COBALT AND NICKEL CONCENTRATIONS IN THE KOMATIITE COMPONENT OF APOLLO-16 POLYMICT SAMPLES - REPLY TO KOROTEV,R.L.

We agree with Korotev that the abundances of siderophiles in Apollo 16 breccias are anomalously large but do not accept his hypothesis that they were supplied by the impact of a hitherto unknown type of meteorite possessing a grossly non-chondritic composition. Co, Ni, and other siderophiles in Apollo 16 breccias display systematic relationships which imply that a substantial proportion of these siderophiles were present as oxidised species in the lunar crust prior to the impacts of chondritic-type meteorites with the lunar surface. These pre-existing ( = indigenous) lunar siderophiles were reduced to the metallic state, to varying degrees, during impact bombardment and became alloyed with (chondritic) meteoritic metal. In this manner, the metallic particles of complex and unusual compositions now present at the A16 site were formed. The indigenous siderophiles present in the A16 crust prior to meteoritic bombardment may themselves have been derived from two distinct sources, one endogenous and the other exogenous. The first corresponds to basic and ultrabasic magmas formed by partial melting of the Moon's deep interior which were erupted and then and intimately mixed into the lunar crust. The latter represents basic and ultrabasic material ejected from the Earth's primitive crust by early meteorite bombardment, which then fell upon the lunar surface. The siderophiles present in each of these components display "terrestrial-like" abundance patterns because they are both ultimately of terrestrial origin. The anomalously high abundances of siderophiles at the A16 site may reflect an important contribution from ejected terrestrial material. Overall, the siderophile abundance patterns present in lunar highland breccias represent mixtures of terrestrial and chondritic components. © 1990.