In my polar wander theory this area had a pole at one stage. So the underground ice would be residual from that pole. The pole would have accumulated dust and ice, and when the pole moved on the ice would have sublimated and the dust covered the ground. This dust need not have come into contact with water if at the time Mars had low air pressure.

The pole may have moved to this position though when the air pressure was higher, or Olympus Mons may have been active. The Mons may have heated the edge of the pole and outgassed CO2 keeping the air pressure up and allowing ice on the edge of the pole to make the various water channels in Lucus Planum, Gusev, and west of Tharsis. The pole would also account for the glacial signs on Olympus Mons and Tharsis.

If Olympus Mons had gone colder the pole when it started moving, the ice would have sublimated directly to vapor not altering the olivine. The pole then moved to north west of Olympus Mons and then to Alba Patera.

The mix of olivine and water channels was an expected part of the model, which is good so far. The other rover lands in Meridiani which would be the opposite pole so it should find the same conditions. That pole is not near a volcano and so wouldn't have the water channels, and may have less salts. It does have the hematite and that may imply when the pole moved to both these positions the air pressure was higher, and water was melting off the pole cap.

The interesting thing about this theory is it makes very specific predictions, which fingers crossed may pan out. Isidis would have been one of the oldest and driest parts of Mars and probably would have been bad for astrobiology.