Journal of Petrology | Volume 39 | Number 5 | Pages 953-971 | 1998
© Oxford University Press 1998
Evolution of Galápagos Magmas: Mantle and Crustal Fractionation without Assimilation
1 Department of Geology and Geological Engineering, University of Idaho Moscow, ID 83844, USA
2 Department of Geology, Washington State University Pullman, WA 99164, USA
Received April 8, 1997; Revised typescript accepted December 2, 1997
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Abstract |
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Galápagos magmas evolve by fractional crystallization at systematically different depths in the crust and mantle, which results in strikingly different volcano morphologies. Every Galápagos volcano has erupted some lavas that are saturated with olivine + plagioclase + augite, especially 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 Galápagos, magmas equilibrate in the crust, at pressures between 1 and 3 kbar. The withdrawal of magma from these shallow chambers results in calderas, and there appears to be a direct relation between the depth of fractionation and caldera morphology. Magmas of the volcanoes with deep calderas stage and fractionate at very shallow depths. Magmas of the volcanoes with broad, shallow calderas cool and crystallize in the lower crust. There is no evidence for high- or low-18O sources in the Galápagos plume, as have been observed at other hotspot volcanoes. A combination of O-isotope, He-isotope, and trace-element data indicates that assimilation of oceanic crust is not an important process in the evolution of Galápagos magmas, either in terms of total mass of assimilated material or in producing the characteristic chemistry of the lavas. We suggest two possible explanations for the systematic differences in the depths at which Galápagos magmas cool and fractionate. First, the depth of the magma chambers may be due to the magma supply rate. A second potential control on the depth at which Galápagos magmas cool and crystallize is the regional difference in the lithospheric structure. The lithosphere thickens to the west across a sharp discontinuity at about 90°30'W which approximately separates the western islands with shallow fractionation and the central islands with deep fractionation.
KEY WORDS: Galápagos; assimilation; ocean-islands; crystal fractionation; calderas
* Corresponding author. Telephone: 208-885-6491. Fax: 208-885-5724. e-mail: dgeist{at}uidaho.edu
Present address: Department of Geoscience, UNLV, Las Vegas, NV 89154, USA.
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