Lead isotopic heterogeneities within alkali feldspars: Implications for the determination of initial lead isotopic compositions

Abstract

The mobility of uranium during metamorphism and alteration places constraints on the use of lead isotopic compositions as petrogenetic tracers in older rocks. The problem of uranium mobility may be partially circumvented by analysis of alkali feldspar which, because of its low U:Pb ratio, should have a lead isotopic composition similar to the initial lead isotopic composition of the rock. Previous leaching and volatilization experiments, however, have demonstrated that alkali feldspars contain radio-genic lead which can be removed preferentially to nonradiogenic lead. This paper reports on the results of progressive acid leaching of a single alkali feldspar phenocryst and on albitic perthites separated from the phenocryst. This experiment suggests that a number of distinct lead isotopic reservoirs are present within the alkali feldspar, including a nonradiogenic reservoir approximating the initial lead isotopic composition of the system, a radiogenic reservoir corresponding to albitic exsolution lamellae within the alkali feldspar, loosely bound labile lead presumably associated with uranium and thorium occupying fractures, and finally a radiogenic component corresponding to unidentified uranium-thorium rich phases. The commonly observed decrease in radiogenicity with progressive leaching of alkali feldspars is interpreted to be largely the result of preferential leaching of the albitic perthites relative to the host alkali feldspar containing the nonradiogenic reservoir. The results of this experiment confirm that initial lead isotopic compositions may be estimated from alkali feldspars through severe acid leaching; however, this experiment also illustrates the complexity of the lead isotopic systematics of alkali feldspars. The determination of the initial lead isotopic composition of an alkali feldspar requires experimental procedures which recognize and compensate for the presence of multiple lead reservoirs within typical alkali feldspars.