U-Th-Pb fractionation in Archaean lower continental crust: Implications for terrestrial Pb isotope systematics

Pb isotopic compositions are reported for leached feldspars and whole rock samples of felsic to ultramafic, amphibolite to granulite facies xenoliths from the Bearpaw Mountains in the northern Wyoming Craton, Montana, USA. Two-point dates obtained for leached feldspar-whole rock pairs provide a record of Pb isotopic homogenisation at 1.6-2.1 Ga, during an inferred Palaeoproterozoic thermo-tectonic event. Model ages for leached feldspars, assuming single-stage melt extraction from depleted mantle, indicate formation of the protoliths from 2.8 to 4.0 Ga. The inferred timing of the Palaeoproterozoic high-grade metamorphism and extended Archaean crust formation is in agreement with available geochronological data for the Wyoming Craton. Following Palaeoproterozoic isotope resetting, the second stage of Pb isotope evolution suggests prolonged residence in a low U/Pb environment. Isotope resetting was apparently accompanied by a significant decrease in 238U/204Pb (μ-values), reflecting substantial loss of the highly incompatible and mobile element U and, by inference, other heat-producing elements. Geothermal considerations suggest that unstratified continental crust, as approximated by a “reconstituted” average xenolith composition from the Wyoming Craton, would have been thermally unstable at lower crustal levels at the time of formation and metamorphism, if abundances of heat-producing elements had been unaffected by the U-Th-Pb fractionation event. In contrast, differentiated continental crust comprising a chemically depleted lower layer and an enriched upper layer would have reached thermal stability at the time of high-grade metamorphism. On this basis, the Wyoming Craton xenoliths data are proposed as an approximation to the lower continental crust. In 207Pb/204Pb vs. 206Pb/204Pb space the xenoliths plot to the left of the meteorite isochron and above the depleted mantle evolution curve, in contrast to most previous data from lower crustal xenolith suites. Simple mass balance, using an average of the studied xenoliths from the Wyoming Craton as approximation to lower continental crust and published compositions for upper continental crust, yields a Pb isotopic composition of bulk continental crust that plots close to the meteorite isochron. Hence, it is proposed that Archaean lower crust, as exemplified by the Wyoming Craton xenoliths, displays the essential geochemical and isotopic requisites to be a major reservoir to balance the Pb isotope composition of bulk silicate earth.