Cryptic evolved melts may be common in basaltic shield volcanoes: xenoliths from Volcan Fernandina

In 1968, Fernandina volcano experienced a VEI 4 hydromagmatic eruption that resulted in 350 m of catastrophic collapse. In addition to abundant ash, lithic material, and scoria, the eruption ejected olivine-gabbro blocks. These xenoliths provide evidence of the differentiation processes that otherwise are constrained only by the compositions of erupted lavas and geophysical observations of the geometry and location of the magma chambers. The xenoliths are cumulates: they contain more olivine than exists in any lavas exposed at Fernandina. Several contain miarolitic cavities, indicating emplacement at a relatively shallow level and probably at the top of or above the 1 km-deep subcaldera sill that has be identified geodetically. Some of the blocks contain epidote and pyrite, indicating alteration by Fernandina's active hydrothermal system. Average olivine compositions range from Fo54-61, which are much more evolved than those observed in submarine basalts (Fo72-88) and subaerial lavas (Fo65-82; Allan and Simkin, 2000). Average plagioclase compositions in the xenoliths range from An58-72, in comparison to subaerial phenocryst cores (An70-90). Phenocryst cores in submarine basalts range from An71-90, whereas those in rare evolved rocks erupted from the submarine SW rift range from An55-60. Because the mineral compositions are much more evolved than those in erupted basalts, we conclude that the gabbros are not the cumulate complement to the erupted melts. Those more primitive cumulates must lie in a deeper part of the system and were unsampled by the explosive eruption. Instead, the gabbros must have crystallized from evolved melts that never erupt from subaerial vents, and only rarely erupt from deep submarine vents. The relation of these evolved melts to the dominant erupted basalts is elusive. Cryptic evidence for the existence of strongly evolved melts residing at shallow depths has also recently been discovered at Volcan Wolf. So far as we know, these evolved melts always hybridize with basaltic melts before eruption in the subaerial environment and only rarely leak out of the submarine flanks. In some ways, the concealed evolved melts in Galapagos volcanoes may be analogous to the rhyolites found in the subsurface at Kilauea that never erupt.