fluid inclusions

fluid inclusions Fluid inclusions are bubbles of liquid trapped inside crystals, and were among the first new features to be discovered with the introduction of microscopic studies of thin sections of rocks. A seminal paper by H. C. Sorby, published in the Quarterly Journal of the Geological Society in 1858, recognized many of the major inclusion types and showed how useful information could be obtained from them. Strictly speaking liquid, vapour, or supercritical fluid (e.g. methane) may be present in fluid inclusions. Most workers would include melt inclusions in igneous minerals in the same general category, on the grounds that they were liquid when trapped in the host crystal.

Fluid inclusions are particularly common in vein minerals; myriad fine inclusions give rise to the milky appearance of much vein quartz. However, fluid inclusions occur in a wide range of rock-types, including diagenetic overgrowths in sediments, veins in active geothermal systems, hydrothermal ore deposits, metamorphic rocks and veins, granites, pegmatites, and some other types of igneous rock. Melt inclusions are particularly common in volcanic phenocrysts, especially in andesites. In addition to quartz, fluid inclusions can occur in carbonates, gypsum, halite, fluorite, feldspar, garnet, pyroxene, amphibole, tourmaline, olivine, and sphalerite; indeed in most minerals except phyllosilicates. Fluid inclusions have also been studied in opaque ore minerals, using infra red microscopy.

Most fluid inclusions are small, only a few microns to tens of microns in diameter, but large inclusions visible to the naked eye are known. Fluid inclusions can be trapped in crystals as they grow (primary inclusions), in which case they often define zones reflecting the morphology of the growing crystal, or can form during the resealing of cracks in a pre-existing crystal (secondary inclusions). Water is the most common fluid present, but carbon dioxide, methane, heavier hydrocarbons, and nitrogen can also occur. If the fluid carried a large dissolved load, material may precipitate as daughter crystals during cooling to surface conditions. Halite is the most common daughter mineral, but many other salts and some silicate minerals also occur in the same way.

The value of fluid inclusions is twofold: they preserve samples of the fluids present during mineral-forming processes and, in favourable circumstances, they also preserve the density of these fluids, from which the pressure or temperature of mineral growth can be estimated. Rocks formed in the deep crust normally contain multiple generations of fluid inclusions, formed at different times and under different conditions. This makes it difficult to relate specific inclusions to particular events in the history of the rock.

The approximate composition of fluid inclusion contents is normally determined by microthermometry, that is, by studying their freezing behaviour (in thin, doubly polished slices) using a microscope with a heating–freezing stage. Carbon dioxide occurs in liquid form in fluid inclusions, reflecting a significant internal pressure, and its purity can be estimated from the melting point. Likewise, the salinity of aqueous inclusions is determined from the temperature at which ice melts. More precise chemical information is obtained by laser Raman spectroscopy, for measurement of gas compositions of individual inclusions, or by crushing and leaching of bulk samples for cation and anion analysis. Elemental analysis of individual inclusions is now being developed using laser and ion beam techniques.

Fluid inclusion densities are also measured by microthermometry. The hole in the host crystal that the fluid occupies has an almost constant volume, and so the density of the filling remains unchanged during uplift and cooling, unless the inclusion leaks. If an inclusion is originally trapped at the pressure–temperature conditions of point A shown in Fig. 1, then on cooling, the internal pressure of the fluid contents may vary along a line of constant density, or isochore, irrespective of the external pressure acting on the rock. At point B, the isochore intersects the liquid–vapour curve of the fluid. With further cooling, a vapour bubble develops in the inclusion, and the internal pressure lies along the liquid– vapour curve. As the liquid portion becomes denser, the proportion of vapour to liquid increases. In microthermometric studies, this history is reversed under the microscope, and the temperature of homogenization, that is, where the vapour bubble vanishes, is measured. At this temperature, the density of liquid coexisting with vapour (which is well known for a range of fluids) corresponds to the density of the inclusion filling as a whole, and hence an isochore can be constructed which passes through the P–T conditions at which the inclusion formed, or last leaked. Synthetic fluid inclusions, made under known condition of composition, pressure, and temperature, have been used to obtain primary data on the volume of fluids over a wide range of physical conditions.

Bruce W. D. Yardley

Bibliography

Roedder, E. (1984) Fluid inclusions. Reviews in Mineralogy, 12, Mineralogical Society of America, Washington D. C.