The Archean isotope record of Hadean geodynamic evolution

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Within the Hadean Eon, several key processes that shaped our planet took place, i.e., the Earths growth via asteroidal collisions, formation of the first proto-crust, and the delivery of volatiles. Nevertheless, it is still a fairly open question how the Earth evolved during the Hadean Eon in terms of its geodynamic regime and its chemical inventory. This is because virtually all accessible rock archives are younger than 4 billion years, with the exception of some rare older zircon grains. In the absence of a Hadean rock record, this proposal will assess dynamics of the Hadean Earth by employing diagnostic isotope tracers in Archean rocks. The proposed study is of particular relevance for two of the major SPP 1833 goals. These are the reconstruction of the Earths building material and the understanding of the Earths early differentiation. The effect of core formation and late meteorite bombardment on the Earths chemical inventory will be studied by measuring isotope compositions of the moderately siderophile elements W and Mo at high precision. Both elements were depleted in the silicate Earth during core formation, but were likely replenished during later meteorite bombardment and slowly mixed into the mantle. Molybdenum displays strong nucleosynthetic anomalies between different meteorite groups with the present day silicate Earth being an endmember to all known meteorite groups. Tungsten comprises two radiogenic isotopes (182 and 180) from the decay of Hf and Os, and by combining the information from both it may be possible to put new time constraints on core formation and late veneer addition. Our group is among the few worldwide that can perform high precision 182W measurements (to better than 10 ppm) and was the first group to establish an analytical protocol for sufficiently precise 180W measurements. The early history of the crust-mantle system and of the Earths volatile inventory will be assessed by analysing U-Th-Pb and Rb-Sr compositions of fresh minerals preserved in Archean rocks in order to obtain sufficiently robust initial radiogenic Sr and Pb isotope compositions. Amongst these elements, Rb and Pb are volatile whereas all other elements are refractory. By better constraining initial Sr and Pb isotope compositions of Archean rocks, the timing of volatile addition to the young Earth can be modelled. In particular for mafic Archean rocks, there is little robust initial Sr-Pb isotope information due to the mobility of Rb-Sr and U-Pb during metamorphism, rendering a robust age correction difficult. This issue will be overcome by analysing fresh pyroxenes from our collection of Archean samples, complemented by measurements in crustal apatites and barites. From past projects and collaborations, we have acquired a nearly unique collection of well preserved Archean rock samples and also of complementing extraterrestrial samples.