Molybdenum in eucrites: Evidence for a metal core in the eucrite parent body

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Abstract
Molybdenum concentrations from 15 to 70 ppb in eucrites have been determined with a new method of neutron activation. These values are very low compared to chondrites and are in agreement with the large depletions of other siderophile elements in eucrites. The eucrites have a mean Mo/La ratio of 0.0066, which is a factor of 570 lower than the CI chondritic ratio (3.75). The depletions of siderophiles in eucrites were presumably caused by segregation of metal. Molybdenum, however, is fractionated as an incompatible element, similar to La, among the individual eucrites. The variable concentrations, but roughly constant ratio, of Mo to La indicate that metal was absent during the fractionation that produced the eucrites and imply that core formation occurred before the fractionation. The need for a relatively large degree of partial melting to completely segregate metal argues for a fractional crystallization model for the production of the eucrite magmas. The amount of metal in the Eucrite Parent Body (EPB) required to obtain the observed depletion of the Mo/La ratio depends on the degree of partial melting during metal‐silicate equilibrium. The depletion of Co has a dependence on the degree of partial melting different from Mo and W, and using Co together with Mo and W the metal content and degree of partial melting can be estimated. A metal content of 30% to 50% and metal segregation at approximately 50% partial melting of the silicates is consistent with the data. If the EPB had a ratio of Co to refractory lithophile elements less than CI chondrites, a metal content as low as 20%, with metal segregation at 20% partial melting, is possible. A large metal core in the EPB (Vesta?) may therefore be detectable from a spacecraft.