Volcanism on Earth is known to occur in three tectonic settings: divergent plate boundaries (such as mid-ocean ridges), convergent plate boundaries (such as island arcs), and hot spots. We report volcanism on the 135 million-year-old Pacific Plate not belonging to any of these categories. Small alkalic volcanoes form from small percent melts and originate in the asthenosphere, as implied by their trace element geochemistry and noble gas isotopic compositions. We propose that these small volcanoes erupt along lithospheric fractures in response to plate flexure during subduction. Minor extents of asthenospheric melting and the volcanoes' tectonic alignment and age progression in the direction opposite to that of plate motion provide evidence for the presence of a small percent melt in the asthenosphere.

The Hawaiian-Emperor volcanic island and seamount chain is usually attributed to a hot mantle plume, located beneath the Pacific lithosphere, that delivers material sourced from deep in the mantle to the surface. The shield volcanoes of the Hawaiian islands are distributed in two curvilinear, parallel trends (termed 'Kea' and 'Loa'), whose rocks are characterized by general geochemical differences. This has led to the proposition that Hawaiian volcanoes sample compositionally distinct, concentrically zoned, regions of the underlying mantle plume. Melt inclusions, or samples of local magma 'frozen' in olivine phenocrysts during crystallization, may record complexities of mantle sources, thereby providing better insight into the chemical structure of plumes. Here we report the discovery of both Kea- and Loa-like major and trace element compositions in olivine-hosted melt inclusions in individual, shield-stage Hawaiian volcanoes--even within single rock samples. We infer from these data that one mantle source component may dominate a single lava flow, but that the two mantle source components are consistently represented to some extent in all lavas, regardless of the specific geographic location of the volcano. We therefore suggest that the Hawaiian mantle plume is unlikely to be compositionally concentrically zoned. Instead, the observed chemical variation is probably controlled by the thermal structure of the plume.

We compare two intraplate, Pliocene-Pleistocene volcanic fi elds in different tectonic settings-the central Basin and Range and the northwest Pacifi c Ocean. Both fi elds are characterized by widely scattered, small-volume, alkali basaltic volcanoes; within the fi elds, each volcano apparently originates from a separate, volatile-enriched parental melt from the upper mantle. There is no evidence at either fi eld for locally anomalous heat fl ow or ongoing introduction of new fl uids into the upper mantle such as might occur above a subducting slab. We conclude that the volcanic fi elds refl ect deformation-driven collection of already existing partial melts in a heterogeneous upper mantle. Deformation-driven melt collection may be an important mechanism for other diffuse intraplate volcanic fi elds, and this is consistent with a tectonically controlled, low-fl ux end member for intraplate fi elds where magmatism is a passive response to regional deformation. Differences in the degree of fractionation and contamination between the two fi elds are inferred to be related to fl exure-induced vertical variations in the orientation of principal stresses in the northwest Pacifi c Ocean, which cause stalling of ascending dikes in the lithosphere.