Chemical Evolution of Archean Mantle and Crust: Constraints From Hafnium, Neodymium, and Lead Isotopes

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9814885VervoortThe aim of this proposal is to constrain the isotopic evolution of the crust-mantle system during the Archean, along two subparallel paths: 1) definition of the Hf and Nd isotopic record of the Archean depleted mantle; and 2) integration of Hf and Nd isotopic records of silicate rocks with the Pb isotopes from Archean sulfides. Integrating Hf, Nd, and Pb isotopic systems will allow better constraints to be placed on the isotopic evolution of the depleted mantle, the growth and development of continental crust, and the interactions between crust and mantle during the first two billion years of Earth's history. The first aspect of the work will focus on determining the Hf isotopic composition in a range of well-dated and geologically constrained samples from the depleted mantle, 4.0 to 2.7 Ga. This will define the Hf isotopic record of depleted mantle evolution in the Archean, and by comparison, evaluate whether the current heterogeneity in the Nd isotopic record is a real feature of the Archean mantle or is an artifact caused by disturbances in the Sm-Nd isotopic system. U-Pb dating of zircons will provide better age constraints where they are lacking. To achieve the second goal, integrated Hf, Nd and Pb isotopic studies will be conducted on well-constrained volcanogenic massive sulfide (VMS) deposits, such as in the 2.7 Ga Abitibi greenstone belt. These deposits show evidence for retention of mantle heterogeneity from differentiation events in the 4.3 to 4.5 interval of early Earth history. It is not clear if this was related to U-Pb fractionation during core formation or during crust-mantle differentiation. Pb isotopes on VMS deposits, coupled with Hf and Nd isotopes on cogenetic volcanic rocks, will show whether early differentiation events were core-related, or caused by silicate melting/differentiation processes connected with magma and primitive crust formation.