Two series of anhydrous experiments have been performed in an
end-loaded piston cylinder apparatus on a primitive, mantle-derived
tholeiitic basalt at 1.0 GPa pressure and temperatures in the range
1060-1330degreesC. The experimental data provide constraints on phase
equilibria, and solid and liquid compositions along the liquid line of
descent of primary basaltic magmas differentiating in storage
reservoirs located at the base of the continental crust. The first
series are equilibrium crystallization experiments on a single basaltic
bulk composition; the second series are fractionation experiments where
near-perfect fractional crystallization was approached in a stepwise
manner using 30degreesC temperature steps and starting compositions
corresponding to the liquid composition of the previous,
higher-temperature glass composition. Liquids in the fractional
crystallization experiments evolve with progressive SiO2 increase from
basalts to dacites, whereas the liquids in the equilibrium
crystallization experiments remain basaltic and display only a moderate
SiO2 increase accompanied by more pronounced Al2O3 enrichment. The
principal phase equilibria controls responsible for these contrasting
trends are suppression of the peritectic olivine + liquid = opx
reaction and earlier plagioclase saturation in the fractionation
experiments compared with the equilibrium experiments. Both
crystallization processes lead to the formation of large volumes of
ultramafic cumulates related to the suppression of plagioclase
crystallization relative to pyroxenes at high pressures. This is in
contrast to low-pressure fractionation of tholeiitic liquids, where
early plagioclase saturation leads to the production of troctolites
followed by (olivine-) gabbros at an early stage of differentiation.