PACROFI VI - Electronic Program

Plastic flow vs. brittle deformation: Do fluid inclusions in high temperature metamorphic environments decrepitate?

Maxim O. Vityk and Robert J. Bodnar

Fluids Research Laboratory, Department of Geological Sciences, Virginia Polytechnic Institute & State University, Blacksburg, VA, U.S.A., 24061

In the laboratory, the response of fluid inclusions to stress usually involves two types of behavior: elastic behavior, where inclusion strain is recoverable, and permanent deformation which occurs when the stress exceeds some critical value. Permanent deformation accompanied by fracturing of the host due to high effective and/or differential pressure is well known as inclusion decrepitation or brittle deformation. Permanent deformation whereby strain is distributed uniformly throughout the inclusion walls is termed stretching or plastic deformation. There is a common belief that fluid inclusions reequilibrated under conditions of high internal overpressure deformed by decrepitation. In nature, however, deformation of fluid inclusions at conditions of high temperature and low loading rates includes considerable plasticity accommodated by dislocation mechanisms (glide, climb, multiplication and interaction of dislocations). Plastic deformation of fluid inclusions might allow the internal stress of the fluid inclusions to be reduced gradually, thus preventing the inclusions from decrepitating. To test this idea we conducted a series of decompressional high temperature (600-700oC) and pressure (2-5 kbar) experiments using synthetic aqueous inclusions in natural quartz. With up to 1 kbar of internal overpressure the inclusions deform elastically (Fig. 1A). Once the yield point (elastic limit) for the inclusion is passed the inclusion walls start to flow. Under conditions of internal overpressure, inclusion flow causes a permanent increase in the inclusion volume (increase in strain) (Fig. 1B). The increase in inclusion strain reduces the internal stress of the fluid inclusion (Fig. 1C), shifting (reducing) inclusion density towards the final re-equilibration isochore. We found that this partial "recovery" of the inclusions from high internal stresses is a relatively rapid process and can be achieved over one month duration (primary recovery, Fig. 1C). Each reequilibration experiment produced a unimodal Th histogram with low variability and no correlation between inclusion size and density. After each run, the estimated internal overpressure for the inclusions at the conditions of reequilibration was about 1 kbar. This amount of internal overpressure closely approximates the amount of overpressure at which the inclusions start to flow (Fig. 1C). Additional time (up to 270 days at the reequilibration conditions) had little effect on inclusion density (secondary recovery, Fig. 1C), suggesting that some apparent state of volumetric equilibrium had been achieved by most fluid inclusions after about 30 days.

Our results clearly indicate that even fast experimental loading rates allow the inclusion volume to adjust such that internal stresses never reach the point where the inclusions fail in a brittle manner. The internal overpressure required to initiate plastic deformation of fluid inclusions corresponds to the internal overpressure that the inclusions can hold over geological time. The driving force for recovery of the inclusions from the internal stresses arises from plastic deformation of the inclusion walls. Assuming that inclusion deformation occur in a similar way in nature, we conclude that for most of high P and T natural conditions, decrepitation of the inclusions is unlikely to occur. The only condition where decrepitation might take place is in the low temperature, high strain metamorphic environment.

Figure 1. Deformation behavior of aqueous inclusions in natural quartz under conditions of internal overpressure. A. At low amounts of internal overpressure, the inclusion deforms elastically. Once the elastic limit is reached, further increase in internal pressure causes the inclusion to deform plastically. B. Typical form of a strain-time curve for fluid inclusions. Once the elastic limit is reached, strain (volume) increases with time until some maximum value is reached. Additional time has little effect on strain (volume). C. Internal overpressure - time deformation curve. When the internal overpressure exceeds the elastic limit, the inclusion volume increases with time, thus reducing the internal pressure to approach the elastic limit of the host.