Dramatic effects of stress on metamorphic reactions

Abstract

Thermodynamic calculations predict mineralogy from temperature and pressure and vice versa. Such calculations assume that stress is isotropic despite the fact that differential stresses prevail in Earth, resulting from large-scale tectonics and/or differences between fluid and rock pressures in porous rocks. New calculations show that differential stress can have significant effects on thresholds for metamorphic reactions, depending on the grain-scale reaction pathways. A differential stress may, depending on the reaction pathway, have an effect equivalent to a pressure difference on the order of (assemblage volume)/(reaction volume change) × (differential stress). The multiplying factor is typically 10 or more. For example, the onset of a garnet + clinopyroxene breakdown reaction may be offset, up or down, by the equivalent of 500 MPa in pressure for a 50 MPa differential stress. The effect is equivalent to a temperature difference on the order of (assemblage volume)/(reaction entropy change) × (differential stress). For example, the onset of muscovite + quartz breakdown may be offset, up or down, by the equivalent of 130 °C for a 50 MPa differential stress. Much of Earth is under differential stress, so the new calculations invite a reappraisal of metamorphic mineralogy and microstructure, indicating that new insights into stresses and fluid pressures on Earth can be gained.