HR: 1330h
AN: V12D-0618    [PDF]
TI: Petrology and Geochemistry of Volcan Darwin, Isabela Island, Galapagos Archipelago
AU: * Naumann, T R
EM: aftrn@uaa.alaska.edu
AF: Univ of Alaska Anchorage Anchorage, Department of Geology 3211 Providence Dr, Anchorage, AK 99508 United States
AU: Krebs, L K
EM: aslkk2@uaa.alaska.edu
AF: Univ of Alaska Anchorage Anchorage, Department of Geology 3211 Providence Dr, Anchorage, AK 99508 United States
AB: The Galapagos Islands represent a hotspot system where the distribution and number of simultaneously active volcanoes, their petrologic and morphologic diversity, and the lack of a chemical evolutionary pattern distinguish them from the more familiar example of Hawaiian hotspot volcanism. The large western Galapagos shield volcanoes share many geomorphic and chemical attributes, yet in detail each is unique. 88 new major and trace element analyses document that Volcan Darwin, one of the large shield volcanoes of Isabela island, has erupted a range of compositions from basalt to andesite. The diverse compositions of Darwin are unlike those of the neighboring volcanoes of Wolf and Alcedo. On average, lavas of Darwin and Alcedo are much more highly evolved than the lavas of Wolf. For example, at Darwin, total alkalies (K2O+Na2O) range from 2.5 to 4.6 wt percent, SiO2 ranges from 47.6 to 57.0 wt percent and MgO ranges from 2.9 to 6.2 wt percent. Geochemical modeling shows that this compositional range is due mostly to crystal fractionation involving olivine, plagioclase and clinopyroxene. Petrographically, the population of Darwin lavas are strongly bimodal. Eighty percent of the samples collected are extremely crystal-rich and contain up to forty percent plagioclase phenocrysts and zoned megacrysts up to 8 mm in diameter. Conversely, about twenty percent of the samples collected are aphyric or only sparsely phyric. These differences in crystallinity and the occurrence of highly evolved compositions suggest that the thermal flux and residence times for Darwin's magmas are highly variable. We believe that at Darwin, as has been shown for other Galapagos volcanoes, most batches of magma are mixed in a large, thermally regulated chamber which commonly results in the eruption of very uniform tholeiitic basalt. However, the structure and longevity of Darwin's lithospheric storage and transport system must be highly variable, as some isolated magmas have cooled and evolved to much greater degrees than most Galapagos magmas, resulting in the eruption of andesite.
DE: 8434 Magma migration
DE: 8439 Physics and chemistry of magma bodies
DE: 8499 General or miscellaneous
DE: 9360 South America
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting