A mechanism for preferential H2O leakage from fluid inclusions in quartz,based on TEM observations

Preferential leakage of H₂O from fluid inclusions containing multiple gas components has been suspected in natural metamorphic rocks and has been demonstrated experimentally for synthetic H₂O-CO₂-rich inclusions in natural quartz. Knowledge of the physical and chemical characteristics of the leakage mechanism, which may be very complex, increases the value of natural fluid inclusions to metamorphic geology. It is proposed that crystal defects play a major role in nondecrepitative preferential H₂O leakage through quartz, and remain effective during metamorphism. Inclusions with either an internal overpressure or underpressure produce strain in the adjacent quartz crystal via the nucleation of many dislocations and planar defects (like Dauphiné twin boundaries). These defects allow preferential loss of H₂O from H₂O-CO₂-rich inclusions at supercritical conditions. The transport capacity of this leakage mechanism is enhanced by nucleation of small bubbles on defect structures. The nucleation of these bubbles seems to be a recovery process in strained crystals. Solubility gradients of quartz in water in a crystal with internally underpressurized inclusions may result in optical visible implosion halos in a three dimensional spatial arrangement, caused by the growth of small bubbles at the expense of the larger original fluid inclusion. Natural fluid inclusions from Naxos (Greece) are always associated with numerous interlinked dislocations. These dislocations may have been produced by plastic derormation or by crystal growth related processes (e.g. crack healing). The presence of small bubbles on these dislocations indicates that a similar leakage mechanism for H₂O must have occurred in these rocks.