Collaborative Research: Evolution of an Early Enriched Reservoir in the Northern Wyoming Province: Evidence from Ancient Zircons

知识价值。限制地球早期贫化和富化地球化学储层特征的元素分馏的大小、空间分布和程度一直是地球和行星地球化学家的一个基本长期目标。对于Sm-Nd和Lu-Hf系统，2.5 Ga之后的地壳-地幔演化被认为是地幔内放射性成因子同位素的普遍单调增加，这反映了大陆地壳的提取，产生了一个“熔融”耗尽的上地幔储层，其特征是父/子比大于整体硅酸盐土的值。在该层段上互补的富集储层是大陆地壳。然而，对于最早的4.5到2.5 Ga的地球历史，岩石记录更难破译——部分原因是如此古老的岩石暴露有限。现有数据表明，这一时期的地壳-地幔演化在熔体提取过程方面可能有很大不同，导致与更年轻的Nd-Hf同位素记录“异常”偏离。近年来，对保存完好的古老锆石中Lu-Hf分系统的分析为解析早期地球演化的同位素记录提供了新的途径。该方法将通过分析怀俄明州北部变质沉积岩和变质岩中保存的一套3.2至4.0 Ga锆石，来深入了解地球早期富集储层的发育情况。该研究将由3个主要部分组成：[1]U-Pb测年和痕量元素分析，通过离子探针（SHRIMP-RG），[2]更精细的元素分析和电子探针成像，[3]通过LA-ICP-MS对单个锆石定年部分的Lu-Hf系统测量。pi先前已经记录了该地区地壳的古代，并认为了解其演化将为了解地球早期的地球化学分化提供基本的见解。更广泛的影响。该项目将包括培训研究生和博士后研究人员的地质和分析技能，这些技能将用于未来的研究和/或私营部门的就业。本科生也将参与这个项目，向他们介绍地质研究的方法。这个小组先前的研究成果已经被收录在流行书籍和美国自然历史博物馆的教育展览中（例如，“地球里外”，Gottesman的行星地球展厅展览）。同样，拟议的研究也将纳入正规和非正规教育和外联活动。以路易斯安那州立大学岩石学教学中使用的“宠物岩石”方法为例。这项研究的所有结果将通过专业会议和同行评审出版物的传统报告传播。

Intellectual Merit. Constraining the size, spatial distribution, and extent of elemental fractionation that characterized both depleted and enriched geochemical reservoirs in the early Earth has been a fundamental long-term goal of terrestrial and planetary geochemists. For the Sm-Nd and Lu-Hf systems, crust-mantle evolution subsequent to 2.5 Ga has been viewed in terms of a generally monotonic increases in the radiogenic daughter isotopes within the mantle that reflects extraction of continental crust to produce a 'melt'-depleted upper mantle reservoir characterized by parent/daughter ratios that are greater than the values for the bulk silicate earth. The complementary enriched reservoir over this interval is the continental crust. However, for the earliest 4.5 to 2.5 Ga of Earth history, the rock record is more difficult to decipher - in part owing to limited exposures of rocks this old. Available data indicate that crust-mantle evolution in this period may have been quite different in terms of melt-extraction processes, leading to 'anomalous' departures from the younger Nd-Hf isotopic record. Recently, analysis of Lu-Hf systematics in old, well-preserved zircons have provided a new approach to resolving isotopic records for early earth evolution. This approach will be used to gain insights into the development of Earth's early enriched reservoirs through analysis of a suite of 3.2 to 4.0 Ga zircons preserved in both metasedimentary and metaigneous rocks from the northern Wyoming Province. The proposed research will consist of 3 primary components: [1] U-Pb dating and trace element analyses via ion probe (SHRIMP-RG), [2] finer scale elemental analyses and imaging via electron probe, and [3] measurement of Lu-Hf systematics via LA-ICP-MS on dated portions of individual zircons. The PIs have previously documented the antiquity of the crust in this region, and argued that understanding its evolution will provide fundamental insights into the early geochemical differentiation of the Earth. Broader Impacts. This project will involve training of graduate students and post-doctoral researchers in geologic and analytical skills that will serve in future research and/or private sector employment. Undergraduate students will also be involved in this project - introducing them to methods of geologic research. Prior research results of this group have been included in popular books and in educational displays at the American Museum of Natural History (e.g., "The Earth Inside and Out", Gottesman's Hall of Planet Earth exhibit). Likewise, the proposed research will also be incorporated into formal and informal education and outreach. An example is provided by the "pet rock" approach utilized for petrology instruction at LSU. All results of this research will be disseminated via traditional presentations at professional meetings and peer reviewed publications.