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Tracing continental weathering using lithium and magnesium isotopes: Insights from the chemical weathering of Columbia River basalts and mass balance modeling.

机译：使用锂和镁同位素追踪大陆风化：哥伦比亚河玄武岩化学风化和质量平衡模型的见解。

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摘要

Chemical weathering is an important mechanism that changes the mass and composition of the continental crust and regulates the global CO2 cycle over geological time scales. Basalt, in particular, is the main building block of the juvenile continental crust, and it is estimated that basalt weathering currently accounts for more than 30% of global CO2 consumption due to silicate weathering. Here I explore Li and Mg isotopes as tracers of chemical weathering. The primary objective of this dissertation is to understand Li and Mg isotopic fractionation mechanisms during chemical weathering of basalts by analyzing Li and Mg isotopic compositions in weathering-related reservoirs. In addition, I perform mass balance modeling of Li to place constraints on the amount of continental crust that has been removed by chemical weathering. The samples include different flows of the fresh Columbia River Basalts (CRBs), drill cores through bauxites developed on the basalts, eolian dust, streams and ground waters draining only/mainly the CRBs.;The results show that: 1) leaching, secondary mineral formation, and eolian addition are the main processes controlling Li and its isotopic composition in weathered basalt, 2) gibbsite in the basalt regolith generates large Mg isotopic fractionation and δ26Mg is also influenced by eolian addition, 3) the variations of δ7Li in streams are likely to reflect weathering intensity, with lower δ7Li corresponding to higher chemical weathering intensity, and 4) the mass of juvenile continental crust lost due to chemical weathering is at least 15% of the original crustal mass. Collectively, the results imply that: 1) chemical weathering may decrease the magnitude of the Nb anomaly of the bulk continental crust, 2) chemical weathering produces isotopically light Mg in water, leaving behind an isotopically heavy regolith and driving the bulk continental crust composition to heavier values, 3) Li and Mg isotopes have the potential to be useful in tracing continental chemical weathering, 4) chemical weathering played a significant role in changing the bulk continental crustal composition from basaltic to andesitic.
机译：化学风化是一种重要的机制，它可以改变大陆壳的质量和组成，并在地质时间尺度上调节全球CO2循环。特别是玄武岩，是青少年大陆壳的主要构造块，据估计，由于硅酸盐的风化，玄武岩的风化目前占全球二氧化碳消费量的30％以上。在这里，我探讨了锂和镁同位素作为化学风化的示踪剂。本文的主要目的是通过分析与风化有关的储层中的Li和Mg同位素组成，来了解玄武岩化学风化过程中Li和Mg同位素的分馏机制。此外，我对Li进行了质量平衡建模，以限制化学风化已去除的大陆壳的数量。样品包括新鲜的哥伦比亚河玄武岩（CRB）的不同流量，通过玄武岩上发育的铝土矿的岩心，风尘，溪流和仅/主要排放CRB的地下水;结果表明：1）浸出，次生矿物风化玄武岩中形成锂和风成岩是控制锂及其同位素组成的主要过程，2）玄武岩长石中的三水铝石产生大量的镁同位素分馏，风成石也影响δ26Mg，3）河流中δ7Li的变化很可能为了反映风化强度，较低的δ7Li对应较高的化学风化强度，并且4）由于化学风化而损失的幼年大陆壳质量至少是原始地壳质量的15％。总的来说，这些结果暗示：1）化学风化作用可能会降低大块陆壳Nb异常的程度，2）化学风化作用会在水中产生同位素轻的Mg，留下同位素重的重块岩，并驱使大块陆壳组成更重的数值，3）锂和镁同位素有可能用于追踪大陆化学风化，4）化学风化在将整个大陆地壳组成从玄武岩变为安山岩方面发挥了重要作用。

著录项

  • 作者

    Liu, Xiao-Ming.;

  • 作者单位

    University of Maryland, College Park.;

  • 授予单位 University of Maryland, College Park.;
  • 学科 Geology.;Geochemistry.
  • 学位 Ph.D.
  • 年度 2012
  • 页码 165 p.
  • 总页数 165
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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