HR: 0830h
AN: V41C-0315    [PDF]
TI: Composition of Olivine, Melt Redox State, and Oxygen Fugacity
AU: * Barton, M
EM: barton.2@osu.edu
AF: The Ohio State University, Department of Geological Sciences, Columbus, OH 43021 United States
AB: The compositions of ferro-magnesian silicates in equilibrium with melts are a function of oxygen fugacity. This was demonstrated experimentally in the case of olivine by Roeder and Emslie (1970). In principle, therefore, the composition of coexisting olivine and melt (with total Fe as FeO) allows melt redox state and hence oxygen fugacity to be calculated if the equilibrium distribution of Mg and Fe between olivine and melt is known. It has been found that the expression given by Gee and Sack (1988) accurately describes this distribution over a wide range of liquid compositions, so that melt redox state and oxygen fugacity can be calculated from the compositions of olivine-melt pairs. Oxygen fugacities have been calculated from published data for olivine-melt pairs produced in 160 experiments at 10$^{5}$ Pa at known oxygen fugacities (from 3 log units below to 7.6 log units above QFM) over a range of T from 1080 to 1339$^{O}$C. Melt compositions in these experiments include tholeiitic and alkali-olivine basalts, picrites, andesites, and highly alkaline lavas (nephelinites, leucitites, ugandites). Forsterite compositions of olivines range from 0.98-0.55, whereas melt Mg-no's range from 0.75-0.25 (all Fe as FeO). There is excellent correlation (0.99) between calculated and experimental values. The maximum difference between calculated and experimental values is about 0.5 log units, but the average uncertainty (calculated at the 1-sigma level) is 0.24 log units. The results indicate that the expression given by Kress and Carmichael (1991) better describes the relationship between melt redox state and oxygen fugacity than that given by Kilinc et al. (1983). The results suggest that oxygen fugacities can be retrieved from coexisting olivine-melt pairs in natural samples. However, the non-linear relationship between olivine composition, melt ferric-ferrous iron ratio, and (log) oxygen fugacity indicates relatively large uncertainties at oxygen fugacities more reducing than about one log unit below QFM. In addition, it is not clear that this method works for Fe-rich liquids (Mg-no's less than about 0.3). On the other hand, the method does provide a rigorous test for equilibrium between experimentally produced olivines and melts.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3655 Major element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting