HR: 0800h
AN: V51C-0597    [Abstracts]
TI: Lithium Isotopic Composition of the Deep Continental Crust
AU: * Teng, F
EM: tfz@geol.umd.edu
AF: Geochemistry Laboratory, Department of Geology, Univ. of Maryland, College Park, MD 20742 United States
AU: McDonough, W F
EM: mcdonough@geol.umd.edu
AF: Geochemistry Laboratory, Department of Geology, Univ. of Maryland, College Park, MD 20742 United States
AU: Rudnick, R L
EM: rudnick@geol.umd.edu
AF: Geochemistry Laboratory, Department of Geology, Univ. of Maryland, College Park, MD 20742 United States
AU: Gao, S
EM: sgao1962@vip.sina.com
AF: Department of Geology, Northwest Univ,, Xi'an, 710069 China
AB: We have investigated the Li isotopic composition of the deep continental crust by measuring composite samples from Archean high-grade metamorphic terranes in East China and granulite-facies xenoliths from East China (Hannuoba suite) and Queensland, Australia (Chudleigh and McBride suites). The 30 composite samples, including TTG gneiss, amphibolites and felsic to mafic granulites, have a narrow range of $\delta$$^{7}$Li values from +1.7 to +7.5$\permil$, with an average of +4 $\pm$ 1.4$\permil$ (1$\sigma$), which is indistinguishable from the upper mantle. In contrast, the three granulite xenolith suites display a much larger range in $\delta$$^{7}$Li, from +15.7 to -17.9$\permil$ with average values decreasing in the order: Hannuoba (-0.7 $\pm$ 4.9$\permil$ (1$\sigma$, 18 samples)), Chudleigh (-2.5 $\pm$ 5.5$\permil$ (1$\sigma$, 14 samples)) and McBride (-3.8 $\pm$ 7.6$\permil$ (1$\sigma$, 12 samples)). The xenoliths are, on average, lighter than the high-grade metamorphic terrane composites. The Li concentrations are also variable with xenoliths having lower Li concentration than high-grade metamorphic terrane composites (5 $\pm$ 4 ppm vs. 13 $\pm$ 6 ppm, 1$\sigma$). $\delta$$^{7}$Li correlates positively with H$_{2}$O for 12 granulite composites; mafic samples have the highest H$_{2}$O contents and $\delta$$^{7}$Li values while felsic ones have the lowest. This, together with an excellent positive correlation between Li concentration and Mg# for 13 TTG gneiss composites, suggests that both metamorphic dehydration and protolith lithology play important roles in determining the Li isotopic composition of metamorphic rocks. For the Hannuoba xenoliths, $\delta$$^{7}$Li correlates positively with Al$_{2}$O$_{3}$/CaO and negatively with Li, FeO, MgO, CaO and Co, suggesting that the protolith composition primarily controls the $\delta$$^{7}$Li values. No such correlations are found for the other granulite xenoliths. The difference in $\delta$$^{7}$Li between terranes and granulite xenoliths may reflect evolving lighter $\delta$$^{7}$Li with depth in the crust, since the former equilibrated at middle to upper lower crustal depths, while the latter equilibrated in the deep lower crust. Not surprisingly, the Li isotopic composition of the deep continental crust is very heterogeneous. Assuming the high-grade metamorphic terranes represent the middle to upper lower continental crust and the xenoliths represent the lowermost crust, we estimate the $\delta$$^{7}$Li of the lower crust to be +1 $\pm$ 2$\permil$ (1$\sigma$), which is similar to the upper continental crust and slightly lighter than the mantle. This probably is the result of metamorphic dehydration of different types of deep crustal rocks, which, together with surface weathering and low-T magmatic differentiation, drives the hydrosphere ($>$+30$\permil$) to heavier and bulk continental crust to lighter $\delta$$^{7}$Li values (0) than the mantle (+4$\permil$).
DE: 3660 Metamorphic petrology
DE: 1020 Composition of the crust
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting