Ion microprobe study of intragrain micropermeability in alkali feldspars

Alkali feldspar cleavage fragments from the Klokken layered syenite, South Greenland, were heated to 700°C at 0.1 GPa in 99% H₂¹⁸O for 75 h. These samples were then polished and imaged by ion microprobe for ¹⁸O. The feldspars were known to contain areas of pristine, braid micro-perthite which were not turbid and areas of deuteric patch perthite which were turbid. Turbidity is related to the presence of micropores in the feldspars. On imaging the grain, it was found that the ¹⁸O had penetrated into the parts of the grain which were microporous and not into the pristine areas. Micropores are therefore responsible for rendering the feldspars permeable as well as porous. The implications of micropermeable feldspars in several areas of geology are discussed.