Abstract

The Juan Fernandez Ridge (JFR) is an age-progressive volcanic chain (similar to 800 km long) related to a fixed mantle plume in the Southeast Pacific offshore central Chile. The high He-3/He-4 ratio (up to 18 times higher than that of the atmosphere) and spatiotemporal Ar-40/Ar-39 geochronology suggest that the source material of the JFR is derived from the lower mantle and transferred to the surface by a relatively stationary plume. We used new whole-rock geochemical data (major-element, trace-element, and Sr-Nd-Pb isotopic data) for representative samples from the shield-stage of volcanism in the JFR (from O'Higgins Guyot, Alpha Seamount, Robinson Crusoe Island, and Alejandro Selkirk Island) to develop a petrogenetic model with the goal of understanding the temporal and spatial evolution of magmatism along the JFR. The shield-building lavas of JFR consist primarily of tholeiitic to alkalic basalts. Their compositional differences are explained by the fractional crystallization of olivine and clinopyroxene +/- plagioclase, magmatic recharge, melt mixing, and olivine accumulation. Radiogenic Sr-Nd-Pb isotopes show a narrow field within the compositional range of the common FOZO mantle. The (206)pb/(204)pb and Pb-207/Pb-204 ratios of JFR lavas are similar to those of other islands on the Nazca Plate( e.g., San Felix and San Ambrosio). However, the JFR lavas are more radiogenic and show a narrower compositional range compared to the Easter Seamount Chain. The low CaO content at a given MgO content, moderate Ti-Ta-Nb 'TITAN' anomaly, fractionated heavy rare earth element values, and isotopic composition of JFR lavas are consistent with the presence of pyroxenite (recycled oceanic crust) in the mantle source. To estimate source parameters, we used OBS1 software (Kimura and Kawabata, 2015) to calculate the potential temperature (1316-1412 degrees C), total degree of melting (3.4-19.2 wt%), and pyroxenite fraction (0.6-18.4 wt%) of the mantle beneath JFR. The temporal changes in the thermal, compositional, and lithologic characteristics of the source material can explain the chemical differences observed between different JFR volcanoes. We propose that shield-stage volcanism in JFR is largely generated by the melting of pyroxenite in a relatively low-temperature mantle plume. As a result, this weak plume containing low mantle He is difficult to image using seismic tomography. (C) 2019 Elsevier Ltd. All rights reserved.