Petrology and geochemistry of the Galápagos Islands: Portrait of a pathological mantle plume

We report new major element, trace element, isotope ratio, and geochronological data on the Galápagos Archipelago. Magmas erupted from the large western volcanos are generally moderately fractionated tholeiites of uniform composition; those erupted on other islands are compositionally diverse, ranging from tholeiites to picritic basanitoids. While these volcanos do not form a strictly linear age progressive chain, the ages of the oldest dated flows on any given volcano do form a reasonable progression from youngest in the west to oldest in the east, consistent with motion of the Nazca plate with respect to the fixed hotspot reference frame. Isotope ratios in the Galápagos display a considerable range, from values typical of mid-ocean ridge basalt on Genovesa (⁸⁷Sr/⁸⁶Sr: 0.70259, ɛ_Nd: +9.4, ²⁰⁶pb/²⁰⁴Pb: 18 44), to typical oceanic island values on Floreana (⁸⁷Sr/⁸⁶Sr: 0.70366, ɛ_Nd: +5.2, ²⁰⁶pb/²⁰⁴Pb: 20.0). La/Sm_N ranges from 0.45 to 6.7; other incompatible element abundances and ratios show comparable ranges. Isotope and incompatible element ratios define a horseshoe pattern with the most depleted signatures in the center of the Galápagos Archipelago and the more enriched signatures on the eastern, northern, and southern periphery. These isotope and incompatible element patterns appear to reflect thermal entrainment of asthenosphere by the Galápagos plume as it experiences velocity shear in the uppermost asthenosphere. Both north-south heterogeneity within the plume itself and regional variations in degree and depth of melting also affect magma compositions. Rare earth systematics indicate that melting beneath the Galápagos begins in the garnet peridotite stability field, except beneath the southern islands, where melting may occur entirely in the spinel peridotite stability field. The greatest degree of melting occurs beneath the central western volcanos and decreases both to the east and to the north and south. Si_8.0, Fe_8.0, and Na_8.0 values are generally consistent with these inferences. This suggests that interaction between the plume and surrounding asthenosphere results in significant cooling of the plume. Superimposed on this thermal pattern produced by plume-asthenosphere interaction is a tendency for melting to be less extensive and to occur at shallower depths to the south, presumably reflecting a decrease in ambient asthenospheric temperatures away from the Galápagos Spreading Center.