Sm-Nd, Rb-Sr, and ^(18)O/^(16)O isotopic systematics in an oceanic crustal section: Evidence from the Samail Ophiolite

Creators: McCulloch, Malcolm T.; Gregory, Robert T.; Wasserburg, G. J.; Taylor, Hugh P., Jr.

Abstract

The Sm-Nd, Rb-Sr, and ^(18)O/^(16)O isotopic systems have been used to distinguish between the effects of seafloor hydrothermal alteration and primary magmatic isotopic variations. The Sm-Nd isotopic system is essentially unaffected by seawater alteration, while the Rb-Sr and ^(18)O/^(16)O systems are sensitive to hydrothermal interactions with seawater. Sm-Nd mineral isochrons from the cumulate gabbros of the Samail ophiolite have an initial ^(143)Nd/^(144)Nd ratio of ε_(Nd) = 7.8 ± 0.3, which clearly substantiates the oceanic affinity of this complex. The initial ^(143)Nd/^(144)Nd ratios for the harzburgite, plagiogranite, sheeted diabase dikes, and basalt units have a limited range in ε_(Nd) of from 7.5 to 8.6, indicating that all the lithologies have distinctive oceanic affinities, although there is also some evidence for small isotopic heterogeneities in the magma reservoirs. The Sm-Nd mineral isochrons give crystallization ages of 128 ± 20 m.y. and 150 ± 40 m.y. from Ibra and 100 ± 20 m.y. from Wadi Fizh, which is approximately 300 km NW of Ibra. These crystallization ages are interpreted as the time of formation of the oceanic crust. The ^(87)Sr/^(86)Sr initial ratios on the same rocks have an extremely large range of from 0.7030 to 0.7065 and the δ^(18)O values vary from 2.6 to 12.7. These large variations clearly demonstrate hydrothermal interaction of oceanic crust with seawater.

Additional Information

© 1981 by the American Geophysical Union. Received August 9, 1979; accepted December 6, 1979. We are particularly indebted to R. G. Coleman and C. A. Hopson who conceived and organized the U.S. Geological Survey/National Science Foundation Oman project. Field work was also conducted with E. H. Bailey and J. S. Pallister. The samples from northern Oman were provided by Bob Coleman who also reviewed the manuscript. This work has benefitted from discussion with M. A. Lanphere, S . B. Jacobsen, R. E. Criss, E. Stolper, and J. Chen. We express our gratitude to the Ministry of Agriculture, Fisheries, Petroleum, and Minerals, Sultanate of Oman, whose hospitality enabled us to conduct field work in Oman. This work has been supported by NSF grants PHY 76-83685, EAR 76-21310, and EAR 78-16874, and indirectly by a grant to C. A. Hopson. Division of Geological and Planetary Sciences, California Institute of Technology, contribution 3303 (317).

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