Lithium Isotopic Investigations of Crustal Evolution

知识价值。有几条证据表明，化学风化在形成大块大陆地壳的过程中起着重要作用：[1]其组成“安山”,虽然今天的地壳,并可能太古代以来,增长了玄武岩,[2]这是耗尽在某些fluid-mobile元素相对于幔源岩浆(如老,老的地壳的低/ Nd), 3)它有一个比地幔的重氧同位素组成(分别d18O ~ 9和6,由于水与岩石轻氧),和4)有一个轻的锂同位素组成比地幔(d7Li ~ 1和4,分别是由于水中的锂比岩石中的重)。然而，通过化学风化作用剥离的地壳质量受到的约束很差。本文建议通过两个平行的博士项目来量化风化对地壳组成的影响，这些项目主要关注Li同位素和主微量元素地球化学：[1]定量化了玄武岩大陆地壳的风化通量，以太平洋NW的哥伦比亚河玄武岩（CRB）为重点；[2]利用锂浓度（[Li]）和d7Li建立了整体大陆地壳的质量平衡模型，确定了风化作用下地壳的质量损失。大陆的风化通量及其伴随的玄武岩向更富硅的风化层的转化将通过原位风化和来自CRB的河流通量的耦合研究来评价。喀斯喀特山脉强烈的雨影效应导致了该山脉东部和西部CRB玄武岩风化强度的巨大变化。大量红土和铝土矿形成于喀斯喀特山脉湿润的西侧。相比之下，东部的土壤发育和/或保存是不同的，这是由于上一个冰河时期发生的大规模洪水造成的不同暴露年龄。对选定的风化剖面的分析，加上对只排出CRB的集水区的河流通量的分析，将用于量化玄武岩风化过程中的化学通量和相关反应。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基性下地壳剥离和可溶成分风化作用的综合作用的结果，但它们的相对作用却不太明确。利用Li及其同位素建立了求解大陆地壳风化组分时间积分化学通量的质量平衡模型，并进行了灵敏度测试，结果表明，计算的质量损失对大陆地壳整体和幼年地壳添加物中Li浓度高度敏感。这些参数将通过对深部地壳岩石以及太古宙富钠花岗岩（长闪长岩、闪长岩、花岗闪长岩- TTG =假定的大陆地壳的板块熔体）的分析来估计。通过时间对页岩中Li的平行研究将探索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。几名本科生将参与研究，期望至少有一篇毕业论文将从这个项目产生。该项目将支持实验室设施作为众多国内和国际游客的资源。

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.