Geochemistry of near-EPR seamounts: importance of source vs. process and the origin of enriched mantle component

Niu and Batiza [Earth Planet. Sci. Lett. 148 (1997) 471^483] show that lavas from the seamounts on the flanks of the East Pacific Rise (EPR) between 5 ‡ and 15‡N vary from extremely depleted tholeiites to highly enriched alkali basalts. The extent of depletion and enrichment exceeds the known range of seafloor lavas in terms of the abundances and ratios of incompatible elements. New Sr^Nd^Pb isotope data for these lavas show variations (87Sr/ 86Sr = 0.702362^0.702951; 206Pb/204Pb = 18.080^19.325 and 143Nd/144Nd 0.512956^0.513183) larger than observed in lavas erupted on the nearby EPR axis. These isotopic ratios correlate with each other, with the abundances and ratios of incompatible elements, with the abundances of measured major elements such as MgO, CaO, Na2O and TiO2 contents, and with the abundances and ratios of major elements corrected for crystal fractionation to Mg#=0.72 (Ti72, Al72, Fe72, Ca72, Na72, and Ca72/Al72). These coupled correlations and the spatial distribution of seamounts require an EPR mantle source that has long-term (s 1 Ga) lithological heterogeneities on very small scales [Niu and Batiza, Earth Planet. Sci. Lett. 148 (1997) 471^483]. Mid-ocean ridge basalt (MORB) major element systematics are, to a great extent, inherited from their fertile sources, which requires caution when using major element data to infer melting conditions. The significant correlations in elemental and isotopic variability (defined as RSD%=1c/ meanU100) between seamount and axial lavas suggest that both seamount and axial volcanisms share a commo