PETROLOGY AND GEOCHEMISTRY OF THE GALAPAGOS-ISLANDS - PORTRAIT OF A PATHOLOGICAL MANTLE PLUME

We report new major element, trace element, isotope ratio, and geochro nological data on the Galapagos Archipelago. Magmas erupted from the l arge western volcanos are generally moderately fractionated tholeiites of uniform composition; those erupted on other islands are compositio nally diverse, ranging from tholeiites to picritic basanitoids. While these volcanos do not form a strictly linear age progressive chain, th e ages of the oldest dated flows on any given volcano do form a reason able progression from youngest in the west to oldest in the east, cons istent with motion of the Nazca plate with respect to the fixed hotspo t reference frame. Isotope ratios in the Galapagos display a considera ble range, from values typical of mid-ocean ridge basalt on Genovesa ( Sr-87/Sr-86: 0.70259, epsilon(Nd): +9.4, Pb-206/Pb-204: 18.44), to typ ical oceanic island values on Floreana (Sr-87/Sr-86: 0.70366, epsilon( Nd): +5.2, Pb-205/Pb-204: 20.0). La/Sm(N) ranges from 0.45 to 6.7; oth er incompatible element abundances and ratios show comparable ranges. Isotope and incompatible clement ratios define a horseshoe pattern wit h the most depleted signatures in the center of the Galapagos Archipel ago and the more enriched signatures on the eastern, northern, and sou thern periphery. These isotope and incompatible element patterns appea r to reflect thermal entrainment of asthenosphere by the Galapagos plu me as it experiences velocity shear in the uppermost asthenosphere. Bo th north-south heterogeneity within the plume itself and regional vari ations in degree and depth of melting also affect magma compositions. Rare earth systematics indicate that melting beneath the Galapagos beg ins in the garnet peridotite stability field, except beneath the south ern islands, where melting may occur entirely in the spinel peridotite stability field. The greatest degree of melting occurs beneath the ce ntral western volcanos and decreases both to the east and to the north and south. Si8.0, Fe8.0, and Na8.0 values are generally consistent wi th these inferences. This suggests that interaction between the plume and surrounding asthenosphere results in significant cooling of the pl ume. Superimposed on this thermal pattern produced by plume-asthenosph ere interaction is a tendency for melting to be less extensive and to occur at shallower depths to the south, presumably reflecting a decrea se in ambient asthenospheric temperatures away from the Galapagos Spre ading Center.