EVOLUTION OF GALAPAGOS MAGMAS - MANTLE AND CRUSTAL FRACTIONATION WITHOUT ASSIMILATION

Galapagos magmas evolve by fractional crystallization at systematicall y different depths in the crust and mantle, which results in strikingl y different volcano morphologies. Every Galapagos volcano has erupted some lavas that am saturated with olivine + plagioclase + augite, espe cially those magmas With MgO <6 Wt %. Magmas of the central volcanoes cool and fractionate below the Moho at pressures >5 kbar resulting in transient chambers and no calderas. In the Western Galapagos, magmas e quilibrate in the crust, at pressures between I and 3 kbar. The withdr awal of magma from these shallow chambers results in calderas, and the re appears to be a direct relation between the depth of fractionation and caldera morphology. Magmas of the volcanoes With deep calderas sta ge and fractionate at very shallow depths. Magmas of the volcanoes wit h broad, shallow calderas cool and crystallize in the lower crust. the re is no evidence for high-or low-O-18 sources in the Galapagos plume, as have been observed at other hotspot volcanoes. A combination of O- isotope, He-isotope, and trace-element data indicates that assimilatio n of oceanic crust is not an important process in the evolution of Gal apagos magmas, either in terms of total mars of assimilated material o r in producing the characteristic chemistry of the lavas. We suggest t wo possible explanations for the systematic differences in the depths at which Galapagos magmas cool and fractionate. First, the depth of th e magma chambers may be due to the magma supply rate. A second potenti al control an the depth at which Galapagos magmas cool and crystallize is the regional difference in the lithospheric structure. The lithosp here thickens to the west across a sharp discontinuity at about 90 deg rees 30'W which approximately separates the western islands with shall ow fractionation and the central islands with deep fractionation.