Li Isotopes as a Tracer of Earth Processes

Details Li Isotopes as a Tracer of Earth Processes Li Isotopes as a Tracer of Earth Processes

Dec 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.v52b..11m&link_type=abstract

American Geophysical Union, Fall Meeting 2001, abstract #V52B-11

Physics

3670 Minor And Trace Element Composition, 5455 Origin And Evolution, 8125 Evolution Of The Earth

Scientific paper

The δ 7Li composition of the bulk Earth is not well established, although it can be estimated from that of mantle-derived magmas and peridotites to be about +4. Presently, there are next to no δ 7Li data for meteorites and this lack of cosmochemical constraint for the Earth needs to be considered in our models. Most upper crustal rocks and sediments have δ 7Li compositions greater than this mantle value and are generally of the order +5 to +15. Continental surface waters typically have δ 7Li values ranging from +15 to +32, while seawater has a nearly constant composition of +32. Collectively, these observations point to a uni-directional evolution in δ 7Li values from an initial mantle composition. Recently, we (Zack et al, this conference) determined that subduction-zone related eclogites and their meta-sediments possess dominantly negative δ 7Li values, ranging down -10. Additionally, the flood basalt sample BCR has a δ 7Li value of +2, which is lower than typical MORB/OIB/arc basalt values and indicates a contribution from an isotopically lighter source component. Collectively, these observations raise 2 interesting questions: (1) What is the physics controlling the fractionation of 7Li from 6Li? In contrast to an anion like O, the heavier isotope of Li is enriched in the fluid not the solid during low P-T processes. (2) Are eclogitic residues from subduction zones stored in the deep mantle as highly negative Li isotope reservoirs? The latter question raises the possibility of finding greater Li isotopic diversity in OIBs than that presently observed.

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