Hydrothermal Processes at Seafloor Spreading Centers: Application of Basalt-Seawater Experimental Results

Abstract

Both the chemistry of seafloor hot springs and the chemical changes exhibited by basalts during alteration to greenschist facies assemblages have been accurately predicted by laboratory experiments reacting seawater with basalt. Although the experiments were run as an isothermal, closed-system, batch process, they largely succeeded in duplicating the products of the natural open-system, continuous flow process. For the solutions, this resulted mainly from rapid reaction rates at high temperature, relative to flow rates in the natural systems, so that equilibrium with the secondary mineral assemblage represented a significant control on solution composition both in the experiments and in nature. For the rocks, it resulted from a similar alteration history in which largely unreacted seawater reached greenschist facies temperatures before reacting with the basalts, and from element exchanges between rock and solution which were coupled via charge balance constraints so that the batch process in the experiments simulated the incremental process in nature.

The key concept in relating the batch process to the incremental process is the seawater/rock ratio, which because of the nature of the chemical exchanges involved can best be estimated from the uptake of seawater Mg by the altered rock. The experiments predict a systematic change in rock chemistry and mineralogy as alteration proceeds to higher seawater/rock ratios. The prediction is borne out for the fluxes of Mg and Ca, the flux directions of Na, Si, and Mn, and the mineral abundances of chlorite, quartz, and actinolite. It is not borne out for the Fe flux, the magnitude of the Na flux, and the abundances of albite and epidote, because the experiments failed as batch processes to allow for local redistribution of elements via diffusion. This latter process is important in altered rocks from the natural systems for Fe²⁺, which diffuses into zones where chlorite is forming preferentially due to influx of seawater Mg, and for Na⁺, which accumulates as albite in zones of lesser Mg influx, in exchange for Ca.