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

Abstract

Lawsonite equilibria are predicted to occur over a broad P-T spectrum developed during subduction, yet lawsonite-bearing assemblages are rare. In the context of mafic mineral equilibria modelled for the range of common crustal metamorphism (4-23kbar, 400-750 °C) using the system Na2O-CaO-K2O-FeO-MgO-Al2O3 -SiO2-H2O and the software thermocalc, unusually high water contents are demanded by lawsonite assemblages. As a consequence, lawsonite assemblages are predicted to have difficulty forming and lawsonite equilibria to be uncommon. Metabasalt undergoing cooler subduction may experience substantial periods involving the metastable persistence of mineral assemblages because of water under-satur..