On August 25, 2012, approximately 1100 mountain daylight time, a Cessna 140, N140XX, registered to the student pilot, sustained substantial damage during a forced landing after a partial loss of engine power in the vicinity of the Black Hills Airport - Clyde Ice Field (SPF), Spearfish, South Dakota. The student pilot, who was on a cross country solo flight, was not injured. Visual meteorological conditions prevailed in the vicinity and a flight plan was not filed. The flight was conducted under the provisions of 14 Code of Federal Regulations Part 91. The final portion of the multi-airport flight originated at 1000 from the Philip Airport (PHP), Philip, South Dakota. The pilot reported that he was nearing the end of his uneventful cross country student solo flight and set up for a downwind at SPF. As he applied carburetor heat, the engine RPM dropped below 1000. Unable to continue to the planned runway, the pilot elected to land the airplane in an adjacent field. During the landing roll, the airplane struck a barb wired fence and nosed over inverted, resulting in substantial damage to the vertical stabilizer and rudder. The pilot exited the airplane uninjured. Post accident examination of the engine revealed foreign object debris (FOD) lodged in the throttle body beneath the carburetor venturi. The debris appeared to be remnants of aluminum tape/foil. Also found was what appeared to be a small piece of exhaust insulation/asbestos wrap within the carburetor heat box. No other anomalies with the engine or airframe were discovered. It is unknown how long the foreign object debris was present below the carburetor venturi. The airplane and engine had operated normally for about 3 flight hours after its last annual inspection, dated May 3, 2012. The tachometer time at the time of the annual inspection was 1,473 hours. The tachometer time at the time of the accident was 1,476 hours.