Plume-ridge interaction in the Easter-Salas y Gomez seamount chain-Easter microplate system: Pb isotope evidence

Abstract

Lead isotope ratios are reported on fresh basalt glasses from 28 seamounts located along the western end of the Easter-Salas y Gomez seamount chain (ESC), southeast Pacific for the purpose of testing the mantle plume source-migrating ridge sink (MPS-MRS) model for the origin of this extensive intraplate neovolcanic seamount chain. Most of these basalts west of 103°W are less than 1.5 m.y. old. Results reveal a 1100-km-long westward gradient decreasing in radiogenic Pb from Salas y Gomez to the Easter microplate (EMP). The results confirm the previously established four-point average trend based on data from Salas y Gomez, Easter Island, the east rift, and the west rift of the Easter microplate [Hanan and Schilling, 1989]. Tight linear arrays in Pb isotope space of the combined EMP and ESC data sets indicate binary mixing between the radiogenic isotope-rich Salas y Gomez plume and the depleted asthenosphere source is the dominant factor in controlling Pb isotopic variation in the region. Mapping of the plume component mass fraction reveals not only a westward dilution of the plume component from Salas y Gomez to the EMP but also dilution with depleted material across the ESC on both flanks. The mapping is consistent with the MPS-MRS model which calls for a subhorizontal, sublithospheric plume channel connecting the mantle plume conduit near Salas y Gomez with the East Pacific Rise, where plume flow is confined along a thermal groove. Mixing by entrainment of surrounding depleted material takes place progressively downstream. A numerical thermal lithosphere cooling model reveals a rheological boundary layer (i.e., the base of the lithosphere) shoaling by 1.3° to 3.5° from Salas y Gomez to about 100 km east of the EMP. The slope magnitude is consistent with those suggested by laboratory and numerical fluid dynamic models for hot buoyant plume-ridge channeled flow to develop and be maintained.