Wolf Volcano, Galápagos Archipelago: Melting and Magmatic Evolution at the Margins of a Mantle Plume

Abstract

Wolf volcano, an active shield volcano on northern Isabela Island in the Galapagos Archipelago, has undergone two major stages of caldera collapse, with a phase of partial caldera refilling between. Wolf is a typical Galapagos 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 He/He) indistinguishable from midocean ridge basalt erupted from the Galapagos Spreading Center (GSC) 250–410 km away from the peak of influence of the Galapagos 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 Galapagos plume. The lower degrees of melting and extraction from deeper levels result from a thicker lithospheric cap at Wolf than exists at the GSC.