Argon-loss by alkali feldspars

摘要: Abstract Ar-loss is generally thought to preclude the use of alkali feldspars for dating old rocks by K-Ar method, and has been occur via structural features such as boundaries perthite lamellae or twins. We have done a 40 Ar- 39 Ar step-heating study suite from Klokken syenite intrusion (U-Pb age 1166 ± 4 Ma ) which fully characterized transmission electron microscopy. All are low albite-low microcline intergrowths, but some non-turbid, ‘strain-controlled’ cryptoperthites, while others turbid, coarse microperthites. Geological textural evidence suggests that turbidity developed fluid-rock interactions during initial cooling pluton, in stability field ( 5 after crystallization. The finest cryptoperthites numerous coherent lamellar (lamellar periodicity ~ nm) twin composition planes. retained radiogenic yield total degassing ages high 1125 16 maximum on spectra 1162 Ma. turbid microperthites, with patchy intergrowths twins mm-scale, gave 650 (maximum 750 Ma). Argon leakage occurs least most interfaces, proportion degree turbidity, be caused large numbers micropores. Strain-controlled perthites allow recognition volumes feldspar crystals not interacted deuteric fluids. These retain argon can give reliable rocks. Saddle-shaped appear result preferential degassing, at 1000°C, part structure lost more its nature. laboratory does manner analogous occurred naturally.