The carbonate detected by TES and mini-TES has been attributed to small amounts of carbonate in dust but not to large-scale carbonate deposits.
But look at the images here from the Pathfinder mission:

Space Sciences
re: Huge Moomaw article on the current views of mars
Posted by Robert Clark on 12/18/2003 5:00:27 PM
In Reply to: Huge Moomaw article on the current views of mars posted by alcaray on 12/18/2003 9:49:09 AM
http://habitablezone.com/space/messages/305743.html

Note that though some surfaces are relatively free of dust, there is still some lingering amount of dust even in the relatively clear areas. How can you be sure that when you see "2% carbonate" that what you are seeing is not in fact the portion of the large-scale deposits that are the most free of dust, the rest being obscured by the dust? That is, the obscuring dust contains no carbonate at all. It is just that the amount of dust required to completely obscure detection of carbonate is so small that it is only the 2% of the carbonate surface that is *completely* free of dust that can be detected.
Here's a report that says a layer of dust only 300 microns thick, less than a third of a millimeter, is enough to obscure mineral detection on Mars:

EFFECTS OF PALAGONITIC DUST COATINGS ON THERMAL EMISSION SPECTRA OF ROCKS
AND MINERALS: IMPLICATIONS FOR MINERALOGICAL CHARACTERIZATION OF THE
MARTIAN SURFACE BY MGS-TES.
T. G. Graff1, R. V. Morris2 and P. R. Christensen1, 1Dept. of Geological Sciences, Arizona State University, Tempe, AZ (tgraff@asu.edu), 2SN/NASA Johnson Space Center, Houston, TX.
Lunar and Planetary Science XXXII (2001) 1899.pdf
http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1899.pdf

Robert Clark