The role of hydrothermal cooling of the oceanic lithosphere for ocean floor bathymetry and heat flow

摘要

We investigate the influence of hydrothermal circulation on cooling of oceanic lithosphere. We include hydrothermal convection in a 1-D lithosphere cooling model by using a scaling law which relates the Rayleigh to the Nusselt number. This law allows calculating an effective thermal conductivity accounting for the extra heat transport. Based on a global data set for bathymetry and surface heat flow for ocean floor ages between 0.7 and 160Ma, we perform a joint inversion based on a downhill simplex algorithm to constrain characteristic parameters for hydrothermal cooling (beside the classical parameter mantle temperature, thickness of the lithosphere, and thermal expansivity of lithosphere rocks). Hydrothermal cooling parameters are crack aspect ratio which controls the hydrothermal penetrations depth, characteristic cementation time at which cracks become closed and hydrothermal circulation ceases, and sealing time when enough sedimentary cover on top of the lithosphere has accumulated to prevent hydrothermal fluid escaping into the ocean. Best fitting parameter sets predict mantle temperatures between 1350 and 1450 degrees C and lithosphere thickness between 70 and 90km and further suggest that (1) the fit to the data is strongly improved if hydrothermal cooling effects are considered, (2) hydrothermal cooling is important for up to 10Ma old lithosphere and leads to a significant deviation from the 1/square rootlaw with an exponent closer to 1/3, (3) sedimentary sealing is completed for similar to 1.5Ma old lithosphere, and (4) fitting of the data is improved for an apparently 1.4 times higher value of thermal expansivity which may account for effects of melt solidification.