The Archean isotope record of Hadean geodynamic evolution

在冥古宙，几个塑造我们星球的关键过程发生了，即，地球通过小行星碰撞而成长，第一个原始地壳的形成，以及挥发物的输送。尽管如此，地球在冥古宙期间是如何演化的，就其地球动力学机制和化学物质而言，这仍然是一个相当开放的问题。这是因为除了一些罕见的更古老的锆石颗粒外，几乎所有可获得的岩石档案都小于40亿年。在没有冥古宙岩石记录的情况下，本提案将通过在太古宙岩石中使用诊断同位素示踪剂来评估冥古宙地球的动力学。提出的研究与SPP 1833的两个主要目标特别相关。这是对地球构造材料的重构和对地球早期分化的认识。通过高精度测量中度亲铁元素W和Mo的同位素组成，研究岩心形成和晚期陨石轰击对地球化学库存的影响。在地核形成过程中，这两种元素在硅酸盐地球中都被耗尽了，但很可能在后来的陨石轰击中得到了补充，并慢慢混合到地幔中。钼在不同的陨石群之间表现出强烈的核合成异常，而现在的硅酸盐地球是所有已知陨石群的末端成员。钨由Hf和Os衰变产生的两种放射性同位素（182和180）组成，通过结合这两种同位素的信息，可能会对岩心形成和后期贴面添加施加新的时间限制。我们的团队是世界上为数不多的能够进行高精度182W测量（优于10 ppm）的团队之一，并且是第一个建立足够精确的180W测量分析方案的团队。通过分析太古宙岩石中保存的新鲜矿物的U-Th-Pb和Rb-Sr组成，以获得足够可靠的初始放射性成因Sr和Pb同位素组成，评估壳幔系统和地球挥发性储量的早期历史。在这些元素中，Rb和Pb是易挥发的，而其他元素都是难熔的。通过更好地约束太古宙岩石的初始Sr和Pb同位素组成，可以模拟年轻地球挥发性添加的时间。特别是对于基性太古宙岩石，由于变质过程中Rb-Sr和U-Pb的流动性，使得初始Sr-Pb同位素信息很少，使得稳健的年龄校正变得困难。这个问题将通过分析我们收集的太古宙样品中的新鲜辉石来解决，并辅之以地壳磷灰石和重晶石的测量。从过去的项目和合作中，我们获得了几乎独一无二的保存完好的太古宙岩石样本，以及补充的外星样本。

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.