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

锆石中的锂同位素行为，对冥宙地球的影响

• 批准号: 1551388
• 负责人: Roberta Rudnick
• 金额: $ 13.3万
• 依托单位: University of Maryland, College Park
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-01-01 至 2018-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1551388&HistoricalAwards=false
• 关键词: Li; Isotope; Behavior; Zircons; Implications

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.

1)锂同位素越来越多地被用来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--西澳大利亚州杰克山古老沉积岩中40亿年前的锆石--的锂同位素组成进行了一项具有挑战性的研究，报告了高度不同的同位素组成。特别是，李在其中一些锆石中观察到的非常轻的物质被认为反映了地球上最早的地壳的强烈化学风化，这意味着地表温度高，可能是酸性水和强降雨。然而，这种解释建立在这样的假设之上，即锂在锆石中忠实地记录了它们结晶的岩浆的同位素组成，并且没有受到锂在锆石中扩散等过程的影响。在这个项目中，该团队试图量化锂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)原位分析天然锆石中的锂同位素，这些锆石显示出不同的化学核心和边缘；b)通过对在不同条件下加热的合成的含锂锆石进行实验研究。这一结果应该会导致更广泛地使用锆石中的锂同位素作为地质过程和追踪岩浆来源的速度计。2)研究人员试图调查锂在什么情况下在矿物锆石中扩散。古锆石中异常轻的锂同位素被解释为反映了高度风化的物质并入花岗岩浆的来源，推测锆石在花岗岩浆中结晶。言下之意是，哈迪亚大陆的海拔高于海平面，由于地表温度高、酸性水和强降雨，这块地壳暴露在强烈的化学风化中。另一种解释得到了实验测定的锂在锆石中的扩散系数的支持，即轻锂是在锂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Li扩散的证据，例如，它们在Li同位素中没有显示出渐进的分带(在离子探针斑点的10-25µm分辨率内)，并且它们在Li中显示出突然的浓度阶跃。解开这些解释中的哪一种是正确的，对于理解赫迪亚地球具有重要的意义。该小组将对天然和合成的锆石进行调查，以解决锂是否容易在锆石中扩散的问题。使用飞行时间(ToF)SIMS绘制了锂分布图的天然锆石将使用NanoSIMS分析其同位素组成，以确定是否有任何证据表明锂在尖锐的锂浓度边界上扩散。在不同的压力、温度和稀土元素浓度下合成的锆石将被表征其P和REE的浓度和分布。然后将这些合成的锆石暴露在不同的Li浓度下，并进行加热实验，然后使用NanoSIMS分析它们的同位素组成以及Si、Sc、Ti和Y的浓度。该项目的结果将使人们更好地了解锂在锆石中的扩散，这反过来将为利用锆石中的锂来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)、岩浆作用的时间尺度(如果锂扩散经常发生)，或者如果锂在某些情况下发生扩散而不是在其他情况下两者都有可能打开大门。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原子Li/REE比率。