Excess argon, crustal fluids and apparent isochrons from crushing K-feldspar |
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| Authors: | Grenville Turner Wang Songshan |
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| Institution: | aDepartment of Geology, University of Manchester, Manchester M13 9PL,UK bInstitute of Geology, Academia Sinica, Beijing ,China |
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| Abstract: | Using a combination ofin-vacuo crushing and stepped heating, the40Ar 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 excess40Ar (and palaeoatmospheric36Ar) trapped in moderately saline fluid inclusions in quartz and possibly the K-feldspar. The very high40Ar/Cl ratios of these fluids (2.7 × 10?4 and1.7 × 10?4) is interpreted as indicating a metamorphic source for the40Ar. More surprisingly, crushing also released, from the K-feldspar, a K-correlated component with a very well defined40Ar/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 yielded40Ar/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 distinct40Ar/K ratio is obscure, although several effects involving microporosity and a combination of39Ar 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 a40Ar39Ar thermochronometer. |
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