Insights Into the Origins and Compositions of Mantle Plumes: A Comparison of Galapagos and Hawai'i

The Galapagos and Hawai'i archipelagos are formed by mantle plumes originating at the large low shear velocity province (LLSVP) boundary. We report new high-precision Pb, Sr, Nd, and Hf isotopic analyses on 83 Galapagos samples and compare them with those of Hawai'i. The data confirm that like Hawai'i, Galapagos is a bilaterally asymmetric plume whose compositional boundary trends NW-SE. On their northeast sides, the plumes share a common source, Pacific lower mantle, whose intermediate isotopic signature may be common to many plumes. The Hawaiian and Galapagos plumes' southwestern sides are anchored in the Pacific LLSVP and are compositionally distinct; in Hawai'i, Loa trend lavas reflect contributions from the EM1 mantle end-member, whereas in Galapagos, HIMU is dominant, suggesting that the Pacific LLSVP is compositionally heterogeneous and includes different types of recycled material. Furthermore, the surficial expression of a bilaterally asymmetric plume is strongly influenced by its tectonic setting: (a) Thick Hawaiian lithosphere supports a volcano evolution process, including rejuvenated volcanism, whereas the thin Galapagos lithosphere inhibits Hawai'i-style rejuvenated-stage eruptive activity, instead causing extended, widespread volcanism; (b) the proximity of the Galapagos to a mid-ocean ridge causes entrainment of the depleted upper mantle, overwhelming depleted material intrinsic to the plume and affecting volcanoes' magmatic architecture; and (c) the geometric relationship between the LLSVP boundary and plate motion influences geochemical patterns at the surface. Thus, despite striking differences in surficial expression of the Galapagos and Hawai'i plumes, they share a common generation mechanism, supplied by the Pacific LLSVP and the lower mantle.