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.