An Experimental Investigation of the Influence of Water and Oxygen Fugacity on Differentiation of MORB at 200 MPa

Crystallization experiments were performed at 200MPa in the temperature range 1150–950C at oxygen fugacities corresponding to the quartz–fayalite–magnetite (QFM) and MnO–Mn3O4 buf-fers to assess the role of water and fO2 on phase relations and differentiation trends in mid-ocean ridge basalt (MORB) systems. Starting from a primitive (MgO 98 wt %) and an evolved MORB (MgO 649 wt %), crystallization paths with four different water contents (035–47 wt % H2O) have been investigated. In primi-tive MORB, olivine is the liquidus phase followed by plagioclase þ clinopyroxene. Amphibole is present only at water-saturated condi-tions below 1000C, but not all fluid-saturated runs contain amphibole. Magnetite and orthopyroxene are not stable at low fO2 (QFM buffer). Residual liquids obtained at low fO2 show a tholeiitic differentiation trend. The crystallization of magnetite at high fO2 (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 950C 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 frac-tionation processes are necessary to reach typical compositions of natural oceanic plagiogranites.