Die Entwicklung des Krusten Mantel Systems im Früharchaikum

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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.