As part of the trade, the Jets are also sending a seventh-round pick in 2019 and a fourth-round pick in 2020 to Montreal. In return, Montreal is sending AHL defenceman Simon Bourque to the Jets.
Mason, 30, is coming off a disappointing first season with the Jets, who signed him to a two-year contract last summer that carries a $4.1 million cap hit. Mason was expected to be the starter in Winnipeg last season, but a slow start gave Connor Hellebuyck an opportunity and he took it. By the end of the season, Hellebuyck was a Vezina Trophy finalist and Mason was left to be the backup.
Mason also missed some time with concussions, which limited him to only 13 games with the Jets. He posted a 5-6-1 record with a 3.24 goals-against average and .906 save percentage.
Shortly after the trade was announced, the Canadiens placed Mason on waivers for the purpose of a buyout.
Armia, 25, is a winger who was originally a first-round pick by the Buffalo Sabres and was acquired by Winnipeg as part of the Evander Kane trade. In 180 NHL games he has 26 goals and 58 points.
Bourque, 21, was a sixth-round pick by the Canadiens in 2015. In his first AHL season last year, Bourque had three assists in 46 games.