Phase relations and partial melt compositions have been determined for a suite of high-alumina basalts and andesites from the Aleutian island of Atka and from the Mariana islands. Experimental conditions varied from anhydrous, at pressures from 1 atm to 8 kbar, to hydrous (2% H2O in the melt) at 2 and 5 kbar, although not all compositions were studied at all conditions.

Partial melt compositions at 1 atm, projected onto pseudotemary phase diagrams, define phase fields for plagioclase, augite, olivine, and orthopyroxene. Hydrous partial melts, 2 wt% H2O in the melt, at 2 and 5 kbar define equivalent phase fields. At all those conditions, the four phases coexist with melts of andesitic composition, and olivine reacts with those melts to produce orthopyroxene. Anhydrous partial melt compositions at 8 kbar, however, define phase fields for plagioclase, olivine, augite, and pigeonite. Phase compositions indicate the presence of thermal divides on both the plagioclase-olivine-augite and plagio-clase-augite-pigeonite phase boundaries.

Crystallization fractionation at deep-crustal pressures of 8 kbar can only account for a portion of Atka basalt compositions, but it cannot account for the total array of Atka rock compositions, because of the presence of a thermal divide involving pigeonite cyrstalli-zation. Even though the olivine reaction relation has been demonstrated to occur, lower-pressure anhydrous fractionation cannot account for andesitic and dacitic melts. In reality, all Atka rock compositions are considerably more aluminous (more plagioclase-rich on pseudotemary diagrams) and less Fe-enriched than low- or high-pressure anhydrous multiply saturated melts. That mismatch can be alleviated, at least for andesites and dacites, on the hypothesis that andesite and dacite melts containing about 2% H2O have evolved from basaltic andesite parents in crustal magma chambers (2–5-kbar pressure). The array of Atka basalt to andesitic basalt compositions is also consistent with crystallization fractionation at 5-8–kbar pressures and low H2O contents, although experimental details are currently lacking.

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