The Lead, Neodymium and Strontium Isotopic Structure of Ocean Ridge Basalts

Abstract

Pb-, Nd- and Sr-isotope compositions and U, Pb, Sm, Nd, Rb and Sr concentrations are reported for samples of basaltic glass and altered substrates from spreading centres in the Atlantic, Indian and Pacific Oceans. Correlations are shown to exist between 208, 207, 206Pb/204Pb ratios, and 87Sr/86Sr and 143Nd/144Nd ratios in basaltic glasses, but they are dominated by samples from the Mid-Atlantic Ridge. Whereas basaltic glasses from East Pacific spreading centres exhibit smaller variability in Pb, Sr and Nd isotope compositions than Atlantic samples, seamount samples from the E. Pacific have a similar range of Pb-isotope compositions as Mid-Atlantic Ridge glasses. Contamination of basaltic magmas by altered oceanic crust or sediments is not considered to be of prime importance in determining the isotopic structures of MORB glasses. It is proposed that the isotopic heterogeneity in the mantle beneath the Pacific and Atlantic is similar, but magma generation processes associated with fast spreading ridges of the East Pacific more effectively eradicate heterogeneities in the erupted basalts. Alteration of oceanic crust is further investigated with respect to the relative response of the U−Pb, Sm−Nd and Rb−Sr systems, and the role of recycled oceanic crust in producing the mantle heterogeneities is evaluated.