Seawater alteration of the Karmøy Ophiolite Complex, SW Norway: Nd and Sr isotopic evidence

Sm-Nd and Rb-Sr isotopic data are presented for two magmatic components of the Karmøy Ophiolite Complex, SW Norway. The 493 Ma old Visnes High Level Complex has initial ?Nd = +6.6, suggesting a primitive island are setting. The slightly older East Karmøy Layered Series has initial ?Nd = +4.3, indicating a more mature island are setting. Initial 87Sr/86Sr ratios indicate seawater alteration of the Visnes High Level Complex, particularly the upper crustal members, but no detectable interaction of seawater with the East Karmøy Layered Series. Published data reveal that ophiolitic igneous rocks have typically exchanged more of their Sr with seawater than have young MOR-generated oceanic crust. The difference is ascribed to greater circulation of hydrothermal fluids in the subduction-related environments in which many ophiolites formed.