Currently preferred versions of the "giant impact" hypothesis of lunar origin imply that the Moon was derived mainly from the mantle of a giant (Martian-sized) planetesimal which struck the Earth. This hypothesis also implies that the depletions of Cr, V, and Mn which are observed in the Moon were inherited from the mantle of the impactor. Experiments have been undertaken to determine whether the formation of an iron core within a differentiated giant planetesimal could have caused depletions of Cr, V, and Mn in the planetesimal mantle, owing to siderophile behavior of these elements during core formation. Partition coefficients of Cr, V, and Mn between metallic iron and the principal mineral phases present in the mantle of a giant planetesimal have been determined at 1500-2000°C and at 3-25 GPa using an MA-8 apparatus. Cr, V, and Mn were found to remain lithophile (D silicate/metal >1) under these conditions. It follows that the formation of an iron core within a giant planetesimal would not have caused any depletion of Cr, V, and Mn in its mantle. Depletions of Cr and V (relative to Mg) cannot be attributed to selective volatilization in the solar nebula, prior to planetesimal and planet formation because V is less volatile than Mg, whilst the condensation temperature of Cr is similar to that of Mg and higher than that of Si. Accordingly, it is concluded that the depletions of Cr and V (and probably Mn) in the Moon were not inherited from the mantle of a giant planetesimal. Cr, V, and Mn are depleted (relative to Mg) in the Earth's mantle and the terrestrial depletions for these elements are similar to the lunar depletion factor. This similarity suggests that protolunar material was derived mainly from the Earth's mantle. Depletions of Cr and V (and perhaps Mn) in the terrestrial mantle are believed to have been connected with core-formation processes within the Earth which occurred at much higher pressures and temperatures than those prevailing during core formation in giant planetesimals.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science