Petrology and Geochemistry of Volcan Cerro Azul: Petrologic Diversity among the Western Galapagos Volcanoes

Cerro Azul, one of the large shield volcanoes in the westernGalápagos archipelago, has erupted a wide range of tholeiiticto alkalic basalts. These diverse compositions include someof the most primitive yet reported from the western archipelagoand are unlike those of the other, well-studied, neighboringvolcanoes of Sierra Negra and Alcedo, which have erupted basaltof fairly uniform composition. Major- and trace-element modelingshows that Cerro Azul, Alcedo and Sierra Negra share a similardepth of melting and source composition. Modeling also revealsthat there are small, systematic differences in the extent ofpartial melting between the volcanoes that can be related totheir distance from the proposed plume center below the westernmostisland of Fernandina. However, even though melts segregatingfrom the plume in the western Galápagos reflect a narrowrange of temperatures and source compositions, there are widevariations in the enrichments of major and trace elements betweenCerro Azul, Alcedo and Sierra Negra that cannot be attributedto mantle processes. We believe the observed intershield geochemicaldifferences result from magma supply and cooling rates thatare unique to each volcano, and reflect the variations in lithospherictransport and storage processes across the western archipelago. KEY WORDS: basalt; Galápagos; magma supply; mantle plume; ocean island