Summary: Mathematical Geology, Vol. 37, No. 8, November 2005 ( C 2005)
Tetrahedral Plots of the Phase Relations for Basalts1
and Richard N. Abbott, Jr.3
The phase relations of quaternary systems are generally represented by projections onto ternary
compositional planes. Such projections often obscure relationships that would only be evident in a
three-dimensional tetrahedral plot. The tetrahedral plot requires that compositions of the minerals
and melts be transformed into Cartesian coordinates. It is shown here how this transformation is
carried out. The application is demonstrated by tetrahedral plots of experimental melt compositions of
partially molten lherzolite. Furthermore, the plot can be used to evaluate whether or not a particular
basaltic composition represents a primary melt. The methods are applicable to any four-component
KEY WORDS: phase diagrams, basalts, primary magmas.
The compositions of primary basaltic magmas are controlled by the phase re-
lations of lherzolite and the way melt accumulation takes place in the mantel.
Characterization of the phase relations of lherzolite is therefore fundamental for
understanding the genesis of basalts. Phase relations for partial melting of lherzo-
lite involve olivine, orthopyroxene, clinopyroxene and spinel or garnet. Accessory