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Mariah Carey has a brand new piece of jewelry -- a massive engagement ring -- 'cause her billionaire boyfriend James Packer just popped the question ... and Mariah said YESSSSS!!!!

It all went down in NYC -- where Packer secretly rounded up a bunch of Mariah's close friends for a private dinner at Eleven Madison Park ... so he could get down on one knee and propose in front of the most special people in her life.

Carey and Packer have only been dating since June 2015 -- but their relationship heated up real quick ... with the two famously spending several consecutive weeks together on his yacht over the summer.

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The relationship blossomed months after Carey split from husband numero dos, Nick Cannon ... the father of her two children.

Carey had been married once before Cannon to music mogul Tommy Mottola ... so this will be her 3rd marriage.

As for Packer, the 48-year-old has also been married twice before -- and has 3 children of his own ... so, this is kind of like a celebrity 'Brady Bunch' situation ... with a lot more money.

Packer is Australia's 4th richest man -- thanks to a combination of family money and success in real estate, casinos and entertainment.