An experimental investigation of the influence of water and oxygen fugacity on differentiation of MORB at 200 MPa

Crystallization experiments were performed at 200 MPa in the temperature range 1150-950degreesC at oxygen fugacities corresponding to the quartz-fayalite-magnetite (QFM) and MnO-Mn3O4 buffers to assess the role of water and f(O2) on phase relations and differentiation trends in mid-ocean ridge basalt (MORB) systems. Starting from a primitive (MgO 9.8 wt %) and an evolved MORB (MgO 6.49 wt %), crystallization paths with four different water contents (0.35-4.7 wt % H2O) have been investigated. In primitive MORB, olivine is the liquidus phase followed by plagioclase + clinopyroxene. Amphibole is present only at water-saturated conditions below 1000degreesC, but not all fluid-saturated runs contain amphibole. Magnetite and orthopyroxene are not stable at low f(O2) (QFM buffer). Residual liquids obtained at low f(O2) show a tholeiitic differentiation trend. The crystallization of magnetite at high f(O2) (MnO-Mn3O4 buffer) results in a decrease of melt FeO*/MgO ratio, causing a calc-alkaline differentiation trend. Because the magnetite crystallization temperature is nearly independent of the H2O content, in contrast to silicate minerals, the calc-alkaline differentiation trend is more pronounced at high water contents. Residual melts at 950degreesC in a primitive MORB system have compositions approaching those of oceanic plagiogranites in terms of SiO2 and K2O, but have Ca/Na ratios and FeO* contents that are too high compared with the natural rocks, implying that fractionation processes are necessary to reach typical compositions of natural oceanic plagiogranites.