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地球表面暴露的岩石所经历的化学风化的严重程度，这是一项对地球上最古老矿物（来自古代沉积岩的 40 亿年前的锆石）的锂同位素组成的研究。澳大利亚西部的杰克山报告了高度可变的同位素组成，特别是，在其中一些锆石中观察到的非常轻的李被认为反映了强烈的化学风化作用。然而，这种解释基于这样的假设：锆石中的锂忠实地记录了它们结晶的岩浆的同位素组成，并且没有受到此类过程的影响。在这个项目中，研究小组试图量化锂是否在天然锆石中扩散以及在什么条件下进行扩散：a）对锂进行原位分析。 b) 通过对在不同条件下加热的合成含锂锆石进行实验研究，发现锆石中的锂同位素具有不同的化学性质。用于地质过程和追踪岩浆起源的速度计 2) 研究人员试图调查锂在何种情况下会在矿物锆石中扩散。地球历史上的冥古宙（40 亿年）中的古代锆石被解释为反映了高度风化的物质融入了花岗岩岩浆的来源，而锆石被推断在花岗岩岩浆中结晶，这意味着冥古宙地球曾经存在过。大陆上升到海平面以上，并且由于高表面温度、酸性水和强降雨，该地壳暴露于强烈的化学风化作用。另一种解释得到了实验确定的锂扩散系数的支持。然而，大多数天然锆石没有显示出锂扩散的证据，例如，它们没有显示出锂同位素的渐进分区（在离子探针的 10-25 µm 分辨率内）。点），并且它们显示了 Li 中的突然集中步骤。弄清楚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。为了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，以查看是否有任何锂扩散的证据。在各种压力、温度和稀土元素浓度下合成的锆石的P和REE浓度和分布将被表征。可变的 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。该项目的结果将有助于更好地了解锆石中的 Li 扩散，反过来，将为使用锆石中的锂来追踪演化火成岩的来源（如果发现锂扩散微不足道）、岩浆过程的时间尺度（如果锂扩散经常发生）或可能两者（如果发生锂扩散）打开大门在某些情况下，但在其他情况下，两种情况都可能发生，具体取决于感兴趣的锆石的原子 Li/REE 比率。