TEMPORAL AND GEOCHEMICAL VARIABILITY OF VOLCANIC PRODUCTS OF THE MARQUESAS HOTSPOT

The Marquesas archipelago is a short, NW-SE trending cluster of island s and seamounts that formed as a result of volcanic activity over a we ak 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 s ubmerged portions of volcanoes along the Marquesas lineament decrease in age from northwest to southeast. The new sample age distribution yi elds a volcanic migration rate significantly slower than that expected for Pacific plate motion over a stationary Marquesas hotspot. This an d the aberrant orientation of the chain indicate deflection of the plu me by westward upper mantle flow. The interaction of this weak plume w ith upper mantle flow accounts for the temporal and spatial patterns i n Marquesan volcanism. The compositions of subaerial and submarine bas alts reflect the mixing of at least two mantle sources, distinguished by Sr, Nd, and Pb isotope and trace element compositions. There is a c onsistent evolutionary pattern at each volcano, from early tholeiitic to later alkalic basalt eruptions. Tholeiitic and transitional lava co mpositions can be derived by variable degrees of partial melting of a source composed of depleted mid-ocean ridge basalt mantle (DMM) and ma ntle 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 lowe r lithosphere or upper mantle (DMM+HIMU) entrained within a sheared, t hermally buoyant plume (EM II) could produce the tholeiitic and transi tional basalts found in the main shields of the volcanoes, while alkal ic basalts could result from melting of mantle of EM II composition at the edges of the hotspot.