PLUMES, PLATES, AND PROTOCONTINENTS: A PLUMBOLOGICAL PERSPECTIVE

Most crustal growth models indicate that a majority of combined extant and recycled continental crust formed in the Archean. Proposals for the environments in which this crust formed include duplicates or analogs of modern environments such as mantle upwellings (e.g., plumes, heat pipes, LIPs, etc.) and plate tectonic subduction systems, as well as planetary analogs such as stagnant lid convection regime(s). Evaluation of the viability of any modern analog model begins with understanding the geochemical differences between mantle melts in these systems (e.g., MORB vs. island arc basalt) and identifying predictable differences between the products of these systems in modern vs. ancient environments. Today two geochemical signatures consistently distinguish mafic rocks produced in dry (e.g., MORB) vs. wet (e.g., subduction zones) melting environments. Anhydrous melting of peridotite leads to high U/Pb without depletion of HFSE relative to LILEs. Hydrous melting produces HFSE depletions relative to LILEs and U/Pb tends to be lower than in dry melts. In addition, the low redox Archean exosphere will inhibit conversion of insoluble U⁴⁺ to more soluble U⁶⁺. The result is that U/Pb of modern surface waters will exceed that of Archean surface waters by ~10⁵. Consequently, the high U/Pb in the modern ocean, which contributes to the high U/Pb of altered, subducted, oceanic lithosphere (>>crustal average), will not exist. These differences in U/Pb combined with the relatively rapid decay of the ²³⁵U/²⁰⁷Pb system (700 m.y.) should lead to significant differences in Pb isotopic evolution. For example, if the mafic “proto” stage of a continent’s evolution formed from anhydrous mantle melting, that crust will be characterized by higher U/Pb than crust produced by hydrous melting. The difference in U/Pb will be >2x and distinct differences in Pb-isotopic compositions will develop rapidly in the Hadean-Archean. Once produced, this characteristic Pb isotopic composition can be transferred completely to TTG and more granitic rocks produced from melting such crust. Significantly, this scenario can be readily confirmed by comparing the ages of secondary Pb isochrons to U-Pb zircon ages in TTGs and related rocks. Examples of Archean crust that show evidence of this high U/Pb heritage include the Wyoming Province and Limpopo Belt.