4.4 billion years of maturation of the continental crust?

地球在结构上分为几个不同的壳体，一个富含铁的核心，以及硅酸盐(岩石)地幔和地壳。由于密度和成分的不同，大陆地壳一般地势较高，而洋壳地势较低。这种双模地壳结构在太阳系中是独一无二的。地球表面的游离水(水圈)也是独一无二的，它是地球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之发生反应。地球物理方法，如地震波的传播，主要揭示了地球的广泛结构，但地球化学(地质样品的化学研究)揭示了地壳形成过程的许多信息。大块地球的成分与母体陨石相似，但地球不同部分的成分非常不同。所谓的“富集型”大陆地壳与所谓的“贫化”地幔具有互补的成分，这与地球历史早期从地幔中提取出来的作为熔融产物的地壳相一致。越来越多的证据表明，大陆地壳的形成是古老的，其中很大一部分形成于地球历史的前半部分。然而，地幔部分熔融形成地壳的过程细节仍存在很大争议。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，其组成可能在地球历史上发生了变化。由于大部分地壳是古老的，并不断地被沉积和板块构造过程改造，因此大部分地壳已经发生了变化或风化。覆盖目前地球表面大部分的土壤就是这种风化的现代表现。在大陆风化过程中，许多可溶元素(例如矿泉水中的元素)被转移到海水中。一些元素从海水循环回到岩石中。许多海洋生物以方解石壳的形式沉淀钙，最终形成石灰石。其他元素，如镁，在大洋中脊岩浆产生后冷却高温大洋地壳的流体对流单元中，在水-岩石相互作用期间返回大洋地壳。在很长的时间尺度上，镁从大陆转移到洋壳。与地幔岩石相比，镁(一种可溶元素)在大陆地壳中特别贫化，重要的是要限制这反映地壳风化或熔融过程的程度。该项目建议通过利用元素同位素比率的极小差异(低至10000份中的1份)来评估大陆地壳的变化程度，例如镁(大陆地壳中第五大丰度元素)的同位素比率。在化学反应过程中，这种差异被赋予了岩石记录。特别是，与高温反应(熔化)相比，低温反应(如风化)产生了明显且更大的特征。质谱学的最新进展使同位素比率的细微差别变得可检测出来。通过检测陆壳与地幔中不同的镁同位素比值，将能够量化陆壳风化造成的镁的损失。这将使大陆地壳形成模型的改进成为可能，因为初始成分将得到更好的约束。

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.