I would recommend that each party's project manager have the authority to approve a variation. The scientists should not have that authority. It is usually the scientists who propose variations. It would be inappropriate to give them the authority to approve their own proposals. Besides, scientists are more inclined to go off in tangents, thanks to their ability to think laterally, without worrying about the consequences.
Scientists can be their own worst enemies.
They get so caught up in the excitement of the project. They think of the project as the pursuit of knowledge. They may have little concern for staying within the terms of the project plan. That's why project managers are so vitally important. Without project managers, you may end up with a chaotic situation.