Natalia C. Del Cogliano has a very interesting post at The Monkey Cage about Argentina’s “Open, Simultaneous and Compulsory Primaries.”
Voters were able to select presidential candidates from across party lines, hence the “open” and “simultaneous” parts. However, this was not much of a primary: each party had only one presidential (pre-)candidate! In other words, this turned out to be nothing more than an early dry-run of the general election. (President Cristina F. de Kirchner won just over half the votes cast, with no other candidate even close.)
For other offices besides the presidency, there was intra-party competition.
As for all Argentine elections, voting was obligatory.