Other
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.v43b1375d&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #V43B-1375
Other
1020 Composition Of The Continental Crust, 1040 Radiogenic Isotope Geochemistry, 1115 Radioisotope Geochronology, 1213 Earth'S Interior: Dynamics (1507, 7207, 7208, 8115, 8120), 8103 Continental Cratons
Scientific paper
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 ɛHf values of -61 to -58 (initial ɛHf 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 ɛHf 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 ɛHf values imply little material exchange between Archean crust and SRP melts (typical ɛHf 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.
Dufrane Andrew S.
Leeman William P.
Vervoort Jeff D.
Wolf Dietrich E.
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