4.4 billion years of maturation of the continental crust?

地球在结构上被分为几个不同的壳层，一个富含铁的地核，以及硅酸盐（岩石）的地幔和地壳。大陆地壳在地形上通常是高的，而海洋地壳是低的，这是它们不同的密度和组成的结果。这种双模式地壳结构在太阳系中是独一无二的。地球表面的自由水（水圈）也是独一无二的，它是地球上生命形成的重要组成部分，它不断地塑造地球表面的岩石，并与岩石发生反应。地球物理方法，如地震波的传播，主要揭示了地球的大致结构，但地球化学（对地质样品的化学研究）揭示了地壳形成过程的许多情况。地球的整体组成与母陨石的组成相似，但地球的不同部分有很大的不同。所谓的“富集”大陆地壳与所谓的“枯竭”地幔具有互补成分，这与地壳在地球历史早期作为熔融产物从地幔中提取相一致。越来越多的证据表明，大陆地壳的形成是古老的，其中很大一部分是在地球历史的前半期形成的。然而，关于地幔部分熔融形成地壳的过程的细节仍然存在很大的争议。建立和测试从地幔中提取地壳模型的一个困难是，地壳是古老的，它的成分可能在地球的历史中发生了变化。由于地壳的大部分是古老的，并且在沉积和板块构造过程中不断被改造，因此地壳的大部分已经被改变或风化。如今覆盖地球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（例如在矿泉水中发现的那些）被转移到海水中。一些元素从海水中循环回到岩石中。钙被许多海洋生物沉淀成方解石壳，最终变成石灰岩。其他元素，如镁，在流体对流细胞中的水-岩相互作用过程中返回到海洋地壳中，在海洋中脊岩浆生成后，流体对流细胞冷却了热的海洋地壳。在很长的时间尺度上，镁从大陆转移到海洋地壳。与地幔岩石相比，镁（一种可溶性元素）在大陆地壳中的消耗尤为严重，因此限制镁在多大程度上反映了地壳的风化作用或熔融过程是很重要的。本项目拟利用镁（大陆地壳中含量第5高的元素）等元素同位素比值的极小差异（低至1 / 10000）来评价大陆地壳的蚀变程度。这些差异在化学反应过程中被传递到岩石记录中。特别是，与高温反应（熔化）相比，低温反应（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探测到。探测到大陆地壳中与地幔中不同的Mg同位素比值，可以量化大陆地壳因风化作用而损失的Mg。这将使大陆地壳形成的模型得以改进，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.