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

摘要

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.