Summary: The petrological interpretation of the variation of K2O content with depth to the Benioff zone in orogenic magmas is complicated by several factors: (1) Magmas may not be true primary liquids but have under-gone differentiation. K2O contents, at a specific SiO2 level, will depend upon the SiO2 content of the parental magma and the natureof the crystallizing phases more than on original K2O content. (2) The depth to the Benioff zone may not give a true reflection of the depth of melting. Magmas may not be derived from the uppermost layer of subducted oceanic crust but rather the overlying upper mantle. Earthquakes may not occur in the uppermost layer of the subducted plate, but in its colder, central parts. Hence, earthquake depth may not be related to the source of the magma. (3) It is difficult to envisage how the temporal increase in K2O may occur in certain areas (e.g. Aleutian I., Fiji) if it is primarily controlled by the depth to the Benioff zone.
There may be some correlation between K2O content of magmas and earthquake depth, but its cause is difficult to determine. None of the existing explanations is entirely satisfactory, and it is suggested that varying degrees of fractional crystallization may play an important role in controlling K2O contents.