Composition of the Cayley formation at Apollo 16 as inferred from impact melt splashes
Authors: Morris Richard V. , See Thomas H. , Hörz Friedrich 
Published: February 2008

Chemical data for impact melt splashes (IMS) collected at the Apollo 16 landing site show that Ni concentrations and FeO‐Ni, Cr‐Ni, and Co‐Ni correlations are consistent with a chondritic meteoritic component present at levels between approximately 1.5 and 11 wt %. Like Apollo 16 soils, the IMS are intermediate in composition to Apollo 16 rocks. The chemical data for the IMS are consistent with two main compositional groups. The Group A IMS (28 samples) are chemically distinct from local Apollo 16 soils and are very similar to mixtures of anorthosite and low‐Sc VHA (LScVHA) impact melt rocks. Calculated to a meteorite‐free basis, their average Sc concentration, La/Sc, MgO/Sc, and TiO2/MgO ratios, and mg′ value are 6.1 ug/g, 1.94, 9800, 0.066, and 0.74, respectively. The Group B IMS (seven samples) have chemical equivalents among the soils. Five samples are chemically very similar to mature Apollo 16 soils from the central and southern parts of the landing site; two are more similar to soils 67601 and 67941 from North Ray Crater. Generally, the Group B IMS and soils are enriched in a composition similar to anorthositic norite (67215) or mixtures thereof with anorthosite as compared to the Group A IMS. The average Sc concentration, La/Sc, MgO, and TiO2/MgO ratios, and mg′ value for the subgroup of five are 9.3 ug/g, 1.41, 5780, 0.124, and 0.66, respectively. The corresponding values for the subgroup of two are 7.5 ug/g, 1.04, 7400, 0.090, and 0.71, respectively. The Group B IMS (and most soils) have a minor high‐Ti component not present in the Group A IMS. If this component is high‐Ti mare basalt, its average concentration is approximately 1.6 wt %. The seven Group A IMS that have been dated by rare gas methods all have approximately 2 m.y. exposure ages, an age generally associated with South Ray Crater. These seven and potentially most Group A IMS are likely quenched samples of the melt volume produced by the South Ray impact. The chemical signature of the impacting projectile is chondritic, and its average concentration for the Group A IMS is 7.3 wt %. Like large, coherent melt sheets from terrestrial impacts, the impact melt volume of the South Ray impact as represented by the Group A IMS is homogeneous with respect to target lithologies. Target lithologies at South Ray Crater, which are represented by the host rocks for the IMS, include anorthosite, LScVHA impact melt rocks, HScVHA impact melt rocks, and dimict breccias. With the exception of 61016, the melt portion of analyzed dimict breccias has the HScVHA composition, implying that the dimensional scales of LScVHA‐type of rocks and anorthosite in the South Ray target do not extend to the 1–10 cm scale observed for the HScVHA dimict breccias. The compositional difference between Group A IMS and surface soils from the southern part of the landing site implies stratigraphic variation in the dominant type of VHA impact melt rock within the Cayley Formation. Chemical data and mixing‐model results are consistent with a model in which HScVHA rocks dominate LScVHA rocks in the surface layer. The LScVHA rocks dominate HScVHA rocks in the layer beneath. The South Ray impact penetrated both layers, as both types of VHA impact melt rocks are traceable as ejecta material. Its impact melt volume was formed primarily from the lower layer to give the observed composition of the Group A IMS. That the North Ray impact also sampled the lower layer is indicated by the chemical affinities of soils 67601 and 67941 with the LScVHA composition. Away from North Ray Crater, most soils have chemical affinities with the HScVHA composition, reflecting soil development primarily from the surface layer. Estimated average compositions give TiO2, MgO, Sc, and La concentrations of 0.69 and 7.1 wt % and 10.5 and 21.2 ug/g, respectively, for the surface Cayley layer and 0.38 and 5.9 wt % and 6.1 and 11.8 ug/g, respectively, for the subsurface Cayley layer. An average composition estimated for the Descartes Formation from soil compositions has TiO2, MgO, Sc, and La concentrations of 0.25 and 3.5 wt % and 7.7 and 2.2 ug/g, respectively. The Cayley Formation is more KREEPy, more magnesian, and less aluminous than the Descartes Formation.