HR: 1340h
AN: V43B-1375    [Abstracts]
TI: Hafnium Isotope Composition of Archean Zircons from Xenoliths of the Snake River Plain, Idaho
AU: * DuFrane, S A
EM: dufrane@wsu.edu
AF: School of Earth and Environmental Sciences,Washington State University, Webster Physical Sciences 1228, Pullman, WA 99164, United States
AU: Vervoort, J D
EM: vervoort@wsu.edu
AF: School of Earth and Environmental Sciences,Washington State University, Webster Physical Sciences 1228, Pullman, WA 99164, United States
AU: Leeman, W P
AF: Earth Science Division, National Science Foundation, 4201 Wilson Blvd., Arlington, VA 22230, United States
AU: Wolf, D E
AF: Department of Earth & Atmospheric Sciences, Cornell University, Snee Hall, Ithaca, NY 14853, United States
AB: The composition, structure, spatial extent, and history of Archean crust buried beneath the Snake River Plain (SRP) are important for assessing the role of the lithosphere in regional igneous and tectonic activity. We report the U-Pb age and Hf isotope composition of Archean zircons from xenoliths entrained in Snake River Plain basalts. The xenoliths come from three localities on the SRP: Square Mountain (SM), Craters of the Moon National Monument (COM) and the Spencer-Kilgore (SK) volcanic field. Cathodoluminescence imaging and previous age dating of the zircons show that many are complexly zoned and for this reason a majority of the Hf isotope data was determined via laser ablation MC-ICPMS. We used a New Wave UP-213 Nd-YAG laser interfaced with a Thermo-Finnigan Neptune MC-ICPMS and Element II HR-ICPMS for Hf isotope determinations U-Pb age dating, respectively. Previous U-Pb age dating has shown that the zircons vary from having simple age systematics (e.g., SM xenolith DM103 has a single zircon age population of ~ 2.58 Ga) to highly complex (e.g., COM xenoliths have zircons with ages populations from 2.7 to 3.2 Ga). All these zircons are Archean in age, although some have young low-U overgrowths jacketing their Archean cores which yield ²⁰⁶Pb/²³⁸U ages of ~19-25 Ma. There is no evidence in any of the xenoliths for zircon growth between late Archean and these young ages which attests to the stability of the lithosphere during this span of time. The Hf isotopic compositions of the Archean zircon grains are extremely unradiogenic consistent with their old age. For example, a SM xenolith (DM103) has present-day εHf values of -61 to -58 (initial εHf values at 2.56 Ga of -5 to -2), comparable with data determined by conventional solution-based methods. Another zircon from a COM xenolith (COM22) has an Archean core (2.7 Ga) and a large overgrowth with an age of ~20 Ma. Remarkably the present day εHf values of the core and overgrowth are identical within analytical uncertainties, -76 and -73, respectively. This indicates the young overgrowth consists entirely of recycled Archean crust with no detectable involvement of mantle derived Hf. The low εHf values imply little material exchange between Archean crust and SRP melts (typical εHf between -10 and +10). The ages (~20 Ma) and the lack of a mantle component of the zircon overgrowths seem to preclude that young zircon growth was produced during SRP magmatism. However, the recent transfer of heat into the Archean basement may provide important clues concerning the processes operative in the lithosphere prior to and perhaps during SRP magmatism.
DE: 1020 Composition of the continental crust
DE: 1040 Radiogenic isotope geochemistry
DE: 1115 Radioisotope geochronology
DE: 1213 Earth's interior: dynamics (1507, 7207, 7208, 8115, 8120)
DE: 8103 Continental cratons
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting