Fluid flow in crystalline rocks in the absence of fractures or ductile shear zones dominantly occurs by grain boundary diffusion, as it is faster than volume diffusion. It is, however, unclear how reactive fluid flow is guided through such pathways. We present a microstructural, mineral chemical, and thermodynamic analysis of a static fluid-driven reaction from dry granulite to ‘wet’ eclogite. Fluid infiltration resulted in re-equilibration at eclogite-facies conditions, indicating that the granulitic protolith was out of equilibrium, but unable to adjust to changing P–T conditions. The transformation occurred in three steps: (1) initial hydration along plagioclase grain boundaries, (2) complete breakdown of plagioclase and hydration along phase boundaries between plagioclase and garnet/clinopyroxene, and (3) re-equilibration of the rock to an eclogite-facies mineral assemblage. Thermodynamic modelling of local compositions reveals that this reaction sequence is proportional to the local decrease of the Gibbs free energy calculated for ‘dry’ and ‘wet’ cases. These energy differences result in increased net reaction rates and the reactions that result in the largest decrease of the Gibbs free energy occur first. In addition, these reactions result in a local volume decrease leading to porosity formation; i.e., pathways for new fluid to enter the reaction site thus controlling net fluid flow. Element transport to and from the reaction sites only occurs if it is energetically beneficial, and enough transport agent is available. Reactive fluid flow during static re-equilibration of nominally impermeable rocks is thus guided by differences in the energy budget of the local equilibrium domains.

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Aims. A major goal in the search for extraterrestrial life is the detection of liquid water on the surface of exoplanets. On terrestrial planets, volcanic outgassing is a significant source of atmospheric and surface water and a major contributor to the long-term evolution of the atmosphere. The rate of volcanism depends on the interior evolution and on numerous feedback processes between atmosphere and interior, which continuously shape atmospheric composition, pressure, and temperature. Methods. We present the results of a comprehensive 1D model of the coupled evolution of the interior and atmosphere of rocky exoplanets that combines central feedback processes between these two reservoirs. We carried out more than 280 000 simulations over a wide range of mantle redox states and volatile content, planetary masses, interior structures and orbital distances in order to robustly assess the emergence, accumulation and preservation of surface water on rocky planets. To establish a conservative baseline of which types of planets can outgas and sustain water on their surface, we focus here on stagnant-lid planets. Results. We find that only a narrow range of the mantle redox state around the iron-wüstite buffer allows forming atmospheres that lead to long-term habitable conditions. At oxidizing conditions similar to those of the Earth's mantle, most stagnant-lid planets transition into a runaway greenhouse regime akin to Venus due to strong CO 2 outgassing. At more reducing conditions, the amount of outgassed greenhouse gases is often too low to keep surface water from freezing. In addition, Mercury-like planets with large metallic cores are able to sustain habitable conditions at an extended range of orbital distances as a result of lower volcanic activity.

<p>The long-term evolution of the atmospheres or rocky planets depends on several different factors, including (but not limited to) volcanic outgassing by partial melting of the rocky interior. Uprising melt may contain different assembladges of volatiles (CHONS) depending on the general mantle composition and redox state, the local volatile inventory, as well as melting depth. All these factors are assumed to vary for mobile-lid planets with an active surface recycling mechanism (e.g. plate tectonics or convective mobile resurfacing) when compared to stagnant-lid planets. </p> <p>In addition, the strength of volcanic activity varies for stagnant-lid and mobile-lid planets, with stronger activity and hence volcanic outgassing expected for the latter case. Atmospheres with pressures above Earth values also severely influence which volatiles will be further outgassed into the atmosphere depending on the solubility of the individual gas species. The outgassing fluxes therefore strongly depend on the evolution of the atmosphere, including atmosphere losses to space or by condensation or weathering. Ultimately, different atmospheric compositions will evolve for planets with low-pressure atmospheres (i.e. low-mass planets, planets without an active surface mobilization process, or planets with efficient atmosphere sinks/losses) and high-pressure atmospheres.</p> <p>Our studies allow us to predict ranges of likely atmospheric properties depending on planet mass and the surface mobility regime, that can then be compared to observations (i.e. with JWST or in the more distant future with the proposed LIFE mission).</p>