HYDROTHERMAL CIRCULATION, JUAN-DE-FUCA RIDGE EASTERN FLANK - FACTORS CONTROLLING BASEMENT WATER COMPOSITION

Pore water has been analyzed from sediment cores taken from three area s on the eastern flank of the Juan de Fuca Ridge as part of FlankFlux 90, a study of hydrothermal circulation through mid-ocean ridge flanks . Seismic reflection and heat flow surveys (Davis et al., 1992a) indic ate that the three areas differ in sediment thickness, basement topogr aphy, abundance of outcrops, basement temperature, and fraction of hea t lost by advection versus conduction. Area 1 is on 0.6 Ma crust with nearly continuous basement outcrop, area 2 is on 1.3 Ma crust over the first buried ridge parallel to the present ridge axis, and area 3 is on 3.5-3.8 Ma crust over two axis-parallel buried ridges that penetrat e the sediment cover in three locations. Each area includes a hydrothe rmal system in which seawater flows into basement, reacts with crustal basalt, and then exits basement either through the sediment or direct ly into the overlying water column. As constrained by concentrations o f sulfate and lithium in the pore waters, at least some seawater enter s basement in all three areas without reacting fully with the overlyin g sediment, even where no outcrops are known nearby. Speeds of upwelli ng of pore water through the sediment have been estimated by fitting p rofiles of dissolved magnesium and chlorinity, which behave conservati vely in these areas, to numerical time-dependent transport models. The estimated velocities range from < 0.1 to 7.4 cm/yr; faster flows prob ably occur but were not sampled. Upwelling speed correlates positively with heat flow and basement highs and negatively with sediment thickn ess. The correlation with heat flow differs from area 2 to area 3 alon g with differences in physical properties of the turbidite sediment. W e have documented pore water upwelling through sediment up to 100 m th ick. We estimate that upwelling continues at decreasing speeds through sediment up to 160 m thick, corresponding to a heat flow of 0.44 W/m2 in area 2 and 0.3 W/m 2 in area 3. Concentrations of magnesium and ch lorinity in the altered seawater upwelling from basement are uniform w ithin each area but differ from one area to the next. Both species rem ain at the bottom seawater concentration in area 1, where basement is cooled to < 10-degrees-C at the base of the sediments mainly by advect ion. The concentration of magnesium decreases with increasing basement temperature in areas 2 and 3 to a minimum of 2.5 mmol/kg at about 90- degrees-C in area 3. The transition from largely advective to largely conductive heat loss occurs over only 20 km between areas 1 and 2 and corresponds to a dramatic change in the composition of fluid circulati ng through basement, as the uppermost basement is heated from < 10-deg rees to 40-50-degrees-C. Chlorinity of the basement fluid increases ab ove the present-day bottom seawater concentration in areas 2 and 3 and in nearly all other mid-ocean ridge flanks studied to date, as a resu lt of rock hydration and the higher chlorinity of bottom seawater duri ng the last glacial period. While chlorinity generally correlates posi tively with uppermost basement temperature in various ridge flank hydr othermal systems, it reaches a maximum in area 2 at only 40-degrees-C, probably because alteration there occurs at a lower water/rock ratio than elsewhere. For all mid-ocean ridge flanks studied to date, the te mperature at the basement interface correlates better with the fractio n of heat lost by advection versus conduction and with the average thi ckness of the sediment cover than with crustal age.