Abstract

Microprobe analysis of the near liquidus minerals in the crystallization interval of basalt +5 percent H2O and andesite +5 percent H2O at 30 kb, combined with similar published data from other studies of hydrous and anhydrous calc-alkaline rock compositions, defines compositional trends that permit calculation of the compositions of minerals through the crystallization intervals of calc-alkaline liquids. Using the calculated mineral compositions and estimates of mineral proportions, the compositions of successive liquids through the crystallization intervals of hydrous and anhydrous basalt and andesite are calculated. The calculated liquids diverge from the average calc-alkaline trend, more so for hydrous than for anhydrous conditions. These data support the conclusion that magmas produced by partial melting of subducted oceanic crust at 100 km must undergo fractionation or other modifications at shallower depths, which bring them to the calc-alkaline composition trend.

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