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陨石坑的原始(原始)地壳，而地球上显然完全没有超过40.5亿年的岩石。然而，已经发现了长达43.5亿年的矿物，证明了以前陆地原壳的存在。地壳的形成和破坏是地球分化过程中最神秘的步骤。然而，在已知的最古老的岩石中保存着放射性同位素线索，这些岩石是从古老但已经消失的地壳中继承下来的，要么是地幔枯竭，要么是地壳再循环。学会阅读这些线索将破译原始地壳的历史。在这里，我建议对其中之一的意义进行第一次系统的研究，即所谓的高Mu(U/Pb)特征，它体现在包括加拿大在内的几个地块的前寒武纪岩石的铅同位素系统学中。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、温尼伯河地体)是地球上研究人员也发现了已知最古老矿物的地方，在所有情况下，这些矿物都大大超过了岩石的年龄。提议的方法背后的理由有两个。首先，我建议测试一块岩石的高Mu值特征的强度与带有该特征的岩石中古锆石的数量之间是否存在关系。这意味着两者都是从不再保存的原壳继承而来的。这项研究将在CFI资助的一家最先进的设施中对从最高亩的晚太古代片麻岩和变沉积中挑选出来的矿物进行快速、原位的锆石U/Pb定年。其次，无论锆石的产量如何，该提案还旨在研究铅同位素指纹是否存在任何化学变化，就像月球(也具有高亩地壳)一样。这将沿着跨同位素梯度的系统横断面进行研究，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接受该项目的培训，该项目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.