Composition of fluids in the lower crust inferred from metamorphic salt in lower crustal rocks

Knowledge of the rheological properties of the lower crust and the metamorphic processes that operate there is important for our understanding of orogenic processes and granite genesis. The rheological properties critically depend on whether fluids are present in the lower crust1,2 and, if present, on their composition3,4,5,6. Fluid-inclusion7,8,9 and phase-equilibria4,10 studies of lower crustal granulites have shown that fluids with low water activities (due to the presence of dissolved components such as CH4, N2, CO, CO2 and chlorides)11 are present at least episodically in the lower crust. Here we report the occurrence of a solid salt solution (NaCl–KCl) found together with chlorine-rich amphibole and biotite in lower crustal granulites. A desiccation mechanism explains how salt and chlorine-rich minerals formed from an originally water-rich fluid through a short-lived series of hydration reactions in the granulites, during which chlorine was progressively enriched in the fluid. Consequently, it would appear that fluid was present in the lower crust in only small amounts and was not stable over geologically long periods of time, leading to the conclusion that the lower crust is devoid of a free fluid phase during most of its history.