Oxygen and Hydrogen Isotope Studies of Hydrothermal Interactions at Submarine and Subaerial Spreading Centers

Oxygen and hydrogen isotope studies of three subaerial spreading centers, East Greenland (50–55 m.y.), Red Sea (22 m.y.) and Iceland ( 1), and much of the alteration and exchange takes place at very high temperatures (500°–1000°C) at depths of a least 5–10 km. The sheeted dike complexes themselves produce large-scale convective circulation of hydrothermal fluids that overlaps the hydrothermal systems produced by the gabbro plutons. In all these areas, assimilation and/or partial melting of hydrothermally altered country rocks in the vicinity of the magma chambers is an important petrological process. Detailed analysis of the differentiated Skaergaard layered gabbro body in East Greenland proves that only very minor amounts of H2O diffused directly into the liquid magma, even though the hydrothermal system was initiated immediately after intrusion and operated for the entire 130,000-year period of crystallization. However, blocks of low −18 0 roof rocks fell into the Skaergaard magma; these had undergone hydrothermal depletion in 180 above the magma chamber prior to their assimilation. Most of the the 180 depletion observed in these environments took place after crystallization, with plagioclase becoming much more strongly depleted in 180 than coexisting clinopyroxene. Essentially all of the above processes have also been documented in the submarine equivalents of such spreading centers, namely the ophiolite complexes. In particular, at the Samail ophiolite in Oman, the data of Gregory and Taylor (1981) indicate that pervasive subsolidus hydrothermal exchange with seawater occurred throughout the upper 75% of this 8-km-thick oceanic crustal section; locally, the H2O even penetrated down below the Moho into the tectonized peridotite. Pillow lavas (δ180 = 10.7 to 12.7) and sheeted dikes (4.9 to 11.3) are typically enriched in 180, and the gabbros (3.7 to 5.9) are depleted in 180. Integrating δ 180 as a function of depth for the entire ophiolite establishes (within geologic and analytical error) that the average δ180 (5.7 ± 0.2) of the oceanic crust did not change as a result of all these hydrothermal interactions with seawater. Therefore the net change in δ180 of seawater was also zero, indicating that seawater is buffered by MOR hydrothermal circulation. Under steady-state conditions the overall bulk 180 fractionation (Δ) between the oceans and primary mid-ocean ridge basalt magmas is calculated to be +6.1 ± 0.3, implying that seawater has had a constant δ180 ≈ −0.4 (in the absence of transient effects such as continental glaciation). The δ180 data and the geometry of the mid-ocean ridge (MOR) magma chamber require that two decoupled hydrothermal systems must be present during much of the early spreading history of the oceanic crust (approximately the first 106 years); one system is centered over the ridge axis and probably involves several convective cells that circulate downward to the roof of the magma chamber, while the other system operates along the sides of the chamber in the layered gabbros. Upward discharge of 180-shifted water into the altered dikes from the lower system, just beyond the distal edge of the magma chamber, combined with the effects of continued low-T hydrothermal activity, produces the 180 enrichments in the dike complex and pillow lavas. Whereas the dilute meteoric fluids have had relatively little influence on the chemical compositions of rocks and magmas in the subaerial spreading centers, this is not the case in the ophiolite complexes, where more saline, NaCl-rich fluids were involved. Oceanic plagiogranites are the equivalents of the low−180 potassic granophyres in subaerial spreading centers; the chemical differences between the two types of rocks are due to subsolidus hydrothermal exchange and to the fact that the hydrothermally altered precursors in the oceanic environments were spilites with very high Na/K, Na/Ca, and Na/Rb ratios.