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3.7 h. Measurement of oxide activities in silicate melts in the CaO-MgO-Al2O3-SiO2 system by equilibration with Pd (L. Chamberlin)

It is possible to determine directly the activities of the oxide components MgO, Al2O3, or SiO2 in melts and minerals through analysis of Pd alloys equilibrated with them at fixed T and fO2. The activity of each component relative to the pure solid oxide is a simple function of the ratio of the Mg, Al, or Si dissolved in Pd equilibrated with the sample to the Mg, Al, or Si in Pd equilibrated with the corresponding oxide. Previous experimentation suggested a dependence of the activity coefficients of MgO, Al2O3, and SiO2 on composition for simple silicate liquids in the system CaO-MgO-Al2O3-SiO2-TiO2 (CMAST) that were synthetic analogs of Ca,Al-rich inclusions in carbonaceous chondrites. Measurement of oxide activities have now been performed on compositions in the "basalt tetrahedron" (forsterite-diopside-CaTschermaks-silica) in the system CaO-MgO-Al2O3-SiO2 (CMAS), with the intent of extending the study to more silicic compositions and lower Al:Si ratios, and providing a simple model system in which changes in thermodynamic properties as a function of differentiation in an Fe-free basaltic system may be observed. Comparison between activities determined experimentally at 1250° - 1400 °C and those predicted by published models shows good agreement overall, although the SiO2 activities are systematically high. Plots were made of the activity coefficients γi of the various oxide components as a function of composition using both the prior data on the meteoritic liquids and the new data on the Fe-free basaltic liquids. A strong positive correlation exists between γSiO2 and XSiO2, which probably indicates the controlling influence of SiO2 as a network former (Fig. 3.7-7). There is also a good negative correlation between γAl2O3 and the number of non-bridging oxygens per total oxygens in the melt (NBO/Σ O), implying that

Fig. 3.7-7: Activity coefficient of SiO2 in silicate melt as a function of mole fraction of SiO2.
depolymerization of the melt results directly in the breakup of Al - O bonds. However, γSiO2 increases either with decreasing NBO/Σ O or decreasing Al:Si ratio, such that there is no correspondingly simple relationship between γSiO2 and NBO/Σ O. This implies that network modifiers preferentially attack Al - O bonds as they break up the silicate framework. The activity coefficient of MgO exhibits a well-defined negative correlation with XSiO2 and a positive correlation with XCaO, implying an overall negative relationship between gMgO and the degree of polymerization. However, there is no clear correlation between γMgO and NBO/Σ O, perhaps due to the presence of the second network modifier CaO. The effects of differentiation in these liquids may be observed in the increase of γSiO2 as the silica-poor phases forsterite and pigeonite crystallize from the melt.

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