Volcanism on the flanks of the East Pacific Rise: Quantitative constraints on mantle heterogeneity and melting processes

We present major and trace element and Sr, Nd and triple-spike Pb isotope data for 17 fresh volcanic glasses from Seamount 6, a 10-km diameter seamount located 140 km east of the East Pacific Rise at 12°45′N. Geological and geochronological evidence show that magma compositions evolved from tholeiitic basalts to alkalic basalts and basaltic trachyandesites during the 1–2 Ma active lifetime of the seamount. Major and trace element compositions in Seamount 6 lavas vary systematically with isotope ratios; the youngest lavas with the highest incompatible trace element concentrations have the highest La/Yb, Nb/Zr, K2O/TiO2, 87Sr/86Sr, 206Pb/204Pb and the lowest 143Nd/144Nd, MgO and CaO. The range in element concentrations, incompatible element ratios, and isotope compositions in Seamount 6 lavas exceeds that observed in lavas erupted at the adjacent ridge axis, and is comparable to the range in lava compositions reported from all near-ridge seamounts studied to date. The observed range in lava compositions is consistent with mixing between enriched and depleted melts at shallow levels in the crust. The inferred difference in composition between these mixing endmembers cannot be explained by variable degrees of melting of a single source composition, and requires that the upper mantle is extremely heterogeneous on the scale of the melting region beneath a single seamount.We can show that the range in composition of EPR seamount lavas cannot be generated by melting of variably heterogeneous mantle in which enriched and depleted materials contribute equally to melting (source mixing). Instead, the trace element and isotope compositions of seamount lavas can be reproduced by melting models in which more enriched, fertile mantle lithologies are preferentially melted during mantle upwelling. At progressively lower degrees of melting, erupted lavas are thus more enriched in incompatible trace elements, have higher La/Yb, K/Ti, 87Sr/86Sr ratios and lower 143Nd/144Nd. If this is a common process, then mantle-derived magmas are unlikely to inherit the average incompatible trace element and isotope composition of their mantle source, which is likely to be significantly more depleted, nor will they display the full range of compositions present in the mantle melting region. These results have implications for the way in which oceanic basalts can be used to infer the composition of the upper mantle.

► Geochemical variability of Seamount 6 lavas exceeds that of the adjacent EPR.
► Major and trace element compositions vary systematically with isotope ratios.
► Enriched, fertile mantle lithologies are preferentially melting.
► Magmas are unlikely to inherit the average chemical composition of their source.