Pour Champagne into each compartment of 2 ice-cube trays, and freeze until almost set, at least 6 hours. (Cubes will not be completely firm.)
Combine seltzer water, tangerine juice, and liqueur in a pitcher, stirring to combine. Divide Champagne ice cubes among 6 cocktail glasses, and top with tangerine juice mixture. Garnish with tangerine slices, if desired.