d upward curvature y exposed LRM and ixtures of HRP with he assumpt coarse-grain ld require T dances too high to be consistent with MESSENGER X-Ray Spectrometer measurements. Three or mixtures of phases that could be responsible for the low reflectance of LRM are consistent w analyses. Graphite, in amounts consistent with upper limits from the Gamma-Ray Spectromet be consistent with geochemical models of Mercury’s differentiation calling for a graphite-enriched pri-mary flotation crust from an early magma ocean and impact mixing of that early crust before or during the late heavy bombardment (LHB) into material underlying the volcanic plains. The grain size of preexisting iron or iron sulfide could have been altered to a mix of nanophase and microphase grains

co ow udfl the cha anic volc nel e co fo ter nical and thermal processes involving lavaflowswith depths of 5–20 m and dynam-omina s effusi rsky, 1

was covered with several different plains deposits. The earliest geological interpretation of images from the first M10 flyby (Murray et al., 1974) included a regional map showing three dis-tinct geologic units: plains material, hilly and lineated terrain, and heavily cratered terrain. The intercrater plains unit shortly cratered surfaces uest, 1975; Trask,