The origin of volatile elements in the Earth-Moon system

Lars E Borg; Gregory A Brennecka; Thomas S Kruijer

doi:10.1073/pnas.2115726119

The origin of volatile elements in the Earth-Moon system

Proc Natl Acad Sci U S A. 2022 Feb 22;119(8):e2115726119. doi: 10.1073/pnas.2115726119.

Authors

Lars E Borg¹, Gregory A Brennecka¹, Thomas S Kruijer^{1

2}

Affiliations

¹ Nuclear and Chemical Science Division, Lawrence Livermore National Laboratory, Livermore, CA 94550; borg5@llnl.gov brennecka2@llnl.gov kruijer1@llnl.gov.
² Department of Solar System, Impacts & Meteorites, Museum fur Naturkunde, Berlin 10115, Germany.

Abstract

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.

Keywords: Giant Impact; Moon; Moon-forming impact; volatile elements.