Geochemical and geophysical estimates of lithospheric thickness variation beneath Galápagos

Active volcanism in Galapagos is far more widespread (> 40,000 km2) than in other hotspot-related archipelagos, such as Hawaii (~ 20,000 km2). Here we employ both geochemical and geophysical models to constrain the causes of this large spatial extent of melt generation and the diverse compositions of erupted basalts. Insight in to the physical as well as the chemical nature of the melting regime beneath Galapagos –– and the cause of the relatively widespread, non-linear age-progressive distribution of volcanism – is provided by incompatible-trace-element ratios of basaltic magmas. Whilst variations in these (and isotopic) ratios of basalts from individual Galapagos volcanoes are limited, considerable differences have been observed in basalts erupted across the archipelago. We have used rare-earth-element inversion modelling for basalts dominated by “plume” and depleted MORB mantle components to constrain the depth to the top of the melt column beneath different Galapagos volcanoes. By converting S-wave data from a recently published tomographic experiment [Villagomez, D.R. et al., 2007. Upper mantle structure beneath the Galapagos Archipelago from surface wave tomography. J. Geophys. Res. 112] to temperature we have been able to map the base of the Galapagos lithosphere, i.e. where the geotherm, with a mantle potential temperature of 1315 °C, intersects the anhydrous peridotite solidus. An excellent correlation exists between the results of our geophysical and geochemical models. These predict that lithospheric thickness varies from ~60 km beneath islands in the south west of the Galapagos Archipelago (e.g. Fernandina and Isabela) to ~ 45 km below those in the northeast (e.g. Genovesa, Marchena, eastern Santiago and northern Santa Cruz). The thinner lithosphere away from the postulated site of the present-day plume axis, combined with the lateral deflection of the plume, is responsible for active volcanism over a relatively large area. Non-uniform variations in lithospheric thickness relative to distance from the Galapagos Spreading Centre are consistent with the complex nature of the oceanic lithosphere beneath this part of the Pacific, perhaps due to ridge jumps.