INTRODUCTION Cerro Azul, one of the large shield volcanoes in the western Galapagos archipelago, has erupted a wide range of tholeiitic to alkalic basalts. The Galapagos archipelago is a hotspot-generated collection of island volcanoes that differs in many respects These diverse compositions include some of the most primitive yet from more familiar examples, such as Hawaii. The arreported from the western archipelago and are unlike those of the chipelago is located adjacent to the Galapagos Spreading other, well-studied, neighboring volcanoes of Sierra Negra and Center and constructed atop a shallow volcanic platform Alcedo, which have erupted basalt of fairly uniform composition. on young, thin ocean lithosphere (Fig. 1). Although it Majorand trace-element modeling shows that Cerro Azul, Alcedo has been long recognized that the Galapagos islands and Sierra Negra share a similar depth of melting and source reflect intraplate volcanism resulting from a mantle plume composition. Modeling also reveals that there are small, systematic (Morgan, 1971), a number of tectonic and petrologic differences in the extent of partial melting between the volcanoes features conflict with a simple mantle plume model. that can be related to their distance from the proposed plume center These include: (1) a non-linear arrangement of volcanoes; below the westernmost island of Fernandina. However, even though (2) a wide distribution of simultaneously active volcanoes; melts segregating from the plume in the western Galapagos reflect a (3) a distinct geographic pattern to the petrologic and narrow range of temperatures and source compositions, there are geochemical diversity of the volcanoes; (4) a lack of wide variations in the enrichments of major and trace elements any consistent petrological and geochemical evolutionary between Cerro Azul, Alcedo and Sierra Negra that cannot be patterns like those documented for Hawaii (Macdonald attributed to mantle processes. We believe the observed intershield & Katsura, 1964; Frey & Clague, 1983) Chen & Frey, geochemical differences result from magma supply and cooling rates 1985; and Reunion (Albarede et al., 1997). Much of that are unique to each volcano, and reflect the variations in the geochemical variability between volcanoes in the lithospheric transport and storage processes across the western Galapagos has been ascribed to mantle processes conarchipelago. nected with a well-defined, archipelago-wide pattern of depleted and enriched mantle source compositions (Geist et al., 1988; Geist, 1992; Graham et al., 1993; White et al., 1993; Harpp, 1995; Kurz & Geist, 1999; Harpp & White, 2001). These geochemical patterns are proposed to result from both intrinsic differences in plume com