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 5degrees and 15degreesN 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 (Sr-87/Sr-86 = 0.702362-0.702951; Pb-206/Pb-204 = 18.080-19.325 and Nd-143/Nd-144 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 (Ti-72, Al-72, Fe-72, Ca-72, Na-72, and Ca-72/Al-72). These coupled correlations and the spatial distribution of seamounts require an EPR mantle source that has long-term (> I 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%=1sigma/mean X 100) between seamount and axial lavas suggest that both seamount and axial volcanisms share a common heterogeneous mantle source. We confirm previous interpretations [Niu and Batiza, Earth Planet. Sci. Lett. 148 (1997) 471-483; Niu et al., J. Geophys. Res. 104 (1999) 7067-7087] that the geochemical variability of lavas from the broad northern EPR region results from melting-induced mixing of a two-component mantle with the enriched (easily melted) component dispersed as physically distinct domains in a more depleted (refractory) matrix prior to the major melting events. The data also allow the conclusion that recycled oceanic crust cannot explain elevated abundances of elements such as Ba, Rb, Cs, Th, U, K, Pb, Sr etc. in enriched MORB and many ocean island basalts. These elements will be depleted in recycled oceanic crust that has passed through subduction zone dehydration reactions. We illustrate that deep portions of recycled oceanic lithosphere are important geochemical reservoirs hosting these and other incompatible elements as a result of metasomatism taking place at the interface between the low velocity zone and the cooling and thickening oceanic lithosphere. (C) 2002 Elsevier Science B.V. All rights reserved.