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Fluid InclusionsFluid inclusions are microscopic bubbles of liquid and gas that are trapped within crystals. As minerals often form from a liquid or aqueous medium, tiny blebs of that liquid can become trapped within the crystal structure or in healed fractures wirhin a crystal. These small inclusions range in size from 0.1 to 1 mm and are usually only visible in detail by microscopic study. These inclusions occur in a wide variety of environments. For example they are found within 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 Antarctic ice caps. The inclusions can provide information about the conditions existing during the formation of the enclosing mineral. 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. An inclusion often contains two or more phases. If a vapor bubble is present in the inclusion along with a liquid phase, simple heating of the inclusion to the point of resorption of the vapor bubble gives a likely temperature of the original fluid. If minute crystals are present in the inclusion, such as halite, sylvite, hematite, or sulfides are present, they provide direct clues as to the composition of the original fluid. Paleoclimate applications 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. The analysis of these trapped bubbles of air provides a record of atmosphere composition going back 140 million years. The data indicate that the oxygen content of the atmosphere reached a high of nearly 35% during the Cretaceous Period and then plummeted to near present levels during the early Tertiary http://minerals.cr.usgs.gov/gips/na/0amber.htm#amber. The abrupt decline corresponds to or closely follows the Cretaceous-Tertiary extinction event and may be the result of a major meteorite impact that created the Chicxulub Crater. Air bubbles trapped within the deep ice caps can also be analyzed for clues to ancient climate conditions. References
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