Six vent fields sampled at 13¿--11 ¿N, East Pacific Rise (EPR) in May 1984 exhibit large interfield variations and a much wider range of chemical compositions than previously observed at 21 ¿N. Measured pH at 25 ¿C are acidic, ranging from 3.1 to 3.7. Sodium and chloride vary from 40% lower to 30% higher than seawater. Iron concentrations range from 2 to 10 mmol/kg, compared with 0.7--2.5 mmol/kg at 21 ¿N. Other sulfide-forming metals (Cu, Zn, Cd, and Pb) are generally lower at 11¿--13 ¿N than at 21 ¿N. Reliable temperature measurements were obtained at only two of the six vents and both were 350¿¿5 ¿C. The vent fields at 21 ¿N, EPR were resampled in August 1985, thus extending to almost 6 years the time period over which they have been monitored (previous expeditions were made in November 1979 and November 1981). Campbell et al. (this issue) have shown that the chemistry of the hydrothermal fluids from these fields has been very stable over the period of repeated observation. Equilibrium calculations for the fluids from the fields at 13 ¿N and 21 ¿N, using a greatly improved thermodynamic data base, are described in this report. They indicate that the chemistry is rock buffered and that the stability of these systems over time is a result of equilibrium control with respect to a greenschist-type mineral assemblage at depth. Calculated high-temperature pH of the fluids range from 4.1 to 4.7 with those from 13¿--11¿N at the more acidic end of the range. Calculated affinities show that the fluids are close to, or at, saturation with respect to quartz, albite, muscovite, smectite/chlorite, epidote, and pyrrhotite.

The computations imply that lower-temperature vents such as National Geographic Smoker (NGS) (273 ¿C) may have their silica concentrations controlled by equilibrium with respect to a phase other than quartz. A comparison of fluid chemistry between NGS and vent 5 at 11¿N suggests that the latter vent may also have a temperature <300 ¿C. While the fluid chemistry of a given field is compositionally stable, the composition themselves are unique for a particular field and cover a wide range between fields. The processes controlling this variability probably include the depth of reaction, i.e., pressure, which is difficult to treat thermodynamically at present. In addition, increasing depth in the crust may be accompanied by decreasing permeability and hence a probable decrease in the water-rock ratio. It may be difficult to separate these two effects.

If the age differences between the fields (at present unknown) are of the order of decades, then their temporal histories may have an important influence on their particular chemistries. ¿ American Geophysical Union 1988