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Aims: Ices in dense star-forming cores contain the bulk of volatile moleculesapart from H2 and thus represent a large fraction of dark cloud chemistrybudget.To directly constrain the freeze-out profile of CO, the formation routeof CO2 and the carrier of the 6.8 micron band, the spatial distribution of theCO/CO2 ice system and the 6.8 micron band carrier are measured in a nearbydense core.  Methods: VLT-ISAAC, ISOCAM-CVF and Spitzer-IRS archival mid-infrared (3-20micron) spectroscopy of young stellar objects is used to construct a map of theabundances of CO and CO2 ices in the Oph-F star-forming core, probing coreradii from 2 10^3 to 14 10^3 AU or densities from 5 10^4 to 5 10^5 cm^-3 with aresolution of ~ 3000 AU.  Results: The line-of-sight averaged abundances relative to water ice of bothCO and CO2 ices increase monotonously with decreasing distance to the corecenter. The map traces the shape of the CO abundance profile between freeze-outratios of 5-60% and shows that the CO2 ice abundance increases by a factor of 2as the CO freezes out. It is suggested that this indicates a formation route ofCO2 on a CO ice surface to produce a CO2 component dilute in CO ice, inaddition to a fraction of the CO2 formed at lower densities along with thewater ice mantle. It is predicted that the CO2 bending mode band profile shouldreflect a high CO:CO2 number ratio in the densest parts of dark clouds. Incontrast to CO and CO2, the abundance of the carrier of the 6.8 micron bandremains relatively constant throughout the core. A simple freeze-out model ofthe CO abundance profile is used to estimate the binding energy of CO on a COice surface to 814+/-30 K.Comment: 5 pages, accepted for publication in A&A letter

Spatial mapping of ices in the Ophiuchus-F core