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EUR-12 Cenozoic volcanism in Europé
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CO2 fluid inclusions in mantle xenoliths from the Lower Silesia, SW Poland - investigations of fossil pressures in the upper mantle and decompression history of volcanic events
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Anna Ladenberger, Uppsala University (Sweden)
Peter Lazor, Uppsala University (Sweden)
Marek Michalik, Jagiellonian University (Poland)
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The Raman spectroscopy has been applied in order to determine the density and pressure of CO2 fluid inclusions in mantle xenoliths which were carried to the surface by alkaline magmas in early Miocene in southwestern Poland. Wilcza Góra is major locality with large amount of upper mantle and crustal xenoliths. The magmas were generated by partial melting at the transition zone between spinel and garnet stability region. Determination of densities of CO2 inclusions allows calculations of partial pressures and reconstruction of the depth of xenoliths origin and their history en route to the surface. Fluid inclusions from xenoliths from Wilcza Góra were re-equilibrated at magmatic temperatures. Their entrainment was followed by rapid ascent towards the surface which resulted in large range of low densities in the upper crust. The density of CO2 inclusions varies from 0.06 to 1.10 g/cm3 for olivines and 0.17 to 1.11 g/cm3 for orthopyroxenes. Only inclusions with density above 0.8 g/cm3 reflect lower crust and upper mantle conditions at depths between 14 and 37 km. All calculated densities should be taken with caution; the values are underestimated and high density inclusions represent minimum pressures and depths of xenoliths entrainment at depth between 31 and 38 km. Slight differences in density of inclusions among olivines and orthopyroxens can be attributed to their different rheological properties during the magma ascent. Modeling of pressure and depths within the range of temperatures in which xenoliths equilibrated with magmas gives the complex information about the eruption history of basanitic volcanoes. High density inclusions originate from the Moho region and possibly indicate underplating processes at that level due to density and rheology contrast. They define the minimum pressures of trappment up to 1.1 GPa, which is in agreement with values calculated from other intracontinental areas. Our estimates are consistent with seismic data suggesting the existence of region with velocities up to 8 km/s attributed to presence of mafic and ultramafic lithologies at the Moho border (c. 30-35 km deep).
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