Way to early to rule out carbonates or sedementary rocks, all one need do is read:

Spectroscopic Identification of Carbonates in the Martian Dust
Bandfield, J. L.; Glotch, T. D.; Christensen, P. R.
34th Annual Lunar and Planetary Science Conference, March 17-21, 2003, League City, Texas

The spectral signature of surface dust isolated from the MGS-TES instrument indicates that a small amount of carbonate is present within a fine-particulate silicate matrix.

"Alternatively, the carbonate present in the Martian dust may also have
been widely distributed in the source lithology from
which the dust was created. Several Martian meteorites
have small, disseminated concentrations of Mg,
Ca, and Fe carbonate that formed by precipitation from
groundwater in fractures. A concentrated source may also have been present and mechanically weathered and mixed with the other
components of the dust. Extensive searches with TES
and other spectroscopic measurements indicate that
high concentrations of carbonates are not widely exposed
on the Martian surface at the ~10 km scale [16].
A putative source region may have been subsequently
buried however, and not detected."

http://www.lpi.usra.edu/meetings/lpsc2003/pdf/1723.pdf