Volcanic evolution in the Galápagos: The dissected shield of Volcan Ecuador
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 Volcan Ecuador, a young, active shield volcano at the northwest tip of Isabela Island, Galapagos, experienced at least one sector collapse event that removed its western half. The compositions of lavas exposed in the old caldera wall and fault scarps dissecting the outer shield together with young post-collapse lavas provide insights into the history of this volcano. Ar-40/Ar-39 and cosmogenic He-3 dating reveal that sector collapse occurred at <100 Ka. Prior to sector collapse most magma erupted from circumferential dikes that fed the summit carapace and other dikes that fed vents on the caldera floor. Almost all of the erupted magmas cooled and underwent fractional crystallization in the lower crust or upper mantle, where clinopyroxene crystallization dominated. Magmas then rose to a small (&SIM;0.1 km(3)), steady state, subcaldera magma chamber where plagioclase, along with lesser amounts of olivine and clinopyroxene, crystallized. Sector collapse perturbed the plumbing system in a way that now allows some magmas, particularly those erupted on the East Rift, to bypass the magma chamber. Sector collapse also opened new pathways to the surface that enabled magma to erupt at low elevation, effectively cutting the magma supply to the summit. The paucity of summit eruptions over the past 100,000 years accounts for the old, eroded appearance of the shield. Subtle variations in isotopic and trace element composition occurred over the past 100,000 years, with the net change being toward slightly more depleted compositions. These variations most likely reflect small-scale (<5 km) heterogeneity superimposed on more regular regional patterns of mantle composition beneath the Galapagos. Overall, however, the compositional variation at Volcan Ecuador has been minimal, suggesting that, unlike Hawaii, the compositions of magmas do not substantially change while the volcano is in its shield-building phase.