The Effect of Seawater SR Concentration on the Hydrothermal Alteration of Oceanic Crust: SR Isotopes in Dikes of the Early Paleozoic Bay of Islands Ophiolite

Oceanic crust is composed of basaltic rocks that have been hydrothermally altered at mid-ocean ridges by circulating heated seawater. Alteration changes the [[sup]]87[[/sup]] Sr/[[sup]]86[[/sup]] Sr of oceanic crust, especially in sheeted dike sections, because seawater has high [[sup]]87[[/sup]] Sr/[[sup]]86[[/sup]] Sr and sufficient dissolved Sr to affect the rocks. The inference that paleoseawater Sr concentrations have been higher than in the modern ocean suggests that ancient examples of altered oceanic crust should record evidence of elevated seawater Sr concentrations, a possibility supported by comparison of Cretaceous ophiolites and Neogene oceanic crust. To further evaluate this relationship, we measured Sr isotopes in sheeted dikes and other oceanic crustal samples of the 488 Ma Bay of Islands (BOI) ophiolite, which formed when seawater may have had the highest Sr concentration of any time during the Phanerozoic, 7 times higher than modern oceans and ∼2.5 times higher than mid-Cretaceous oceans. Our results are consistent with the inferred high Ordovician seawater Sr concentrations. The Sr isotope effects in the BOI ophiolite are not much larger than those in the Cretaceous ophiolites, but the BOI rock Sr concentrations are unusually high, suggesting that a high seawater Sr concentration is needed to account for the Sr isotope effects.