Figure 3. Competing models for input-output studies. A. The grass control model indicates that the kind of milk (dark vs. light) is controlled by the kind of grass (dark vs. light) that the cow eats. B. In the cow control model the breed of the cow is the important control in determining the kind of milk produced. Different cows (Guernsey vs. Jersey) eating the same kind of grass may still produce different kinds of milk (dark vs. light). These models are directly analogous to subduction recycling studies, where the grass is the subducted input (sediments and oceanic crust), the milk is the volcanic arc output and the cow is the subduction zone.

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