For sticking two lumps of worktop together, I've used "cascamite" in the past which is a waterproof and water based resin adhesive (used in boat building and timber laminating). You mix it from powder and it's very strong and forms an effective seal to the join - but unlike resin products such as Araldite, it can be wiped off the surface more readily when it oozes out.
I just ripped my kitchen out before christmas after 15 years - the laminate has never lifted around the two joints, and the joints were so strong I couldn't break them with a lump hammer, nor split them with a hammer/chisel and I ended up having to cut the worktop out in situ.
I suppose the question here is WHY the joint has come apart and that suggests movement. So if that isn't dealt with (e.g. with carcase levelling, wall support, repair plates underneath etc.), a little bit of cascamite forced into the seam may still crack, and silicone (as suggested by John) may also pull away and let moisture in again. Both would probably do the job otherwise given it's only a small section.
Midge