A laboratory and theoretical study of the growth of “black smoker” chimneys

Abstract

Observational evidence suggests that black smoker chimneys are formed by the precipitation of anhydrite from seawater producing a solid framework which is replaced successively by iron, zinc and copper sulfides. We have demonstrated the feasibility of this process using a laboratory model in which KNO 3 is first crystallized from a warm, nearly saturated solution round an inflowing plume of cold K 2CO 3. The chimney grows in length at a nearly constant rate and, at the same time, it thickens as heat conduction causes further crystallization. The dynamic replacement process has been modelled separately, with CuSO 4 passed through a previously formed chimney of KNO 3 and flowing out through the porous walls when the flow rate, and hence the pressure difference, is increased. The formation of chimneys at a line or slit source has also been investigated in the laboratory. It has been shown that, in this case, the slit is quickly blocked off by crystallization over most of its length and that the growth is concentrated at just a few points to form a small number of nearly axisymmetric chimneys. A theory has been developed which predicts both the diameter of the outlet vent and the sign of the pressure difference between the inside and the outside of an axisymmetric chimney of constant internal diameter for a specified flow rate and density difference. It suggests that changes in flow rate or in the internal diameter of the chimney can cause fluid to flow in or out through the porous wall, leading to changes in the position of mineral stability fields within the evolving chimney. The theory has been extended to describe the pressure distribution in tapering interior conduits and it leads to the conclusion that the direction of flow through a porous chimney can reverse along its length.