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Toward an understanding of chemical and isotopic heterogeneity in the Earth's mantle.

机译：了解地球地幔中的化学和同位素异质性。

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摘要

We present models of the evolution of isotopic heterogeneity in the Earth's mantle to develop an understanding of the relationship between modern geochemical observables and physical processes through time.; Our basic model is an extension of the conventional geochemical reservoir model for the evolution of the Earth's crust-mantle system in which we calculate not only the mean isotopic ratios, but also the distribution of those ratios within the reservoirs. Owing to low chemical diffusion rates, subreservoirs that are created by mass transport into and out of the mantle effectively exist as distinct geochemical entities for all time. By tracking these subreservoirs, we obtain a model of the full range of isotopic values represented in the mantle. Using results from numerical calculations of mixing, we also track the length scales associated with each subreservoir. Applying simple statistics, we obtain the distribution of expected measurements as a function of the stirring time, effective melt fraction, sampling volume, and mass transport history.; In developing the model, we focus on the samarium-neodymium and rubidium-strontium parent-daughter systems, as these are the best behaved of the commonly-used systems. We then focus on the uranium-thorium-lead system, which introduces a significant degree of complexity not present in the simpler systems. We find that the removal of lead from the oceanic crust by subduction zone processes is critical in obtaining the observed slopes in lead-isotopic space.; We derive an internally consistent model of the Earth's isotopic evolution both in the bulk and statistical sense. We obtain a model of the Earth's mantle, heterogeneous on all length scales, which successfully reproduces the spectrum of heterogeneity observed in mid-ocean ridge basalts. This model leads us to a new version of the plum-pudding mantle in which relatively young, (1 Gyr) depleted residua from continental crustal extraction form the plums and everything else mixes to form the pudding. This latter component may be identified with FOZO or C, the intermediate mixing endmember suggested previously by other authors.
机译：我们提出了地球地幔中同位素异质性演化的模型，以加深对现代地球化学可观测物与物理过程之间的关系的了解。我们的基本模型是常规地球化学储层模型的扩展，用于地球地幔-幔系统的演化，在该模型中，我们不仅计算平均同位素比，而且还计算储层内这些比例的分布。由于化学扩散速率低，由质量运入和运出地幔所形成的子储层一直有效地作为独特的地球化学实体存在。通过跟踪这些子储层，我们获得了地幔中代表的同位素值的整个范围的模型。使用混合数值计算的结果，我们还可以跟踪与每个子储层相关的长度尺度。应用简单的统计数据，我们获得了预期测量值的分布，该分布是搅拌时间，有效熔体分数，采样量和传质历史的函数。在开发模型时，我们重点关注on钕和rub锶母女系统，因为它们是常用系统中性能最好的。然后，我们将重点放在铀-铅系统上，该系统引入了在较简单系统中不存在的高度复杂性。我们发现，通过俯冲带过程从大洋地壳中除去铅对于获得铅同位素空间中观察到的坡度至关重要。我们从体积和统计意义上推导了地球同位素演化的内部一致模型。我们获得了在所有长度尺度上都是非均质的地球地幔模型，该模型成功地再现了在洋中脊玄武岩中观察到的非均质光谱。该模型将我们引向新版的李子布丁地幔，其中相对年轻的（<1 Gyr）枯竭的大陆地壳提取物残留物形成了李子，其他所有物质混合后形成了布丁。后者的成分可以用FOZO或C来识别，FOZO或C是其他作者先前建议的中间混合末端成员。

著录项

  • 作者

    Kellogg, James Brewster.;

  • 作者单位

    Harvard University.;

  • 授予单位 Harvard University.;
  • 学科 Geophysics.; Geochemistry.
  • 学位 Ph.D.
  • 年度 2004
  • 页码 130 p.
  • 总页数 130
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 地球物理学;地质学;
  • 关键词

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