CSEDI Collaborative Research: The nature and timing of Earth's accretion

地球是由较小的岩石天体之间的一系列碰撞形成的。在这个过程中的某个时刻，地球也获得了组成大气和海洋的挥发物元素。但地球是如何以及何时形成的，以及它是如何以及何时获得挥发物的，仍然非常不确定。这很重要，因为我们知道，挥发性元素对生命至关重要；如果我们能了解地球是如何获得挥发物的，这将有助于我们了解太阳系和其他地方的其他行星是如何获得挥发物的。为了解决这个问题，研究人员使用了两个主要工具。一个是一系列天然的“时钟”，来自于衰变的放射性元素；这些告诉我们事情发生得有多快。第二个是实验，以确定挥发性元素是否被隔离在地球的铁芯中，还是留在岩石和大气中。作为这项调查的一部分，该小组将培训研究生和本科生——其中一些来自代表性不足的少数民族——掌握实验和分析技术，从而增加受过技术培训的劳动力。在本提案中，研究人员探讨了挥发性损失和岩心封存对一系列中度挥发性和难熔性元素的综合影响。他们将使用四个同位素计时器（Hf-W, Pd-Ag， U-Pb和I-Pu-Xe），它们具有不同的半衰期和化学特性，来分离这两种效应。建模工作使用n体吸积模型，可以跟踪来源和同位素演化；他们还建议对地幔的分配行为和Xe同位素组成进行必要的实验测量。研究小组将使用四种同位素系统来回答三个主要问题：1)大Tack理论和传统吸积理论哪个更符合观测结果？2)随着吸积的进行，加入地球的物质的组成是如何变化的？3)在吸积期间和之后发生了多少挥发性损失？在回答这些问题的过程中，研究人员将为地球的形成和早期演化提供一个更集中的图景。拟议的研究涉及建模师、同位素地球化学家和高压矿物学家之间的跨学科合作。因此，它跨越了传统的学科界限，并将为三个小组提供相互教育和整合实验，测量和建模的机会。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The Earth formed by a series of collisions between smaller rocky bodies. At some point during this process, the Earth also acquired the elements, known as volatiles, that make up its atmosphere and oceans. But how and when the Earth formed, and how and when it acquired its volatiles, are still very uncertain. This matters, because the volatile elements are essential for life as we know it; if we can understand how the Earth acquired its volatiles, that will help us understand how other planets did so, in this solar system and elsewhere. To solve this problem the investigators use two main tools. One is a series of natural “clocks”, derived from radioactive elements that decay; these tell us how fast things happened. The second is experiments to determine whether the volatile elements were sequestered into the Earth’s iron core, or whether they were left behind in the rocks and atmosphere. As part of this investigation the team will train graduate and undergraduate students – some from under-represented minorities - in experimental and analytical techniques, adding to the technically-trained workforce.In this proposal the investigators explore the combined effects of volatile loss and core sequestration on a range of moderately volatile and refractory elements. They will use four isotopic chronometers (Hf-W, Pd-Ag, U-Pb and I-Pu-Xe), with different half-lives and chemical characteristics, to disentangle these two effects. The modeling efforts use N-body accretion models, allowing provenance to be tracked and isotopic evolution to be tracked; they also propose to carry out necessary experimental measurements on partitioning behavior and mantle Xe isotopic compositions. The team will use the four isotopic systems to answer three major questions: 1) are the Grand Tack or conventional accretion scenarios more consistent with the observations? 2) how did the composition of material added to the Earth change as accretion proceeded?; 3) how much volatile loss happened during and after accretion itself? In answering these questions the investigators will provide a more focused picture of the formation and earliest evolution of the Earth. The proposed research involves an inter-disciplinary collaboration between a modeler, an isotope geochemist and a high-pressure mineralogist. As such, it cuts across traditional disciplinary boundaries and will provide an opportunity for the three groups to educate each other and integrate experiments, measurements and modeling.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.