Fluid inclusions

Fluid inclusions are small amounts of fluids trapped in minerals within rocks. They contain valuable clues regarding many geologic processes. The study of fluid inclusions also has a number of practical applications in the exploration for mineral and petroleum resources, the study of gemstones, and the search for a storage site for nuclear wastes.

Primary and Secondary Fluid Inclusions

During its history, almost every rock will come in contact with at least one fluid. Geologists have long recognized that fluids play an important role in shaping and altering rocks and in determining the earth's geologic history. It has been said that the fluid phase is the critical “missing” phase in petrology (the study of rocks). Behind this statement is the widespread belief that the fluids eventually leave rock systems, so that only indirect evidence can be used to deduce their nature. There is a growing recognition, however, that small amounts of these fluids are often left behind, trapped in small cavities, as fluid inclusions. In most cases, these inclusions represent the only available direct samples of fluids active deep within the earth or in the distant past.

Fluid inclusions can form in a variety of ways, though two are most common. Fluids can be trapped during mineral growth, to yield primary fluid inclusions. Most of these inclusions probably form when fluid fills pits on the surface of a growing crystal. New material added to the crystal grows over the tops of these cavities, trapping the fluid. These inclusions provide information about the nature of the fluids during the growth of the host minerals. Fluids trapped after the growth of their host minerals are secondary fluid inclusions, which form when fluid enters a crack within a mineral. As the ends of the crack grow together, the fluid is trapped. Secondary fluid inclusions originally form a thin envelope with a high surface-area-to-volume ratio. With its very high surface energy, this envelope is very unstable. Therefore, the host mineral will frequently recrystallize around the inclusion, causing it to break up into a swarm of smaller but thicker inclusions with a smaller surface-area-to-volume ratio. Such recrystallization occurs when the fluid inside the inclusion dissolves material from some parts of the inclusion wall and precipitates new material on other parts. Secondary fluid inclusions are sources of information about the fluids that have interacted with the rock after the growth of the host mineral.

Size, Shape, and Appearance

Fluid inclusions vary greatly in their size, shape, and appearance. Those that have diameters larger than a few tenths of a millimeter can be seen with the naked eye, but they are rare. Fluid inclusions with diameters between 1 and 100 microns are common and can be studied with a microscope. Inclusions with diameters as small as 0.01 micron have been observed with electron microscopes.

Fluid inclusions occur in almost any shape, but particularly noteworthy are negative crystals—cavities shaped like a crystal of the host mineral. This is the shape with the lowest surface energy and in many cases appears to be the final result of the recrystallization of the host mineral around the inclusion. Many inclusions contain only one phase, liquid or gas. However, gas bubbles within a liquid are also common. If these bubbles are small enough, they will move vigorously back and forth. This motion is a consequence of their relatively high surface tension. Some fluid inclusions contain immiscible liquids such as oil and water. In this situation, one of the liquids generally lines the walls of the inclusion, and the other liquid forms a droplet inside it. Solids can precipitate out of the fluids trapped within an inclusion to form tiny crystals known as daughter minerals. Trapped magma will solidify upon cooling to form either a glass or a mass of tiny mineral grains. Although such inclusions may not be fluid now, the material was trapped as a fluid, and hence they are generally classified as fluid inclusions.

Types of Inclusions

The composition of fluid inclusions depends on the environment in which they are trapped. Many different kinds of inclusions have been found in materials formed at the surface or in the upper levels of the earth's crust. Glacial ice and amber (fossilized tree sap) contain gas inclusions, which serve as modified samples of the atmosphere. Studies of amber inclusions show that the oxygen content of the atmosphere fell from 35% to 20% about 65 million years ago at the time that the dinosaurs disappeared. Minerals in evaporite deposits (rocks formed by precipitation from evaporating water) may contain samples of the concentrated brines from which they formed. Inclusions of groundwater can be found in the mineral deposits formed on walls, ceilings, and floors of caves. Fluid inclusions can be found in the cements that hold grains together in sedimentary rocks, as well as in the minerals that line the walls of vugs (roughly spherical cavities) and fractures in these rocks. Two kinds of inclusions most frequently occur in these situations. Water-bearing inclusions generally contain fairly high amounts of dissolved salts. Hydrocarbon inclusions can contain natural gas (most commonly methane) or crude oil.

Volcanic rocks can contain a number of different kinds of inclusions. Glass inclusions may represent trapped samples of the silicate liquid from which the rock crystallized. In some cases, inclusions represent immiscible liquids present as droplets in the main magma. These droplets may have consisted of another silicate-rich liquid now present as a glass or a fine-grained mixture of minerals, or they may have been a nonsilicate liquid rich in sulfur and iron, now represented by tiny sulfide mineral grains. Water-rich inclusions in volcanic rocks tend to be filled with either a concentrated brine or water vapor. Carbon dioxide is the other gas most commonly found in volcanic rocks.

Composition at Greater Depths

Fluid inclusions are also found in rocks formed at greater depths in the earth. The rock portions of the earth that surround the iron core consist of the relatively dense mantle surrounded by the much thinner, less dense crust. Carbon dioxide is the most common fluid in inclusions in rocks from the upper mantle and in metamorphic rocks from the lower crust. Because these fluids are trapped at high pressures, they are often very dense, and the carbon dioxide is usually present as a liquid, which may or may not contain a gas bubble. Variable amounts of nitrogen and methane can be dissolved in these inclusions, and it appears that compositions range from pure carbon dioxide to pure nitrogen or methane. At low temperatures, dense liquid carbon dioxide and water form immiscrible liquids. Thus, when water occurs in these dense carbon-dioxide-rich inclusions, it forms a separate liquid phase that generally lines the wall of the inclusions.

Water is the most abundant fluid in inclusions in metamorphic rocks formed in the upper levels of the crust, although carbon dioxide, methane, and nitrogen also occur. The kinds and amounts of dissolved solids vary greatly in these aqueous solutions and in many cases appear to reflect the nature of the rocks in which they were trapped. In the lower crust, the most abundant inclusions are rich in carbon dioxide. This change is coincident with a decrease in the abundance of water-bearing minerals in the deeper levels of the earth. Intrusive igneous rocks cool and solidify slowly within the earth. Under these conditions, inclusions of the silicate liquid from which the rock forms will become not glass but rather clusters of small mineral grains. Many other kinds of fluids can be trapped during or after the crystallization of an intrusive rock. Although inclusions rich in carbon dioxide, hydrocarbons, sulfide minerals, and nitrogen have been found, the most common inclusions in these kinds of rocks are water rich and in some cases contain large amounts of dissolved solids.

Changes Undergone by Fluid Inclusions

Fluid inclusions can undergo a variety of changes after they are trapped. Most inclusions form at temperatures significantly above those normally found on the earth's surface. As an inclusion cools, the volume of liquid will decrease much more rapidly than will the volume of the surrounding solids. This differential shrinkage frequently results in the formation of a vapor bubble from the liquid. If the inclusion is heated, this bubble will disappear. For inclusions trapped near the earth's surface, the temperature at which the bubble disappears will be close to the temperature at which the inclusion was trapped. For fluids trapped at elevated pressures deeper within the earth, the temperature at which the bubble disappears (in the laboratory) will be lower than the initial temperature of trapping. As temperature decreases, the fluid may become saturated with one or more solid compounds; the result is the precipitation of daughter crystals.

The thermal expansion or contraction of solids is low enough that temperature changes have a negligible effect on the volume of most inclusions. Thus, inclusions can generally be treated as constant volume systems. Unless material leaks out of the inclusions, their density will usually not change after trapping. Dense fluids trapped at elevated pressures will exert pressure on the walls of the inclusion. If this pressure exceeds the strength of the host mineral, the inclusion will burst, and much of the fluid will leak out. Fluid inclusions may also leak slowly instead of catastrophically. One way this leakage can occur is by slow diffusion of molecules through the host mineral. Certain molecules have a greater tendency to diffuse through the host; leakage by diffusion, then, can change the composition of an inclusion. Recrystallization of the host mineral can cause changes in the shape of an inclusion, as well as cause a larger fluid inclusion to break into smaller ones. If the larger inclusion contains two or more fluids (for example, a liquid and a vapor), generally they will not split evenly between the new smaller inclusions. Thus, the compositions of the new inclusions will differ from each other and from the compositions of the original inclusion. The effects of these and many other possible secondary changes have to be carefully evaluated during any fluid inclusion study.

Extraction of Fluid from Inclusions

Two major problems confront anyone trying to study the composition of a fluid inclusion: separating the fluid in the inclusion from the rest of the sample (including other inclusions) and obtaining an analysis of a very small sample. Most attempts to analyze fluid inclusions by conventional chemical methods have involved the extraction of fluid from many different inclusions in the same rock. One way to do this is to heat the rock. As temperature increases, so does the pressure exerted by the fluid on the walls of the inclusion. When the pressure exceeds the strength of the host mineral, the inclusion will burst open, and the fluid will escape—a process known as decrepitation. Another possibility is to crush the rock, thereby releasing the fluid. The fluid given off during crushing or decrepitation is collected, and its composition is determined by any of a number of different analytical techniques. Unfortunately, most samples contain different kinds of fluids trapped at different times. Analyses done by the methods above give the composition of a mixture of these fluids, which usually differs significantly from the composition of the fluid in any of the individual inclusions.

Analysis of Individual Inclusions

A number of techniques have been developed which will permit a partial chemical analysis of a single inclusion. Most of these involve hitting the inclusion with a small, tightly focused beam of light—usually a laser. Under these conditions, radiation will be emitted or absorbed; its wavelengths will be characteristic of the kinds of molecules present, while the intensity of the radiation will be proportional to their concentration. Measuring the spectra, then, makes it theoretically possible to obtain the composition of the inclusion. Most attempts to do this have used the Raman spectrum, emitted when the inclusion is struck by a laser beam. Methods using the infrared spectrum have also been used.

Most information on the composition of individual inclusions has come from observations made under the microscope. For such studies, a wafer of rock is ground to a millimeter or less in thickness and then polished on both sides. Light can be transferred through many minerals at this thickness. Much information can be obtained by careful observation at room temperature.

Heating-Cooling Stages

Even more information can be obtained by observing changes in an inclusion as it is heated or cooled. For example, if a solid forms upon cooling and then melts at 0 degrees Celsius, when the sample is heated, the inclusion contains water with no dissolved solids (0 degrees is the melting point of pure water). To make these kinds of observations, the rock wafer is placed in the sample chamber of a heating-cooling stage, which is in turn placed onto the microscope stage. Most heating-cooling stages are capable of cooling a sample to about −180 degrees Celsius or heating it to 600 degrees while it is being observed under the microscope. Observations made over this temperature range usually allow scientists to identify the major fluid species present (for example, water, carbon dioxide, or methane) and put some limits on the amounts and kinds of dissolved solids. Moreover, heating-cooling stages are relatively cheap and easy to use and give results quickly. Finally, they provide important information on the density of inclusions. Thus, the heating-cooling microscope stage has been, and is likely to continue to be, the major tool of fluid inclusion studies.

At low temperatures, most fluid inclusions contain both a gas and a liquid. As the inclusion is heated, the fluid will homogenize; the volume of one of the fluids increases, and the other completely disappears. The temperature at which this homogenization occurs is a function of the density of the fluid inclusion. Thus, by observing the temperature at which homogenization occurs on the heating-cooling stage, researchers can often obtain the density of the inclusion. Such density data may supply important information about the temperatures and pressures at the time the fluid was trapped.

Search for Economic Resources

The principal use of fluid inclusions has been in the study of ancient fluids and their interaction with solid earth materials. Although this research has made important contributions to the understanding of the earth's geologic history, the interest has been largely academic. Nevertheless, a number of practical applications of fluid inclusion studies have also been found. Much of this application has involved the search for mineral resources and the study of petroleum migration.

Fluid inclusions have contributed to the search for mineral resources in many different ways. First, studies of these inclusions have contributed enormously to the understanding of the processes by which ore deposits form. Many ore deposits have been formed by hot, water-rich solutions (hydrothermal solutions), which circulate through the rocks, dissolving and removing elements from some areas and precipitating them as ore minerals in others. This understanding has in turn guided the selection of areas in which to search for ore deposits. Fluid inclusion studies can also contribute to knowledge of the geologic history of a specific area, including an understanding of the development of features most likely to control the emplacement of an ore body. Finally, fluid inclusions can be one of the telltale signs in the search for ore. The strategy is to increase the size of the “target”—the small area containing a valuable mineral—by looking for secondary effects that accompanied ore deposition but affected a larger area. The improvement in the chances of finding ore with this strategy can be compared to the increase in the chances of finding a nail rather than a needle in a haystack. Fluid inclusions surrounding an ore body can show special properties related to the development of the ore, and these anomalies can extend well beyond the deposits. A search for such anomalies has been used in ore exploration.

Petroleum originates in a fine-grained source rock, moves into a more permeable reservoir rock, and then migrates into a petroleum trap. Hydrocarbon inclusions in old reservoir rocks are potential clues to the process of petroleum migration. This is a relatively new area of research, but it shows good promise of increasing the understanding of the movement of oil underground, leading to improved strategies in the search for this oil.

Other Practical Applications

The study of fluid inclusions has also been applied to the establishment of the authenticity of gemstones and investigation into long-term storage sites for nuclear waste. Establishing the authenticity of gemstones is one of the more important jobs of a gemologist. Many gemstones contain fluid inclusions; the nature of these inclusions often allows the expert to distinguish between synthetic and natural stones. In some cases, the fluid inclusions can be used to identify the source of the gem. Thus, for example, Colombian emeralds can often be recognized by fluid inclusions containing a concentrated brine and large daughter crystals of sodium chloride.

Because of its dangerous, highly radioactive nature, nuclear waste must be isolated for periods of time ranging from thousands to hundreds of thousands of years. Most proposals of ways to bring about such isolation involve the underground burial of the waste. The principal hazard with burial is that the nuclear waste could be dissolved in and carried away by fluids circulating through the local rocks. The study of fluid inclusions has been used to evaluate this danger. These studies are based on the realization that fluid inclusions represent a partial record of the fluids that moved through the rocks in the past and hence provide some basis for extrapolating into the future. Thus, for example, studies of fluid inclusions from salt beds have indicated that some of these beds probably have been penetrated by circulating groundwater. This finding is especially significant, since salt deposits have often been mentioned as possible repositories of nuclear waste.

Principal Terms

brine: water with a higher content of dissolved salts than ordinary seawater

fluid: a material capable of flowing and hence taking on the shape of its container; gases and liquids are both examples of fluids

glass: a solid without a periodic ordered arrangement of atoms; it frequently forms when molten material is rapidly cooled

igneous rocks: rocks formed by the crystallization of magma

immiscible fluids: two fluids incapable of mixing to form a single homogeneous substance; oil and water are common examples

intrusive rocks: igneous rocks formed from magmas that have cooled and crystallized underground

magma: molten material capable of yielding a rock upon cooling

metamorphic rocks: rocks that have transformed from their original condition as a result of changes in physical or chemical conditions within the earth

mineral: a naturally occurring, inorganic substance with a regular periodic arrangement of atoms

ore: any concentration of economically valuable minerals

sedimentary rocks: rocks formed by the consolidation of material transported by and deposited from wind or water

volcanic rocks: igneous rocks formed at the surface of the earth

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