Author name: Henrik Svensen

Carbon dioxide is produced by metamorphic reactions in orogenic belts and high-heat-flow systems. Part of this carbon is ultimately released to the atmosphere, but the long timescale of regional metamorphism implies that the short-term effects on the environment are minor. However, contact metamorphism around igneous sill intrusions in organic-rich sedimentary basins has the potential to generate huge volumes of CH4 and CO2, and these gases are rapidly released to the atmosphere through vertical pipe structures. The high flux and volume of greenhouse gases produced in this way suggest that contact metamorphic processes could have a first-order influence on global warming and mass extinctions.

The evolution of the Earth’s lithosphere is affected in a major way by metamorphic processes. Metamorphism affects the lithosphere’s chemical and mineralogical composition, as well as its physical properties on scales ranging from a nanometer to the size of tectonic plates. Studies of metamorphism during the last couple of decades have revealed that fluids are as important in a changing lithosphere as water is in the biosphere. History-dependent characteristics of metamorphic rocks, such as their microstructure, compositional variation, and deformation features, reflect the dynamics of fluid–rock interactions. Migration of the fluids produced during prograde metamorphic processes or consumed during retrogression links metamorphism at depth to the evolution of the hydrosphere, the atmosphere, and the biosphere.

Metamorphic rocks make up a substantial portion of the Earth’s evolving lithosphere. Understanding metamorphism is central to interpreting large-scale geodynamic processes and interactions among the geosphere, the hydrosphere, the atmosphere, and the biosphere. In this issue of Elements, we emphasize the critical role of fluids in controlling the rates and mechanisms of metamorphic processes. The patterns observed over a wide range of scales in metamorphic rocks are not just passive recorders of tectonic events. They also reveal that the complex coupling of chemical reactions, transport, and deformation processes that constitute metamorphism sometimes operates surprisingly far from equilibrium.