We compile a composite oceanic crustal section for strontium, oxygen, and sulfur isotope tracers of seawater interaction, including new oxygen isotopic data for a 1 km section of lower ocean crust, and summarize the main similarities and differences between hydrothermal alteration in ophiolites and ocean crust. Ophiolitic crust is consistently more intensely recrystallized and isotopic tracers exhibit greater shifts in ophiolites than in-situ ocean crust. This requires 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 answers 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 ocean crust, many important similarities exist, and the ophiolite analogy remains invaluable to understand the structure of ocean 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 lithological transitions that coincide with changes in fluid circulation and alteration.
Reprinted with permission from the Geological Society of America.