40AR-39AR ANALYSIS OF PERTHITE MICROTEXTURES AND FLUID INCLUSIONS IN ALKALI FELDSPARS FROM THE KLOKKEN SYENITE, SOUTH GREENLAND

Laser probe and in vacuo crushing have been used to apply the 40Ar-39Ar technique to microtexturally well-characterised pristine and turbid alkali feldspar from the 1166 Ma Klokken syenite. Individual crystals are several millimetres in size, but SEM and TEM studies reveal complex microstructures which define a range of sub-grain sizes over three orders of magnitude, from > 100-mu-m in pristine areas with fine-scale cryptoperthite, down to < 200 nm in turbid areas with coarsened perthites and numerous micropores. Laser-probe 40Ar-39Ar measurements of turbid feldspar gave low apparent ages indicating significant 40Ar loss (40% average), which can be accounted for by sustained heating of the small sub-grains (less-than-or-equal-to 1-mu-m) for 1166 Ma at low temperature (< 150-degrees-C). Pristine feldspar gives high apparent ages indicating the presence of excess 40Ar. This component was readily lost from the feldspar by laboratory heating at low temperature (600-degrees-C) and was also released by crushing, which indicates that it was released from fluid inclusions. During crushing, release of excess 40Ar is correlated with Cl release and the 40Ar*/Cl ratio of the fluid is similar to that of mantle fluids. Turbid feldspar also released a fluid during crushing, but the 40Ar*/Cl ratio was lower, probably as a result of evolution of the original fluid during boiling. Anomalously high apparent ages are often a feature of low-temperature Ar release in stepped-heating age spectra of feldspars. The presence of excess 40Ar, in fluid inclusions, may substantially affect the lower-temperature release in feldspar stepped-heating spectra. The character of the microtextures in the host feldspar provides information on the temperature of fluid trapping because inclusions formed above the coherent alkali feldspar solvus have little or no effect on the exsolution textures, whereas fluids trapped at low temperatures are associated with major structural rearrangements leading to development of patch perthite.