Well, if this is the same 2006 Mariner you just bought on C/L for a song, then the frame is at least 4130 chromoly and weldable .... from the pictures (might be the angle) there seems to be quite a bit of 'pinching' at the top of the slot .. does it have a plastic or aluminum seat post sleeve? I'd pull the seatpost and sleeve out and see if the crack is all the way through to the inside ... you might be able fill and grind the crack early..