CONSTRAINING THE ROLES OF ARCS AND PLUMES IN ARCHEAN-HADEAN CRUSTAL GENESIS USING U-PB SYSTEMATICS

If precursors to continental crust are forming today, they are most likely in oceanic LIPs (e.g., Ontong java) or island arcs (e.g., IBM). Bulk compositions and structures of these arc- and plume-systems are quite variable, yet none correspond to the composition and structure of modern continents, which are typically cored by Precambrian crust. These observations beget two key questions: 1) Did analogues of current arc and plume systems play a role in the formation of ancient proto-continents and 2) If so, can we distinguish their relative contributions? Because processes of crustal maturation have obscured much of the earliest physical record of crustal genesis, understanding the role of either system in forming Hadean-Archean crust requires a geochemical approach that distinguishes time-dependent from time-independent processes. Because of its temporal and redox sensitivity, the U-Pb system can provide unique constraints on early crustal genesis, and its impact on the evolution of the depleted mantle (DM). For example, the uranogenic Pb isotopic compositions of the DM sampled by mid-ocean ridge basalts (MORB) require an integrated ²³⁸U/²⁰⁴Pb (µ) greater than its current value (<8) and greater than that of the bulk silicate earth (BSE, ~8.5) for the last ~4.5 Ga (Pb-paradox). These relationships require formation of, and isotopic exchange between, the DM and high-µ reservoirs formed in the Hadean-Archean in order to generate the elevated ²⁰⁷Pb/²⁰⁴Pb compositions of the DM; low-µ reservoirs must form simultaneously. Either or both of these reservoirs may exist today, and several proposals place the early, enriched (high-µ) reservoir in the deep mantle. In contrast, we show that some remnants of these ancient low- and high-µ reservoirs are preserved in sections of Archean continental crust that exhibit enriched Pb isotopic compositions and low µ values. As such, they also reflect time-independent aspects of peridotite melting, e.g., hydrous systems (arcs) tend to generate less fractionation of U/Pb and lower µ-values than anhydrous systems (plumes and MORB). Consequently, early crustal genesis may have involved formation of high-µ proto-continents over zones of mantle upwelling (anhydrous) followed by higher-volume, subduction-driven (low-µ) crustal growth centered on these proto-continental nuclei.