4.4 billion years of maturation of the continental crust?

地球在结构上分为几个不同的外壳，一个富含铁的核心，硅酸盐（岩石）地幔和地壳。大陆地壳一般地势较高，而洋壳地势较低，这是由于它们的密度和组成不同。这种双模地壳结构在太阳系内是独一无二的。地球表面的自由水（水圈）也是独一无二的，它是地球上生命产生的重要组成部分，不断地塑造并与地球表面的岩石发生反应。地球物理学方法，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）揭示了地壳形成的过程。大部分地球的成分与母陨石相似，但地球的不同部分有非常不同的成分。所谓的“富集”大陆地壳与所谓的“贫化”地幔具有互补的成分，这与地壳在地球历史早期作为熔融产物从地幔中提取出来是一致的。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部分形成的。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，地壳是古老的，它的成分可能在地球的历史上发生了变化。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，大部分地壳已经被改变或风化。覆盖着现在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。钙被许多海洋生物沉淀为方解石壳，最终成为石灰石。其他元素，如镁，在洋中脊岩浆生成后，在冷却热洋壳的流体对流单元中的水-岩石相互作用期间返回到洋壳。在很长的时间尺度上，镁从大陆转移到海洋地壳。镁（一种可溶性元素）在大陆地壳中比地幔岩中特别贫乏，重要的是要限制这反映地壳风化或熔融过程的程度。该项目提出利用镁（大陆地壳中第五丰富的元素）等元素的同位素比率的微小差异（低至1/10000份）来评估大陆地壳的蚀变程度。这种差异在化学反应过程中被赋予岩石记录。特别地，与高温反应（熔化）相比，低温反应（例如风化）产生明显且更大的特征。质谱分析的最新进展使得同位素比率的微小差异可以被检测到。大陆地壳中不同的镁同位素比值的检测相比，地幔将使镁的损失从大陆地壳风化的量化。这将有助于改进大陆地壳形成的模型，因为初始组成将得到更好的限制。

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.