Lithium Isotopic Investigations of Crustal Evolution

地壳演化的锂同位素研究

• 批准号: 0948549
• 负责人: Roberta Rudnick
• 金额: $ 44.62万
• 依托单位: University of Maryland, College Park
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-03-15 至 2016-02-29
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0948549&HistoricalAwards=false
• 关键词: Lithium; Isotopic; Investigations; Crustal; Evolution

Intellectual Merit. Several lines of evidence point to an important role for chemical weathering in producing the bulk continental crust: [1] its composition is "andesitic", although the crust today, and likely since the Archean, grew by addition of basalt, [2] it is depleted in certain fluid-mobile elements relative to mantle-derived magmas (e.g., Sr, as seen in the crust's low Sr/Nd), 3) it has an oxygen isotopic composition heavier than that of the mantle (d18O ~9 vs. 6, respectively, due to lighter oxygen in water vs. rock), and 4) its has a lithium isotopic composition lighter than that of the mantle (d7Li ~1 vs. 4, respectively, due to heavier Li in water vs. rock). However, the mass of crust removed through chemical weathering is poorly constrained.It is proposed here to quantify the influence of weathering on the crust's composition through two parallel PhD projects that focus on Li isotopes and major and trace element geochemistry: [1] quantification of the weathering flux from basaltic continental crust, focusing on the Columbia River Basalts (CRB) in the Pacific NW, and [2] construction of a mass balance model for the bulk continental crust using lithium concentration ([Li]) and d7Li to determine the mass lost to the crust via weathering. The weathering flux from the continents and the attendant transformation of basalt to more Si-enriched regolith will be evaluated through a coupled study of in situ weathering and the riverine flux from the CRB. The strong rain shadow effect of the Cascade Mountains causes large variations in the weathering intensities of CRB basalts east and west of this range. Extensive laterites and bauxites formed on CRB from the wet, western side of the Cascades. In contrast, soil development and/or preservation is variable to the east due to differential exposure ages as a result of the massive flooding that occurred during the last ice age. Analyses of selected weathering profiles coupled with analyses of the fluvial flux in catchments that drain only CRB will be used to quantify the chemical flux and associated reactions attending basalt weathering. These data will be used to model the influence of weathering on continental crust composition. The non-basaltic composition of the continental crust was likely the consequence of the combined effects of delamination of mafic lower crust and removal of soluble components by weathering, but their relative contributions are poorly constrained. A mass balance model constructed to solve for the time-integrated chemical flux of weathering components from the continental crust using Li and its isotopes and accompanying sensitivity tests show that the calculated mass lost is highly sensitive to Li concentration in the bulk continental crust and juvenile crustal additions. These parameters will be estimated via analyses of deep crustal rocks as well as Archean Na-rich granites (trondhjemites, tonalites, granodiorites - TTG = putative slab melts additions to the continental crust). A parallel study of Li in shales through time will explore their utility as indicators of continental weathering intensity over geologic time.Broader Impacts. This project will support two PhD projects that are just underway. Several undergraduates will be involved in the research with the expectation that at least one senior thesis will result from this project. The project will support laboratory facilities used as a resource by numerous national and international visitors.

智力优势。几条证据表明化学风化在形成大块大陆地壳中起着重要作用：[1]它的成分是“安山质”，尽管今天的地壳，可能从太古代开始，通过添加玄武岩而增长，[2]它相对于地幔衍生的岩浆缺乏某些流体流动元素（例如，Sr，如地壳的低Sr/Nd所示），3）它的氧同位素组成比地幔重（d18 O ~9与6，分别是由于水与岩石中的氧较轻），4）它的锂同位素组成比地幔轻（d 7 Li ~1与4，分别是由于水与岩石中的Li较重）。然而，通过化学风化作用去除的地壳质量受到很大的限制。这里建议通过两个平行的博士项目来量化风化作用对地壳成分的影响，这些项目侧重于锂同位素和主要和微量元素地球化学：[1]玄武岩大陆地壳风化通量的量化，重点是太平洋西北部的哥伦比亚河玄武岩（CRB），[2]利用锂浓度（[Li]）和d 7 Li建立块体陆壳的质量平衡模型，以确定通过风化作用损失到陆壳中的质量。大陆的风化通量和随之而来的玄武岩转化为更富硅的风化层将通过原位风化和河流流量的耦合研究从CRB进行评估。喀斯喀特山脉强烈的雨影效应导致了该山脉东西部CRB玄武岩风化强度的巨大变化。广泛的红土和铝土矿形成的CRB从潮湿的，西部的小瀑布。相比之下，土壤的发展和/或保存是可变的东部，由于不同的暴露年龄的结果，在最后一个冰河时代发生的大规模洪水。选定的风化剖面的分析，再加上河流流量的集水区，排水仅CRB的分析将被用来量化的化学通量和相关的反应参加玄武岩风化。这些数据将用于模拟风化对大陆地壳组成的影响。大陆地壳的非玄武岩成分可能是镁铁质下地壳分层和风化去除可溶组分的综合作用的结果，但它们的相对贡献受到很大限制。利用Li及其同位素建立的陆壳风化组分的时间积分化学通量的质量平衡模型和相应的敏感性试验表明，计算的质量损失对大块陆壳和幼年陆壳中Li浓度的增加非常敏感。这些参数将通过分析深部地壳岩石以及太古代富钠花岗岩（奥长花岗岩、英云闪长岩、花岗闪长岩- TTG =推定的板状熔体添加到大陆地壳）来估算。对页岩中锂含量随时间变化的平行研究将探索其作为地质时期大陆风化强度指标的实用性。该项目将支持正在进行的两个博士项目。几名本科生将参与该研究，预计该项目将产生至少一篇高年级论文。该项目将支持实验室设施，作为众多国家和国际访问者的资源。