Abstract
The Marquesas archipelago is a short, NW-SE trending cluster of islands and seamounts that formed as a result of volcanic activity over a weak hotspot. This volcanic chain lies at the northern margin of a broad region of warm and compositionally diverse mantle that melts to build several other subparallel volcanic lineaments. Basalts dredged from submerged portions of volcanoes along the Marquesas lineament decrease in age from northwest to southeast. The new sample age distribution yields a volcanic migration rate significantly slower than that expected for Pacific plate motion over a stationary Marquesas hotspot. This and the aberrant orientation of the chain indicate deflection of the plume by westward upper mantle flow. The interaction of this weak plume with upper mantle flow accounts for the temporal and spatial patterns in Marquesan volcanism. The compositions of subaerial and submarine basalts reflect the mixing of at least two mantle sources, distinguished by Sr, Nd, and Pb isotope and trace element compositions. There is a consistent evolutionary pattern at each volcano, from early tholeiitic to later alkalic basalt eruptions. Tholeiitic and transitional lava compositions can be derived by variable degrees of partial melting of a source composed of depleted mid-ocean ridge basalt mantle (DMM) and mantle characterized by radiogenic Pb (HIMU). Alkalic lava compositions appear to be dominantly the result of smaller degrees of melting of a more radiogenic mantle source (EM II). Large-scale melting of the lower lithosphere or upper mantle (DMM+HIMU) entrained within a sheared, thermally buoyant plume (EM II) could produce the tholeiitic and transitional basalts found in the main shields of the volcanoes, while alkalic basalts could result from melting of mantle of EM II composition at the edges of the hotspot. Copyright 1993 by the American Geophysical Union.