The nature of the meteoritic components of Apollo 16 soil, as inferred from correlations of iron, cobalt, iridium, and gold with nickel

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Abstract
Concentrations of Co, Ni, Ir, and Au in small samples of submature and mature soil from Apollo 16 are highly variable. Correlations of Fe, Co, Ir, and Au with Ni indicate that the variation in siderophile element concentrations results from variation in the concentration of Fe‐Ni metal. From these correlations, the mean composition of the metal is inferred to be 5.6% Ni and 0.36% Co for a Ni/Co ratio of 15.5 ± 0.7, i.e., dissimilar to the chondritic ratio of 20–21. The Fe‐Ni metal is also characterized by low Ir/Ni and Ir/Au ratios compared to metal in ordinary chondrites. Metal with a similar composition is also found in metal‐rich, noritic impact melt breccias that occur as discrete rocks as well as small clasts in ancient regolith breccias at Apollo 16. Mass‐balance models using lithophile element concentrations indicate that about 35% of the soil is noritic impact melt. The Fe‐Ni metal associated with this component is sufficient to supply virtually all the ancient Fe‐Ni metal found in the typical Apollo 16 soil (0.4–0.5%), i.e., no other source is required. The soil also contains 1–2% carbonaceous chondrite. The concentration of this component appears to be relatively constant from sample to sample and does not contribute significantly to the variation in siderophile element abundances. These two meteoritic components contribute nearly equal concentrations of Ni to the typical Apollo 16 soil.

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