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

锂同位素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。对地球上最古老的矿物锂同位素组成的一项具有挑衅性的研究——来自西澳大利亚杰克山古代沉积岩的40亿年前的锆石，报告了高度变化的同位素组成。特别是，在这些锆石中观察到的非常轻的Li被认为反映了地球最早地壳的强烈化学风化，这意味着高表面温度，可能是酸性水和强降雨。然而，这种解释是基于这样一个假设：锆石中的Li忠实地记录了它们结晶的岩浆的同位素组成，并且没有受到锆石中Li扩散等过程的影响。在这个项目中，研究小组试图量化Li是否在天然锆石中扩散以及在什么条件下扩散。他们将采取双管齐下的方法：a)对天然锆石中的锂同位素进行原位分析，显示出化学上不同的核心和边缘；b)对合成的含锂锆石进行实验研究，这些锆石在不同条件下加热。这一结果将使锆石中的Li同位素更广泛地用作地质过程的测速仪和追踪岩浆起源的测速仪。2)研究人员试图研究在何种情况下Li在矿物锆石中扩散。来自地球历史（40亿年）冥古宙的古代锆石中异常轻的Li同位素被解释为反映了高度风化物质与花岗岩岩浆来源的结合，锆石被推断是在花岗岩岩浆中结晶的。这意味着冥古宙的地球上有上升到海平面以上的大陆，由于地表高温、酸性水和强降雨，这些地壳暴露在强烈的化学风化中。由实验确定的锆石中Li的扩散系数支持的另一种解释是，轻Li是在Li扩散过程中通过动力学分馏产生的。然而，大多数天然锆石没有显示出Li扩散的证据，例如，它们在Li同位素中没有递进分带（在离子探针点10-25µm分辨率范围内），并且它们在Li中显示出突变的浓度步骤。解开这些解释中哪一个是正确的，对于理解冥古宙地球具有重要意义。这个团队将对天然锆石和合成锆石进行调查，以解决Li是否容易在锆石中扩散的问题。利用飞行时间（ToF） SIMS绘制了天然锆石的Li分布，将使用NanoSIMS分析其同位素组成，以确定是否有任何证据表明Li在尖锐的Li浓度边界上扩散。在不同压力、温度和稀土元素浓度下合成的锆石，其P和REE的浓度和分布将被表征。然后将这些合成锆石暴露于不同浓度的Li中，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。这个项目的结果将导致对锆石中Li扩散的更好理解，反过来，这将为利用锆石中的Li来追踪演化火成岩的来源（如果Li扩散被发现是微不足道的），岩浆过程的时间尺度（如果Li扩散经常发生）打开大门，或者如果Li扩散在某些情况下发生，而在其他情况下没有发生，则可能两者都有。可以想象，这两种情况都可能发生，这取决于所研究锆石的原子Li/REE比率。

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.