Magmatic crystallization experiments at 1 bar in systems closed to oxygen:a new/old experimental approach

Many controversial discussions on the control of the oxidation state of cry stallizing magmas are hampered by the fact that there are no available expe rimental data gained in systems closed to all elements, inclusive oxygen. T o fill this gap the old technique of conducting experiments in evacuated si lica-glass ampoules at 1 bar has been revived and adapted to perform equili brium crystallization experiments with basaltic melts at high temperatures under closed-system conditions. The experiments are conducted in two steps. Step 1: in order to fix the ini tial oxygen fugacity (f(O2)), small charges of the glassy starting material s are either pressed onto a loop of thin Pt-wire or into a small AgPd cruci ble and equilibrated at super-liquidus temperatures (>1180 degreesC) with C O/CO2 gas mixtures. Step 2: to achieve equilibrium crystallization under cl osed-system conditions, the charges are subsequently placed together with t heir ml:tal holder/container in evacuated silica-glass ampoules and re-equi librated under sub-liquidus conditions (1050-1170 degreesC). To test whether this Experimental approach really ensures closed-system con ditions, a series of experiments was conducted at near-liquidus temperature s with a synthetic ferro-basaltic starting composition. Within the analytic al uncertainties, the bulk ferrous iron contents of the samples remain cons tant during the step-2 experiments, pointing to systems closed to oxygen. T here are, however, indications for a slight oxidation related to a small lo ss of iron from the sample to the AgPd container. Using the same synthetic ferro-basaltic composition, preliminary equilibriu m crystallization experiments under closed-system conditions were performed at 1091-1146 degreesC with an initial superliquidus f(O2), corresponding t o FMQ. The crystallization sequence of the mineral phases is the same as un der open system conditions but magnetite(ss) appears at higher temperatures . The FeOtot content of the residual melt shows the same increase with decr easing temperature under closed and open system conditions down to about 11 00 degreesC. At lower temperatures, however, the values drop drastically un der closed-system conditions, in contradiction with previous modelling. Thi s exemplifies the need fur further experiments in closed systems.