Thematic Articles

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.

Three-quarters of global magmatism and one-quarter of global heat loss are associated with tectonomagmatic and hydrothermal processes governing oceanic lithosphere accretion and the aging of the lithosphere from ridge to trench. Hydrothermal reactions between seawater and oceanic lithosphere under zeolite to granulite facies conditions are linked with magmatic and deformation processes, but they differ in nature depending on spreading rates. Fast-spreading ridges with frequent eruptions have telescoped metamorphic gradients and short-lived hydrothermal systems. Less magmatically robust, slow-spreading ridges are commonly cut by normal faults that expose ultramafic rocks on the seafloor and sustain long-lived hydrothermal systems with distinct vent fauna and fluid compositions.

Metamorphic devolatilization generates fluid and grain-scale porosity. Evidence for high fluid pressure indicates that devolatilization occurs under poorly drained conditions. Under such conditions, fluid expulsion is limited by the capacity of the reacted rocks to resist compaction or by the rate at which deformation modifies the permeability of the overlying rocks. In the former case, the compaction timescale must be greater than the metamorphic timescale, and flow patterns are dictated by details of rock permeability. The alternative is that compaction processes are fast relative to metamorphism. In this case, flow is compaction driven and accomplished by waves of fluid-filled porosity.

A fundamental question in metamorphism is: What is the mechanism that converts one mineral assemblage into another in response to a change in the physical and/or chemical environment? The fact that aqueous fluids must be involved in such large-scale re-equilibration has been demonstrated by petrological, mineralogical, micro-structural and isotopic data. Fluid–mineral reactions take place by dissolution–precipitation processes, but converting one rock into another requires pervasive transport of reactive fluid through the entire rock. The generation of reaction-induced porosity and the spatial and temporal coupling of dissolution and precipitation can account for fluid and element transport through rocks and the replacement of one mineral assemblage by another.

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.