Synthetic fluid inclusions. XV. TEM investigation of plastic flow associated with reequilibration of fluid inclusions in natural quartz

The nature and abundance of dislocations in quartz surrounding fluid inclus ions were studied to obtain a better understanding of processes associated with fluid inclusion reequilibration. Synthetic fluid inclusions containing 10 wt% NaCl aqueous solution were formed in three samples at 700 degrees C and 5 kbar. One of the samples was quenched along an isochore to serve as a reference sample. The other two samples were quenched along a P-T path th at generated internal pressures in excess of the confining pressure. The tw o samples were held at the final reequilibration P-T conditions of 625 degr ees C and 2 kbar for 30 and 180 days, respectively. Following the experimen ts, microstructures associated with fluid inclusions were examined with the TEM. Quartz in healed fractures in the reference sample that was quenched isochorically shows a moderate dislocation activity. Quartz adjacent to ree quilibrated fluid inclusions in the other two samples, however, showed a ma rked increase in dislocation activity compared to the un-reequilibrated sam ple. Deformation of the inclusion walls occurred anisotropically by expansi on of mobile dislocations in their slip systems. Dislocation expansion was controlled by glide in the rhombohedral planes {1 0 1 1} that was restricte d to narrow zones (less than or equal to 3 mu m) in the immediate vicinity of the fluid inclusion walls outside of the healed fracture plane. These pl astic zones were observed after both short term (30 days) and long term (18 0 days) experiments and are attributed to hydrolytic weakening of quartz ar ound fluid inclusions owing to diffusion of water into the quartz matrix du ring the experiment. The close spatial association of submicroscopic water bubbles with dislocations, and the rarity of water bubbles in the reference sample, show clearly that in both the 30 and 180 day experiments reequilib ration involves water loss from the fluid inclusions. Our results indicate that synthetic fluid inclusions in this study recover (chemically and volum etrically), even at relatively fast experimental loading rates, such that i nternal stresses never reach the point of brittle failure. The driving forc e for fluid inclusion deformation involves two related mechanisms: plastic deformation of hydrolytically weakened wet quartz in the healed fracture, a nd water leakage associated with preexisting and strain-induced dislocation s.