Abstract
Redox-sensitive elements figure prominently in studies of the evolution of Earth's surface redox state, including the first major rise in atmospheric O(2), the Paleoproterozoic Great Oxidation Event. Most Precambrian rocks endured multistage tectonothermal histories, however, adding ambiguity to interpretation of their chemistry. Here, we apply U-Th-Pb isotope geochronology to the highly oxidized ~2.06 Ga Kuetsjärvi Volcanic Formation, Pechenga Greenstone Belt, Russia, to constrain the age and extent of U oxidation. By contrasting the relative mobility of U and Th using Pb isotopes, we find that complete to near-complete oxidation and removal of U occurred shortly after eruption. We argue that this likely indicates relatively high atmospheric O(2), where oxidative weathering and alteration produced a global pulse of U to the oceans. Such a pulse could explain widespread shifts in the U-Th-Pb isotope character of mantle reservoirs at ~2 Ga, including a decrease in the (232)Th/(238)U ratio of the mid-ocean ridge basalt source and inception of the high-(238)U/(204)Pb (HIMU) source to ocean island basalts, underscoring the connections between the redox character of the Paleoproterozoic surface and deep Earth. Using (207)Pb-(206)Pb, (238)U-(206)Pb, (235)U-(207)Pb, and (232)Th-(208)Pb geochronology, ~2.06 Ga oxidative loss of U may be distinguished from reintroduction of U at ~1.8 Ga during regional metamorphism, as well as Pb loss during a Phanerozoic tectonothermal event. Our results therefore establish the complex history of redox-sensitive element behavior in the rocks, highlighting the fact that elemental abundances, by themselves, are unlikely to capture straightforward proxy information in rocks that have seen multistage geologic histories.