DYNAMICS OF FLUID INCLUSION ALTERATION IN SEDIMENTARY-ROCKS - A REVIEW AND DISCUSSION

Fluid inclusions represent small volumes of interstitial fluids trapped in crystal imperfections (flaws) during crystal growth and/or deformation. The entrapped fluids are commonly believed to preserve information on the composition, pressure and temperature of fluid inclusion formation and later diagentic alterations. It is now known that fluid inclusions can be dynamic structures, responsive to thermal and tectonic stresses by plastic and brittle deformation of the entrapping mineral wall. These dynamic responses may include both morphologic and volumetric changes. A comparison of measured fluid inclusion homogenization temperatures with sample burial depths for diagenetic studies from around the world show a strong positive correlation. This correlation suggests that fluid inclusion stretching, which results in increased homogenization temperatures, is a common phenomenon associated with burial diagenesis. Burial diagenesis shifts (and spreads) the measured homogenization temperatures towards maximum burial conditions. Confining mineralogy, shape, size and composition of the fluid inclusions are important factors in determining the mode and degree of fluid inclusion deformation (stretching). Classical fracture mechanics theory predicts that plastic and brittle deformation of the surrounding mineral crystal will occur with burial and that annealing (retrograde stretching) of the strain in the inclusion walls will not occur in low temperature regimes. Experimental, theoretical and field studies all indicate that fluid inclusions will react dynamically to burial diagenesis by increases in measured homogenization temperatures with increased burial. It is suggested that this relationship can be selectively used to help determine the approximate maximum thermal exposures experienced by a sedimentary rock.