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 crystallizing magmas are hampered by the fact that there are no available experimental data gained in systems closed to all elements, inclusive oxygen. To fill this gap the old technique of conducting experiments in evacuated silica-glass ampoules at 1 bar has been revived and adapted to perform equilibrium 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 initial oxygen fugacity (fO₂), small charges of the glassy starting materials are either pressed onto a loop of thin Pt-wire or into a small AgPd crucible and equilibrated at super-liquidus temperatures (>1180 °C) with CO/CO₂ gas mixtures. Step 2: to achieve equilibrium crystallization under closed-system conditions, the charges are subsequently placed together with their metal holder/container in evacuated silica-glass ampoules and re-equilibrated under sub-liquidus conditions (1050–1170 °C).

To test whether this experimental approach really ensures closed-system conditions, a series of experiments was conducted at near-liquidus temperatures with a synthetic ferro-basaltic starting composition. Within the analytical uncertainties, the bulk ferrous iron contents of the samples remain constant during the step-2 experiments, pointing to systems closed to oxygen. There are, however, indications for a slight oxidation related to a small loss of iron from the sample to the AgPd container.

Using the same synthetic ferro-basaltic composition, preliminary equilibrium crystallization experiments under closed-system conditions were performed at 1091–1146 °C with an initial superliquidus fO₂) corresponding to FMQ. The crystallization sequence of the mineral phases is the same as under open system conditions but magnetite_ss appears at higher temperatures. The FeO_tot content of the residual melt shows the same increase with decreasing temperature under closed and open system conditions down to about 1100 °C. At lower temperatures, however, the values drop drastically under closed-system conditions, in contradiction with previous modelling. This exemplifies the need for further experiments in closed systems.