Circum-Pacific andesite types

If geophysical evidence and geochemical inference that andesites are erupted from the mantle as juvenile sial are valid, areal variations among andesitic suites may reflect variable conditions within the mantle. Characterization and comparison of different andesitic suites as an array of lines with varying position and slope is successful with the use of a simple plot of the potash-versus-silica variation. Relative potash contents of andesitic suites rise away from trenches across island arcs in a transverse direction and also vary from arc to arc. Intra-oceanic arcs typically erupt lavas of lower potash content than marginal continental ranges do, but some arcs that pass longitudinally from oceanic to continental settings display no change in the levels of potash content. The potash content of arc lavas is closely correlated with the depth from the volcanos to the inclined seismic zones within the mantle beneath the arcs. The slope of the potash-versus-depth curve apparently predicts the trend of the over-all distribution coefficient for potash between solid and liquid during partial melting or fractional crystallization at varying pressures within the mantle. If the extraction of andesite magmas depletes the mantle in potash, patterns of andesitic volcanism in time and space may reflect an irreversible geochemical evolution of the mantle related to fundamental tectonic conditions.