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

众所周知，今天的地球由一个致密的金属核心组成，周围环绕着一个部分熔融的地幔，在地幔上有一个浮力的外壳。为了让这种分离发生，我们的星球必须在某个时间点发生物理上的分化。我们还知道，在其他类地行星上也发生了分化。增强我们对类地行星演化的理解的关键是分化的时间。来自陨石的证据有力地表明，在太阳系形成的3000万年内，其他类地行星分化为核心、地幔和地壳，有一些证据表明地球上也是如此。然而，虽然非常古老的地壳岩石似乎存在于地球的月球上，可能也存在于火星上，但地球上暴露的最古老的地壳岩石要年轻得多。太阳系大约有4.57亿年的历史，而地球上发现的最古老的地壳岩石大约有4.03亿年的历史。这就提出了一个问题，即是否有任何证据表明，地球地壳岩石是由于对更古老的先前存在的地壳进行改造而形成的，或者地壳实际上是以一种更零碎的方式在较长的时间尺度上形成的。回答这个问题是这项拟议研究的主要重点，以便与其他类地行星相比，更有信心地了解地球的演化。要做到这一点，我们可以使用同位素技术以几种方式监测从地幔中提取地壳的情况。元素Nd的同位素组成可以确定非常古老的地壳是否是由已经因分异而枯竭的地幔形成的，而钙(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，氧的同位素(O)可以进一步确定这些先前存在的地壳岩石的性质。此外，还需要用铀铅(U-铅)放射性衰变法来解释同位素数据。问题的一部分是识别一个非常古老的样本，该样本没有经过后来的同位素重置。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，可以经受住后来的重新加工。矿物锆石以前曾被使用过，通过这一点，地球上最古老的地壳岩石的证据已经追溯到44亿年前。但锆石中不含丰富的Nd和Ca。因此，我们建议使用矿物钛铁矿，它可以用U-Pb方法测定年龄，并含有丰富的Nd，Ca和O。钛铁矿不如锆石那么坚固，但一个绕过这个问题的方法是分析单晶中的小区域。由于单独的钛铁矿颗粒可以包含不同的核心，即使在后来的地壳再加工过程中也能保留重要的同位素信息，因此我们可以使用一种新的方法来提取这些信息。我们计划对来自格陵兰岛西部的样本进行研究，在那里我们可能会在泰坦石中找到地壳最古老残留物的证据。来自这些岩石的同位素证据将证实地球是否很早就分化了。然后，我们计划研究一系列来自世界各地的岩石，这些岩石大约有3000到25亿年的历史。如果地球上的早期地壳确实形成并存活了下来，这些岩石应该包含一个独特的钙同位素特征，样本覆盖将使我们能够确定这个早期地壳的范围。结合这些岩石中的钕同位素，也将有可能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幅准确的图像，说明地壳何时形成，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。

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.