Chemical Evolution of Archean Mantle and Crust: Constraints From Hafnium, Neodymium, and Lead Isotopes

太古代地幔和地壳的化学演化：来自铪、钕和铅同位素的限制

• 批准号: 9814885
• 负责人: Jeffrey Vervoort
• 金额: $ 19.85万
• 依托单位: University of Arizona
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 1999
• 资助国家: 美国
• 起止时间: 1999-01-01 至 2001-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9814885&HistoricalAwards=false
• 关键词: Chemical; Evolution; Archean; Mantle; Crust

9814885VervoortThe aim of this proposal is to constrain the isotopic evolution of the crust-mantle system during the Archean, along two subparallel paths: 1) definition of the Hf and Nd isotopic record of the Archean depleted mantle; and 2) integration of Hf and Nd isotopic records of silicate rocks with the Pb isotopes from Archean sulfides. Integrating Hf, Nd, and Pb isotopic systems will allow better constraints to be placed on the isotopic evolution of the depleted mantle, the growth and development of continental crust, and the interactions between crust and mantle during the first two billion years of Earth's history. The first aspect of the work will focus on determining the Hf isotopic composition in a range of well-dated and geologically constrained samples from the depleted mantle, 4.0 to 2.7 Ga. This will define the Hf isotopic record of depleted mantle evolution in the Archean, and by comparison, evaluate whether the current heterogeneity in the Nd isotopic record is a real feature of the Archean mantle or is an artifact caused by disturbances in the Sm-Nd isotopic system. U-Pb dating of zircons will provide better age constraints where they are lacking. To achieve the second goal, integrated Hf, Nd and Pb isotopic studies will be conducted on well-constrained volcanogenic massive sulfide (VMS) deposits, such as in the 2.7 Ga Abitibi greenstone belt. These deposits show evidence for retention of mantle heterogeneity from differentiation events in the 4.3 to 4.5 interval of early Earth history. It is not clear if this was related to U-Pb fractionation during core formation or during crust-mantle differentiation. Pb isotopes on VMS deposits, coupled with Hf and Nd isotopes on cogenetic volcanic rocks, will show whether early differentiation events were core-related, or caused by silicate melting/differentiation processes connected with magma and primitive crust formation.

本文的目的是沿着两条平行的路径来约束太古代壳幔系统的同位素演化：1）确定太古代亏损地幔的Hf和Nd同位素记录; 2）整合硅酸盐岩石的Hf和Nd同位素记录和太古代硫化物的Pb同位素记录。 综合Hf、Nd和Pb同位素体系将更好地制约地球历史最初20亿年期间亏损地幔的同位素演化、大陆地壳的生长和发展以及壳幔之间的相互作用。工作的第一个方面将集中在确定Hf同位素组成的范围内的一系列的良好的日期和地质约束的样品从亏损地幔，4.0至2.7 Ga。 这将确定太古代亏损地幔演化的Hf同位素记录，并通过对比，评价现今Nd同位素记录的不均匀性是太古代地幔的真实的特征，还是Sm-Nd同位素体系扰动造成的假象。锆石U-Pb测年将在缺乏锆石的地方提供更好的年龄限制。为实现第二个目标，将对约束良好的火山成因块状硫化物（VMS）矿床（如2.7 Ga Abitibi绿岩带）进行Hf、Nd和Pb同位素综合研究。这些沉积物表明，在早期地球历史的4.3至4.5间隔的分异事件的地幔异质性保留的证据。 目前尚不清楚这是否与核形成或壳幔分异过程中的U-Pb分馏有关。 VMS矿床上的Pb同位素，再加上同生火山岩上的Hf和Nd同位素，将显示早期分异事件是否与核有关，或与岩浆和原始地壳形成有关的硅酸盐熔融/分异过程引起的。