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Pb and Sr isotopic systematics of some basalts and sulfides from the East Pacific Rise at 21°N (project RITA)

Earth and Planetary Science Letters
Publication Date
DOI: 10.1016/0012-821x(81)90103-5


Abstract Tholeiitic basalts and sulfide deposits from the “Cyana” and “Alvin” diving programs (RITA project) on the East Pacific Rise were analyzed for Pb and Sr isotopes. The basalt data plot within the field defined previously by other East Pacific Rise basalts ( 206Pb/ 204Pb: 18.35–18.58; 207Pb/ 204Pb: 15.48–15.53; 208Pb/ 204Pb: 37.8–38.1; 87Sr/ 86Sr: 0.7022–0.7025). Pb, U and Sr contents (∼0.5, ∼0.05 and ∼110 ppm, respectively) and μ values (∼6) are typical of MORB, whereas Th/U ratios (∼3.5) are significantly higher. The Pb isotopic ratios of the sulfide samples are very homogeneous ( 206Pb/ 204Pb∼18.47 207Pb/ 204Pb∼15.49 208Pb/ 204Pb∼37.90), and plot in the middle of the basalt field. This indicates that (1) the sulfide Pb was derived from the basaltic crust without any significant contribution from either seawater or hemipelagic sediments, and (2) the solutions from which the sulfides were deposited had uniform Pb isotopic composition. The Pb contents of three sulfide samples is relatively high (170–1310 ppm). The Sr contents of five sulfide samples are widely scattered from 12 to 210 ppm, with 87Sr/ 86Sr ratios intermediate between basaltic and seawater values (0.70554±0.00005 to0.70795±0.00011). Leaching experiments show that both basalt-derived Sr and seawater Sr were present in the solutions which deposited the sulfides. In some cases, Sr was also adsorbed from seawater onto the sulfides following their deposition. Basalt-derived Sr and seawater Sr are also present in associated non-sulfide phases.

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