The origin of the depletions of V, Cr and Mn in the mantles of the Earth and Moon

摘要

The reasons for the similarity of the depleted abundances of V, Cr and Mn in the mantles of the Earth and Moon, which may throw light on the origin of the Moon, have been controversial. In order to test if a single-stage equilibrium core formation model can explain these depletions in the Earth, we investigated the partitioning of V, Cr, Mn, Ni and Co between liquid metal and magnesiowustite, an important liquidus phase of the peridotite mantle at high pressure. The experiments were performed mostly at 2200 ℃, 5-23 GPa and constrained oxygen fugacities. Metal-magnesiowustite partition coefficients for V, Cr and Mn increase very weakly with increasing pressure, indicating only a slight increase in siderophile behaviour, whereas temperature has a relatively large effect on their partitioning. Ni and Co partition coefficients decrease as a function of pressure as previously reported. Because magnesiowustite/silicate melt partition coefficients are close to unity, these results can be applied to metal/silicate partition coefficients. The results demonstrate that high pressure alone during core formation cannot explain the depleted mantle abundances of V, Cr and Mn, in contrast to those of Ni and Co. However, taking all variables into account enables the mantle abundances of V, Cr, Mn, Ni and Co to be explained by metal segregation in a deep magma ocean above ～3300 ℃, 35 GPa and an oxygen fugacity that is consistent with the present FeO content of the Earth's mantle. Such conditions can be reached in a magma ocean on a giant impactor, if it is large enough (i.e. several times the mass of Mars), or on the Earth. Therefore, the Moon is likely to have formed largely from material that was ejected either from the mantle of a large impactor of from the Earth's mantle.