Effect Of Oxygen Fugacity Research Articles

Oxygen diffusion in olivine: Effect of oxygen fugacity and implications for creep

Oxygen self‐diffusion experiments on single crystals of San Carlos olivine (∼Fo92) at 1200° ≤ T ≤ 1400°C, oxygen fugacities (ƒO2) along the Ni‐NiO and Fe‐FeO buffers, and silica activity at the olivine‐orthopyroxene buffer yielded results that follow the relationship D = 2.6 × 10−10 ƒO20.21±0.03 exp [−266±11 (kJ mol−1/RT)], where D is the diffusion coefficient in m2 s−1 and ƒO2 is given in pascals. The activation energy compares reasonably well with results for pure forsterite. The positive dependence of ƒO2 implies that the oxygen defect responsible for diffusion is an interstitial rather than a more sterically reasonable oxygen vacancy. Diffusion of oxygen in other close‐packed oxides has also shown a positive dependence on ƒO2. The rate of creep of single‐crystal olivine at fixed orthopyroxene activity also shows a positive ƒO2 dependence. If oxygen interstitials should be shown to be unimportant in oxygen diffusion in oxides, then coupled mechanisms such as countervacancy diffusion must be appealed to in order to explain the positive ƒO2 dependence. Such processes are rate‐limited by the diffusion of metal vacancies which also display a positive ƒO2 dependence in olivine. Compared with data for silicon diffusion in forsterite, our data indicate that oxygen is not the slowest diffusing species in olivine. The activation energy for oxygen diffusion is also low compared to that obtained in a majority of measurements of creep in single‐crystal olivine. Hence if oxygen diffusion contributes to the control of creep in olivine, it must be coupled to another process having a nonzero activation energy. A subinvestigation of the rate of sublimation from polished olivine surfaces, using a high‐resolution thermal balance, showed surface erosion rates at 1300°C of 0.13±0.10 nm/h. Such values are far too slow to noticeably affect our diffusion measurements at temperatures ≤1400°C.

Effects of oxygen fugacity and temperature on sulfur isotope composition in igneous rocks

Effects of oxygen fugacity and temperature on sulfur isotope composition in igneous rocks

Effect of oxygen fugacity on sulfur isotope compositions and magnetite concentrations in the Kirkpatrick Basalt, Mount Falla, Queen Alexandra Range, Antarctica

The δ 34S-values of total sulfur in the Jurassic tholeiite flows on Mt. Falla in Antarctica range from −1.45 to +11.73‰. The concentrations of sulfur range from 80 to 480 ppm, which is typical of subaerial lava flows that lose varying proportions of sulfur by out-gassing of SO 2. The concentrations of magnetite range from less than 1% to more than 4% and appear to correlate inversely with the total Fe content of the flows. However, the five flows which are anomalously enriched in 34S also have elevated magnetite concentrations. We suggest that the elevated magnetite concentrations and the 34S enrichment were both caused by high oxygen fugacities ( f O 2) in the melt. The magnetite concentrations are affected by the fugacity of oxygen through equilibrium in the FMQ buffer whereas the enrichment of the flows in 34S resulted from outgassing of SO 2 at f O 2 greater than ∼ 10 −8 atm. The dependence of δ 34S and the magnetite concentrations of the flows on f O 2 is supported by the stratigraphic variation of these parameters and by their direct linear correlation.

Phase Equilibria of Nuclear Waste Ceramics: The Effect of Oxygen Fugacity

Wasteform phase assemblages and radionuclide immobilization are described for a Synroc‐D bulk composition processed at 1200°C under oxygen fugacities (f O2) ranging from air to those approximating graphite‐CO‐CO2 buffer. All processing conditions produce assemblages of titanate and aluminate crystalline phases plus alkali aluminosilicate glass. In all cases the important radionuclides are immobilized in crystalline phases which have been shown to be highly leach‐resistant.

The effect of oxygen fugacity on liquid immiscibility in iron-bearing silicate melts

The effect of oxygen fugacity on liquid immiscibility in iron-bearing silicate melts

The Solubility of Water and Effects of Oxygen Fugacity and Water Content on Crystallization in Mafic Magmas

The solubility of water in a basaltic and in an andesitic melt has been determined in the pressure range from approximately 1,000 to 6,000 bars at 1,100° C. The solubility in basaltic melt is 3.1 weight percent at 1,000 bars and 9.4 weight percent at 6,000 bars; in the andesitic melt it is 4.5 weight percent at 1,000 bars and 10.1 weight percent at 5,300 bars. The temperatures of appearance of the primary, secondary, and tertiary phases in the basalt have been determined at 1,000 bars water pressure and at the fo2's of the magnetite+hematite (MH), magnetite+fayalite+quartz (MFQ) and magnetite+wustite (MW) buffers. Results are as follows: Buffer Pyroxene Plagioclase Ore mineral MH 1,095°C 1,065°C 1,230°C MFQ 1,040° 1,015° 1,010° MW 1,020° 1,010° 995° A comparison of the solubility of water at 1,100°C and up to approximately 6,000 bars pressure in several silicate melts, ranging in composition from granitic to gabbroic, indicates that the spread of solubility is narrower than has been supposed. The marked effect of fo2's on the crystallization sequence in the Columbia River basalt confirms the importance of this factor in determining liquid lines of descent. In experiments with low fo2's (MW buffer) and 1,000 bars water pressure, the basalt was completely liquid at the relatively low temperature of 1,020° C.