Deep underground, fluids circulating in Earth’s crust and mantle transport carbon, sulfur and other volatiles, as well as rare and precious metals. As they move through rocks, fluids react with minerals to form ores. These fluids also change the chemistry of the rocks, influencing deep volatile cycles, volcanic eruptions, and the composition of Earth’s atmosphere. We know these processes take place, however they are so deep that they challenge our ability to observe and understand them.
A team led by DCO’s Matthieu Galvez (Branco Weiss Fellow at ETH Zurich, Switzerland), including DCO Extreme Physics and Chemistry Chair Craig Manning (University of California Los Angeles, USA), set out to address this problem by focusing on a critical and yet overlooked characteristic of deep fluids — their pH. Their work is published in the journal Nature .
Fluid pH, along with pressure, temperature and redox state, is a crucial chemical parameter defining how molecules and elements behave. However, directly measuring the pH of Earth’s mantle, in this case 100km below the surface where temperatures may reach 900°C and pressures top 3 GPa, is impossible.
Galvez and his colleagues therefore turned to thermodynamic and novel chemical modeling approaches to investigate how fluid pH changes in deep Earth environments. They used newly refined models for minerals, solvents, and solutes (using an extension of H2O  and COH fluid  dielectric properties to mantle conditions) in order to address the global effects of mineral composition on fluid pH.
“The novelty of our study is in its focus on the sensitivity of a critical and rarely examined fluid property, pH, to a variety of parameters rather than on its absolute values,” the author said. “We discuss a range of mechanisms that control those variations. For example, pH variations may be dramatic in typical mantle rocks with little alkali metals that prove critical in our models. We also draw attention to the potential consequences of Earth’s aging (i.e. cooling) on temporal trends in subduction zone fluid properties and global element cycles, in particular C and O”
With this model in hand, Galvez and colleagues can now investigate how specific elements behave in deep Earth fluids. Certain elements, such as gold and other chalcophile elements, will respond more dramatically to pH fluctuations than others, they predict.
In addition, although bulk C flux from subducting slabs does not depart substantially from previous models, Galvez et al’s suggest that the efficiency of carbon transport from the surface to deep Earth may have changed over geologic time for both tectonic and geochemical reasons. They suggest that deep carbonate subduction may undergo a significant acceleration as Earth continues cooling.
“There is still work to do,” noted Galvez. “For example, we need to perform a range of lab-based experiments to improve the accuracy of our predictions. Overall, this study lays important groundwork on key controlling factors of the physics and chemistry of Earth’s mantle, and its connection to surface processes over geological time.”
See also a News & Views article in the same issue of Nature by David Dolejš.
Image: Gold Ore from the Nalunaq Gold Mine, southern Greenland. Source: Wikimedia Commons