Contrib Mineral Petrol (1994) 116:7-20 Contributions to Mineralogy and Petrology 9 Springer-Verlag 1994 A mechanism for preferential H20 leakage from fluid inclusions in quartz, based on TEM observations Ronald J. Bakker, and J. Ben H. Jansen* Department of Geochemistry, Institute for Earth Sciences, Utrecht University, P.O.Box 80.021, NL-3508 TA Utrecht, The Netherlands Received: May 5, 1993 / Accepted October 15, 1993 Abstract. Preferential leakage of H20 from fluid inclu- sions containing multiple gas components has been sus- pected in natural metamorphic rocks and has been demonstrated experimentally for synthetic H20-CO 2- rich inclusions in natural quartz. Knowledge of the phys- ical and chemical characteristics of the leakage mecha- nism, 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 non- decrepitative preferential H20 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 nu- cleation of many dislocations and planar defects (like Dauphin~ twin boundaries). These defects allow prefer- ential loss of H20 from HzO-CO2-rich inclusions at su- percritical 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 crys- tals. 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 bub- bles 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 deforma- tion or by crystal growth related processes (e.g. crack healing). The presence of small bubbles on these disloca- tions indicates that a similar leakage mechanism for H20 must have occurred in these rocks. Introduction Natural aqueous fluid inclusions in metamorphic rock may have preferentially leaked H20. Water leakage may Correspondence to: R.J. Bakker, CREGU, BP 23, F-54501 Van- d0euvre-les-Nancy Cedex, France *Present address: BOWAGEMI BV, Prinses Beatrixlaan 20, NL-3972 AN Driebergen, The Netherlands be implied from fluid inclusions in metamorphic rocks which experienced P-T conditions which do not corre- spond with the specific isochore of the trapped fluid at peak metamorphic conditions. Selective H20 leakage is not necessarily uniform within both grains and single trails, and some H20-rich inclusions may not have leaked at all, which indicate that the mechanism is complex (e.g. Roedder 1984). Synthetic H20-CO 2 fluid inclusions have been pro- duced in quartz by Bakker and Jansen (1990, 1991). They showed that non-decrepitative H20 leakage may occur during re-equilibration at conditions differing from the original conditions of entrapment. Inclusions were syn- thesized through crack-healing processes in gem-quality Brazilian quartz. The re-equilibration experiments mimic the changing P-Tconditions experienced by rocks, fol- lowing real metamorphic P-T-paths. Non-uniform pref- erential leakage of H20 occurred, without evidence of decrepitation or inclusion-volume adjustments, during re-equilibration experiments which simulated isothermal compression and isothermal decompression at 835 K. It is not possible to identify routes for leakage using optical microscopy. Small open cracks or channels connected with inclusions appear to be absent. Crystal defects in natural quartz have been studied using Transmission Electron Microscopy (TEM) since in- vestigations suggested that plastic deformation in quartz is accommodated by dislocation movement (e.g. McLaren and Phakey 1965; Doukhan and Trepied 1985). To explain the hydrolytic weakening phenomenon in nat- ural and synthetic quartz, water-related point defect bulk diffusion through the crystal and water enhanced mobili- ty of dislocations were intensively studied. The main ar- gument for water-related point defect bulk diffusion in synthetic quartz and natural amethyst is the formation of small bubbles during annealing experiments (McLaren et al. 1983; Gerretsen et al. 1989). A similar phenomenon has already been described by Wolff (1845) and Ferguson (1914), who observed that natural quartz became milky white after annealing, due to the formation of numerous very small fluid inclusions. Water-related point defects consist mainly of 4(H)s~ substitutes, H20 interstitials, and