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Published January 1980 | public
Journal Article

A neodymium, strontium, and oxygen isotopic study of the Cretaceous Samail ophiolite and implications for the petrogenesis and seawater-hydrothermal alteration of oceanic crust


In the Samail ophiolite, ^(147)Sm-^(143)Nd,^(87)Rb-^(87)Sr, and ^(18)O/^(16)O isotopic systems have been used to distinguish between sea-floor hydrothermal alteration and primary magmatic isotopic variations. The Rb-Sr and ^(18)O/^(16)O isotopic systems clearly exhibit sensitivity to hydrothermal interactions with seawater while the Sm-Nd system appears essentially undisturbed. Internal isochrons have been determined by the ^(147)Sm-^(143)Nd method using coexisting plagioclase and pyroxene and give crystallization ages of 130 ± 12m.y. from Ibra and 100 ± 20 m.y. from Wadi Fizh. These ages are interpreted as the time of formation of the Samail oceanic crust and are older than the inferred emplacement age of 65–85 m.y. The initial ^(143)Nd/^(144)Nd ratios for a tectonized harzburgite, cumulate gabbros, plagiogranite, sheeted dikes and a basalt have a limited range in ε_(Nd) of from 7.5 to 8.6 for all lithologies, demonstrating a clear oceanic affinity and supporting earlier interpretations based on geologic observations and geochemistry. The ^(87)Sr/^(86)Sr initial ratios on the same rocks have an extremely large range of from 0.70296 to 0.70650 (ε_(Sr) = −19.7 to +30.5) and the δ^(18)O values vary from 2.6 to 12.7. These large variations are clearly consistent with hydrothermal interaction of seawater with the oceanic crust. A model is presented for the closed system exchange of Sr and O, that in principle illustrates how the Sr isotopic data may be utilized to estimate the water/rock ratio and subsequently used to evaluate the temperature of equilibration between the water and silicates from the ^(18)O/^(16)O water-rock fractionation.

Additional Information

© 1980 Elsevier Scientific. Received August 17, 1979. Revised version received October 18, 1979. Field work was conducted in collaboration with the National Science Foundation-U.S. Geological Survey Oman project organized by R.G. Coleman and C.A. Hopson. Their help and support have been invaluable. This work has benefited from discussions with R.G. Coleman, C.A. Hopson, E.H. Bailey, J.S. Pallister, J. Chen, and M.A. Lanphere. R.G. Coleman provided additional samples from northwest of Ibra which greatly broadened the scope of this study. S.B. Jacobsen and the reviewers provided helpful comments on the manuscript. We appreciate Marvin Lanphere's indulgence in matters regarding an easily exchanged element. We express our gratitude to the Directorate General of Petroleum and Minerals, Sultanate of Oman, particularly Mohammed Kassim and Ismail M. El Boushi, for their support of this project. This work has been supported by N.S.F. grants PHY 76-83685, EAR-76-21310, EAR-7816874, and also indirectly by an N.S.F. grant to C.A. Hopson.

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