Chemical and isotopic constraints on the generation and transport of magma beneath the East Pacific Rise

Interpretation of U-series disequilibria in midocean ridge basalts is highly dependent on the bulk partition coefficients for U and Th and therefore the mineralogy of the mantle source. Distinguishing between the effect of melting processes and variable source compositions on measured disequilibria (U-238-Th-230-Ra-226 and U-235-Pa-231) requires measurement of the radiogenic isotopes Hf. Nd, Sr, and Pb. Here, we report measurements of U-238-Th-230-Ra-226 and U-235-Pa-231 disequilibria; Hf. Nd, Sr, and Pb isotopic and major and trace element compositions for a suite of 20 young midocean ridge basalts from the East Pacific Rise axis between 9degrees28' and 9degrees52'N. All of the samples were collected within the axial summit trough using the submersible Alvin. The geological setting and observational data collected during sampling operations indicate that all the rocks are likely to have been erupted from 1991 to 1992 or within a few decades of that time. In these samples. Th-230 excesses and Ra-226 excesses are variable and inversely correlated. Because the eruption ages of the samples are much less than the half-life of Ra-226. this inverse correlation between Th-230 and Ra-226 excesses can be considered a primary feature of these lavas. For the lava suite analyzed in this study, Ra-226 and Th-230 excesses also vary with lava composition: Ra-226 excesses are negatively correlated with Na-8 and La/Yb and positively correlated with Mg#. Conversely, Th-230 excesses are positively correlated with Na8 and La/Yb and negatively correlated with Mg#. Th/U, Th-230/Th-232, and Th-230 excesses are also variable and correlated to one another. Pa-231 excesses are large but relatively constant and independent of Mg#, La/Yb, Th/U, and Na-8. The isotope ratios Nd-143/Nd-144, Hf-176/Hf-177. Sr-87/Sr-86. and Pb-208/Pb-206 are constant within analytical uncertainty, indicating that they were derived from a common source. The source is homogeneous with respect to parent/daughter ratios Lu/Hf. Sm/Nd, Rb/Sr, and Th/U; therefore, the measured variations of Th/U, Th-230, and Ra-226 excesses and major and trace element compositions in these samples are best explained by polybaric melting of a homogeneous source, not by mixing of compositionally distinct sources. Copyright (C) 2002 Elsevier Science Ltd.