ACR LGT ENCOUNTERED POSSIBLE FUEL CONTAMINATION IN ONE ENGINE AND DECLARED EMERGENCY FOR EXPEDITIOUS LNDG.

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ACFT SAT ON RAMP AT DSM OVERNIGHT WITH TEMP GETTING DOWN TO -6 DEGS F. ACFT WAS DEICED 30 MINS PRIOR TO SCHEDULED DEP TO REMOVE FROST. IT STARTED SNOWING JUST AFTER DEICING; THEN ATC DELAYED OUR DEP 35 MINS DUE TO FLOW CTL INTO ORD. I HAD ACFT DEICED A SECOND TIME PRIOR TO TKOF AT XA29. CRUISE ALT FOR THE LEG TO ORD WAS FL230. SHORTLY AFTER PASSING DBQ VOR THE F/E RPTED HE HAD A FUEL ICING WARNING LIGHT ON #3. FUEL TEMP WAS APPROX -6 DEGS C. HE APPLIED FUEL HEAT BUT WARNING LIGHT REMAINED ILLUMINATED. WE KNOW FUEL HEAT WAS OPERATING NORMALLY BECAUSE WE GOT THE RISE IN #3 OIL TEMP. FUEL HEAT WAS LEFT ON FOR REST OF FLT BUT WARNING LIGHT NEVER DID GO OUT. IT DID FLICKER SOME DURING DSCNT FOR LNDG. ICING OF THE FUEL DIDN'T SEEM TO ME TO BE THE PROB SINCE THE FUEL HEAT DIDN'T PUT THE WARNING LIGHT OUT. I SUSPECTED THE FUEL MAY BE CONTAMINATED. AFTER REVIEWING THE LGT OPERATING MANUAL I THOUGHT IT SAID IF THE LIGHT DIDN'T GO OUT 'LAND AT THE NEAREST SUITABLE ARPT.' UPON LATER REVIEW (AFTER LNDG) THE BOOK REQUIRES THAT ACTION IF 2 ICING LIGHTS ARE ILLUMINATED. ANYWAY; I DIDN'T KNOW THE EXTENT OF THE SUSPECTED FUEL CONTAMINATION PROB SO EXERCISED MY EMER AUTHORITY AND REQUESTED PRIORITY HANDLING FROM CHI CENTER. I DECLARED AN ACFT EMER TO GET THE ACFT ON THE GND ASAP. A VECTOR DIRECT TO THE O'HARE ARPT WAS RECEIVED AND A VISUAL APCH TO RWY 27L WAS ACCOMPLISHED. AN UNEVENTFUL LNDG WAS MADE AND TAXI TO THE GATE WAS NORMAL.