Earth’s Leaky Core: Identifying Signatures of Core Materials in the Lithosphere
Jan Render | 21-LW-046
The Earth's core hosts the vast majority of our planet's iron (Fe), nickel (Ni), carbon, and many other elements, but is generally assumed to be isolated from other terrestrial reservoirs. Geophysical evidence, in contrast, points towards materials from the outer core frequently percolating into the surrounding mantle, providing opportunities for exchange at one of Earth's most dramatic discontinuities. One way to test this hypothesis is by investigating the isotopic compositions of mantle-derived rocks, as the huge thermal gradient at the core–mantle boundary is expected to result in substantial isotopic fractionation of a few specific elements.
To this end, we developed methods at LLNL to analyze Ni isotopic compositions at state-of-the-art precision. These techniques were applied to samples from thermodiffusion experiments, as well as to various basaltic rocks from all over the globe. Whereas the thermodiffusion experiments demonstrate that heavy isotopes of both Ni and Fe are concentrated towards the colder ends of temperature profiles, similar isotopic signatures do not appear to be recorded in natural samples, highlighting that our current understanding of mantle-convection models requires revision. Instead, we show that small-scale isotope variations throughout our sample set of basalts reflects magmatic processes, an insight that is crucial for better defining the Ni isotopic composition of the bulk Earth and other planetary bodies. As a continuation of this work, we intend to apply the herein established methods to various lithologies from the Moon to shed light on lunar-formation mechanisms.
In addition to addressing fundamental research questions, the capability of measuring Ni isotopic compositions at state-of-the-art precision could be similarly applied to programmatic purposes at LLNL. This is not only because Ni is abundant in most alloys, soils, and rock formations, but also because of the short-lived isotope 63Ni (T1/2 ≈ 100 yr). Moreover, a better understanding of 63Ni production could inform us about iron-peak element nucleosynthesis in massive stars.
The techniques developed during this project will be applied to specimens from the Ryugu and Bennu asteroids returned by the Hayabusa 2 and Osiris Rex missions to address fundamental research questions in planetary science. Furthermore, we established a collaboration with Smith College to investigate the volatile-rich source of the Bermuda archipelago, which will further elucidate the processes that drive isotopic fractionation in the lithosphere.
Publications, Presentations, and Patents
Sio, C.K.I. et al. "Nickel Isotope Fractionation in Fe-Ni and Fe-Ni-S Alloys by Thermodiffusion." id. V13A-03. AGU Fall Meeting New Orleans, LA. 2021.
Sio, C.K.I. et al. "A Search for Isotopic Evidence of Thermodiffusion at the Core–Mantle Boundary." Goldschmidt Conference, Honolulu, HI. 2022. https://doi.org/10.46427/gold2022.10271.
Mazza S.E., Render J., and Wimpenny J. "Determining the Volatile-Rich Source for Bermuda Using Zn Isotopes." Goldschmidt Conference, Honolulu, HI. 2022. https://doi.org/10.46427/gold2022.10701.