The distribution of geothermal and geochemical gradients near Site 501/504: implications for hydrothermal circulation in the ocean crust (ODP, Leg 111, Costa Rica Rift)

Abstract

A detailed survey of heat flow and geochemical gradients in pore water within a 10 × 10 km area surrounding Site 501/504 revealed a broad and undulating variation in heat flow. The highest values were found in 3 localized zones. Gradients of Ca2+ and Mg2+ in pore waters of the upper 12 m of sediment were found to be extremely high within these high heat-flow zones. Flux rates as high as 6 mm/yr were estimated from the shape of the curves. The discovery of significant flux through the sediments implies that there are reduced pressure variations in the top of the igneous crust on the order of a few bars and that the pressure field relative to hydrostatic probably drives lateral flow in upper basement at rates of 10-30 cm/yr. The survey and subsequent drill data verify that this flow attenuates variations in temperature and pore-water chemistry in the upper few 100 m of basement. The variations in heat flow, pore-water chemical gradients, and implied fluxes of pore water are associated with hydrothermal convection in basement at a lateral scale of 3.5-4 km that may penetrate deeply into the crust. -Authors