Synthetic fluid inclusions ‐ VII. Re‐equilibration of fluid inclusions in quartz during laboratory‐simulated metamorphic burial and uplift
Authors: STERNER S. M. , BODNAR R. J. 
Published: May 2007

ABSTRACT P‐T conditions inferred from fluid inclusions in metamorphic rocks often disagree with the values predicted from mineral equilibria calculations. These observations suggest that inclusions formed during early stages of regional metamorphism continue to re‐equilibrate during burial and subsequent uplift in response to differential pressure. P‐T conditions accompanying burial and uplift were experimentally simulated by initially forming pure H2O inclusions in quartz at elevated temperatures and pressures, and then re‐equilibrating the inclusions in the presence of a 20 wt% NaCl solution such that final confining pressures ranged from 5 kbar above to 4 kbar below the initial internal pressure of the inclusions at the temperature of re‐equilibration.

In all samples re‐equilibrated at confining pressures below the internal pressure, some inclusions were formed that had compositions of 20 wt% NaCl and densities in accord with the final P‐T conditions. Additionally, some inclusions were observed to contain fluids of intermediate salinities (between 0 and 20 wt% NaCl). Densities of these inclusions were also consistent with formation at the re‐equilibration P‐T conditions. The remainder of the fluid inclusions observed in these samples contained pure H2O and their homogenization temperatures corresponded to densities intermediate between the initial and final P‐T conditions. In short‐term experiments (7 days) where the initial internal overpressure exceeded 1 kbar, no inclusions were found that contained the original density and none were found to have totally re‐equilibrated. Instead, most H2O inclusions re‐equilibrated until their internal pressures were between ∼750 and 1500 bars above the confining pressure, regardless of the initial pressure differential. In a long‐term experiment (52 days), inclusions re‐equilibrated at a lower confining pressure than the initial internal pressure displayed homogenization temperatures corresponding to a range in final internal pressures between 0 kbar (i.e. total re‐equilibration) and 1.2 kbar above the confining pressure.

In experiments where the confining pressure during re‐equilibration exceeded the initial internal pressure, densities of pure H2O inclusions increased to values intermediate between the initial and final P‐T conditions. Additionally, these inclusions were generally surrounded by a three‐dimensional halo of smaller inclusions, also of intermediate density, resulting in a texture similar to that previously ascribed to decrepitation from internal overpressure. In extreme cases where confining pressures were 4–5 kbar above the initial pressure, the parent inclusion almost completely closed leaving only the three‐dimensional array of small (5 μm) inclusions, the outline of which may be several times the volume of the original inclusion. Groups of such inclusions closely resemble textures commonly observed in medium‐ to high‐grade metamorphic rocks.

Inclusions containing 10 and 42 wt% NaCl solutions trapped at 600 °c and 3 kbar were re‐equilibrated at 600 °c and 1 kbar for 5 days in dry argon to evaluate the importance of H2O diffusion as a mechanism of lowering the inclusion bulk density. Salinities of re‐equilibrated inclusions obtained from freezing point depressions and halite dissolution temperatures indicate that original compositions were preserved. Density changes similar to those previously described were noted in these experiments, in inclusions showing no visible microfractures. Therefore, density variations observed in inclusions in this study, re‐equilibrated under rapid deformation conditions, are considered to result from a change in the inclusion volume, without significant loss of contents by diffusion or leakage.