The low-pressure liquidus boundaries of natural basaltic liquids have been parameterized as quasi-linear empirical functions of projection coordinates in the Ol-Pl-Wo-Qtz model system. Secondary variables are simple chemical parameters such as Mg' and the albite and orthoclase fractions of the normative feldspar. These empirical expressions lend themselves to rapid construction of liquidus diagrams as well as forming an integral part of quantitative models of fractional and equilibrium crystallization. The position and reaction relations of the liquidus boundary between olivine (ol) and low-Ca pyroxene (lpyx) are strongly dependent on Mgl such that olivine reacts with magnesian liquids along the boundary, but precipitates from more ferroan liquids. The transition from a reaction to a crystallization relationship in liquids with intermediate Mg' is complex, and it is possible for olivine to precipitate during equilibrium crystallization and react during fractional crystallization of the same liquid on the ol-lpyx boundary. A survey of the silicateJiquidus equilibria appropriate to mare, midocean ridge, and calc-alkaline basalts shows that the different alkali contents of these magma types are responsible for the different proportions of mafic and feldspathic components observed in cotectic liquids of each type. Different alkali contents can even produce important differences, such as silica enrichment vs. silica depletion, in the liquid lines of descent (LLD) of parent liquids that have the same initial Mg' and crystallization sequence. Liquid lines of descent are in general different from the equilibrium crystallization paths that petrologists have employed to construct liquidus diagrams. If one were to use only the plagioclase-component projection, there is the potential for confusing a low-pressure LLD with a high-pressure trend.

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