pH buffering by metastable mineral-fluid equilibria and evolution of carbon dioxide fugacity during burial diagenesis
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Lunar armaloclites, (Fe, Mg)Ti2O5, contain appreciable amounts of Ti(3+) (less than 1 to 17% of Ti mole fraction). This is a function of the oxygen fugacity occurring at the time of its formation, with lower fugacities being reflected in higher Ti(3+) contents. Controlled cooling-rate and isothermal experimentation on synthetic analog and natural specimens of 70017 and 74275 have been used to calibrate an oxygen geobarometer. Most lunar rocks have followed crystallization paths in oxygen fugacity/T space such that the prevailing oxygen fugacity can be represented by a curve near parallel to the I/W buffer curve. The oxygen fugacity estimates derived from Ti(3+) considerations of armalcolites range from the iron/wustite curve to about 1.5 log units below.
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The T-oxygen fugacity equilibrium for the system has been developed for a semi-quantitative T-oxygen fugacity petrogenic grid applicable to both basic igneous rocks and to meteorites. Olivine-silica-pyroxene-metallic iron-liquid exist at 1305 degrees C and 10 (super -11.8) atm (1 atm total), and in the system Fe-O-SiO ~2~ at 17.5 kb, 1280 degrees C, and 10 (super -10.8) atm oxygen fugacity. Subsolidus Mg enrichment of the silicate phases occurs with decreasing T and increasing oxygen fugacity in the magnetite-bearing assemblages at 1 atm. For iron-bearing assemblages, a T increase and oxygen fugacity decrease leads to Mg enrichment of the silicate phase. These phases can be divided by a region in which neither Mg nor metallic iron are stable, and transition between the two is unlikely.
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The ore-forming metallogenic specialization of felsic rocks is closely related to oxygen fugacity. In this paper, however, oxygen fugacity is considered to be one of the necessary but not sufficient conditions for ore formation, and the analytical thinking concerned is partly based on the authors' geological field work. The metallogenic significance of oxygen fugacity is divided into metallogenic specialization and ore-bearing potential. Two aspects of understanding have been otained: 1 Porphyry Cu-Au deposits derived from the reduced I-type granitoid should have a high oxygen-fugacity origin; different sub-regions within the covariogram of oxygen fugacity versus other geochemical parameters, such as degree of magmatic evolution, temperature, pressure, pH, sulfur fugacity and rock-type, correspond to different metals' geochemical behavior or mineralization, respectively, and oxygen fugacity versus rock-type seems to be of significant specialization; 2 The evolution from metallogenic specialization to real metallogenesis is the evolution from magma to hydrothermal solution and then from hydrothermal solution to ore;during such a process, oxygen fugacity is also an essential controlling factor for elemental geochemical behaviors,whose influence on mineral precipitation, however, should not be overemphasized, because some kinds of precipitation might have had nothing to do with oxygen fugacity.
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The kinetics of rock/water interactions are sufficiently rapid that most hydrothermal systems in nature will be in equilibrium with the adjacent rock mass. The bulk rock chemistry buffers the fugacity of oxygen, which in turn fixes the fugacities of water and of hydrogen for a given pressure and temperature. Systems in which only water, oxygen, and hydrogen are present as fluid phases are considered here. Variations in the fugacity of oxygen by several orders of magnitude are possible locally, controlled by variations in local rock chemistry; these lead to relative small variations in the fugacity of water. Incorporation of a hydrogen defect that is capable of acting as an acceptor into silicates leads to a strong dependence of point defect chemistry upon the fugacities of both water and oxygen. The strong dependence on the fugacity of water is capable of explaining the hydrolytic weakening effect, but in view of the strong dependence on oxygen fugacity, the question should also be raised whether it is an oxygen effect that is observed in the classical hydrolytic weakening process or solely a dependence on changes in the fugacity of water. Examples are given for impure natural quartz, olivine, and albite with trace amounts of calcium.
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