Lithium Isotopic Investigations of Crustal Evolution

智力上的功绩。有几条证据表明，化学风化在大陆块体地壳的形成中发挥了重要作用：[1]它的成分是“安山岩”，尽管今天的地壳，很可能是自太古代以来，通过添加玄武岩而生长的；[2]相对于地幔岩浆，它缺乏某些可流动的流体元素(例如，在地壳低的锶/钕中看到的锶)，3)它的氧同位素组成比地幔的重(分别为18O~9比6，这是由于水和岩石中的氧较轻)，4)其锂同位素组成比地幔轻(分别为d7Li~1和4，这是由于水和岩石中的Li较重)。本文建议通过两个平行的PHD项目来量化风化对地壳组成的影响，这两个项目集中在锂同位素和主元素和微量元素地球化学上：[1)玄武岩陆壳风化通量的量化，重点是太平洋西北部的哥伦比亚河玄武岩(CRB)；[2]建立陆壳质量平衡模型，利用锂浓度([Li])和d7Li来确定陆壳风化损失的质量。来自大陆的风化通量和伴随的玄武岩向更富硅的风化层的转变将通过现场风化和CRB的河流通量的耦合研究来评估。喀斯特山脉强烈的雨影效应导致该山脉以东和以西CRB玄武岩的风化强度差异很大。广泛的红土和铝土矿形成于CRB上，形成于喀斯喀特潮湿的西侧。相比之下，由于上一次冰河时期发生的大规模洪水造成的暴露年龄不同，东部的土壤发展和/或保存是不同的。对选定的风化剖面的分析，以及对只排出CRB的集水区河流通量的分析，将用于量化参与玄武岩风化的化学通量和相关反应。这些数据将被用来模拟风化对陆壳成分的影响。陆壳的非玄武岩成分可能是镁铁质下地壳拆沉和风化去除可溶性组分共同作用的结果，但它们的相对贡献很少受到限制。利用Li及其同位素建立了陆壳风化组分随时间变化的化学通量的质量平衡模型，并进行了敏感性试验，结果表明，计算的质量损失对陆壳块体和年轻地壳中Li的浓度高度敏感。这些参数将通过对地壳深部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)的分析来估计-TTG=推定的板片熔体添加到陆壳中。页岩中锂随时间的平行研究将探索它们作为地质时期大陆风化强度的指示器的有效性。该项目将支持两个刚刚进行中的博士项目。几名本科生将参与这项研究，期望至少有一篇高级论文能从这个项目中产生。该项目将支持许多国内和国际访问者将实验室设施用作资源。

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.