Chemical properties and evolution of mid-ocean ridge hydrothermal systems-flow system approach.

Abstract

Low water/rock ratios (about 1 in weight) in discharge zone appear to be an important characteristic of submarine hydrothermal systems associated with axial spreading centers. These ratios, which are stable over time, are linked with the chemistry of end-member hydrothermal solutions (original hydrothermal solution before mixing with ambient seawater) and the chemistry and distribution of greenstones. These features are considered to result from the properties of an open flow system. This paper is an attempt to analyze a submarine hydrothermal system, using an ideal open flow model. In this model, seawater is percolating through a rock column which is divided into a number of cells. In each cell, both rocks and solutions come to equilibrium, based on strontium isotope exchange. The fundamental features of water/rock interaction in this flow system differ from those inferred from closed system; (1) chemical composition of the discharged solution can be kept constant for some while although large volume of recharged solution brings large change in the bulk chemical composition of the system. (2) The difference between chemical composition of recharged and discharged solutions is compensated by the large change of rock composition near recharge zone at earlier stage of hydrothermal system. (3) The values of water/rock ratio deduced from rock or solution chemistry are, in general, different from the integrated volume water/rock ratio. These results are applied to the natural subseafloor hydrothermal system. The features are classified in relation to three evolutionary stages: (1) Early stage: chlorite-quartz-(CQ-) and CQ-rich greenstones occur only in recharge zone while CQ-poor greenstones occur in the rest of the system. (2) Intermediate stage: the increase of fluid flow promotes replacement of CQ-poor with CQ-rich and some of CQ-rich with CQ-greenstones, respectively. (3) Late stage: CQ-poor greenstones are completely replaced by CQ- and CQ-rich greenstones. The integrated volume water/rock ratio ([W/R]FLOW) constrained by the energy of heat source is estimated to be up to 4. So it is suggested that hydrothermal activity dies out by the intermediate stage. The chemical composition of end-member hydrothermal solutions stays constant through the early and intermediate stage. An increase of integrated water volume circulating through the system does not necessarily lead to change the chemistry of hydrothermal solution and host rock through the system.