportion to decreasing degree of incompatibility of the trace elements. This complexity can be explained by fractional partial melting effects of a two-component solid mantle mixture.