The Earth is encircled by a network of mid-ocean ridges (MOR) where new seafloor forms as part of plate tectonics. This seafloor spreading process is accompanied by hydrothermal processes where seawater seeps down into the oceanic crustal rocks, is heated by the magma upwelling beneath the ridge, reacts with the basaltic rocks of the oceanic crust by dissolving minerals and forming replacement minerals, and results in the exchange of chemical elements between the solid Earth and oceans. The hydrothermal systems are located beneath the seafloor and consequently are difficult of characterize. What has been done is to sample hot fluids emanating from seafloor "vents," drilling into older oceanic crust that has moved away from the ridge and cooled, and studying on-land examples of old oceanic crust. 

This project was initiated to begin the process of developing numerical models of the hydrothermal systems to better understand how the chemical processes - dissolving and precipitating minerals and exchanging chemical elements - relates to the flow and heating of seawater through the rocks. To do this requires a thermo-hydro-chemical (THC) computer code that can simultaneously account for heat and fluid flow and mineral-fluid chemical reactions. This project used the code TOUGHREACT produced by the Lawarence Berkeley National Laboratory. Our simulations are greatly simplified versions of MOR hydrothermal systems but show features that are difficult to infer from fleld studies. An example of the temperatures achieved at steady state flow are shown in Figure 1, which represents half of a 2-dimensional slice across a mid-ocean ridge.  It shows that the region of high temperatures above 150°C is restricted and associated with very fast flow of fluid through fractures (Figure 2).

A key issue is where the secondary mineral anhydrite forms, because its formation removes sulfate from seawater, and also reduces permeability in the fractures. Anhydrite forms at temperatures above 150°C, but because of kinetic limitations and rapid fluid flow it occurs in rocks at temperatures up to 300°C. Anhydrite forms only deep in the crust and close to the ridge axis. As the crust moves away from the ridge there is a tendency for anhydrite to be redissolved.

Another issue is how the hydrothermal processes affect the ⁸⁷Sr/⁸⁶Sr ratio of the fluid. In general this ratio decreases from the seawater value of 0.70918 to the rock values of 0.7028, but the model shows that only the highest temperature upwelling fluid has values less than 0.704. Much of the other upwelling fluid has higher values. This project also investigated how this Sr isotope exchange process changes with different compositions of seawater. Ancient seawater is believed to have had much higher concentrations of Ca and Sr, and lower concentrations of Mg and sulfate. Those conditions result in smaller changes to the seawater ⁸⁷Sr/⁸⁶Sr ratio and larger changes to the rock ⁸⁷Sr/⁸⁶Sr ratios

As the project evolved, the modeling was done with increasingly refined versions of the code that captures flow better near the critical point of water, and with a newer thermodynamic database that extends to higher temperature and pressure. Models of the oceanic crust were changed from 1-layer, to 3-layer (basalt, dikes, gabbro). 

The project showed that the THC code, guided by observations of the natural systems, can help fill in knowledge gaps relating to MOR hydrothermal systems. This capability is particularly important for understanding how MOR processes may have worked on the ancient Earth when many of the conditions may have been different, including seawater chemistry, water temperature and pH, seafloor spreading rates, and ocean depth.

Figure 1: Steady state temperature in circulating fluid for a model of a mid-ocean ridge hydrothermal system where fluid circulates to a depth of 1200 meters below the sea floor. Permeability is set to 1.5 x 10^-14 m².

Figure 2: Steady state flow velocity in fractures for the same model as that in Figure 1. Upward flowing fluid is restricted to the region to the left of the dashed line.  Maximum fluid velocity in the model is 1350 meters per year.

Figure 3: Distribution of fracture-filling anhydrite.

Figure 4: Sr isotope ratio in fluid flowing in fractures.

Figure 5: Fluid temperature in a 3-layer model using the supercritical version of the code. Left side scale gives tempreature in upwelling fluid at the axis. Alteration of dikes near the spreading axis occurs at temperature between 220°C and 400°C.

Last Modified: 03/07/2024
Modified by: Donald J Depaolo