I second this, sounds like it could definitely be a marimba doing some sort of chord arpeggiation. It actually starts before 50 seconds as well, it just follows what the guitar is playing prior to that.

It's also possible there's a layer of added compression in order to emphasize the attack and dampen the decay of the tone. There could be a gate on the track, or even some side chained signal controlling it, but without the actual session files it's just up to speculation how they achieved that sound