Episodic growth models for the continental crust; new tests from Hf and O isotopes in zircon

大陆地壳是地球独有的，分析技术和方法的最新发展使解决有关其起源和演化的根本问题成为可能。这些问题包括大陆的岩石是什么时候形成的？是否有大量火成岩形成的时期和其他形成较少岩石的时期，为什么会这样？这些火成岩在多大程度上代表了新的大陆地壳，或者它们是由较老的地壳岩石重熔而来的？大陆地壳是在什么构造背景下形成的，这些构造背景是否在地球历史的不同时期发生变化，例如，在地壳快速增长和缓慢增长之间？从沉积物获得的地壳演化记录与从火成岩获得的地壳演化记录如何比较，两者如何协调？关键的一步是获得大陆地壳的代表性记录，这些记录在矿物锆石中可用。新的是从同位素和微量元素中获得的信息，现在可以从这些微小的矿物中获得。锆石存在于大陆地壳上部的大多数岩石中，它们非常坚固，因此它们在反复的侵蚀和沉积过程中存活下来，甚至重新熔融和产生新的火成岩。它们包含的生长带可以用U-Pb同位素精确测定年龄，因此它们提供了无与伦比的岩浆条件变化的时间序列，现在可以用Hf和O同位素和微量元素丰度来确定。这些分析是使用离子束或激光在现场进行的，Hf和O同位素的耦合可以唯一地揭示即使是锆石中的单个生长带也是从地壳形成期间的年轻岩浆结晶出来的，还是来自先前存在的沉积岩改造而来的岩浆。火成岩形成于受空间和时间限制的岩浆岩省份。相比之下，沉积物倾向于取其源区可利用的各种岩石的平均值。因此，它们保存了截然不同的大陆地壳演化记录，现在，第一次有可能将保存在沉积物中的记录与保存在火成岩中的记录分开。这是使用氧同位素比率来完成的，因为在沉积物中的氧同位素比率比在没有涉及沉积物熔融的火成岩中要高。因此，氧同位素可以用来识别从岩浆中结晶出来的锆石，这些岩浆来自于沉积物，而不是来自其他岩浆。初步数据表明，在形成大量地壳的过程中，存在一段受限制的时期，然后，新的地壳花了近10亿年的时间才主导了上覆沉积物的成分。这个项目就是为了验证这些预测。第二个方面是，锆石中的微量元素丰度可以广泛用于区分在与深部岩柱有关的板块环境中产生的结晶元素和在俯冲相关环境中产生的岩浆。关键的问题是评估有多少新的大陆地壳是对反映在地幔热柱中的深层热扰动或俯冲带中表现的浅层构造过程做出的反应，以及这种平衡如何随时间变化。这一方法将适用于在推断的地壳更快速增长时期结束时沉积的碎屑状锆石。这对推断的地壳快速增长高峰的重要性提供了有力的检验，从某种意义上说，它将表明它们不仅仅是今天暴露的岩石的人工制品。待研究的区域包括一些地球上已知的最古老的岩石，这些岩石露出在澳大利亚，以及西非的一个地区，在那里，地幔热柱产生了新的地壳。

The continental crust is unique to the Earth, and recent developments in analytical techniques and in approach now make it possible to tackle fundamental questions over its origins and evolution. The questions include when did the rocks of the continents form? Were there periods when large volumes of igneous rocks were generated and other periods when fewer rocks formed, and why might that be? To what extent did these igneous rocks represent new continental crust, or were they derived by remelting older crustal rocks? In what tectonic setting was the continental crust generated, and did those settings change in different periods of Earth history, as, for example, between periods of rapid and slow crustal growth? How do the records of the evolution of the crust obtained from sediments compare with those obtained from igneous rocks, and how may the two be reconciled? The critical step is to obtain representative records of the continental crust, and these are available in the mineral zircon. What is new is the information from isotopes and trace elements that can now be obtained from these tiny minerals. Zircons occur in most rocks from the upper parts of the continental crust, they are very robust and so they survive through repeated episodes of erosion and sedimentation, and even remelting and the generation of new igneous rocks. They contain growth zones that can be dated precisely using U-Pb isotopes, and so they provide an unparalleled time series of changing magmatic conditions that can now be determining using hafnium and oxygen isotopes and trace element abundances. These analyses are done in situ using ion beams or lasers, and the coupling of hafnium and oxygen isotopes can uniquely reveal whether even the individual growth bands in a zircon crystallised from a juvenile magma during crustal generation, or from a magma derived by reworking of pre-existing sedimentary rocks. Igneous rocks are generated in provinces of magmatic rocks that are restricted in both space and time. Sediments, by contrast, tend to average the various rocks that are available in their source areas. Thus they preserve very different records of the evolution of the continental crust, and now, for the first time, it is possible separately to unpick the record preserved in sediments from those preserved in igneous rocks. This is done using oxygen isotope ratios because they are higher in sediments than they are in igneous rocks that have not involved melting of sediment. Oxygen isotopes can therefore be used to recognise zircons that crystallised from magmas that were derived from sediment, from those that were not. The preliminary data indicate that there were restricted periods when lots of crust was generated, and then it took almost one billion years for the new crust to dominate the composition of the overlying sediments. This project is concerned to test those predictions. The second aspect is the extent that trace element abundances in zircons can be used broadly to distinguish those that crystallized from magmas generated in within plate settings linked to deep seated plumes from those generated in subduction related settings. The key question is to assess how much new continental crust was generated in response to deep-seated thermal disturbances reflected in mantle plumes, or to shallow level tectonic processes manifest in subduction zones, and how that balance changed with time. This approach will be applied to detrital zircons deposited near the end of the inferred periods of more rapid crustal growth. This offers a robust test of the significance of the inferred peaks of rapid crustal growth, in the sense that it will demonstrate that they are not just artefacts of the rocks presently exposed today. The areas to be studied include some of the oldest known rocks on Earth, which outcrop in Australia, and an area where new crust was generated in response to mantle plumes, in west Africa.