The only reason I mentioned the dark streaks was I had a notion that perhaps CO2 could be related to the formation as a solvent for some DHB gunk bubbling up from the presumably warm nether regions under OlyMon. Periodicly busting it's cap and depositing gunk on the slopes, following the gradient of the slope with minimal evidence of erosion.

Sitting there drinking the LPSC coffee, I remembered that a Chem Prof of mine told me that supercritical CO2 is used as a solvent to decaffinate coffee and could in principle dissolve all sorts of hydrocarbons including organnometalic compounds.
I wonder if THEMIS spectra would be helpful here....

Looking at a CO2 phase diagram it's hard to see how the SC phase could be present near the surface.