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

Abstract

Cerro Azul, one of the large shield volcanoes in the western Galápagos archipelago, has erupted a wide range of tholeiitic to alkalic basalts. These diverse compositions include some of the most primitive yet reported from the western archipelago and are unlike those of the other, well-studied, neighboring volcanoes of Sierra Negra and Alcedo, which have erupted basalt of fairly uniform composition. Major- and trace-element modeling shows that Cerro Azul, Alcedo and Sierra Negra share a similar depth of melting and source composition. Modeling also reveals that there are small, systematic differences in the extent of partial melting between the volcanoes that can be related to their distance from the proposed plume center below the westernmost island of Fernandina. However, even though melts segregating from the plume in the western Galápagos reflect a narrow range of temperatures and source compositions, there are wide variations in the enrichments of major and trace elements between Cerro Azul, Alcedo and Sierra Negra that cannot be attributed to mantle processes. We believe the observed intershield geochemical differences result from magma supply and cooling rates that are unique to each volcano, and reflect the variations in lithospheric transport and storage processes across the western archipelago.