The search for the terrestrial protocrust

与月球和火星不同的是，月球和火星上保存着大量的原始地壳，地球上显然完全没有年龄超过40.5亿年的岩石。然而，已经发现了43.5亿年前的矿物，证明了陆地原地壳的存在。地壳的形成和破坏是地球分化过程中最神秘的步骤。然而，在已知最古老的岩石中保存着放射性同位素线索，这些岩石是通过地幔耗尽或地壳再循环从古老但已消失的地壳中继承下来的。学会解读这些线索将破解原始地壳的历史。在这里，我提出了第一个系统的研究的意义之一，所谓的高亩（U/Pb）的签名，这是表示在铅同位素系统的前寒武纪岩石在几个盾牌，包括加拿大。值得注意的是，高亩省份（奴隶，伊尔加恩，津巴布韦，格陵兰西南部，怀俄明州，温尼伯河地）是地球上研究人员还发现最古老的已知矿物的地方，在所有情况下都大大超过了岩石的年龄。所提议的办法背后的理由有两个方面。首先，我建议测试是否存在一个岩石的高亩字符的强度和携带签名的岩石中的古代锆石的量之间的关系。这意味着两者都继承自不再保存的原地壳。该研究将采用快速，在现场U/Pb锆石测年在CFI资助的国家的最先进的设备上的矿物选择从最高的太古代片麻岩和变质沉积物。其次，无论锆石产量如何，该提案还旨在研究是否有任何化学变化伴随着铅同位素指纹，就像月球上一样（也有高mu地壳）。这将在沿着同位素梯度的系统横断面上进行研究，例如在温尼伯河地体中暴露。一名初级博士后研究员和研究生将在该方案中接受培训，旨在加强加拿大在早期地球研究中的地位。

Unlike the Moon and Mars, where heavily cratered original (primordial) crust is preserved, there is an apparent complete absence of rocks older than 4.05 billion years on Earth. Yet minerals as old as 4.35 billion years have been discovered, testifying to the former existence of a terrestrial protocrust. The formation and destruction of this crust are the most cryptic steps of the Earth's differentiation. There are, however, radiogenic isotope clues preserved in the oldest known rocks that were inherited, either via mantle depletion or crustal recycling, from the ancient but lost crust. Learning to read these clues will decipher the history of the primordial crust. Here I propose the first systematic study of the significance of one of these, the so called high mu (U/Pb) signature, which is expressed in the lead isotope systematics of Precambrian rocks in several shields, including Canada. Significantly, the high mu provinces (Slave, Yilgarn, Zimbabwe, SW Greenland, Wyoming, Winnipeg River Terrane) are those places on Earth where researchers also find the oldest known minerals, which in all cases significantly exceed the ages of the rocks. The rationale behind the proposed approach is two-fold. First, I propose to test whether there exists a relationship between the strength of the high mu character of a rock and the amount of ancient zircon in the rocks that carry the signature. The implication being that both were inherited from the no longer preserved protocrust. The research will employ rapid, in situ U/Pb zircon dating in a CFI-funded state-of-the-art facility on minerals selected from the highest mu late Archean gneisses and metasediments. Second, regardless of the zircon yield, the proposal also aims at studying whether there are any chemical variations that accompany the lead isotopic fingerprint, like on the Moon (also with a high mu crust). This will be studied along systematic transects across isotopic gradients, exposed e.g. in the Winnipeg River Terrane. A junior postdoctoral researcher and graduate students will be trained in this program aimed at strengthening Canada's position in early Earth research.