A CAPLESS 350-DEGREES-C FLOW ZONE MODEL TO EXPLAIN MEGAPLUMES, SALINITY VARIATIONS, AND HIGH-TEMPERATURE VEINS IN RIDGE AXIS HYDROTHERMAL SYSTEMS

The physical characteristics of the deeper (> approximately 1.5 km), higher temperature (greater-than-or-equal-to approximately 350-degrees-C) parts of typical ridge axis hydrothermal systems are determined by combining selected observations with equations expressing the conservation of mass, energy, and momentum. The observations are: the approximately 350-degrees-C temperature of venting solutions, the variation of vent salinities from half to twice that of seawater, the < 10-yr residence time of seawater from the time it is heated to temperatures > 150-degrees-C to discharge, the variation in black smoker venting rates from normal to megaplume, and an intrusion geometry consistent with seismic and other data. With these observations the conservation equations predict: a approximately 3.4-m-wide, 330-mD, 350-degrees-C flow zone is separated by approximately 180 m from the hot (1,200-degrees-C) parts of the axial intrusion. High fluid velocities in the thin 350-degrees-C flow zone make it the most resistive element in the convection circuit. Tectonic and magmatic disturbances can increase the permeability of the thin flow zone by two to three orders of magnitude. When this occurs discharge of 350-degrees-C waters increases to megaplume rates for the several days that is required for the width of the flow zone to contract and the heat stored in the formerly wider flow zone to be discharged into the megaplume. The rate of discharge then returns to premegaplume levels. Cracking allows seawater to invade hot areas of the thermal boundary layer and undergo phase separation. Cracking 70 percent of the thermal boundary layer produces sufficient volumes of brine, and returns and recondenses sufficient low-salinity vapor in the thin 350-degrees-C flow zone, to reduce the salinity of the flow zone waters to half seawater levels. Flushing of the thermal boundary layer brines by the later migration of the flow zone into the thermal boundary layer results in a complimentary increase in flow zone (and discharge) salinity to twice seawater levels. Cl-rich veins observed in the Bushveld have lengths, widths, and spacings similar to those required to produce these salinity variations. The model suggests new ways to combine field observations for massive sulfide exploration. Definitive tests of the model could be provided by drilling the near-ridge environment to depths of approximately 2 km.