Starsizer,
If your magazine tube really broke free from it's silver soldered joint, it is not advisable to rely on a Locktite or other sealant to secure it.
The end of the magazine tube provides an attachment for the barrel, and that magazine tube/receiver joint takes considerable pounding from the cycling of the action, even though the bolt locks into the barrel extension.
Either send it back to Remington for repair or have a good gunsmith properly re-solder it for you. Anything less is unreliable.