The Archean isotope record of Hadean geodynamic evolution

太古宙地球动力学演化同位素记录

• 批准号: 276756537
• 负责人: Professor Dr. Carsten Münker
• 金额: --
• 依托单位: Institut für Geologie und Mineralogie
• 依托单位国家: 德国
• 项目类别: Priority Programmes
• 财政年份: 2015
• 资助国家: 德国
• 起止时间: 2014-12-31 至 2020-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/276756537?language=en
• 关键词: Archean; isotope; record; Hadean; geodynamic

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.

在Hadean Eon中，发生了几个关键过程，即我们的星球发生了，即通过小行星碰撞，第一次原始壳的形成以及挥发物的传递。然而，这仍然是一个相当开放的问题，就其地球动力学制度及其化学库存而言，地球在Hadean Eon期间如何发展。这是因为几乎所有可访问的岩石档案都在40亿年以上，除了一些罕见的旧锆石谷物。在没有Hadean Rock记录的情况下，该提案将通过在Archean Rocks中使用诊断同位素示踪剂来评估Hadean Earth的动态。拟议的研究与两个主要SPP 1833目标中的两个特别相关。这些是地球建筑材料的重建和对地球早期差异的理解。通过测量适中的辅助元素W和MO的同位素组成，将研究核心形成和陨石晚期轰击对地球化学清单的影响。在核心形成过程中，这两个元素都在硅酸盐地球中耗尽，但可能在后来的陨石轰击期间得到补充，并慢慢混合到地幔中。钼在不同的陨石基团之间显示出强的核合成异常，而当今硅酸盐地球是所有已知的陨石基团的终点。钨包括HF和OS衰减的两个放射异位素（182和180），并且通过将两者的信息结合在一起，可以将新的时间限制在核心形成和较晚的贴面上加上。我们的小组是少数几个可以执行高精度182W测量值（大于10 ppm）的人之一，并且是第一个为足够精确的180W测量值建立分析方案的组。地壳膜系统和地球挥发性库存的早期历史将通过分析保存在Archean Rocks中的新鲜矿物质的U-TH-PB和RB-SR组成，以获得足够强大的初始放射性SR和PB同位素组成。在这些元素中，RB和PB是挥发性的，而所有其他元素都是难治性的。通过更好地限制Archean Rocks的初始SR和PB同位素组成，可以对年轻地球的挥发性时间进行建模。特别是对于镁铁质的太老子岩石，由于变质过程中RB-SR和U-PB的活动性，很少有强大的初始SR-PB同位素信息，因此难以实现强大的年龄校正。通过分析我们的大将样品收集的新鲜辉石，将克服这个问题，并通过地壳磷灰岩和barites中的测量值进行补充。从过去的项目和合作中，我们获得了几乎保存完好的Archean Rock样本的独特集合，还获得了外星样品的补充。