To add to ceviche's post....gov't online service where one can request a copy of the marriage certificate with annotation of divorce decree.
http://www.buenosaires.gob.ar/tramites/solicitud-de-partidas
To request online:
....The certificate being requested must be registered in the city of BA. Said registry cannot have been done during the course of the current year (2017). If you have the necessary data (tome, act, year, section) the request can be done without a search. If you don't have said data, an additional fee must be paid. If the process is done online, it must be paid online....