The Feo, Nio and Coo Contents of Solar System Basalts and Their Significance for Core Formation in Planetary Bodies

Introduction: In Table 1 we have compiled the chemical compositions, including Ni and Co, of representative basalts from Earth, Moon, Mars and Vesta. The SiO2 and MgO concentrations of all basalts cover only a small compositional range, suggesting a similar extent of fractionation. The Ni and Co contents increase with the radius of the planet, with very low concentrations in Vesta and increasing through Moon and Mars to the high concentrations in the terrestrial basalts. The concentrations of FeO, CoO and NiO in solar system basalts reflect the combined effect of two processes, early metal-silicate equilibration during core formation and fractionation during basalt formation. Metal-silicate fractionation depends on pressure, temperature and oxygen fugacity. The effect of oxygen fugacity can be eliminated by normalizing Co and Ni to Fe, as all three elements have exactly the same dependence on oxygen fugacity [1]. Lunar and Vesta basalts (eucrites) have about twice the FeO content of terrestrial and Venusian basalts (not shown in Table 1), which must have formed at more reducing conditions.