The critical role of stored energy in metamorphism

James Gilgannon, Damien Freitas, Eilidh Vass Payne, Roberto Emanuele Rizzo, Ian Butler, Danielle Duggins, Simon Hunt, Andrew King, Christian M. Schlepütz, Dominik Daisenberger, Neil Bourne, Florian Fusseis

The mechanical behaviour of the Earth is widely assumed to change with increasing timescale as rocks progressively relax imposed stresses [1]. On short timescales, elasticity dominates and its release drives earthquakes [2]; at intermediate timescales, viscoelastic relaxation facilitates processes such as glacial rebound [3]; whereas over geological timescales viscous deformation dominates, allowing the mantle to flow [4]. Energetically, this progression implies that mechanically stored potential energy becomes increasingly irrelevant to long-term geological processes such as metamorphism. Here we challenge this assumption using time-resolved deformation experiments that isolate changes in stored energy during metamorphic reaction. We show that increasing stored energy causes reaction to occur sooner and faster, increases nucleation density and reduces product grain size. Comparison with experiments from crustal and mantle materials shows that this influence persists across different reactions and deformation regimes. These results demonstrate that stored energy can exert a first-order control on geological transformation despite its comparatively small contribution to the total energy budget. Because stress and stored energy are heterogeneous across materials and scales, our results place stored energy alongside water as a driver of metamorphic change and provide experimental support for a view of the Earth in which changes in Helmholtz energy influence the emergence of material instabilities [5-8].