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 3He4He) 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.
Bibliographical noteFunding Information:
D.J.G.’s work on Wolf volcano was supported by NSF Grants EAR-9117640 and EAR-0003367, and work on the DRIFT4 cruise by EAR-0002818. WHOI isotope work was funded by EAR-0126097 and OCE-0002461; we thank J. Bluszstajn and J. Curtice for assistance in the laboratory. We acknowledge the logistical support of the Charles Darwin Research Station and permission of the Galápagos National Park Service, without whom this work could not have been accomplished. Antonio Villema got us up the volcano and saved our lives several times. Thanks go to Scottie and Diane for their proficient running of the WSU geoanalytical lab, and Mike Garcia for help with the UH probe. Kaj Hoernle, Jamie Allan, and Pat Castillo provided highly useful and constructive reviews. We thank Ron Frost and Marjorie Wilson for their editorial handling of the manuscript. Mark Jellinek provided help with boundary layers. Peter Mouginis-Mark kindly provided the DEM from TOPSAR data.
- Partial melting