Phase compositions through crystallization intervals in basalt-andesite-H_2O at 30 kbar with implications for subduction zone magmas

The melting relationships of a gabbro (olivine tholeiite, quartz eclogite composition) and tonalite (andesite composition) with varying amounts of H_2O added were determined at 30 kbar pressure using piston-cylinder apparatus. With 5 percent H_2O, clinopyroxene and garnet are the liquid us minerals for the basalt, and they occur together throughout the crystallization interval from 1260° to 760°C. With 5 percent H_2O, garnet is the liquidus phase for andesite at 1200°C and it crystallizes alone through 100°C; garnet and clinopyroxene occur together through the rest of the crystallization interval from 1100° to 740°C. Compositions of garnets, clinopyroxenes, and glasses were measured with electron microprobe at the following temperatures: tonalite, 1175° (Ga,Gl), 1100° (Ga,Cpx,Gl), 1000° (Ga,Cpx,Gl), 900° (Cpx,Gl); gabbro, 1200° (Ga,Cpx), 1100°C (Ga,Cpx). Published data by T. H. Green and Ringwood on hydrous and anhydrous calc-alkaline rock compositions, combined with these new data, define compositional trends for minerals through the upper parts of crystallization intervals. From these trends and thermodynamic constraints for mineral pairs, the compositions of minerals through the complete crystallization intervals were calculated. Using calculated mineral compositions and estimated mineral proportions, the compositions of equilibrium liquid paths were calculated through the crystallization intervals of hydrous and anhydrous basalts, basaltic andesite, and andesite. Compositions of quenched liquids in the crystallization interval of andesite with 5 percent H_2O measured with the electron microprobe agree well with calculated compositions, supporting the validity of the calculated liquid paths. Equilibrium liquid paths for basalts and andesites at 30 kbar diverge from the average chemical variation trend of typical calc-alkaline rocks (basalt-andesite-dacite-rhyolite). With decreasing temperature and increasing SiO_2 content, the liquid paths increase in Ca/(Mg+ Fe) compared with the average trend, more so for hydrous compositions than for anhydrous. With equilibrium partial fusion of quartz eclogite, the first liquids are richer in Si02 than average andesites, significantly so if H_2O is present. Liquids with SiO_2 content corresponding to andesites occupy no distinctive position such as a thermal valley; they are situated within a continuous sequence of liquid compositions. Partial melting of subducted ocean crust at 100 km depth produces liquids with a range of intermediate SiO_2 contents, but these must be modified by fractionation at shallower depths if they are to reach the surface with chemistry corresponding to average calc-alkaline lavas