A fault‐driven circulation model for the Lost City Hydrothermal Field

The Lost City Hydrothermal Field on the Atlantis Massif is an off-axis hydrothermal system, hosted in serpentinized ultramafic and metagabbroic rocks, that discharges through high-permeability faults and fractures. High-pH fluids vent through carbonate chimneys with temperatures ranging between 24 degrees C and 91 degrees C, and vent fluid geochemistry indicates quantitative removal of magnesium and water-rock reactions consistent with heating to between 250 degrees C and 300 degrees C, conductive cooling, and mixing with seawater. This paper develops a relatively simple, steady state, fault-controlled fluid circulation and crustal heat transfer model that is consistent with these observations. The model results predict that the (1) heat output at Lost City is similar to 4x10(4)W, (2) circulation depth is approximately 2.7km, and (3) permeability of the fault zone is similar to 10(-14)m(2). The model assumes that the hydrothermal system mines heat from the adjacent country rock, and it does not appear that heat from serpentinization reactions or cooling intrusions is needed to drive the system. Because it takes similar to 10(5)a for the current model to reach steady state, earlier episodes of hydrothermal activity at LCHF may have operated differently and vent temperatures may have been higher.