Lavas from volcanoes in the Mariana arc show a range of K2O-SiO2 trends even though the volcanoes are all nearly the same distance above the Benioff Zone. The preferred petrologic explanation for the K2O-SiO2 variations invokes differences in the degree of partial melting required to produce lavas for each volcano from a relatively homogeneous source material. The Mariana arc volcanoes appear to evolve chemically in a manner similar to oceanic volcanoes (e.g., Hawaii) with early and late alkalic stages dominated by an intermediate tholeiitic stage. If volcanoes in other arcs also undergo this evolution, many of the anomalies in K2O-depth relations in modern arcs could be explained.

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