Equilibrium and fractional crystallization experiments at 0.7 GPa;: the effect of pressure on phase relations and liquid compositions of tholeiitic magmas

Two series of anhydrous experiments have been performed in an end-loaded piston cylinder apparatus on a primitive, mantle-derived tholeiitic basalt at 0.7 GPa pressure and temperatures in the range 1060-1270 degrees C. 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 30 degrees C temperature increments and starting compositions corresponding to that of the previous, higher temperature glass. At 0.7 GPa liquidus temperatures are lowered and the stability of olivine and plagioclase is enhanced with respect to clinopyroxene compared with phase equilibria of the same composition at 1.0 GPa. The residual solid assemblages of fractional crystallization experiments at 0.7 GPa evolve from dunites, followed by wehrlites, gabbronorites, and gabbros, to diorites and ilmenite-bearing diorites. In equilibrium crystallization experiments at 0.7 GPa dunites are followed by plagioclase-bearing websterites and gabbronorites. In contrast to low-pressure fractionation of tholeiitic liquids (1 bar-0.5 GPa), where early plagioclase saturation leads to the production of troctolites followed by (olivine) gabbros at an early stage of differentiation, pyroxene still crystallizes before or with plagioclase at 0.7 GPa. The liquids formed by fractional crystallization at 0.7 GPa evolve through limited silica increase with rather strong iron enrichment following the typical tholeiitic differentiation path from basalts to ferro-basalts. Silica enrichment and a decrease in absolute iron and titanium concentrations are observed in the last fractionation step after ilmenite starts to crystallize, resulting in the production of an andesitic liquid. Liquids generated by equilibrium crystallization experiments at 0.7 GPa evolve through constant SiO2 increase and only limited FeO enrichment as a consequence of spinel crystallization and closed-system behaviour. Empirical calculations of the (dry) liquid densities along the liquid lines of descent at 0.7 and 1.0 GPa reveal that only differentiation at the base of the crust (1.0 GPa) results in liquids that can ascend through the crust and that will ultimately form granitoid plutonic and/or dacitic to rhyodacitic sub-volcanic to volcanic complexes; at 0.7 GPa the liquid density increases with increasing differentiation as a result of pronounced Fe enrichment, rendering it rather unlikely that such differentiated melt will reach shallow crustal levels.