The search for records of Earth's earliest crust to test terrestrial planet early global differentiation models.

寻找地球最早地壳的记录来测试类地行星早期的全球分异模型。

• 批准号: NE/D008891/1
• 负责人: Craig Storey
• 金额: $ 32.76万
• 依托单位: University of Bristol
• 依托单位国家: 英国
• 项目类别: Fellowship
• 财政年份: 2006
• 资助国家: 英国
• 起止时间: 2006 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FD008891%2F1
• 关键词: search; records; Earth; earliest; crust

The present-day Earth is known to be composed of a dense metallic core surrounded by a partially molten mantle upon which a buoyant outer crust lies. In order for this separation to have occurred, at some point in time our planet must have physically differentiated. We also know that differentiation occurred on other terrestrial planets. Critical to enhancing our understanding of terrestrial planetary evolution is the timing of differentiation. Evidence from meteorites strongly suggests that other terrestrial planets differentiated into a core, mantle and crust within 30 million years of the formation of the Solar System, and there is some evidence to suggest that that was equally true on Earth. However, whereas very old crustal rocks appear to be present on the Earth's Moon and possibly Mars, the oldest crustal rocks exposed on Earth are much younger. The Solar System is around 4570 million years old, whereas the oldest crustal rocks found on Earth are around 4030 million years old. This raises the question as to whether there is any evidence locked within Earth's crustal rocks to suggest that they formed as a result of reworking of much older pre-existing crust, or whether Earth's crust actually formed in a more piecemeal fashion over an elongated timescale. Answering this question is the main focus of this proposed research so that Earth's evolution can be more confidently compared with that of other terrestrial planets. To do this, we can use isotopic techniques to monitor the extraction of crust from the mantle in several ways. The isotopic composition of the element Neodymium (Nd) can determine whether very old crust was formed from a mantle that had already undergone depletion due to differentiation and Calcium (Ca) isotopes can determine whether early crust was available to contaminate later formed crust. In addition, isotopes of Oxygen (O) can further determine the nature of these pre-existing crustal rocks. Also, dating by Uranium to Lead (U-Pb) radioactive decay methods is required to interpret the isotope data. Part of the problem is identifying a very old sample that has not been subjected to later isotopic resetting. Single minerals within rocks may act as a repository for important isotopic information if they are robust enough to withstand later reworking. The mineral zircon has been used previously and through this, evidence of the oldest crustal rocks on Earth has been extended back to 4400 million years ago. However, zircon does not contain abundant Nd and Ca. Therefore, we propose to use the mineral titanite, which can be dated by U-Pb methods and contains abundant Nd, Ca and O. Titanite is not quite as robust as zircon, but one way of circumventing this problem is analysing small areas within single crystals. Since individual grains of titanite can contain distinct cores that retain important isotopic information even during later reworking of the crust, we can therefore use a novel approach to extract this information. We propose to work on samples from West Greenland where we may find evidence within titanite of the very oldest remnants of Earth's crust. Isotopic evidence from these rocks will confirm whether or not the Earth did differentiate very early. We then propose to work on an array of rocks from around the world that are around 3000 to 2500 million years old. These rocks should contain a distinctive Ca isotopic signature if an early crust on Earth did form and survived and the sample coverage will allow us to determine the extent of this early crust. In combination with Nd isotopes within these rocks, it will also be possible to determine whether new crust was generated throughout this important time period. In this way, it should be possible for the first time to build up an accurate picture of when Earth's crust was formed, whether the early crust survived in large volume and whether later formed crust significantly contributed to the overall growth of Earth's crust.

现今的地球被认为是由一个致密的金属核所组成，其周围是一个部分熔融的地幔，其上是一个有浮力的外壳。为了使这种分离发生，在某个时间点，我们的星球必须有物理上的差异。我们也知道其他类地行星上也发生过分化。对于增强我们对地球行星演化的理解至关重要的是分化的时间。来自陨石的证据有力地表明，其他类地行星在太阳系形成后的3000万年内分化为核心，地幔和地壳，并且有一些证据表明地球上也是如此。然而，尽管非常古老的地壳岩石似乎存在于地球的月球和火星上，但暴露在地球上的最古老的地壳岩石要年轻得多。太阳系大约有45.7亿年的历史，而地球上发现的最古老的地壳岩石大约有40.3亿年的历史。这就提出了一个问题，即是否有任何证据锁定在地球的地壳岩石中，表明它们是由于更古老的先前存在的地壳的改造而形成的，或者地壳是否实际上是在一个更长的时间尺度上以更零碎的方式形成的。解决这个问题是这项拟议研究的主要焦点，以便地球的演化可以更有信心地与其他类地行星进行比较。要做到这一点，我们可以使用同位素技术来监测地壳从地幔中提取的几种方式。元素钕（Nd）的同位素组成可以确定非常古老的地壳是否是由已经由于分异而经历了贫化的地幔形成的，钙（Ca）同位素可以确定早期地壳是否可以污染后来形成的地壳。此外，氧（O）的同位素可以进一步确定这些预先存在的地壳岩石的性质。此外，还需要用铀-铅（U-Pb）放射性衰变法测定年代，以解释同位素数据。问题的一部分是识别一个非常古老的样本，没有受到后来的同位素重置。岩石中的单一矿物可以作为重要同位素信息的储存库，如果它们足够坚固，可以承受以后的改造。矿物锆石以前曾被使用过，通过这个，地球上最古老的地壳岩石的证据已经延伸到44亿年前。但锆石中不含有丰富的Nd和Ca。因此，我们建议使用能用U-Pb法定年的、富含Nd、Ca和O的钛铁矿。钛铁矿不像锆石那么坚固，但解决这个问题的一种方法是分析单晶内的小区域。由于单个的钛铁矿颗粒可以包含不同的核心，即使在后来的地壳改造中也保留了重要的同位素信息，因此我们可以使用一种新的方法来提取这些信息。我们建议对西格陵兰岛的样本进行研究，在那里我们可能会在最古老的地壳遗迹的钛铁矿中找到证据。来自这些岩石的同位素证据将证实地球是否在很早的时候就分化了。然后，我们建议对来自世界各地的一系列岩石进行研究，这些岩石大约有3000到2500万年的历史。如果地球上的早期地壳确实形成并存活下来，这些岩石应该包含独特的Ca同位素特征，样本覆盖率将使我们能够确定早期地壳的范围。结合这些岩石中的Nd同位素，还可以确定在这一重要时期是否产生了新的地壳。通过这种方式，应该可以第一次建立一个准确的地壳何时形成的图片，早期地壳是否大量存活，以及后来形成的地壳是否对地壳的整体增长做出了重大贡献。

项目成果

Frontiers in Geochemistry - Contribution of Geochemistry to the Study of the Earth

地球化学前沿 - 地球化学对地球研究的贡献

• DOI: 10.1002/9781444329957.ch2
• 发表时间: 2011
• 作者: Hawkesworth C

Characterization of magma from inclusions in zircon: Apatite and biotite work well, feldspar less so

• DOI: 10.1130/g32037.1
• 发表时间: 2011-09
• 期刊: Geology
• 影响因子: 5.8
• 作者: E. Jennings;H. Marschall;C. Hawkesworth;C. Storey

Episodic, mafic crust formation from 4.5 to 2.8 Ga: New evidence from detrital zircons, Slave craton, Canada

• DOI: 10.1130/g24861a.1
• 发表时间: 2008-11
• 期刊: Geology
• 影响因子: 5.8
• 作者: A. Pietranik;C. Hawkesworth;C. Storey;A. Kemp;K. Sircombe;M. Whitehouse;W. Bleeker

Provenance of Neoproterozoic and early Paleozoic siliciclastic rocks of the Teplá-Barrandian unit (Bohemian Massif): Evidence from U–Pb detrital zircon ages

Teplá-Barrandian 单元（波希米亚地块）新元古代和早古生代硅质碎屑岩的物源：来自 UâPb 碎屑锆石年龄的证据

• DOI: 10.1016/j.gr.2010.05.003
• 发表时间: 2010
• 期刊: Gondwana Research
• 影响因子: 6.1
• 作者: K. Gerdes;Jeffries;Linnemann;Storey
• 通讯作者: Storey

Zircon record of fractionation, hydrous partial melting and thermal gradients at different depths in oceanic crust (ODP Site 735B, South-West Indian Ocean)

• DOI: 10.1007/s00410-016-1324-y
• 发表时间: 2017-03
• 期刊: Contributions to Mineralogy and Petrology
• 影响因子: 3.5
• 作者: A. Pietranik;C. Storey;J. Koepke;Stéphanie Lasalle;Stéphanie Lasalle