Journal of Petrology Advance Access originally published online on June 13, 2005
Journal of Petrology 2005 46(11):2197-2224; doi:10.1093/petrology/egi052
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Wolf Volcano, Galápagos Archipelago: Melting and Magmatic Evolution at the Margins of a Mantle Plume
1 DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF IDAHO-3022, MOSCOW, ID 83844, USA
2 DEPARTMENT OF GEOLOGY, UNIVERSITY OF ALASKA ANCHORAGE, ANCHORAGE, AK 99508, USA
3 DEPARTMENT OF GEOLOGY AND GEOPHYSICS, MASSACHUSETTS INSTITUTE OF TECHNOLOGY/WOODS HOLE OCEANOGRAPHIC INSTITUTION JOINT PROGRAM, WOODS HOLE, MA 02543, USA
4 DEPARTMENT OF MARINE CHEMISTRY AND GEOCHEMISTRY, WOODS HOLE OCEANOGRAPHIC INSTITUTION, WOODS HOLE, MA 02543, USA
5 DEPARTMENT OF GEOLOGY, COLGATE UNIVERSITY, HAMILTON, NY 13323, USA
6 DEPARTMENT OF GEOLOGICAL SCIENCES, CORNELL UNIVERSITY, ITHACA, NY 14853, USA
RECEIVED JUNE 10, 2004; ACCEPTED APRIL 11, 2005
Wolf volcano, an active shield volcano on northern Isabela Island in the Galápagos Archipelago, has undergone two major stages of caldera collapse, with a phase of partial caldera refilling between. Wolf is a typical Galápagos shield volcano, with circumferential vents on the steep upper carapace and radial vents distributed in diffuse rift zones on the shallower-sloping lower flanks. The radial fissures continue into the submarine environment, where they form more tightly focused rift zones. Wolf's magmas are strikingly monotonous: estimated eruptive temperatures of the majority of lavas span a total of only 22°C. This homogeneity is attributed to buffering of magmas as they ascend through a thick column of olivine gabbroic mush that has been deposited from a thin, shallow (<2 km deep) subcaldera sill that is in a thermochemical steady state. Wolf's lavas have the most depleted isotopic compositions of any historically active intraplate ocean island volcano on the planet and have isotopic compositions (except for 3He/4He) indistinguishable from mid-ocean ridge basalt erupted from the Galápagos Spreading Center (GSC) 250–410 km away from the peak of influence of the Galápagos plume. Wolf's lavas are enriched in incompatible trace elements and have systematic major element differences relative to GSC lavas, however. Wolf's magmas result from lower extents of melting, deeper melt extraction, and a greater influence of garnet compared with GSC magmas, but Wolf and the GSC share the same sources. These melt generation conditions are attributed to melting in a thermal and mechanical boundary layer of depleted asthenosphere at the margins of the Galápagos plume. The lower degrees of melting and extraction from deeper levels result from a thicker lithospheric cap at Wolf than exists at the GSC.
KEY WORDS: caldera; Galápagos; mush; partial melting; plume
* Corresponding author. Telephone: 208-885-6491. E-mail: dgeist{at}uidaho.edu