COMPOSITION AND MATURITY OF APOLLO 16 REGOLITH CORE 60013 14/

Samples from every half-centimeter dissection interval of double drive tube 60013/14 (sections 60013 and 60014) were analyzed by magnetic te chniques for Fe-degrees concentration and surface maturity parameter I (s)/FeO, and by neutron activation for concentrations of 25 lithophile and siderophile elements. Core 60013/14 is one of three regolith core s taken in a triangular array 40-50 m apart on the Cayley plains durin g Apollo 16 mission to the Moon. The core can be divided into three zo nes based both on I(s)/FeO and composition. Unit A (0-44 cm depth) is compositionally similar to other soils from the surface of the central region of the site and is mature throughout, although maturity decrea ses with depth. Unit B (44-59 cm) is submature and compositionally mor e feldspathic than Unit A. Regions of lowest maturity in Unit B are ch aracterized by lower Sm/Sc ratios than any soil obtained from the Cayl ey plains as a result of some unidentified lithologic component with l ow surface maturity. The component is probably some type of mafic anor thosite that does not occur in such high abundance in any of the other returned soils. Unit C (59-62 cm) is more mature than Unit B and comp ositionally equivalent to an 87: 13 mixture of soil such as that from Unit A and plagioclase such as found in ferroan anorthosite. Similar s oils, but containing greater abundances of anorthosite (plagioclase), are found at depth in the other two cores of the array. These units of immature to submature soil enriched to varying degrees (compared to t he mature surface soil) in ferroan anorthosite consisting of approxima tely 99% plagioclase are the only compositionally distinct subsurface similarities among the three cores. Each of the cores contains other u nits that are compositionally dissimilar to any soil unit in the other two cores. These compositionally distinct units probably derive from local subsurface blocks deposited by the event(s) that formed the Cayl ey plains. The ferroan anorthosite with approximately 99% plagioclase, however, must represent some subsurface lithology that is significant on the scale of tens of meters. The compositional uniformity of the s urface soil (0-10 cm depth) over distances of kilometers reflects the large-scale uniformity of the plains deposits; the fine-structure refl ects small-scale nonuniformity and the inefficiency of the impact-mixi ng process at depths as shallow as even one meter.