Metal grains in Apollo 12 igneous rocks

Abstract

The Apollo 12 igneous rocks we have examined contain abundant disseminated grains of metallic iron with a wide range of Ni and Co contents. The compositions of these metal grains vary to a degree not found in terrestrial igneous rocks or in meteorites. Ni and Co are generally higher in metal grains enclosed by the earliest-formed minerals (olivine and chromite) and are lower in those associated with later phases. The metal grains do not indicate the former presence of an immiscible metal liquid but have formed by reduction from the silicate melt. Formation of the metal grains accompanied, and may be a direct result of, the crystallization of the major phases under low oxygen partial pressures. The crystallization of chromite, for example, from a melt containing divalent chromium, may have resulted in the reduction of ferrous iron to metal. The presence of nickel-iron grains in lunar igneous rocks indicates that metal grains in lunar soil need not be exclusively of meteoritic origin.