We　investigate　the　geometric　phase　for　a　quantum　field　driven　by　an　external　source　and　subjected　to　spontaneous　decay　from　coupling　to　the　reservoir.　Starting　from　a　coherent　state,　we　analyze　the　effect　of　decoherence　when　the　system　undergoes　a　nonadiabatic　cyclic　evolution　in　phase　space,　and　show　that　the　lowest　correction　to　the　conventional　geometric　phase　is　quadratic　in　the　decaying　rate　of　the　quantum　field.　This　is　in　distinct　contrast　with　the　nonadiabatic　geometric　phase　associated　with　the　evolution　of　a　spin　system　subjected　to　spontaneous　decay,　which　contains　the　first-order　correction　term.　We　further　show　that　the　unconventional　geometric　phase　is　also　robust　to　field　decay.　©2015　American　Physical　Society.