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The Making of the Earth – Reading the Geochemical Code from Meteorites and the Earth’s Oldest Rocks

Periodic Reporting for period 4 - INFANT EARTH (The Making of the Earth – Reading the Geochemical Code from Meteorites and the Earth’s Oldest Rocks)

Reporting period: 2020-03-01 to 2021-08-31

How Earth could develop towards a habitable, rocky planet is still an open question. The “Infant Earth” project has provided a major advance in constraining the origin of Earth, the Moon and Earth's building blocks by systematically combining information from extraterrestrial samples and the Earth’s oldest rocks at an unprecedented level. Due to the absence of sufficiently old rocks on Earth, our knowledge on the Earth’s first few 100 million years is patchy, and what is known relies on indirect geochemical observations, namely studies of extraterrestrial samples and studies on the Earth’s oldest rocks. First of all, we collected a unique selection of rock samples covering Earth’ major oldest rock units. Further, we developed a number of new, ground breaking analytical methods. The new techniques led to a number of scientific results of which the key findings are listed below.

We could show that Earth formed from inner solar system building blocks that are not present in our meteorite collections anymore. So called siderophile (i.e. metal loving) elements in Earth’s mantle are in part pristine and are not entirely derived from the so called late veneer, the last ca. 0.5 percent of meteorite material that were delivered to Earth after formation of its metal core ceased. A study on Apollo samples from the Moon revised the formation age of the Moon to ca. 40-60 million years after solar system formation, significantly earlier than previously thought. The age also implies that the Earth-Moon system was more or less formed by ca. 60 million years after solar system formation, i.e. by 4.50 billion years ago. We could show that so called volatile elements (i.e. elements with condensation temperatures of less than ca. 800°K in space) on Earth and in some primitive meteorites display a characteristic hockeystick-like pattern, now unambiguously showing that the final 10-15 percent of meteoritic material that was delivered to the Earth had a composition similar to the CI or CM groups of carbonaceous chondrites that originate from the more outer parts of the solar system. The results of this study also allow much improved estimates of volatile element abundances in Earth’s hidden metal core. We could also show that the deficit of the rare element Niobium in Earth’s silicate mantle is likely a vestige of Earth’s early history and was inherited from Earth’s building blocks. The Niobium in Earth’s mantle has therefore been lost to asteroidal cores. Isotope measurements in Earths oldest rocks from Greenland, Australia and South Africa could show that Earth’s upper silicate mantle has lost much more basaltic melts by this time than previously thought.
The first project phase focused on hiring staff and on getting all projects started. We also acquired a unique collection of Earth’s oldest rocks covering most important cratons. Note that for most projects, our challenging method developments in the laboratory have taken several years in some cases. The final two years were mostly dedicated to measuring geological samples and meteorites. All projects were finished within their timeframe and most papers were published. However, some manuscripts are still on their way due to the long review process. Main results are listed in the previous section.
Six final achievements of this project are clearly beyond the state of the art:

1) The first sufficiently precise Ru isotope measurements in terrestrial rocks - Fische Gödde et al. 2020
2) Volatile element measurements in meteorites at unprecedented accuracy - Braukmüller et al. 2019
3) Some of the first Stable W isotope measurements in geological materials - Kurzweil et al. 2019,2020
4) A revised age for formation of the Moon – Thiemens et al. 2019
5) The first high precision measurements of high field strength elements in differentiated meteorites - Münker et al. 2017
6) Firm evidence for an early depleted mantle from Hf-Nd-Ce isotopes in rocks from the Pilbara craton - Hasenstab et al. 2021