Post-metamorphic CO2-rich fluid inclusions in granulites |
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| Authors: | William M Lamb John W Valley Philip E Brown |
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| Institution: | (1) Department of Geology and Geophysics, University of Wisconsin, 53706 Madison, WI, USA;(2) Present address: Department of Geology, Texas A&M University, 77843 College Station, TX, USA |
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| Abstract: | In granulite-facies samples from the Adirondack Mountains, NY, estimates of peak-metamorphic CO2 fugacities based on mineral equilibria are not consistent with estimates based on data for high-density, CO2-rich fluid inclusions. Of the 21 Adirondack samples investigated for this study, all contain CO2-rich inclusions. Inclusions occur in quartz, apatite, and garnet. They range in size from 3 to 50  m and are without visible H2O. In a few of the inclusions, freezing point determinations and preliminary Laser Raman spectroscopy show the presence of small amounts (<3%) of other fluids (N2 and H2S). CO2 liquid-vapor homogenization temperatures are between –46 and +31° C, corresponding to densities between 1.14 and 0.5 gm/cc. Some of these densities are consistent with peak-metamorphic entrapment (1.06 to 1.1 gm/cc).Peak metamorphic fluid compositions in these samples are inferred from fluid-buffering equilibria that restrict the fugacity of CO2 (f CO2) directly (i.e., calcite+quartz+wollastonite) or buffer the fugacity of oxygen (f O2). Assemblages that bufferf O2 are important because knowledge off O2 places an upper limit onf CO2. In 13 of the 21 samples, estimates of peak-metamorphic fluid compositions based on these equilibria show that the mole fraction of CO2 (XCO2) in equilibrium with the rock was low, in some cases less than 0.2.The contradiction of mineral equilibria and fluid inclusion data shows that the inclusions record post-metamorphic conditions. At present, there are no criteria to distinguish these  primary appearing CO2-rich inclusions from those found in other granulite-facies terranes. Therefore, inferences of pressure-temperature conditions and peakmetamorphic fluid compositions based on fluid inclusions must be viewed with caution. |
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