A molecular water pump in quartz dislocations

Abstract

RECENTLY Bakker and Janssen¹ have reported experiments suggesting that water leaks out of bubbles in quartz preferentially along dislocations against thermodynamic gradients in fugacity, chemical potential and pressure. This effect indicates that the density of gas-rich fluid inclusions cannot be calculated from metamorphic pressure–temperature conditions and, conversely, that the density of the fluid cannot be used to infer metamorphic pressure–temperature conditions. Using computer modelling techniques, I show here that water not only diffuses easily along dislocations in quartz, but can also be 'pumped' along them. The simulations indicate that in dislocation cores, water molecules dissociate and the protons and hydroxyl groups become strongly bound to kinks. A shear stress can do work on kinks to move them along a dislocation, dragging with them these water-bearing species and sweeping other, undissociated water molecules before them.