Timothy Demko wrote:


If you look through the older Mars literature, the origin of soil
minerals (including sulfur minerals) seen at the Viking and Pathfinder
sites if often related to something called "palagonitization". This a
garbage can term for various geochemical processes involved in the
weathering and divitrification of basaltic glass (palagonite is an
amorphous to poorly crystalline, almost clay-like substance).
Palagonitization does involve hydration and it is common in hydrothermal
areas where hot thermal waters are circulating through volcanic and
volcaniclastic rocks.

If we read between lines then, it would seem that the MER project
scientists are leaning towards a palagonitized, volcaniclastic
(re-sedimented tuff/ash) origin for the bedrock, and a subsequent
diagenetic/hydrothermal alteration origin for the sulfur anomaly and
hematite-rich spherules.

Note that these processes may all have been related to the same event of
series of events. Maybe something like: 1) rise of magma through the
mantle and crust of Mars; 2) eruption of vents, tuff rings, and maybe
maar volcanoes; 3) melting of permafrost due to higher crustal heat flow
and creation of standing water surface water (lakes, ponds, streams); 4)
deposition of glass-rich, basaltic tuff in lakes and ponds; 5)reworking
of tuff by lacustrine shoreline and fluvial processes; 6) hydrothermal
alteration (palagonitization) of tuffs; 7) leaching of iron-rich
minerals in reworked tuff and re-precipitation as hematite in
spherules/concretions/nodules; 8) heat flow subsides, everything dries
out, winds return and erode lake bottom, various impacts; 9) Opportunity
lands (bunny comes to investigate); 10) speculation ensues.

--
Tim Demko
http://www.d.umn.edu/~tdemko

The description above reads like a textbook description of the formation
of the St. Francois Mountains-- in the anorogenic rhyolite-granite
terrane of the US Midwest. Hence the reason I posted the photos under
the "Earth rock Mars rock" geology thread. Include caldera rise and
collapse, and by jove, you've got it.
Mars would be expected to be slightly different, assuming that it has
always had a high CO2 atmosphere, since atmospheric effects on all this
would be different. The analogy to Yellowstone some have drawn is
incomplete, since much of the rhyolite there is too recent to have been
extensively reworked.
Jo