Europlanet Science Congress 2021
Virtual meeting
13 – 24 September 2021
Europlanet Science Congress 2021
Virtual meeting
13 September – 24 September 2021
EPSC Abstracts
Vol. 15, EPSC2021-495, 2021
https://doi.org/10.5194/epsc2021-495
European Planetary Science Congress 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Atomic-scale mixing between MgO and H2O in the deep interiors of water-rich planets

Taehyun Kim1, Stella Chariton2, Vitali Prakapenka2, Anna Pakhomova3, Hanns-Peter Liermann3, Zhenxian Liu4, Sergio Speziale5, Sang-Heon Shim6, and Yongjae Lee1
Taehyun Kim et al.
  • 1Yonsei University, Earth System Sciences, Korea, Republic of (ppower306@yonsei.ac.kr)
  • 2GeoSoilEnviroCars (GSECARS), University of Chicago, Chicago, Illinois, USA
  • 3Photon Sciences, Deutsches Elektronen-Synchrotron (DESY), Hamburg, Germany
  • 4Department of Physics, University of Illinois at Chicago, Chicago, Illinois, USA
  • 5GFZ, German Research Centre for Geo-sciences, Potsdam, Germany
  • 6School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA

Astrophysical surveys so far have suggested that water-rich planets could be common [1] (including Uranus and Neptune in our Solar System). In the conventional interior model of water-rich planets, it has been assumed to have separate layers of atmosphere, ice/fluid, rocky mantle and metallic core [2]. However, recent studies have proposed the existence of heavy elements in the ice/fluid layer of Uranus, challenging the conventional view [3]. In addition, chemical interaction and thermodynamic processes of major rock-forming minerals at the H2O–rock interface conditions of the water-rich planetary interiors have been scarcely explored.

We have performed laser-heated diamond-anvil cell experiments on two rock-forming minerals, olivine ((Mg0.9,Fe0.1)2SiO4) and ferropericlase ((Mg0.9,Fe0.1)O), in water at the pressure and temperature conditions expected for the water-rich planets. During laser-heating, we collected X-ray diffraction (XRD) data at beamlines 13-IDD of GSECARS at APS and P02.2, the ECB of PETRA III at DESY. Our dataset covers pressures between 20 and 80 GPa. After high-pressure and high-temperature experiments, we conducted chemical and textural analysis using focused ion beam (FIB) and scanning electron microscope (SEM) at Yonsei University.

During laser-heating, Si-rich high-pressure phases were formed, such as bridgmanite ((Mg,Fe)SiO3) and stishovite (SiO2), from the high Mg/Si ratio of starting composition (olivine). The formation of Si-rich phases from Mg-rich starting composition suggests dissolve of MgO into H2O-liquid during laser-heating at high-pressures. This was also found for (Mg0.9,Fe0.1)O ferropericlase starting material. The intensity of the diffraction peak of ferropericlase was dramatically decreased at high-pressure and high-temperature conditions, which indicates that (Mg0.9,Fe0.1)O is soluble in H2O-liquid. From chemical analysis, we found the dome-like structures, which showed that domes are Mg-rich and below the domes is Si-rich. Between Mg-rich and Si-rich regions, porous structures (almost empty) were positioned, meaning that MgO-rich fluid existed at high-pressure and high-temperature conditions. In summary, the textural and chemical analysis combined with XRD data indicates a selective leaching of MgO preferentially from silicate during laser heating, making MgO-dissolved in high-temperature fluid, which peaks between 20 and 40 GPa and above 1,500 K [4].

For water-rich planets with 1–6 Earth masses, the chemical reaction at the deep H2O–rock interface would lead to high concentrations of MgO in the H2O layer. For Uranus and Neptune, our experiments indicate that the top ~3% of the H2O layer of them, the pressure and temperature conditions of which have been achieved in this study, would have a large storage capacity for MgO. If an early dynamic process enables the H2O–rock reaction, the topmost H2O layer may be rich in MgO, possibly affecting the thermal history of the planet.

 

[1] Batalha, N. M. Proc. Natl Acad. Sci. USA 111, 12647–12654 (2014). [2] Guillot, T. Annu. Rev. Earth Planet. Sci. 33, 493–530 (2005). [3] Helled, R., Nettelmann, N. & Guillot, T. Space Sci. Rev. 216, 38 (2020). [4] Kim, T. et al. Nat. Astron. (2021).

How to cite: Kim, T., Chariton, S., Prakapenka, V., Pakhomova, A., Liermann, H.-P., Liu, Z., Speziale, S., Shim, S.-H., and Lee, Y.: Atomic-scale mixing between MgO and H2O in the deep interiors of water-rich planets, European Planetary Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-495, https://doi.org/10.5194/epsc2021-495, 2021.

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