¹Lars E. Borg,¹Gregory A. Brennecka,^1,2Thomas S. Kruijer
Proceedings of the National Academy of Sciences of the United States of America (PNAS) (In Press) Link to Article [https://doi.org/10.1073/pnas.2115726119]
¹Nuclear and Chemical Science Division, Lawrence Livermore National Laboratory, Livermore, CA 94550;
²Department of Solar System, Impacts & Meteorites, Museum fur Naturkunde, Berlin 10115, Germany

The origin of volatile species such as water in the Earth–Moon system is a subject of intense debate but is obfuscated by the potential for volatile loss during the Giant Impact that resulted in the formation of these bodies. One way to address these topics and place constraints on the temporal evolution of volatile components in planetary bodies is by using the observed decay of ⁸⁷Rb to ⁸⁷Sr because Rb is a moderately volatile element, whereas Sr is much more refractory. Here, we show that lunar highland rocks that crystallized ∼4.35 billion years ago exhibit very limited ingrowth of ⁸⁷Sr, indicating that prior to the Moon-forming impact, the impactor commonly referred to as “Theia” and the proto-Earth both must have already been strongly depleted in volatile elements relative to primitive meteorites. These results imply that 1) the volatile element depletion of the Moon did not arise from the Giant Impact, 2) volatile element distributions on the Moon and Earth were principally inherited from their precursors, 3) both Theia and the proto-Earth probably formed in the inner solar system, and 4) the Giant Impact occurred relatively late in solar system history.

¹Rajkrishna Dutta et al. (>10)
Proceedings of the National Academy of Sciences of the United States of America (PNAS) (in Press) Link to Article [https://doi.org/10.1073/pnas.2114424119]
¹Earth and Planets Laboratory, Carnegie Institution for Science, Washington, DC 20015
²Department of Geosciences, Princeton University, Princeton, NJ 08544

Mg₂GeO₄ is important as an analog for the ultrahigh-pressure behavior of Mg₂SiO₄, a major component of planetary interiors. In this study, we have investigated magnesium germanate to 275 GPa and over 2,000 K using a laser-heated diamond anvil cell combined with in situ synchrotron X-ray diffraction and density functional theory (DFT) computations. The experimental results are consistent with the formation of a phase with disordered Mg and Ge, in which germanium adopts eightfold coordination with oxygen: the cubic, Th₃P₄-type structure. DFT computations suggest partial Mg-Ge order, resulting in a tetragonal I4¯2d structure indistinguishable from I4¯3d Th₃P₄ in our experiments. If applicable to silicates, the formation of this highly coordinated and intrinsically disordered phase may have important implications for the interior mineralogy of large, rocky extrasolar planets.