LITHIUM ISOTOPIC SYSTEMATICS OF THE CONTINENTAL CRUST
McDonough, William F
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In order to fully utilize Li isotopes as a geochemical tracer, it is necessary to characterize the Li isotopic compositions of different geological reservoirs, and quantify the magnitude of isotopic fractionations for various conditions and compositions. However, our knowledge of Li isotope geochemistry is mostly limited to the hydrosphere and mantle. Little is known about either the Li isotopic composition of the continental crust or the mechanisms by which Li isotopes are fractionated. The primary objective of this thesis is to characterize the Li isotopic composition of the continental crust. Over 50 upper crustal rocks including loess, shale, granite, and upper crustal composites, have been measured and show a limited range of Li isotopic composition (-5 to +5), with an average (0 ± 2 at 1s) that is lighter than the average upper mantle (+4 ± 2). More than 70 high-grade metamorphic rocks, including granulite xenoliths and composite samples from high-grade metamorphosed terranes have been analyzed to constrain the Li isotopic composition of the deep crust. Thirty composite samples from eight Archean terranes show mantle-like Li isotopic composition (+4 ± 1.4 (at 1&#963;)) while 44 granulite xenoliths display a much larger Li isotopic range from -17.9 to +15.7 with an average of -1± 7 (1&#963;), isotopically lighter than the mantle. These data indicate that the continental crust on average has a lighter Li isotopic composition than the upper mantle from which it was derived. Given that Li isotopes do not fractionate during high-T magmatism, juvenile crust and the mantle should have identical Li isotopic compositions. Therefore, the isotopically light continental crust is likely the result of secondary processes, e.g., weathering, metamorphism and low-T intracrustal melting. Previous studies have shown that weathering can strongly fractionate Li isotopes, with heavy Li leaching into the hydrosphere, leaving the rock residue isotopically light. Studies carried out in this thesis indicate that Li isotopes can be fractionated by diffusion, metamorphic dehydration and granite differentiation. Collectively, these processes shift the continental crust to isotopically lighter and the hydrosphere heavier than the mantle with respect to &#948;7Li.