Irene: Tom128 found my original message for me and I modified my message here with the coords.
Thanks DJ. I will have to spend some time looking at that one much more carefully.
Then I'm officially lost. I figured if it wasn't consistent with a shadow it must be dark volcanic matter. Back to the drawing board.
Thanks for the look-see, Geoff.
Just to clarify, there are 2 types of dark-haloed craters.
1) volcanic vents that have dark halos of pyroclastic material that was emplaced around the vent when lava fountained from the vent.
2) impact craters that have excavated dark mare materials from below a layer of light-coloured ejecta. Sometimes the buried mare material is patchy and so the dark halo can end up patchy too. The image I asked about (from the first quote in this post) was of this type.