The pair, who married in 2008 following a whirlwind romance, have three year old twins Moroccan and Monroe together and are facing the prospect of a custody battle.
If they do make the split permanent, Mariah, 44, is expected to base herself in New York, while Nick, 33, wants to be in Los Angeles.
The source added: "She wants to be in NYC full-time, but he's been wanting to spend more and more time in L.A.
"If and when they split [for good], she intends to stay in NYC with the kids. She will fight for the kids. She is totally focused on them."