Lead isotopic compositions of late Archean lower continental crust

Abstract

Lead isotopes of Earth’s accessible silicate rocks are generally too radiogenic for the planet to have formed from meteoritic material—a problem known as the first Pb paradox. The search for an unradiogenic reservoir to balance Earth’s bulk composition has thus focused, with varying degrees of success, on inaccessible rocks in the core, mantle, and lower crust. Whether the lower continental crust balances the radiogenic depleted mantle and upper crust is debated and largely depends on how unradiogenic the lower crust, in particular the Archean lower crust, is. In this work, granulite terrain, granulite xenoliths, and lower crustal-derived granitoids from the same region of the northern North China craton (NCC) are combined to constrain the Pb isotopic compositions of the late Archean lower crust. The Hannuoba pyroxene-rich mafic granulite xenoliths are interpreted as restites left after partial melting of the late Archean lower crust to produce the Mesozoic granitoids. Both the xenoliths and granitoids have similar Pb isotopic compositions that are much more radiogenic than the granulite facies gneisses from the Huai’an terrain. The results rule out the notion that the lower crust is inherently poor in U and Th relative to Pb and suggest that the Pb isotopic compositions of the granulite facies gneisses are not representative of the Archean lower crust. This rule may be generally applicable to other regions because the Huai’an terrain is chemically and isotopically comparable with several other Archean unradiogenic granulite terrains (e.g., the Lewisian). We argue that the late Archean lower crust may not be so extremely unradiogenic in Pb isotopic compositions as commonly thought and the lower crust appears insufficiently unradiogenic to balance the accessible Earth. We also provide evidence that intracrustal differentiation via lower crustal melting could be an important mechanism causing the elevated Th/U and 208Pb/206Pb in the upper crust.