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.

在冥古宙中，发生了几个塑造我们星球的关键过程，即地球通过小行星碰撞而生长、第一个原地壳的形成以及挥发物的输送。然而，就地球动力学状况和化学物质清单而言，地球在冥古宙期间如何演化仍然是一个相当悬而未决的问题。这是因为除了一些罕见的较古老的锆石颗粒外，几乎所有可获取的岩石档案的年龄都小于 40 亿年。在缺乏冥宙岩石记录的情况下，该提案将通过在太古代岩石中使用诊断同位素示踪剂来评估冥宙地球的动力学。拟议的研究与 SPP 1833 的两个主要目标特别相关。这些是对地球建筑材料的重建和对地球早期分化的认识。将通过高精度测量中等亲铁元素 W 和 Mo 的同位素组成来研究核心形成和晚期陨石轰击对地球化学库存的影响。这两种元素在地核形成过程中在硅酸盐地球中被耗尽，但很可能在后来的陨石轰击中得到补充，并慢慢地混合到地幔中。钼在不同陨石群之间表现出强烈的核合成异常，而当今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，通过结合两者的信息，有可能对核心形成和后期单板添加施加新的时间限制。我们的团队是全球少数能够执行高精度 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 测量分析协议的团队。将通过分析太古代岩石中保存的新鲜矿物的 U-Th-Pb 和 Rb-Sr 成分来评估地壳-幔系统和地球挥发物库存的早期历史，以获得足够可靠的初始放射性 Sr 和 Pb 同位素成分。在这些元素中，Rb和Pb是挥发性的，而所有其他元素都是难熔的。通过更好地限制太古宙岩石的初始 Sr 和 Pb 同位素组成，可以模拟年轻地球中挥发物添加的时间。特别是对于基性太古宙岩石，由于变质作用过程中 Rb-Sr 和 U-Pb 的迁移性，几乎没有可靠的初始 Sr-Pb 同位素信息，使得可靠的年龄校正变得困难。通过分析我们收集的太古代样品中的新鲜辉石，并辅以地壳磷灰石和重晶石的测量，可以解决这个问题。从过去的项目和合作中，我们获得了近乎独特的保存完好的太古代岩石样本以及补充的外星样本。