40Ar39Ar analysis of perthite microtextures and fluid inclusions in alkali feldspars from the Klokken syenite, South Greenland

摘要: Laser probe andin vacuo crushing have been used to apply the40Ar39Ar technique microtexturally well-characterised pristine and turbid alkali feldspar from the 1166 Ma Klokken syenite. Individual crystals are several millimetres in size, but SEM TEM studies reveal complex microstructures which define a range of sub-grain sizes over three orders magnitude, > 100 μm areas with fine-scale cryptoperthite, down < 200 nm coarsened perthites numerous micropores. Laser-probe40Ar39Ar measurements gave low apparent ages indicating significant40Ar loss (40% average), can be accounted for by sustained heating small sub-grains ( ⩽ 1 μm) at temperature 150°C). Pristine gives high presence excess40Ar. This component was readily lost laboratory (600°C) also released crushing, indicates that it fluid inclusions. During release excess40Ar is correlated Cl the40Ar/*Cl ratio similar mantle fluids. Turbid during lower, probably as result evolution original boiling. Anomalously often feature low-temperature Ar stepped-heating age spectra feldspars. The excess40Ar, inclusions, may substantially affect lower-temperature spectra. character microtextures host provides information on trapping because inclusions formed above coherent solvus little or no effect exsolution textures, whereas fluids trapped temperatures associated major structural rearrangements leading development patch perthite.