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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, ϵNd: +9.4, 206pb/204Pb: 18 44), to typical oceanic island values on Floreana (87Sr/86Sr: 0.70366, ϵNd: +5.2, 206pb/204Pb: 20.0). La/SmN 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. Si8.0, Fe8.0, and Na8.0 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.

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Seafloor alteration of the basaltic upper oceanic crust provides one of the major geochemical pathways between the mantle, the ocean/atmosphere and subduction zone regimes. Yet, no reliable mass balances are available, in large part because of the extremely heterogeneous distribution of altered materials in the oceanic crust but also because of the limited availability of high recovery drill cores. In this paper, we document the feasibility of determining the bulk altered and fresh composition of the oceanic crust on a !0-500 m length scale, from a region in the western Atlantic Ocean (DSDP/ODP Sites 417-418). Unaltered compositions were obtained from glass and phenocryst data and altered compositions were determined through analysis of composite samples. Most of the alteration-related chemical inventory resides preferentially in the upper oceanic crust and in highly permeable volcaniclastics. Most major elements (Si, A1, Mg, Ca, and Na) and many trace elements (Sr, Ba, LREE's) experience substantial large scale redistribution, but fluxes are relatively low. Overall, 12 wt % are added to the crust, mostly H20, CO 2, and K, but the distribution varies widely. High field strength elements, Th, Ti and Fe remain essentially immobile during low temperature alteration, while most other elements are affected to some degree. While the total fluxes are relatively small, the re-distribution of alteration -sensitive elements in the ocean crust is much larger, even on length scales exceeding 100m. The bulk composition of the upper 500m at Sites 417/418 can be used to constrain the impact of ocean crust subduction on element recycling to volcanic arcs. Flux balances indicate that the altered domains within the upper basaltic crust may contribute a very large proportion of some element fluxes recycled to the arc (H20, CO 2, K, Rb, U), while other element fluxes require additional contributions from sediments and deeper oceanic crust. whereby approximately 5 x 1() •6 grmns of basalt m'e generated and recycled per yem'. This cycle provides a pathway for •nanfle comlx)nents into the hydrosphere, and lbr sea water derived elements into subduction zones ,and the mm•tle. Chemical fluxes in fl•ese pafl•ways ,'u'e extremely poorly constrained, including the extent of high te•nperature altera-' tion at ridges, as well as off-axis low temperature chemic,'d exchange. This lack of &xta provides a major stumbling block in our understranding of earth chemic,-d dynamics. The alteration or' the ocemfic crust on the se•dloor, ,-red its subsequent met,'unorphism m•d chemical losses during subduction have a major impact on the loci ,'red composition of arc magmatism. Sever,'d recent studies have pointed to the altered b•salfic crust specificsally a,s a major source of elements recycled to volcanic arcs during subduc-fion (e.g. H20 [Peacock, 1990; Ph-mk, 1994]; B [Ishikawa m•d Nakamura, 1993]; Pb [Miller et ,'d, 1992; Peucker-Ehrenbrinck et aL, 1995]). Despite the recognition of the ,altered oceanic 20 GEOCHEMICAL FLUXES DURING SEAFLOOR ALTERATIO...

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