The effect of fluid inclusion size on determination of homogenization ...

American Mineralogist, Volume 94, pages 1569–1579, 2009

The effect of fluid inclusion size on determination of homogenization temperature and density of liquid-rich aqueous inclusions András Fall,* J. Donald Rimstidt, and Robert J. Bodnar† Department of Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall, Blacksburg, Virginia 24061, U.S.A.

Abstract Homogenization temperature variations of several degrees Celsius or more are often observed within a group of fluid inclusions that appear to have all trapped the same homogeneous fluid at the same time and presumably at the same PTX conditions. For inclusions that homogenize at T ≤ ≈230 °C, much of the observed variation can be attributed to the size of the inclusions. Larger inclusions homogenize at higher temperatures compared to smaller inclusions with the same density. The relationship between inclusion size and observed homogenization temperature is predicted by the Young-Laplace equation that relates the stability of a vapor bubble to the surface tension and pressure differential across the vapor-liquid interface. Vapor bubbles instantaneously collapse when the vapor bubble radius becomes less than the critical radius. During heating the critical radius of the vapor bubble is achieved at a lower temperature in the smaller inclusions. The critical vapor bubble radius varies from about 0.01 to ~3 µm for low-temperature aqueous fluid inclusions. The Gibbs surface free energy associated with the growth and collapse of vapor bubbles in pure H2O inclusions with critical radii ranging from 0.01 to 1 μm ranges from about 10–18 to 10–13 J/m2 and increases with both increasing critical vapor bubble radius and homogenization temperature. As a result of surface tension effects, the highest measured homogenization temperature, obtained from the largest inclusion in the group of coeval inclusions, most closely approximate the homogenization temperature that would be expected based on the inclusion density. For inclusions ranging from a few to several tens of micrometers in diameter and having densities such that the homogenization temperatures are approximately rc, the bubble will continue to grow to decrease its surface free energy.

inclusions homogenize at slightly lower temperature compared to larger inclusions. This behavior contradicts the generally held belief that fluid inclusions trapped at the same temperature (and pressure, and having the same composition) should have the same Th. The size dependency is predicted by the Young-Laplace equation that relates vapor and liquid pressures to the surface tension of the liquid-vapor interface and the radius of the vapor bubble. The size dependency of Th decreases with increasing temperature and is not observed for homogenization temperatures higher than about 230 °C. This is expected as the surface tension decreases with increasing temperature, allowing the bubble to collapse with a lower pressure differential across the liquid-vapor interface. Inclusions containing electrolyte solutions should have a critical bubble radius that is greater than that for pure H2O inclusions of

the same size, and should therefore homogenize at a temperature that is further away (lower) from the “nominal” homogenization temperature. Conversely, the critical radius of CO2-H2O and CH4-H2O fluid inclusions would be smaller than that for pure H2O inclusions of similar size, and should homogenize at a temperature that is closer to the “nominal” homogenization temperature, compared to pure H2O inclusions. Results of this study suggest that homogenization temperatures of low-temperature (