EXCESS ARGON, CRUSTAL FLUIDS AND APPARENT ISOCHRONS FROM CRUSHING K-FELDSPAR

Using a combination of in-vacuo crushing and stepped heating, the 40Ar-39Ar technique has been applied to two K-feldspar-bearing sedimentary rocks from the Proterozoic of North China, with a view to studying ancient fluids trapped in these rocks and their chronology. Correlations between natural Ar isotopes and those produced by neutron irradiation permitted a clear distinction to be made between different components of natural argon. Crushing released Cl-correlated excess 40Ar (and palaeoatmospheric Ar-36) trapped in moderately saline fluid inclusions in quartz and possibly the K-feldspar. The very high 40Ar/Cl ratios of these fluids (2.7 X 10(-4) and 1.7 X 10(-4)) is interpreted as indicating a metamorphic source for the 40Ar. More surprisingly, crushing also released, from the K-feldspar, a K-correlated component with a very well defined 40Ar/K ratio, which for both samples corresponded formally to an apparent age comparable to or slightly less than the stratigraphic age. In contrast, stepped heating yielded 40Ar/K ratios corresponding to significantly younger ages which are interpreted as (low-grade) metamorphic ages. The explanation of the K-correlated component released on crushing and the significance (if any) of the distinct 40Ar/K ratio is obscure, although several effects involving microporosity and a combination of 39Ar recoil and diffusion are suggested. There is a contribution of this component to the stepped heating release pattern which may have implications for attempts to use K-feldspar as a 40Ar-39Ar thermochronometer.