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

大陆地壳的幕式生长模型；

• 批准号: NE/E005225/1
• 负责人: Chris Hawkesworth
• 金额: $ 34.74万
• 依托单位: University of Bristol
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2007
• 资助国家: 英国
• 起止时间: 2007 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FE005225%2F1
• 关键词: Episodic; growth; models; continental; crust

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.

大陆地壳是地球所独有的，分析技术和方法的最新发展使人们有可能解决有关其起源和演变的基本问题。问题包括大陆的岩石是什么时候形成的？是否有大量火成岩生成的时期和较少岩石形成的其他时期，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，或者它们是通过重新熔化旧的地壳岩石而形成的？大陆地壳是在什么样的构造背景下形成的，这些背景在地球历史的不同时期是否发生了变化，例如，在地壳快速和缓慢生长的时期之间？从沉积物中获得的地壳演化记录与从火成岩中获得的地壳演化记录相比如何？两者又如何协调？关键的一步是获得大陆地壳的代表性记录，这些记录可以在矿物锆石中获得。新的是现在可以从这些微小的矿物中获得的同位素和微量元素的信息。锆石出现在大多数来自大陆地壳上部的岩石中，它们非常坚固，因此它们经历了反复的侵蚀和沉积，甚至重熔和新火成岩的产生。它们包含生长带，可以使用U-Pb同位素精确地确定年代，因此它们提供了一个无与伦比的岩浆条件变化的时间序列，现在可以使用铪和氧同位素以及微量元素丰度来确定。这些分析是在现场使用离子束或激光进行的，铪和氧同位素的耦合可以唯一地揭示锆石中的单个生长带是从地壳生成期间的幼年岩浆结晶出来的，还是从先前存在的沉积岩改造而产生的岩浆结晶出来的。火成岩是在岩浆岩区中生成的，在空间和时间上都受到限制。相比之下，沉积物倾向于平均其源区的各种岩石。因此，它们保存了非常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，因为它们是在沉积物高于他们在火成岩，没有涉及沉积物的熔融。因此，氧同位素可以用来识别从岩浆中结晶出来的锆石，这些岩浆来自沉积物，而不是来自沉积物。初步数据表明，有一些有限的时期，大量的地壳产生，然后花了近10亿年的时间，新的地壳占主导地位的组成上覆沉积物。该项目旨在验证这些预测。第二个方面是锆石中微量元素丰度的程度，可以广泛地用于区分那些从与深部羽流有关的板块环境中产生的岩浆结晶的锆石和那些在俯冲相关环境中产生的锆石。关键问题是评估有多少新的大陆地壳是由于反映在地幔柱中的深层热扰动或俯冲带中表现出的浅层构造过程而产生的，以及这种平衡如何随时间而变化。这种方法将适用于碎屑锆石沉积的推断更快的地壳生长期结束附近。这为推断出的地壳快速生长峰值的意义提供了一个强有力的检验，因为它将证明它们不仅仅是目前暴露的岩石的人工制品。将要研究的地区包括一些地球上已知的最古老的岩石，这些岩石在澳大利亚露头，以及西非的一个地区，那里的新地壳是由于地幔柱而产生的。

项目成果

Impact melt sheet zircons and their implications for the Hadean crust

• DOI: 10.1130/g30251a.1
• 发表时间: 2009-10
• 期刊: Geology
• 影响因子: 5.8
• 作者: J. Darling;C. Storey;C. Hawkesworth

From sediments to their source rocks: Hf and Nd isotopes in recent river sediments

• DOI: 10.1130/g31785.1
• 发表时间: 2011-04-01
• 期刊: GEOLOGY
• 影响因子: 5.8
• 作者: Dhuime, Bruno;Hawkesworth, C. J.;Cawood, Peter A.
• 通讯作者: Cawood, Peter A.

Aragonite in olivine from Calatrava, Spain-Evidence for mantle carbonatite melts from >100 km depth

西班牙卡拉特拉瓦橄榄石中的文石——超过 100 公里深度的地幔碳酸盐岩熔融的证据

• DOI: 10.1130/g31199.1
• 发表时间: 2010
• 期刊: Geology
• 影响因子: 5.8
• 作者: Humphreys E