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Baker, D.R., 1985

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Baker, D.R., 1985, The compositions of melts coexisting with plagioclase, olivine, augite, orthopyroxene, and pigeonite at pressures from one atmosphere to 20 kbar and application to petrogenesis in intraoceanic island arcs: University Park, Pennsylvania, Pennsylvania State University, Ph.D. dissertation, 239 p., illust.


The anhydrous phase relations of a suite of high-alumina basalts and andesites were determined at pressures from one atmosphere to 20 kbar. The compositions of melts coexisting with plagioclase and various mafic phases were determined at one atmosphere and 8 kbar; additionally, a few melts that coexisted with garnet were analyzed at (TURN)18 kbar. A series of water-undersaturated experiments was performed at 2 and 5 kbar to determine the effect of 2 percent water on the phase relations and the melt compositions. The melt compositions were projected into pseudoternary diagrams in order to visualize the phase relations and to compare the experimental results to natural rock suites from the Aleutian island of Atka, other volcanic centers in the Aleutians, and volcanic centers from other island arcs. Comparison of the anhydrous experimental melt compositions and the natural rocks from the island of Atka indicate that crystal fractionation under anhydrous conditions cannot account for the rock suite found at this volcanic center. Although complicated by iron loss the 2 kbar, 2 weight percent H2)O, experiments produce the same trend on pseudoternary diagrams as observed for the low-SiO2 ((TURN)56 percent) to high-SiO2) ((TURN)63 percent) andesite trend. It therefore appears that the andesites of Atka, and of other Aleutian volcanic centers, can be produced from high-alumina basalts by crystal fractionation at pressures near 2 kbar and water contents near 2 percent. The production of evolved high-alumina basalts ((TURN)53 percent SiO2) from more primitive high-alumina basalts ((TURN)49 percent SiO2) is not well constrained by the experiments. It is suggested that most evolved high-alumina basalts were produced from the more primitive high-alumina basalts by crystal fractionation at pressures near 8 kbar in the presence of less than (TURN)2 weight percent water. These results suggest the presence of both deep magma chambers and shallow magma chambers beneath the Aleutian volcanic centers. The magma chambers appear to form at pressures that correspond to the depth of the boundary between the upper and lower crust and to the depth of the Moho.

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