Other
Scientific paper
Oct 1989
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989gecoa..53.2631d&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 53, Issue 10, pp.2631-2638
Other
2
Scientific paper
Using both high- and low-FeO starting materials, 167 liquids are equilibrated with metallic Fe under controlled oxygen fugacity at 1600 K to contour for a FeO three pseudoternaries in the system [SiO 2 -TiO 2 -KAlO 2 -Ca 0.5 AlO 2 ]-FeO. The pseudoternaries contain synthetic analogs of the most abundant terrestrial and lunar ultramafic and mafic magmas. The data are used to test a simple thermodynamic model in which Ti and Si are assumed to be randomly distributed on the same set of sites in the network of magma. These two elements do not mix ideally on these sites. Instead, they experience a strong "regular-solution type" interaction with each other. Using a short equation, the model permits calculation of the activity of FeO in Ti-bearing melts from data in Ti-free liquids. The experimental data, along with published results for FeO-SiO 2 -TiO 2 melts, clearly support the model. The molar coeffcient ( ) for the Si-Ti interaction is 105 KJ/mole for most of the investigated melts (including synthetic analogs of basalts, basaltic andesites and picrites). The exception is the one set that are both KAlO 2 -free and Ca 0.5 AlO 2 -bearing (containing synthetic analogs of komatiites and lunar basalts) in which drops to 42 KJ/mole. This suggests that the mixing of Si and Ti is a closer approximation to ideality in these alkali-poor melts. The physical reason for this variation in is unknown and warrants further investigation. Finally, the model permits calculation of the effect of Ti substitution for Si on the activity of any component in magma that contains FeO, SiO 2 and/or TiO 2 . The short equation for the activity of fayalite is given as an example.
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