None of the existing explanations for increase in potassium in calc-alkalic rocks away from convergent plate boundaries is entirely satisfactory, and a new model is proposed for future testing based on a demonstrable mobility of incompatible constituents in the upper mantle. It is postulated that hydrous fluids derived from the descending oceanic crust migrate upward into the overlying wedge of mantle, thereby scavenging and zone melting potassium and other incompatible elements from peridotite during ascent. Progressively greater amounts of potassium are abstracted from increasingly thicker vertical sections of the mantle wedge. It is immaterial to the model whether the fluids are hydrous silicate melts derived by partial melting of the crust, which subsequently become calc-alkalic magma, or whether they are subsolidus aqueous solutions derived by dehydration of the crust, which eventually are assimilated by partial melts of the mantle and yield calc-alkalic magma.
The scavenging-zone-melting process is an efficient mechanism for irreversible chemical differentiation of the upper mantle.