Puree the raspberries in a food processor or blender. Strain the puree mixture and set aside. Puree the pears, then add the raspberry puree, yogurt, and maple syrup, and process until thoroughly combined.
Put the mixture in a shallow bowl and freeze overnight. Let the frozen yogurt thaw at room temperature for 30 minutes, or until slightly softened. Scoop the yogurt into 4 tall glasses and add 8 ounces of seltzer to each glass.
