HR: 08:15h
AN: V51A-02    [PDF]
TI: Lithium isotopic composition of the lower continental crust: A xenolith perspective
AU: * Teng, F
EM: tfz@geol.umd.edu
AF: Dept. of Geology, Univ. of Maryland, College Park, MD 20742 United States
AU: McDonough, W F
EM: mcdonough@geol.umd.edu
AF: Dept. of Geology, Univ. of Maryland, College Park, MD 20742 United States
AU: Rudnick, R L
EM: rudnick@geol.umd.edu
AF: Dept. of Geology, Univ. of Maryland, College Park, MD 20742 United States
AU: Tomascak, P B
EM: tomascak@geol.umd.edu
AF: Dept. of Geology, Univ. of Maryland, College Park, MD 20742 United States
AU: Saal, A E
EM: asaal@brown.edu
AF: Dept. of Geological sciences, Brown Univ., Providence, RI 02912 United States
AB: Study of high-grade metamorphic rocks is important for understanding the lithium isotopic composition of the lower crust and determining the degree to which lithium isotopes fractionate during metamorphism. Here we present Li isotopic data for two well-characterized suites of lower crustal xenoliths from North Queesland, Australia [1, 2]. These xenoliths have average major-and trace-element compositions similar to estimates of the bulk lower continental crust, and can therefore be used to place broad constraints on its Li isotopic composition. Furthermore, the Chudleigh xenoliths are a suite of cogenetic cumulates that formed through AFC of Cenozoic basaltic magma that intruded Proterozoic lower crust, and thus may provide insight into the regional average $\delta$$^{7}$Li of the lower crust. The samples display a large range of Li concentrations and isotopic compositions with the average, concentration-weighted $\delta$$^{7}$Li value = 0. McBride xenoliths are enriched in Li (7 $\pm$ 4 ppm versus 3 $\pm$ 2 ppm) and have a larger range of $\delta$$^{7}$Li values (-13 to +7 versus -9 to +6) than Chudleigh xenoliths, consistent with differences in lithology and genetic diversity: McBride xenoliths range from paragneisses to mafic and felsic orthogneisses whereas the Chudleigh xenoliths are mafic cumulates. Mg\#, $^{87}$Sr/$^{86}$Sr and $^{143}$Nd/$^{144}$Nd in the Chudeigh suite correlate with $\delta$$^{7}$Li, reflecting assimilation of isotopically light Proterozoic lower crust ($\delta$$^{7}$Li $<$ -9) by heavier ($\delta$$^{7}$Li $\sim$ +6) mantle-derived basalt. The $\delta$$^{7}$Li values of the lower continental crust are heterogeneous, and its average composition is similar to the upper continental crust [3] or lighter; this suggests that the continents are isotopically lighter than the mantle [4]. The isotopically light continental crust is probably derived from two processes involving fluid-rock interactions: surface weathering and metamorphic dehydration. Both processes drive the hydrosphere to heavier (+30) and bulk continental crust to lighter (-1 to 0) $\delta$$^{7}$Li than the mantle (+4). 1. Rudnick R. L. et al. (1986) GCA 50, 1099-1115. 2. Rudnick R. L. and Taylor S. R. (1987) JGR 92, 13981-14005. 3. Teng F.-z. et al. (2003) submitted to GCA. 4. Chan L. H. and Frey F. A. (2003) G3, 4 art. no.-8707.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1020 Composition of the crust
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting