Volatile loss following cooling and accretion of the Moon revealed by chromium isotopes

Paolo A. Sossi*, Frédéric Moynier, Kirsten Van Zuilen

*Corresponding author for this work

Research output: Contribution to JournalArticleAcademicpeer-review

Abstract

Terrestrial and lunar rocks share chemical and isotopic similarities in refractory elements, suggestive of a common precursor. By contrast, the marked depletion of volatile elements in lunar rocks together with their enrichment in heavy isotopes compared with Earth's mantle suggests that the Moon underwent evaporative loss of volatiles. However, whether equilibrium prevailed during evaporation and, if so, at what conditions (temperature, pressure, and oxygen fugacity) remain unconstrained. Chromium may shed light on this question, as it has several thermodynamically stable, oxidized gas species that can distinguish between kinetic and equilibrium regimes. Here, we present high-precision Cr isotope measurements in terrestrial and lunar rocks that reveal an enrichment in the lighter isotopes of Cr in the Moon compared with Earth's mantle by 100 ± 40 ppm per atomic mass unit. This observation is consistent with Cr partitioning into an oxygen-rich vapor phase in equilibrium with the proto-Moon, thereby stabilizing the CrO2 species that is isotopically heavy compared with CrO in a lunar melt. Temperatures of 1,600-1,800 K and oxygen fugacities near the fayalite-magnetite-quartz buffer are required to explain the elemental and isotopic difference of Cr between Earth's mantle and the Moon. These temperatures are far lower than modeled in the aftermath of a giant impact, implying that volatile loss did not occur contemporaneously with impact but following cooling and accretion of the Moon.

Original languageEnglish
Pages (from-to)10920-10925
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Volume115
Issue number43
Early online date8 Oct 2018
DOIs
Publication statusPublished - 23 Oct 2018

    Fingerprint

Keywords

  • Chromium
  • Equilibrium
  • Evaporation
  • Low temperature
  • Moon

Cite this