Sr-Nd-Pb Isotope Geochemistry of Leg 144 West Pacific Guyots: Implications for the Geochemical Evolution of the "SOPITA" Mantle Anomaly

Abstract

The Sr, Nd, and Pb isotopic compositions of five Cretaceous guyots (Limalok, Lo-En, Wodejebato, MIT, and Takuyo-Daisan) within the West Pacific Seamount Province (WPSP) indicate that they originated from geochemically enriched mantle sources. They are characterized by large isotopic variations in Pb (206Pb/204Pb = 18.58-19.80, 2u7Pb/204Pb = 15.55-15.68, and 208Pb/204Pb _ 38.68-39.54) and by moderate variations in Sr (87Sr/86Sr = 0.70308-0.70486) and Nd ( l43Nd/l44Nd = 0.512670.51307). Data from the Cretaceous guyots studied are within the overall isotopic range of the WPSP and are similar to those of oceanic basalts produced by currently active hotspots of the South Pacific isotopic and thermal anomaly (SOPITA). The WPSP basalts show a large isotopic variance, and they include samples with isotopic signatures of either enriched mantle sources (EMI and EMU), or mantle sources high in 206Pb/204Pb and low in 87Sr/86Sr (HIMU). These observations suggest that the WPSP and SOPITA basaltic rocks may have been produced from the same heterogeneous mantle source: either one that is isotopically distinct and more diverse from other oceanic and continental volcanic regions, or one that is produced by mixing the different mantle components inferred for these other regions. We strengthen previous arguments for the longevity of the isotopically unusual character of the SOPITA hotspots, in particular, for the Rurutu-Ratak-Marcus-Wake Seamount Chain (RRMW). Our data from the WPSP doubles the existing database and allows us to examine the temporal isotopic evolution of the RRMW Chain over a total time span of 110 m.y., excluding the time period between 25 and 50 Ma that has not been sampled yet. The RRMW isotopic data suggest that (1) the overall isotopic variance is constant over time; (2) the Sr and Nd variances are smaller than the total variance for the SOPITA and WPSP, whereas the Pb compositions show similar, large variances; and (3) the isotopic composition of the SOPITA, and specifically the Rurutu volcanics, gradually changed within the last 25 m.y. from a mantle source dominated by the HIMU mantle component to an EM-type mantle source. The latter is indicated by a decrease in 206Pb/204Pb, 7/4Pb, and Δ8/4Pb paralleled by an increase in 87Sr/86Sr. A similar evolution in isotopic composition may also be revealed from the Ratak-Marcus-Wake Chain over the period from 90 through 70 Ma, although this latter trend is obscured by a less dense sampling in the WPSP area.