Li Isotope Behavior in Zircons, with Implications for the Hadean Earth

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1)Lithium isotopes have been increasingly used to track the severity of chemical weathering experienced by rocks exposed at the Earth's surface. A provocative study of the lithium isotopic composition of the oldest minerals on Earth - the 4.0 billion year old zircons from ancient sedimentary rocks in the Jack Hills, western Australia, reported highly variable isotopic compositions. In particular, the very light Li observed in some of these zircons was suggested to reflect intense chemical weathering of the earliest crust on Earth, implying high surface temperatures, possibly acidic waters and intense rainfall. This interpretation, however, rests on the assumption that Li in zircon faithfully records the isotopic composition of the magma from which they crystallized and has not been influenced by processes such as Li diffusion through the zircon. In this project, the team seeks to quantify whether Li diffuses in natural zircons and under what conditions. They will do this with a two-pronged approach: a) in situ analyses of Li isotopes in natural zircons that show chemically distinct cores and rims, and b) by experimental investigations of synthetic, Li-bearing zircons that have been subjected to heating under different conditions. The results should lead to the more general use of Li isotopes in zircon as a speedometer for geologic processes and for tracing the origins of magmas. 2)Researchers seek to investigate under what circumstances Li diffuses in the mineral zircon. Unusually light Li isotopes iin ancient zircons from the Hadean eon of Earth history (4.0 billion years) have been interpreted to reflect incorporation of highly weathered materials into the sources of the granitic magmas in which the zircons are inferred to have crystallized. The implication is that the Hadean Earth had continents that rose above sea level and that this crust was exposed to intense chemical weathering due to high surface temperatures, acidic water and intense rainfall. An alternative interpretation, supported by experimentally determined diffusion coefficients for Li in zircon, is that the light Li was generated via kinetic fractionation during Li diffusion. However, most natural zircons show no evidence for Li diffusion, for example, they show no progressive zoning in Li isotopes (within the 10-25 µm resolution of ion probe spots), and they show abrupt concentration steps in Li. Unraveling which of these interpretations is correct has important implications for understanding the Hadean Earth. This team will undertake an investigation of both natural and synthesized zircons in order to address the question of whether Li readily diffuses in zircon. Natural zircons whose Li distribution has been mapped using time of flight (ToF) SIMS will be analyzed for their isotopic compositions using NanoSIMS to see if there is any evidence of Li diffusion across sharp Li concentration boundaries. Zircons synthesized under a variety of pressure, temperature and rare earth element concentrations, will be characterized for their P and REE concentrations and distributions. These synthetic zircons will then be exposed to variable Li concentrations, and heating experiments followed by analyses of their isotopic compositions, as well as Si, Sc, Ti, and Y concentrations using NanoSIMS. The results of this project will lead to a much better understanding of Li diffusion in zircon, which, in turn, will open the door towards using Li in zircon to trace sources of evolved igneous rocks (if Li diffusion is found to be insignificant), the timescale of magmatic processes (if Li diffusion commonly occurs), or possibly both if Li diffusion occurs in some instances, but not in others. It is conceivable that both may occur, depending on the atomic Li/REE ratios of the zircon of interest.