Equilibrium and Fractional Crystallization Experiments at 0{middle dot}7 GPa; the Effect of Pressure on Phase Relations and Liquid Compositions of Tholeiitic Magmas

Two series of anhydrous experiments have been performed in anend-loaded piston cylinder apparatus on a primitive, mantle-derivedtholeiitic basalt at 0·7 GPa pressure and temperaturesin the range 1060–1270°C. The first series are equilibriumcrystallization experiments on a single basaltic bulk composition;the second series are fractionation experiments where near-perfectfractional crystallization was approached in a stepwise mannerusing 30°C temperature increments and starting compositionscorresponding to that of the previous, higher temperature glass.At 0·7 GPa liquidus temperatures are lowered and thestability of olivine and plagioclase is enhanced with respectto clinopyroxene compared with phase equilibria of the samecomposition at 1·0 GPa. The residual solid assemblagesof fractional crystallization experiments at 0·7 GPaevolve from dunites, followed by wehrlites, gabbronorites, andgabbros, to diorites and ilmenite-bearing diorites. In equilibriumcrystallization experiments at 0·7 GPa dunites are followedby plagioclase-bearing websterites and gabbronorites. In contrastto low-pressure fractionation of tholeiitic liquids (1 bar–0·5GPa), where early plagioclase saturation leads to the productionof troctolites followed by (olivine) gabbros at an early stageof differentiation, pyroxene still crystallizes before or withplagioclase at 0·7 GPa. The liquids formed by fractionalcrystallization at 0·7 GPa evolve through limited silicaincrease with rather strong iron enrichment following the typicaltholeiitic differentiation path from basalts to ferro-basalts.Silica enrichment and a decrease in absolute iron and titaniumconcentrations are observed in the last fractionation step afterilmenite starts to crystallize, resulting in the productionof an andesitic liquid. Liquids generated by equilibrium crystallizationexperiments at 0·7 GPa evolve through constant SiO₂ increaseand only limited FeO enrichment as a consequence of spinel crystallizationand closed-system behaviour. Empirical calculations of the (dry)liquid densities along the liquid lines of descent at 0·7and 1·0 GPa reveal that only differentiation at the baseof the crust (1·0 GPa) results in liquids that can ascendthrough the crust and that will ultimately form granitoid plutonicand/or dacitic to rhyodacitic sub-volcanic to volcanic complexes;at 0·7 GPa the liquid density increases with increasingdifferentiation as a result of pronounced Fe enrichment, renderingit rather unlikely that such differentiated melt will reachshallow crustal levels. KEY WORDS: tholeiitic magmas; experimental petrology; equilibrium crystallization; fractional crystallization