Short wavelength heterogeneity in the Galápagos plume: Evidence from compositionally diverse basalts on Isla Santiago

Abstract

Analyses of basalts from the Galápagos archipelago and adjacent spreading center have shown that the underlying mantle plume is isotopically heterogeneous over length scales of 10s to 100s of km. We show that the convecting mantle is also compositionally heterogeneous on smaller length scales (kms). Our evidence comes from recent small‐volume mildly alkaline and tholeiitic basalts on Isla Santiago, central Galápagos. Flows from volcanic vents <5 km apart are homogeneous in terms of incompatible‐trace element and isotopic ratios but inter‐vent variations in these ratios are large, such that Santiago basalts display some of the most extensive ranges known for any Galápagos island. Geochemical indexes of depth of melting correlate with an eastward decrease in geophysical estimates of lithospheric thickness–from 55 to 43 km over a ∼10 km horizontal distance beneath Isla Santiago–suggesting that melts have not undergone significant lateral transport in the underlying crust. This lithospheric ‘step’ below the center of the island generally results in a greater proportion of enriched mantle melts contributing to basalts from west Santiago, than to those in the east, due to less melting of more depleted mantle as the plume upwells beneath thicker lithosphere. Nevertheless, the sporadic occurrence of isotopically enriched flows with low [Sm/Yb]n, and more isotopically depleted compositions with elevated [Sm/Yb]n, across Santiago suggests that portions of the underlying Galápagos plume are compositionally heterogeneous in terms of enriched and depleted reservoirs over short length scales (kilometers). In this respect the Galápagos mantle plume is similar to plumes beneath Hawaii and Canary Islands.