Die Entwicklung des Krusten Mantel Systems im Früharchaikum

地球上地幔在冥古宙和太古宙早期的演化仍然是一个有争议的问题。有一些相互竞争的模型，一个声称早在4.4 Ga耗尽，而另一些则认为晚耗尽，明显在3.5 Ga之后。为了验证这些模型，我们建议对来自西格陵兰岛南部Isua地区的新鲜3.8 Ga超镁铁质岩石进行微量元素和同位素测量，这些岩石被解释为地球上保存最古老的地幔岩石。伊苏阿地区早太古代（3.6 ~ 3.8 Ga）古绿岩和闪长闪长岩（TTG）套岩在世界范围内是独一无二的，因为它们代表了一个几乎完整且保存完好的早太古代壳幔演化记录。结合这些岩石的放射性成因同位素和微量元素系统，提供了一个独特的机会来了解在硅酸盐地球早期演化过程中活跃的地质过程。在项目的第一阶段，我们对该地区3.70至3.85 Ga的角闪岩相绿岩和ttg进行了最先进的同位素和微量元素测量。结果表明：(1)部分绿岩来自冥古宙形成的高度枯竭的地幔源；(2)拉斑绿岩中Hf-Nd同位素特征存在解耦，反映早在3.8 Gyrs前就存在俯冲带作用。此外，我们还发现与伊苏亚绿岩相关的ttg来自于加厚的基性弧壳的熔融，而不是洋壳的俯冲。在这里提出的项目的第二阶段，我们打算通过直接研究最近在Isua南部地区发现的早期太古代地幔岩石，获得早期硅酸盐地球演化的新见解。这些岩性在ttg中作为新鲜的飞地保存下来。在一次勘察样本活动中，我们从保存着新鲜橄榄石和辉石的超镁铁质飞地中收集了大约40个样本。通过从这些样品中制备矿物分离物，将有可能进行微量元素和放射性成因同位素组成（Sr-Nd-Pb-Hf-Os）的可靠测量，这将首次允许直接表征早期太古宙地幔的微量元素和放射性成因同位素清单。此外，可以直接测试硅酸盐地球中放射性成因同位素组成演化的不同模型，从而有助于更好地理解目前发现的地球上硅酸盐储层的演化。

The evolution of the Earths depleted upper mantle in the Hadean and Early Archean Eons is still a contentious issue. There are competing models, one claiming an early depletion by 4.4 Ga, whereas others argue for late depletion, significantly after 3.5 Ga. To test these models, we propose to perform trace element and isotope measurements on fresh, 3.8 Ga old ultramafic rocks from the Isua region in southern West Greenland, interpreted as the oldest preserved mantle rocks on Earth. The Early Archean (3.6 to 3.8 Ga) old greenstones and tonalite-trondhjemite-granodiorite (TTG) suite rocks from the Isua region are worldwide unique in that they represent a nearly complete and well preserved re-cord of Early Archean crust-mantle evolution. Combining radiogenic isotope and trace elements systematics in these rocks provides a unique opportunity to get insights geological processes active during the earliest evolution of the silicate Earth. In the first phase of our project, we performed state of the art isotope and trace element measurements on 3.70 to 3.85 Ga old amphibolite-facies green-stones and TTGs from the region. We could show that (1) some of the greenstones (originated from highly depleted mantle sources formed in the Hadean, and (2) that there is a decoupling of Hf-Nd isotope signatures in tholeiitic greenstones that reflects the presence of subduction zone processes as early as 3.8 Gyrs ago. Moreover, we found strong evidence that TTGs associated with the Isua greenstones originated from melting of thickened mafic arc crust and not from subducted oceanic crust. During the second phase of the project as proposed here, we intend to gain new insight into the evolution of the early silicate Earth, by directly studying early Archean mantle rocks that were just recently discovered in the region south of Isua. These lithologies are preserved as fresh enclaves in TTGs. During a reconnaissance sample campaign we have collected ca. 40 samples from ultramafic enclaves where fresh olivine and pyroxenes are preserved. By preparing mineral separates from these samples, it will be possible to perform robust measurements of trace elements and radiogenic isotope compositions (Sr-Nd-Pb-Hf-Os) that will for the first time permit to directly characterize the trace element and radiogenic isotope inventory of early Archean mantle. In addition, the different models for the evolution of radiogenic isotope compositions in the silicate Earth can then be directly tested, contributing to a better understanding of the evolution of silicate reservoirs on Earth as found at present day.