Lunar Highland Rock Types: Their Implications for Impact-Induced Fractionation

The first step in a petrologic study must be a classification based on observed textures and mineralogy. Lunar rocks may be classified into three major groups: (I) coarse-grained igneous rocks, (II) fine-grained igneous rocks, and (III) breccias. Group I is interpreted as primitive lunar crustal rocks that display various degrees of crushing and/or annealing. Group II is interpreted as volcanic rocks. Group III is interpreted as resulting from impacts on the lunar surface and is subdivided on the basis of matrix textures into fragmental breccias, crystalline breccias that have been annealed, and crystalline breccias with igneous matrices.

A synthesis of the relevant data concerning lunar highlands polymict breccias from the fields of petrography, chemistry, photogeology, and impact studies compels the prediction that the breccias should have homogenous matrices from rock to rock within regions of the highlands of limited size where impact mixing has been efficient and extensive. But the returned breccias, even from one landing site, display a wide range in composition. This incompatibility between prediction and observation is a paradox that may be resolved by a process that acts after impact mixing to cause a differentiation of the breccia compositions. Partial melting of the local average crustal composition (as modeled by the average soil composition for each site) and separation of melt and residue in ejecta and/or fall-back blankets are compatible with the reviewed data and may resolve the paradox.