Chemical and isotopic effects of retrogression in metamorphic fluid inclusions |
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Authors: | Rolf L Romer Leander Franz Richard Wirth |
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Institution: | 1. GeoForschungsZentrum Potsdam, Telegrafenberg, 14473, Potsdam, Germany 2. Mineralogisch-Petrographisches Institut, Universit?t Basel, Bernoullistrasse 30, 4056, Basel, Switzerland
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Abstract: | Fluid inclusions may provide compositional and isotopic information about fluids from which the host mineral precipitated
as long as the host mineral does not react with the fluid. Our transmission electron microscope (TEM) investigation of grain
boundaries and of fluid inclusions in zoisite and quartz of high-pressure metamorphic rocks from Dabie Shan (eastern China)
demonstrates daughter minerals, such as margarite, muscovite, calcite, and anhydrite. Their precipitation changes (1) the
composition of the fluid by selective and mineral-specific removal of CO2 (carbonates), H2O (sheet silicates, hydration of the walls), or S (gypsum, anhydrite, sulfides), (2) the concentrations and proportions of
ions dissolved in the fluid, and (3) the isotopic composition of the fluid because of isotopic fractionation between mineral
precipitate and fluid and between unmixed fluids. Fluid leakage from overpressurized fluid inclusions with daughter minerals
changes the overall chemical and isotopic composition of the inclusion irreversibly, even when the daughter crystals later
redissolve. Such fluid loss yields a wide range of compositionally and isotopically different fluids from a single starting
fluid. Depending on the relation between mineral reactions in and fluid loss from the inclusion, the fluid remaining in the
inclusion and the fluid lost from the inclusion may appear entirely unrelated. |
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