The complex Pb inventory in Archean K-feldspars

Lead isotope luminaries Ken Ludwig and Leon Silver in 1977 wrote: “… caution must be used in applying total-sample lead (isotopic) analyses to Precambrian feldspars to problems of lead-isotope evolution in crustal rocks”. Despite this, there continues to be published studies which make the assumption that the measured K-feldspar Pb isotopic composition in Archean igneous rock samples is representative of the initial Pb of their host granite ( s.l. ) and the granite source. Based on published Australian examples, this assumption is demonstrably wrong in a significant proportion of Archean cases, and it is likely that some subsequent interpretations proposing sources with unusually high model μ ( 238 U/ 204 Pb) in the Archean will not be confirmed when relevant testing data are available. The Pb inventory in Archean K-feldspars comprises three main components: initial Pb, in-growth Pb from U-Th decay and remobilised Pb attained during later overprint events. The “two-point Pb-Pb isochron” age for the measured K-feldspar and hostrock Pb isotope compositions compared to the granite’s U/Pb age is recommended as a simple test for identifying remobilised Pb in Archean K-feldspar. Comparison of Pb isotope data measured by TIMS and Laser-MC-ICPMS on the same K-feldspar samples, and comparison of both to the formation isochron of their host granite, demonstrate examples where the K-feldspar data do not adhere to the formation isochron: the initial Pb of the granite should lie on the formation isochron. The sensitivity of this test is greatly improved by using optimum precision Pb isotope analyses for both the rock and K-feldspar. The Pb inventory of Archean K-feldspar is complex and should not be used for Pb isotope source characterisation and evolutionary studies without also measuring the Pb isotopes in the host granite and its age. The samples field attributes and petrography should be assessed to identify possible alteration or overprint events, as Pb mobility during alteration hinder preservation of the granite initial Pb isotope composition by K-feldspar in the Archean.