4.4 billion years of maturation of the continental crust?

大陆地壳成熟了44亿年？

• 批准号: NE/G013764/1
• 负责人: Edward Tipper
• 金额: $ 35.95万
• 依托单位: University of Cambridge
• 依托单位国家: 英国
• 项目类别: Fellowship
• 财政年份: 2010
• 资助国家: 英国
• 起止时间: 2010 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FG013764%2F1
• 关键词: 4.4; billion; years; maturation; continental

The Earth is divided structurally into several different shells, an iron rich core, and silicate (rocky) mantle and crust. The continental crust is generally topographically high, and the oceanic crust is low lying, a consequence of their distinct density and composition. This bi-model crustal structure is unique within the solar system. Also unique is the free water at the Earth's surface, (the hydrosphere), and a vital part for the creation of life on Earth, constantly shaping and reacting with the rocks at the its surface. Geophysical methods, such as the transmission of seismic waves have mainly revealed the broad structure of the Earth, but geochemistry (the chemical study of geological samples) has revealed much about the processes by which the crust formed. The bulk Earths composition is similar to that of parent meteorite bodies, but different parts of Earth have very different compositions. The so called 'enriched' continental crust has a complementary composition to the so called 'depleted' mantle, consistent with the crust having been extracted from the mantle as a melt product early in Earths history. There is increasing evidence for the formation of the continental crust to be old, with a large portion of it having formed in the first half of Earth's History. However, the details of the processes by which the crust formed by partial melting of the mantle remain highly controversial. One difficulty with establishing and testing models for the extraction of the crust from the mantle, is that the crust is old, and its composition may have changed over Earth's history. Because much of the crust is old and continually reworked by sedimentary and plate tectonic processes, much of the crust has been altered or weathered. The soil that mantles much of the present surface of the Earth is the modern manifestation of such weathering. During continental weathering, many soluble elements (such as those found in a mineral water for example) get transferred to seawater. Some elements get cycled from seawater back into rocks. Calcium is precipitated as calcite shells by many marine organisms which eventually becomes limestone. Other elements, such as magnesium, are returned to the oceanic crust during water-rock interaction in the fluid convection cells that cools the hot oceanic crust after magma generation at mid-ocean-ridges. Over long time-scales, magnesium is transferred from the continents to the oceanic crust. Magnesium (a soluble element) is particularly depleted in the continental crust compared to mantle rocks, and it is important to constrain the extent to which this reflects weathering of the crust, or melt processes. This project proposes to evaluate the degree of alteration of the continental crust by exploiting very small differences (as low as 1 part per 10000 parts) in the isotope ratios of elements such as magnesium, (the fifth most abundant element in the continental crust). Such differences become imparted to the rock record during chemical reactions. In particular, low temperature reactions (such as weathering) create a distinct and larger signature compared to high temperature reactions (melting). Recent advances in mass spectrometry have made such minute differences in isotope ratios detectable. Detection of distinct Mg isotope ratios in the continental crust compared to the mantle will enable the quantification of the loss of Mg by weathering from the continental crust. This will enable refinement of models for the formation of the continental crust, because the initial composition will be better constrained.

地球在结构上分为几个不同的壳、富含铁的核心、硅酸盐（岩石）地幔和地壳。大陆地壳通常地势较高，而海洋地壳地势较低，这是由于其不同的密度和成分造成的。这种双模型地壳结构在太阳系中是独一无二的。同样独特的是地球表面的自由水（水圈），它是地球上生命创造的重要组成部分，不断地塑造地球表面的岩石并与其发生反应。地震波传输等地球物理方法主要揭示了地球的广泛结构，而地球化学（地质样本的化学研究）则揭示了许多有关地壳形成过程的信息。地球的整体成分与母陨石体的成分相似，但地球的不同部分的成分却截然不同。所谓的“富集”大陆地壳与所谓的“贫化”地幔具有互补的成分，这与地球历史早期作为熔融产物从地幔中提取的地壳一致。越来越多的证据表明大陆地壳的形成很古老，其中很大一部分是在地球历史的前半段形成的。然而，地幔部分熔融形成地壳的过程细节仍然存在很大争议。建立和测试从地幔中提取地壳的模型的一个困难是地壳很古老，并且其成分可能在地球历史上发生了变化。由于大部分地壳都很古老，并且不断受到沉积和板块构造过程的改造，因此大部分地壳已经被改变或风化。覆盖地球大部分表面的土壤就是这种风化作用的现代表现。在大陆风化过程中，许多可溶性元素（例如矿泉水中发现的元素）会转移到海水中。有些元素从海水中循环回到岩石中。许多海洋生物将钙沉淀为方解石壳，最终变成石灰石。其他元素，例如镁，在流体对流单元中的水-岩石相互作用过程中返回到洋壳，该流体对流单元在洋中脊产生岩浆后冷却热洋壳。在很长一段时间内，镁从大陆转移到洋壳。与地幔岩石相比，大陆地壳中的镁（一种可溶元素）特别贫乏，因此限制镁（一种可溶元素）反映地壳风化或熔化过程的程度非常重要。该项目建议通过利用镁（大陆地壳中第五丰富的元素）等元素同位素比率的微小差异（低至万分之一）来评估大陆地壳的蚀变程度。在化学反应过程中，这种差异会被赋予岩石记录。特别是，与高温反应（熔化）相比，低温反应（例如风化）会产生明显且更大的特征。质谱技术的最新进展使得同位素比值的微小差异可以被检测到。检测大陆地壳与地幔中不同的镁同位素比率将能够量化大陆地壳风化造成的镁损失。这将使大陆地壳形成的模型得以完善，因为初始成分将得到更好的限制。

项目成果

Mg isotope systematics during magmatic processes: Inter-mineral fractionation in mafic to ultramafic Hawaiian xenoliths

• DOI: 10.1016/j.gca.2018.02.002
• 发表时间: 2018-04
• 期刊: Geochimica et Cosmochimica Acta
• 影响因子: 5
• 作者: A. Stracke;E. Tipper;S. Klemme;M. Bizimis

Silicate weathering and Si isotope fractionation in a glacial, granitic catchment

冰川、花岗岩流域中的硅酸盐风化和硅同位素分馏

• 发表时间: 2010
• 期刊: GEOCHIMICA ET COSMOCHIMICA ACTA
• 影响因子: 5
• 作者: Reynolds B. C.

Isotope clues on the origin of Mg/Si variations in chondrites and planetary bodies

关于球粒陨石和行星体中 Mg/Si 变化起源的同位素线索

• 发表时间: 2010
• 作者: Bourdon, Bernard

Tracing Mg transfer from rock to the oceans: Insights from Mg isotope ratios in the rivers of the Mackenzie Basin

追踪镁从岩石到海洋的转移：麦肯齐盆地河流中镁同位素比的见解

• 作者: Edward Tipper (Author)

Calcium isotopes in the global biogeochemical Ca cycle: Implications for development of a Ca isotope proxy

• DOI: 10.1016/j.earscirev.2013.10.004
• 发表时间: 2014-02-01
• 期刊: EARTH-SCIENCE REVIEWS
• 影响因子: 12.1
• 作者: Fantle, Matthew S.;Tipper, Edward T.
• 通讯作者: Tipper, Edward T.