Since about the only data but the images we have is that there is a lot of sulfur in the rock, I'm wondering how much that alone tells us.

If there really is a lot of sulfur here, way more presumably than seen in basalt or in the Martian soil, wouldn't that be pretty unusual for unaltered ash or tuff? What is the highest sulfur content one finds in unaltered volcanic rocks?

My sense is that very high S tends to argue against it simply being ash beds (or impact deposits) with no weathering/alteration, but let me know if that's not a good inference.

Hmm, one broader question that raises is, what will be the behavior of the sulfur that comes up with Martian magmas? On earth a lot of the S goes up as SO2 and H2S which become sulfuric acid and get rained out all over the place, and god knows what pathways the S follows from there.

-- Will martian magmas have as much available oxygen and hydrogen, will the sulfur show up as some part of some other gas, or will it be mostly incorporated in the lava?

-- It might be worth noting that both sulfur dioxide and hydrogen sulfide freeze at temperatures commonly found on Mars (-73C for SO2, -85C for H2S).

On Io, you end up with sulfur dioxide snow around vents. I would assume that solid H2S/SO2 would not be stable on the surface of Mars, but even if you had some falling out and sitting on the surface for a little while, maybe mixing in with ash, I wonder if that would produce something we aren't used to seeing here on Earth.