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

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 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.