Experimental systems for calc-alkaline rocks

Phase relations and rock compositions of andesitic and related magmas of the calc-alkaline suite can also be represented by the diopside-olivine-albite-quartz tetrahedron — the silica-saturated part of the Yoder-Tilley (1962) basalt tetrahedron. 

The values are normalized and projected from plagioclase onto the plane quartz-olivine-cEnopyroxene. 

The projections Baker and Eggler (1983) use a modification of the projection procedure of Walker et of Baker and al. (1979) in which the projection is made either from magnetite + plagioclase onto Eggler (1983, olivine-diopside-(quartz + orthoclase) or from magnetite + olivine onto 

The ferric iron content of the rock is calculated from Sack et al (1980) for T -1150 C and yO defined by the Ni-NiO buffer. Under the specific conditions given here, using the temperanire — oxygen fiigacicy relationship given by Eugster and Wones (1962) for the Ni-NiO buffer, the expression reduces to  

Baker and Eggler (1983) note that this projection is very sensitive to small differences in Na20 concentrations. 

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