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.

智力优点。几条证据表明化学风化在产生大块大陆地壳中发挥着重要作用：[1]其成分是“安山岩”，尽管今天的地壳（很可能自太古代以来）是通过添加玄武岩而生长的，[2]它的成分是“安山岩”。相对于地幔岩浆（例如，Sr，如地壳的低 Sr/Nd 中所见），某些流体流动元素被耗尽，3）它的氧同位素成分比地幔（分别为 d18O ~9 vs. 6，因为水与岩石中的氧较轻），4) 其锂同位素组成比地幔轻（分别为 d7Li ~1 vs. 4，因为水中的锂比岩石中的锂重）。然而，通过化学风化去除的地壳质量受到的限制很差。这里建议通过两个平行的博士项目来量化风化对地壳成分的影响，这些项目重点关注锂同位素和主量元素和微量元素地球化学：[1]玄武岩大陆地壳的风化通量，重点关注太平洋西北地区的哥伦比亚河玄武岩（CRB），以及[2]使用锂浓度构建大块大陆地壳的质量平衡模型([Li]) 和 d7Li 来确定地壳因风化而损失的质量。来自大陆的风化通量以及随之而来的玄武岩向富硅风化层的转变将通过对原位风化和来自 CRB 的河流通量的耦合研究进行评估。喀斯喀特山脉强烈的雨影效应导致该范围东西部CRB玄武岩的风化强度变化较大。 CRB 上喀斯喀特潮湿的西侧形成了大量的红土矿和铝土矿。相比之下，由于上一个冰河时期发生的大规模洪水导致暴露年龄不同，东部的土壤发育和/或保存存在差异。对选定风化剖面的分析加上对仅排放 CRB 的集水区河流通量的分析将用于量化化学通量和参与玄武岩风化的相关反应。这些数据将用于模拟风化对大陆地壳成分的影响。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成分综合作用的结果，但它们的相对贡献很少受到限制。为使用 Li 及其同位素求解大陆地壳风化成分的时间积分化学通量而构建的质量平衡模型以及随附的敏感性测试表明，计算出的质量损失对大陆地壳和新生地壳中的 Li 浓度高度敏感补充。这些参数将通过对深层地壳岩石以及太古宙富钠花岗岩（长岩、英云长岩、花岗闪长岩 - TTG = 大陆地壳的推定板片熔融添加物）的分析来估计。对页岩中锂随时间变化的平行研究将探讨它们作为地质时期大陆风化强度指标的效用。更广泛的影响。该项目将支持两个正在进行的博士项目。几名本科生将参与这项研究，预计该项目至少能产出一篇高级论文。该项目将支持众多国内外游客用作资源的实验室设施。