The lawsonite paradox: a comparison of field evidence and mineral equilibria modelling

摘要: Lawsonite equilibria are predicted to occur over a broad P–T spectrum developed during subduction, yet lawsonite-bearing assemblages rare. In the context of mafic mineral modelled for range common crustal metamorphism (4–23 kbar, 400–750 °C) using system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O and software thermocalc, unusually high water contents demanded by lawsonite assemblages. As consequence, have difficulty forming be uncommon. Metabasalt undergoing cooler subduction may experience substantial periods involving metastable persistence because under-saturation with non-occurrence recrystallization. If formed, observed highly unstable; their preservation requires that exhumation accompanied cooling. The amount structurally bound H2O in minerals plays critical role formation assemblages, controlling key changes rocks subduction.