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

1)锂同位素已越来越多地用于追踪暴露在地球表面的岩石所经历的化学风化的严重程度。一项对地球上最古老的矿物--来自澳大利亚西部杰克山古代沉积岩的40亿年前的锆石--的锂同位素组成的挑衅性研究报告了高度可变的同位素组成。特别是，在这些锆石中观察到的非常轻的Li被认为反映了地球上最早的地壳的强烈化学风化，这意味着高表面温度，可能是酸性沃茨和强降雨。然而，这种解释基于这样的假设，即锆石中的锂忠实地记录了它们结晶的岩浆的同位素组成，并且没有受到锂通过锆石扩散等过程的影响。在这个项目中，该团队试图量化Li是否在天然锆石中扩散以及在什么条件下扩散。他们将采用双管齐下的方法：a）对天然锆石中的锂同位素进行原位分析，这些锆石显示出化学上不同的核心和边缘，以及B）对在不同条件下加热的合成含锂锆石进行实验研究。这一结果将使锆石中锂同位素的应用更加广泛，可以作为地质作用和岩浆起源的速度计。2)研究人员试图调查锂在何种情况下扩散到锆石矿物中。来自地球历史的冥古宙（40亿年）的古老锆石中异常轻的Li同位素被解释为反映了高度风化的物质被纳入花岗岩岩浆的来源，其中锆石被推断为结晶。这意味着，冥古宙的地球上有高于海平面的大陆，由于高表面温度、酸性水和强降雨，地壳暴露在强烈的化学风化中。另一种解释，支持实验确定的扩散系数的锂在锆石，是轻锂通过动力学分馏过程中产生的锂扩散。然而，大多数天然锆石没有显示出Li扩散的证据，例如，它们没有显示出Li同位素的渐进分带（在离子探针点的10-25微米分辨率内），并且它们显示出Li的突然浓度台阶。解开这些解释中哪一个是正确的，对于理解古冥宙地球有着重要的意义。该团队将对天然和合成锆石进行调查，以解决锂是否容易在锆石中扩散的问题。使用飞行时间（ToF）西姆斯绘制的Li分布的天然锆石将使用NanoSIMS分析其同位素组成，以查看是否有Li扩散穿过尖锐Li浓度边界的任何证据。在各种压力、温度和稀土元素浓度下合成的锆石，将表征其P和REE的浓度和分布。然后将这些合成锆石暴露于不同的Li浓度，并进行加热实验，然后使用NanoSIMS分析其同位素组成以及Si，Sc，Ti和Y浓度。该项目的结果将导致更好地了解锂在锆石中的扩散，这反过来将打开大门，使用锂在锆石中追踪演化的火成岩的来源（如果锂扩散被发现是微不足道的），岩浆过程的时间尺度（如果锂扩散通常发生），或可能两者，如果锂扩散发生在某些情况下，而不是在其他情况下。可以想象，这两种情况都可能发生，这取决于感兴趣的锆石的Li/REE原子比。

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.