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）锂同位素已越来越多地用于跟踪地球表面暴露的岩石的化学风化体验的严重程度。一项针对地球上最古老的矿物质锂同位素组成的挑衅性研究 - 西澳大利亚州杰克山（Jack Hills）的古代沉积岩石的400亿年历史锆石报道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轻度LI被认为反映了地球上最早地壳的强烈化学风化，这意味着高表面温度，可能的酸性水和强烈的降雨。然而，这种解释基于这样的假设，即锆石中的li忠实地记录了它们从中结晶的岩浆的同位素组成，并且没有受到诸如通过锆石扩散等过程的影响。在这个项目中，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。他们将通过两种依据的方法来做到这一点：a）天然锆石中LI同位素的原位分析，这些锆石表现出了化学上不同的核心和边缘，b）通过对在不同条件下经过加热的合成，含液氧化锆石的实验研究。结果应导致在锆石中更普遍地使用LI同位素作为地质过程的速度表和追踪岩浆的起源。 2）研究人员试图在矿物锆石的哪种情况下进行调查。从地球历史上的Hadean Eon（40亿年）中的古代锆石异常的同位素被解释为反映了将材料掺入的岩浆岩的来源中，其中推断出锆石的结晶。这意味着Hadean Earth继续在海平面上升，并且由于表面温度高，酸性水和强烈的降雨而暴露于强烈的化学风化。通过实验确定的扩散系数支持Li在锆石中的替代解释是，LI是通过LI扩散过程中的动力学分馏产生的。但是，大多数天然锆石都没有表现出LI扩散的证据，例如，它们在LI同位素（在10-25 µM离子探针点分辨率内）中没有进行进行性分区，并且它们在LI中显示出突然的浓度步骤。解散这些解释中的哪一种是正确的，对理解Hadean Earth具有重要意义。该团队将对自然和合成的锆石进行投资，以解决李在锆石中是否容易扩散的问题。将使用纳米菌素分析其同位素组合物的自然锆石的自然锆石，其同位素组合物将被分析，以查看是否有任何证据表明在尖锐的Li浓度边界之间存在LI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将以其P和REE浓度和分布为特征。然后，这些合成锆石将暴露于可变的LI浓度，并进行加热实验，然后分析其同位素组合物以及使用纳米SI，SC，TI和Y浓度进行分析。该项目的结果将导致对锆石中的Li扩散有更好的了解，这反过来，这将为使用氧化锆石的门打开大门来追踪进化的火成岩岩石的来源（如果发现Li扩散是微不足道的），即魔法过程的时间表（如果li扩散通常发生），或者在其他方面可能不会出现在某些Instints的情况下，则可能会出现某些Instintess，但会发生在某些Instintist上。可以想象，这两者都可能发生，具体取决于感兴趣的锆石的原子李/REE比。