Petrogenesis of Alkalic Seamounts on the Galápagos Platform

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In the hotspot-fed Galápagos Archipelago there are transitions between volcano morphology and composition, effective elastic thickness of the crust, and lithospheric thickness in the direction of plate motion from west to east. Through sampling on the island scale it is unclear whether these transitions are gradational or sharp and whether they result in a gradational or a sharp boundary in terms of the composition of erupted lavas. Clusters of interisland seamounts are prevalent on the Galápagos Platform, and occur in the transition zone in morphology between western and eastern volcanoes providing an opportunity to evaluate sharpness of the compositional boundary resulting from these physical transitions. Two of these seamounts, located east of Isabela Island and southwest of the island of Santiago, were sampled by remotely operated vehicle in 2015 during a telepresence-supported E/V Nautilus cruise, operated by the Ocean Exploration Trust. We compare the chemistries of these seamount lavas with samples erupted subaerially on the islands of Isabela and Santiago, to test whether seamounts are formed from melt generation and storage similar to that of the western or eastern volcanoes, or transitional between the two systems. There are no systematic variations between the two seamounts and variability in all samples can be related through < 10% fractional crystallization at 500–900 MPa. Both seamounts are interpreted to represent a single magmatic episode and eruptive event. Trace element compositions indicate they formed downstream of the hotspot center. The calculated extents of melting are consistent with generation of magmas sourcing the seamounts beneath lithosphere of intermediate thickness (~ 56 km). The seamount lavas have compositions that are nearly identical to a subset of lavas erupted subaerially on Santiago Island, suggesting lateral magma transport on the order of 10 km from their source region prior to eruption. The compositional characteristics and, in particular, depth of crystallization suggest that although seamount magmas have a transitional melting signature, they are discretized on the island scale, through homogenization in the lithospheric mantle and redistributed by vertical and horizontal diking in the shallow crust. Due to this homogenization, it remains unclear whether the variation in erupted lava chemistries from west to east are representative of sharp or gradual changes in mantle composition and structure across the archipelago.