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Magmatic crystallization experiments at 1 bar in systems closed to oxygen

Mineralogisches Institut, Universität Heidelberg, INF 236, D-69120 Heidelberg e-mail: dlattard{at}min.min.uni-heidelberg.de

View larger version (58K): [in this window] [in a new window]   Fig. 1. Schematic drawing of two evacuated silica-glass ampoules prepared for step-2 experiments. In the ampoule on the right side, the sample is wetting a loop of Pt-wire placed inside an AgPd crucible. On the left side, the sample was put directly into the crucible. Dimensions of the ampoules: outer diameter 8 mm, inner diameter 6 mm, wall thickness 1 mm, L = 6 mm.

View larger version (124K): [in this window] [in a new window]   Fig. 2. Two silica-glass ampoules after quenching from step-2 experiments (a, b), an empty AgPd crucible (outer diameter: 5 mm) with lid (c) and a sample wetting a Pt-wire loop (d). Conditions of the step-2 experiments: 24 hours at 1180 °C for ampoule (a); 138 hours at 1133 °C for ampoule (b). Note that the ampoules are perfectly transparent and show no apparent crack.

View larger version (13K): [in this window] [in a new window]   Fig. 3. FeO concentrations, determined by wet chemistry (a), and FeOtot contents (calculated from the FeO and Fe2O3 values) (b) in glassy run products quenched either after step-1 runs (open symbols) or after subsequent step-2 experiments (closed symbols). FMQ = logfO2 - log fO2(FMQ) refers to the oxygen fugacity fixed during the step-1 experiments. The squares, resp. the diamonds indicate two consecutive series of experiments, the conditions of which are listed in the legend (see also section "Analytical control of the composition of the samples"). The vertical bars reflect in (a) the estimated uncertainty (±5%) of the FeO determination; in (b) the ± 1 standard deviations (Table 2).

View larger version (18K): [in this window] [in a new window]   Fig. 4. (a) Concentration profiles of the Fe contents through side walls of four Ag60Pd40 crucibles that were used for near-liquidus experiments at differentfo2. Each crucible was used for several step-1 and step-2 experiments and was thus at least 130 h in contact with the SC1 -P melt at temperatures in the range 1178–1189°C. The vertical line on the left marks the contact surface to the sample. (b) Inverse correlation between the mean Fe contents over the walls of the AgPd crucibles depicted in (a) and the oxygen fugacity fixed during the step-1 experiments. (c) Correlation between the Fe and Ag contents in single analyses from the profiles shown in Fig. 4a. The dominant feature is an inverse correlation between the two contents. In the crucible used at FMQ-1, however, the outer rim is strongly depleted in silver due to evaporation of this metal during the step-1 (open system) runs. At FMQ+1 both the silver evaporation and iron absorption are small.

View larger version (14K): [in this window] [in a new window]   Fig. 5. (a) Decreasing silver content of AgPd crucibles with increasing run duration due to evaporation under open-system conditions (step-1 experiments). (b) In contrast, the Ag content is independent of the run duration under closedsystem conditions (step-2 experiments), because no evaporation takes place within the evacuated silica-glass ampoules. The Ag contents are means from single values recorded along a profile through the side wall of a crucible (Fig. 4). Symbols combined with a cross signalize crucibles with thicker walls (600 instead of 300 µm). The run durations are cumulated over several experiments.

View larger version (51K): [in this window] [in a new window]   Fig. 6. FeOtot contents of the quenched residual melt in run products of crystallization experiments as a function of the run temperature. For the closed-system experiments, the fO2 fixed in the step-1 runs corresponds to FMQ. All open-system experiments were also conducted at FMQ conditions. The grey curve shows the FeOtot contents estimated by Toplis & Carroll (1996) for perfect equilibrium crystallization under closed-system conditions, with a starting fO2 at FMQ. The vertical lines point to the upper crystallization temperature for ilmenite, resp. magnetite under closed-system (CS; solid lines) and for magnetite and ilmenite under open-system conditions (OS, dashed line). Olivine, plagioclase and clinopyroxene are present in all run products.