Collaborative Research: Evolution of an Early Enriched Reservoir in the Northern Wyoming Province: Evidence from Ancient Zircons

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Intellectual Merit. Constraining the size, spatial distribution, and extent of elemental fractionation that characterized both depleted and enriched geochemical reservoirs in the early Earth has been a fundamental long-term goal of terrestrial and planetary geochemists. For the Sm-Nd and Lu-Hf systems, crust-mantle evolution subsequent to 2.5 Ga has been viewed in terms of a generally monotonic increases in the radiogenic daughter isotopes within the mantle that reflects extraction of continental crust to produce a 'melt'-depleted upper mantle reservoir characterized by parent/daughter ratios that are greater than the values for the bulk silicate earth. The complementary enriched reservoir over this interval is the continental crust. However, for the earliest 4.5 to 2.5 Ga of Earth history, the rock record is more difficult to decipher - in part owing to limited exposures of rocks this old. Available data indicate that crust-mantle evolution in this period may have been quite different in terms of melt-extraction processes, leading to 'anomalous' departures from the younger Nd-Hf isotopic record. Recently, analysis of Lu-Hf systematics in old, well-preserved zircons have provided a new approach to resolving isotopic records for early earth evolution. This approach will be used to gain insights into the development of Earth's early enriched reservoirs through analysis of a suite of 3.2 to 4.0 Ga zircons preserved in both metasedimentary and metaigneous rocks from the northern Wyoming Province. The proposed research will consist of 3 primary components: [1] U-Pb dating and trace element analyses via ion probe (SHRIMP-RG), [2] finer scale elemental analyses and imaging via electron probe, and [3] measurement of Lu-Hf systematics via LA-ICP-MS on dated portions of individual zircons. The PIs have previously documented the antiquity of the crust in this region, and argued that understanding its evolution will provide fundamental insights into the early geochemical differentiation of the Earth. Broader Impacts. This project will involve training of graduate students and post-doctoral researchers in geologic and analytical skills that will serve in future research and/or private sector employment. Undergraduate students will also be involved in this project - introducing them to methods of geologic research. Prior research results of this group have been included in popular books and in educational displays at the American Museum of Natural History (e.g., "The Earth Inside and Out", Gottesman's Hall of Planet Earth exhibit). Likewise, the proposed research will also be incorporated into formal and informal education and outreach. An example is provided by the "pet rock" approach utilized for petrology instruction at LSU. All results of this research will be disseminated via traditional presentations at professional meetings and peer reviewed publications.