Composition and Surface Properties of the Apennine Bench Formation: Possible Source of the Apollo 15 KREEP Basalts

The Apennine Bench Formation (ABF) is a hilly unit in SE Imbrium Basin with moderate albedo that is interpreted as a volcanic unit [1-4]. Due to occurrence near, or at, the Apollo 15 site, it has been argued that the ABF is represented in the Apollo 15 samples in the form of KREEP basalts [e.g. 2-4]. This work will interpret the surface properties and composition of the ABF within the context of the surrounding geologic units in order to further study KREEP basalts on the lunar surface and their link to the Apollo 15 site and samples. This work synthesizes data from orbital lunar missions (i.e., Clementine, Lunar Prospector, Lunar Reconnaissance Orbiter, and Kaguya) to approach studying the ABF with a variety of instrumentation and the highest resolution data available. Analyses include bulk chemistry, topography, terrain ruggedness index (TRI, a unitless measure of surface topographic heterogeneity), and mineralogy.

This work finds that the ABF is readily distinguished from nearby mare basalts and crater ejecta units when considering both surface properties of the ABF and bulk chemistry. For example, the ABF is low in estimated FeO (12.3 wt.%) compared to nearby mare (16.9 wt.%), and boundaries are typically sharp. However, the ABF FeO concentration is indistinguishable from the surrounding ejecta, primarily from Archimedes crater, (12.4 wt.% FeO). Additionally, the TRI is useful in distinguishing crater ejecta and ABF, as ejecta deposits are significantly more rugged than ABF (TRI of 1115 vs. 300 respectively). TRI is also used to distinguish between ABF and mare, as mare units are consistently less rugged (TRI averages 130) than ABF.

Chemically, the ABF is similar to KREEP basalt samples [3,4]. The ABF has an average FeO of 12.3 wt.% (range = 9.9 - 13.9 wt.%), similar to typical KREEP basalt (average = 11.7 ± 2.5 [6]). Additionally, the ABF is a Th hot-spot, with Th values reaching over 12 ppm (deconvolution done by [5]), again similar to KREEP basalts. Volcanic landforms (i.e. vents) on the ABF, located using LROC NAC imagery, support a volcanic origin for the ABF.

[1] Hackman 1966. [2] Hawke B. R., and Head. J. W. (1978) 9^th LPSC, 3285-3309. [3] Blewett D. T., and Hawke B. R. (2001) MAPS 36, 701-730. [4] Taylor G. J., et al. (2012) MAPS 47, 861-879. [5] Lawrence D. J., et al. (1999) GRL 26, 2681-2684. [6] New Views of the Moon (2009) Ed. B. L. Jolliff.