HYDROTHERMAL CIRCULATION THROUGH MIDOCEAN RIDGE FLANKS - FLUXES OF HEAT AND MAGNESIUM

Thermally driven convection of seawater occurs through oceanic crust of all ages, at the seafloor spreading axis, on mid-ocean ridge flanks, and in the ocean basins. At the ridge axis and on the flanks, circulating seawater produces a major chemical exchange between the oceans and the crust. Based on heat budget constraints and the composition of hot springs, between 10 and 40% of the river flux of Mg can be taken up during high-temperature alteration of the basaltic crust along the ridge axis. Most of the hydrothermal heat loss, however, occurs on the mid-ocean ridge flanks, where the temperatures are lower and the seawater flux correspondingly larger. The estimated heat loss on the flanks is so large that upwelling must occur over a large fraction (5-30%) of the seafloor less than 65 Ma in age, if temperatures are <20-degrees-C and seepage velocities are on the order of 10 to 100 cm/y. The circulating seawater needs to lose on average less than 1-2% of its Mg content in order to solve the Mg mass balance for the oceans. Chemical fluxes through mid-ocean ridge flanks are poorly known because of the wide range of crustal conditions that prevail there and the paucity of study to date. The most critical parameter for characterizing crustal conditions is temperature in basement, which is a function of crustal age, basement topography, and sediment thickness and permeability. Basement temperature, which can usually be inferred from heat flow and seismic reflection surveys, largely determines the change in the composition of seawater circulating through basement. This change can be inferred, in turn, from profiles of sediment porewater chemistry. All sites studied to date with temperatures at the sediment-basement interface less-than-or-equal-to 25-degrees-C have a large component of advective heat loss and show only a small (<10%) loss of Mg from the circulating seawater, whereas all sites with basement temperatures greater-than-or-equal-to 45-degrees-C have a small advective component and show a large (>80%) Mg loss. Both types of sites may be important for chemical fluxes. Whether the cooler sites are important depends on how much the seawater changes in composition as it circulates through the crust; only small changes are needed. Whether the warmer sites are important depends on how much heat is lost by advection in this type of setting. The warmer sites could produce significant chemical fluxes even if they are scarce: they could account for the entire river input of Mg to the oceans even if they represent only 8-20% of the total advective heat loss on ridge flanks.