Hydrothermal alteration and fluid fluxes in ophiolites and oceanic crust

We have compiled a composite oceanic crustal section for strontium, oxygen, and sulfur isotope tracers of seawater interaction, including new oxygen isotope data for a 1-km-thick section of lower oceanic crust, and in this paper, we summarize the main similarities and differences between hydrothermal alteration in ophiolites and that in oceanic crust. Ophiolitic crust is consistently more intensely recrystallized, and isotope tracers exhibit greater shifts in ophiolites than in situ oceanic crust. These differences require greater time-integrated fluid fluxes through the ophiolites (by a factor of ∼3-6), as well as penetration of larger fluid fluxes to greater depths in some ophiolites. The reasons for this fundamental difference must be related in some way to differences in tectonic setting, primary chemical compositions, or heat sources, but the exact reasons remain problematic. The greater fluid fluxes and tracer exchange in ophiolites point out the need for caution when applying data from ophiolites to the global mid-ocean ridge system and estimation of the effects on oceanic and crustal chemistry. Despite significant differences between hydrothermal effects in ophiolites and oceanic crust, many important similarities exist, and the ophiolite analogy remains invaluable to understand the structure of oceanic crust and processes at mid-ocean ridges. A need remains, however, for additional and more continuous sections through ocean and ophiolitic crust in different tectonic settings, in particular through the critical lithologic transitions that coincide with changes in fluid circulation and alteration.