Geochemistry of samples cored from Shatsky Rise, IODP Expedition 324, and their implications for the formation of oceanic plateaus (Invited)

Shatsky Rise, an oceanic plateau located ~1500 km east of Japan, is one of the few large igneous provinces (LIPs) thought to be produced by massive eruptions and intrusion of magma. Previous studies suggested that Shatsky Rise formed along the trace of a triple junction coincident with a mantle plume [e.g., Sager and Han, 1993]. A mantle plume origin was supported by a geochemical study of Nb-Zr-Y data, reporting that some rocks from the plateau have ocean island-like composition [Tatsumi et al., 1998]. In contrast, a recent geochemical study of Nd-Pb-Sr isotopic data for a few basalts cored and dredged from the plateau top shows a Pacific mid-ocean-ridge basalt (MORB)-type signature, not the expected ocean island-type signature of a plume head eruption [Mahoney et al., 2005]. Such isotopic data could suggest a plate-controlled origin of the plateau, with the triple junction tapping fertile mantle to give the excess volcanism. The geochemical paradox could also be caused by sampling problems; both geochemical studies analyzed late stage products that may not be representative of the main plateau. With samples from main plateau-building lava pile, we will address the question of whether Shatsky Rise was formed from a deep-sourced mantle plume or interaction of plate boundaries and lithosphere with the shallow mantle. In order to address this question, Integrated Ocean Drilling Program Expedition 324 has planned to core ~800m of igneous basement at five sites on Shatsky Rise from September to November 2009. During the cruise the recovered basement samples will be petrographically described and their whole rock composition determined by shipboard ICP-AES (all major and selected trace elements). This paper will give a petrological-geochemical overview of the preliminary shipboard data.