A Review of the Geological Constraints on the Conductive Boundary Layer at the Base of the Hydrothermal System at Mid-Ocean Ridges

Models of high-temperature hydrothermal systems at intermediate- to fast-spreading mid-ocean ridges have a conductive boundary layer (CBL) separating the magmatic heat source from the convecting hydrothermal fluid. Paleo-CBLs preserved in the geological record provide a means to test theoretical models of the thermal, mechanical, and petrological evolution of this boundary. CBLs occur as metamorphic contact aureoles at or near the dike-gabbro boundary where axial magma lenses (now plutonic rocks) intruded into the basal dikes, leading to their transformation into low permeability granulite and hornblende hornfels at >800 degrees C. Paleo-CBLs are well documented in the Troodos and Oman ophiolites, and evidence of similar lithologies has been found at every location where the dike-gabbro transition has been mapped and/or sampled in modern fast-spreading crust (Hess and Pito Deeps, IODP Hole 1256D). Evaluation of the geothermometers used in studies of the thermal evolution of the CBL shows a lack of consistency that can be understood in terms of compositional controls. Peak temperatures are >900 degrees C in all areas at the base of the CBL, leading to partial melting, stoping, and disaggregation that facilitates thinning from below as required to maintain the high heat fluxes necessary to drive active black smoker systems. However, such thin CBLs are not the norm; steady state conditions must have thicker CBLs and smaller heat fluxes. In turn, global estimates of properties such as chemical fluxes for normal hydrothermal conditions may lead to substantially different element/heat fluxes than those based on active systems.