Thesis: Fluid Inclusions

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Fluid Inclusions

Until the early 1950s, few people in the scientific community had any idea of the practical applications of fluid inclusion research. After all, Fluid inclusions are bubbles of liquid and gas trapped within crystals. Fluid inclusions generally range from.01 to 1mm in size, which means that they are generally only visible when viewed under a microscope, though some crystals can have visible inclusions.

Therefore, studying them seemed likely to yield a small amount of information, given the small size of the samples involved. However, researchers in the early half of the 20th century, notably H.C. Sorby, came to understand that fluid inclusion research could be helpful because the inclusions "represent trapped portions of the mother liquid from which they grew" (Ryder, 2008). However, Sorby's ideas were not immediately accepted. "His fellow earth scientists did not agree with this proposal and held the view that it was not scientific to study mountains by peering down microscopes" (Ryder, 2008). Their view dominanted geology in the first half of the 20th century, and fluid inclusion research did not begin in earnest until the late 1950s. Since that time, scientists have discovered that these tiny bubbles can reveal a huge amount of information.

Fluid inclusions can occur in a wide variety of crystals, and they form when'd small bleb of a liquid or aqueous medium becomes trapped in the structure or in healed fractures in a crystal. Inclusions can occur in many different types of minerals including the "cementing minerals of sedimentary rocks, in gangue minerals such as quartz or calcite in hydrothermal vein deposits, in fossil amber, and in deep ice cores from the Greenland and Antartic ice caps" (Wikipedia, 2009). When fluid inclusions are "enclosed by minerals that are transparent to visible or infra-red light, fluid inclusions may be observed in a microscope once the host minerals are cut into thin slices and polished" (Bubble, bubble, 2008). If they are enclosed by other types of minerals, various methods can be used to study the inclusions. These studies are based on the three main assumptions that researchers make in dealing with fluid inclusions, which are that: the composition of the fluid has not changed since formation; the density of the trapped fluid has not changed since formation; and the volume of the trapped fluid has not changed since formation (Carey & Parnell, 2004).

Fluid inclusions can tell researchers a significant amount of information about conditions at the time of a crystal's formation. "Hydrothermal ore minerals typically form from high temperature aqueous solutions. The trapped fluid in an inclusion preserves a record of the composition, temperature and pressure of the mineralizing environment" (Wikipedia, 2009). This is because the evidence suggests that fluid inclusions can preserve the original physical and chemical properties of the liquid from which they formed, making them direct samples of the "volatile phases which circulated through the Lithosphere over the course of the Earth's history" (Bubble, bubble, 2009). Moreover, inclusions may contain more than one phase, and each individual phase can provide information about the conditions at the time of the crystal formation. For example, "if a vapor bubble is present in the inclusion along with a liquid phase, simple heating of the inclusion to the point of [reabsorption] of the vapor bubble gives a likely temperature of the original fluid" (Wikipedia, 2009). In addition to temperature, inclusions can give clues about the chemical composition of the original substance. For example, some inclusions contain sulfides or minute crystals, such as halite, sylvite, hematite, which can provide clues as the composition of the original crystallizing fluid. Therefore, fluid inclusion research can help scientists understand "ore transportation and deposit, petroleum generation and migration, explosive volcanism, geothermal energy, earthquake mechanics, petrogenesis of igneous, metamorphic, and diagenetic rocks, contaminant (including radionucleide) transport" and other geological processes (Bubble, bubble, 2009).

Fluid inclusions vary, so that there is not a single approach to studying them. Scientists can use petrographic microscopes to observe fluid inclusions at 100X magnification power, therefore visually observing the contents of the inclusion and the method of formation. Petrographic examination can be used in transparent and in some opaque minerals, where scientists use infrared light. Microthermometry is another important analytical tool for fluid inclusion research. Microthermometry:

involves measuring the temperatures at which phase-transitions are observed to occur in fluid inclusions. If the inclusions have simple compositions (less than 3 or 4 major components) then the microthermometric measurements allow the bulk composition and density of the inclusions to be calculated.... If the inclusions are more complex, then the phase-transition temperatures provide useful constraints on the bulk composition and density, but additional analytical results must be combined to arrive at an exact solution (Bubble, bubble, 2008).

Scientists may also use crushing-stage analysis, which involves the cracking of the inclusions under oil. Scientists microscopically examine the behavior of the gas bubbles that escape from the inclusions to determine the composition and pressure of the gases. Microspectrometry is the final tool used for fluid inclusion analysis, and it permits scientists to identify the covalently-bonded chemical species found within fluid inclusions. Microspectromery involves the focusing of a light source onto a fluid inclusion, and the resulting spectrum emitted from the light reveals the chemical composition of the inclusion.

For fluid inclusions that shrank and separated into a liquid and vapor bubble, microthermometric studies can give a "reasonable estimate of the temperature at which the temperature was formed" (Analysis of fossil fuels, 2008). These studies have revealed interesting information about the environment in which the inclusions and their surrounding crystals formed. For example, inclusions can occur in temperatures ranging from "50°C to over 600°C and at pressures equivalent to what is experienced at the Earth's surface and ranging to what would be found several kilometers deep" (Analysis of fossil fuels, 2008).

One of the difficulties presented by the study of inclusions is that it can be difficult to determine the composition of the trapped liquids:

First, the total amount of dissolved solids is determined by observing with a microscope the freezing/melting points of the inclusions. The sample is then crushed and rinsed with water. The water is recovered and analyzed by using a sensitive analytical technique to determine the ratios of the elements contributed by the trapped fluid. These ratios are used to calculate the composition of the fluid. The compositions range from aqueous solutions with salt content similar to rainwater to fluids with dissolved solid concentrations of over 60% nearly 20 times the amount found in seawater. (Analysis of fossil fuels, 2008).

Fluid inclusion research also involves the study of dissolved gases. The United States Geological Survey has recently designed a gas quadrupole mass spectrometer (QMS) that can analyze the amounts and chemical identify of gas ions in small gas samples (Analysis of fossil fuels, 2008). The QMS can be used in addition to microthermometry, and helps explain the chemical composition of ore-deposits. It can also reveal information about the environment. For example, QMS has been useful in "identifying carbon dioxide as the responsible gas at the Lake Nyos, Cameroon disaster where 2,000 people suffocated in 1986; tracking atmospheric gases from bubbles in climate-study ice cores of Greenland and Antarctica; tracing dispersal of smokestack emissions and gases of geothermal energy wells and springs" (Analysis of fossil fuels, 2008).

One application for fluid inclusion research is ancient climate research. For example, "trapped bubbles of air and water within fossil amber can be analyzed to provide direct evidence of the climate conditions existing when the resin or tree sap formed" (Wikipedia, 2009). Analysis of these bubbles has enabled scientists to study air composition over the last 140 million years, giving insight into some of the major events in pre-history. Like fossilized amber, scientists can study inclusions in the deep ice caps to examine ancient climate conditions. That is because there are several environmental conditions related to crystal formation, including: "temperature, pressure, source of the metals, and composition of any fluids and gases that transported and formed the ore or associated minerals" (Analysis of fossil fuels, 2008). Moreover, the comparison of crystals from various sites can give information about large-scale prehistoric fluid migrations.

In fact, the study of fossilized amber is one of the important aspects of fluid inclusion research. Because of the movie Jurassic Park, many people are probably aware that amber has a unique property as a medium that can trap insect, small animals, and plants, and then, fossilizes, which preserves these organisms for study. In addition to organisms, "minute bubbles of ancient air trapped by successive flows of tree resin during the life of the tree are preserved in the amber" (Air bubbles, 2008). Studying these bubbles has revealed that the earth's atmosphere used to be much-more oxygen rich than in present times. This has led to speculation that dinosaurs had greater oxygen requirements than modern animals, because "their demise was gradual in the transition from the late Cretaceous to early Tertiary times, as was the decrease in oxygen content of the atmosphere" (Air bubbles, 2008).

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