Erosion and aeolian deposition can explain olivine being there. In my theory there was a pole here and the ice underground (not blocks of ice but some percentage of ice remaining in the soil) is from this pole. Material from dust storms including olivine would be deposited in layers on the pole, and when the pole moved on the ice would sublimate and leave the olivine, etc. So a pole would tend to act as a cold trap and accumulate large amounts of dust but not larger particles.

This might also explain the fine grains as if they were originally dust they would be fine, and erosion may have made them finer over time. As the ice sublimated away the grains may have eroded much smaller. The salts may have been leached from the rocks as the pole was moving to there, with water on the pole edge.

In this theory the rocks should be similar to those found in icy areas, like glaciers, but not ground from the movement of ice. There may be some layering but much of it would tend to have fallen apart into dust when the ice and CO2 in the layers sublimated. The rocks probably would predate the soil as they would be from before the pole moved there, and have been broken up partially by the ice and eroded by the wind later.

So the rocks and the soil should be found to be chemically very different according to this theory. The soil if from dust should be very homogenous in terms of the olivine. Salts if from before the pole might accumulate in different concentrations.

I'm not sure of any of this, but the theory I worked out has fairly specific predictions.