HR: 0800h
AN: V51C-0586    [Abstracts]
TI: A Reconnaissance Investigation of Isotopic Heterogeneity in Slowly-Cooled Plutonic Rocks of the Laramie Anorthosite Complex, Wyoming
AU: * Williams, G A
EM: gwalwi@hotmail.com
AF: EOS, University of BC, 6339 Stores Rd, Vancouver, BC V6T1Z4 Canada
AU: Scoates, J S
EM: jscoates@eos.ubc.ca
AF: EOS, University of BC, 6339 Stores Rd, Vancouver, BC V6T1Z4 Canada
AU: Weis, D
EM: dweis@eos.ubc.ca
AF: EOS, University of BC, 6339 Stores Rd, Vancouver, BC V6T1Z4 Canada
AB: The slowly-cooled, unmetamorphosed 1.43 Ga Laramie anorthosite complex in southeastern Wyoming offers a unique opportunity to investigate the possible extent of isotopic heterogeneity or disequilibrium present within plagioclase-rich plutonic rocks. The isotopic and trace element characteristics of four different magnetic fractions of feldspar separates from four different rocks were studied, including plagioclases from an anorthosite, leucogabbro, and troctolite, and mesoperthite from a monzosyenite. The Pb and Sr isotopic compositions were determined by MC-ICP-MS and TIMS, respectively. For the plagioclase, high concentrations of incompatible trace elements and the absence of inclusions in the less magnetic fractions suggest that these grains are from the rims of the crystals, while the inclusion-rich more magnetic fractions with lower concentrations of trace elements represent the cores. Stepwise leaching of the feldspars removed radiogenic Pb produced by the in-situ decay of U, so that the Pb isotopic composition of leached feldspars could be used to approximate the initial Pb isotopic composition of the whole rocks. The Pb and Sr isotopic compositions of the leached feldspars is distinctly lower than the isotopic compositions of the non-leached feldspars. The Pb isotopic compositions for the non-leached separates from the three plagioclase-rich samples span a large range of $^{206}$Pb/$^{204}$Pb (16.9711-17.2909, 16.8404-18.1773, and 17.0903-17.7679). In contrast, there is no significant Pb isotopic variation in the magnetic fractions of mesoperthite from the monzosyenite ($^{206}$Pb/$^{204}$Pb = 16.7365-16.7694). The less magnetic non-leached fractions are systematically more radiogenic than the more magnetic fractions, reflecting the higher concentration of U incorporated into the rims of the grains at the time of crystallization. The Pb-Pb age (1487 $\pm$ 62 Ma) of the non-leached plagioclases from the leucogabbro are within error of the crystallization age (1436.2 $\pm$ 0.6 Ma; U-Pb baddeleyite/zircon), indicating that the plagioclase crystals have remained a closed system since crystallization of the intrusion.
DE: 1020 Composition of the crust
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting