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

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

Oct 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002gecoa..66.3481s&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 66, Issue 19, pp.3481-3504

Mathematics

Logic

31

Scientific paper

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 ( 238 U- 230 Th- 226 Ra and 235 U- 231 Pa) requires measurement of the radiogenic isotopes Hf, Nd, Sr, and Pb. Here, we report measurements of 238 U- 230 Th- 226 Ra and 235 U- 231 Pa 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 9°28' and 9°52'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, 230 Th excesses and 226 Ra excesses are variable and inversely correlated. Because the eruption ages of the samples are much less than the half-life of 226 Ra, this inverse correlation between 230 Th and 226 Ra excesses can be considered a primary feature of these lavas. For the lava suite analyzed in this study, 226 Ra and 230 Th excesses also vary with lava composition: 226 Ra excesses are negatively correlated with Na 8 and La/Yb and positively correlated with Mg#. Conversely, 230 Th excesses are positively correlated with Na 8 and La/Yb and negatively correlated with Mg#. Th/U, 230 Th/ 232 Th, and 230 Th excesses are also variable and correlated to one another. 231 Pa excesses are large but relatively constant and independent of Mg#, La/Yb, Th/U, and Na 8 . The isotope ratios 143 Nd/ 144 Nd, 176 Hf/ 177 Hf, 87 Sr/ 86 Sr, and 208 Pb/ 206 Pb 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, 230 Th, and 226 Ra 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.

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