Igneous units cored during ODP Legs 170/205 were emplaced post approximately 18.2Ma within approximately 24Ma lithosphere generated at the East Pacific Rise on the southern Cocos Plate. New isotope data show that these units are melts of depleted upper mantle previously enriched during transit near the Galapagos hotspot and may represent the northern-most extent of magmatic overprinting from this hotspot yet identified. Widespread post-enrichment melting and emplacement of the igneous complex far from any spreading center or plume may have been triggered by unrelated changes in the regional tectonic stress regime and attest to far off-axis plume-ridge interaction. The lithosphere is also anomalously cool and hydrologically connected to distant regions of ventilation. Downhole geochemical profiles reveal that local maxima or minima in alteration sensitive geochemical signatures, with calculated seawater:rock ratios up to approximately 10, coincide with magmatic contacts that often have high fracture density. Therefore, lithostratigraphy exerts a strong control on secondary alteration and fluid flow along thin horizons. This crust is the oldest within which such active, channelized off-axis fluid flow has been identified; prolonged circulation in crust far from spreading centers has implications for thermal and elemental fluxes between the crust, ocean, and mantle. A detailed geochemical investigation of the changing nature of the slab component with slab-depth is permitted for the first time with new analyses of the incoming crust and volcanoes across the wide Kurile arc. The (super 10) Be flux ratio through the arc is approximately 7-16% and constrains sediment addition to regions of melt generation. Enrichments in sediment-derived cosmogenic (super 10) Be (t1/2=1.5My) in rear-arc lavas are comparable to those in the front, despite longer transit times, which requires a mechanism for extended release of (super 10) Be from the slab. Interpretations of cross-arc geochemical systematics suggest the agent of slab-mantle wedge transfer changes from an aqueous fluid to a partial sediment melt with increasing slab-depth; the latter interpretation is particularly strengthened by combined Be-Nd-Hf systematics. Conditions for progressive distillation of fluid-soluble elements and incipient sediment melting atop the slab have thermal and mineralogical implications for subduction zone models.