Redox reactions in seafloor basalts: possible insights into silicic hydrothermal systems |
| |
| Authors: | John R Holloway |
| |
| Institution: | Departments of Geological Sciences, Chemistry & Biochemistry, Arizona State University, Tempe, AZ 85287-1404, USA |
| |
| Abstract: | Many features of the magma-hydrothermal interface are directly observable in, near seafloor hydrothermal systems at mid-ocean ridges. A striking aspect observed at that interface is the spontaneous redox reaction that occurs during rapid crystallization of basaltic magma, which results in generation of highly reducing hydrothermal fluids. A consequence of the reaction is that the ferric/ferrous ratio observed in the resulting crystalline basalt is much higher than it was in the basaltic magma, the redox state of the magma inferred from the ferric/ferrous ratio in the crystalline rock is considerably higher than it was in the magma Earth Planet. Sci. Lett. 79 (1986) 397]. Magnetite-ulvöspinel is the only ferric iron-containing phase observed in the crystalline portion of the basalt. This suggests that the driving force for that reaction is related to the stability of magnetite relative to the FeO component in the basaltic liquid (or glass), resulting in the oxidation of ferrous iron by H2O dissolved in the melt, yielding magnetite, releasing H2 from the system. This is an auto-oxidation reaction in which all reactants are present internally in the basalt magma prior to crystallization. H2O is the limiting reagent for magnetite formation in the case of MORB magma. If a similar auto-oxidation occurs in the more silicic magmas commonly associated with terrestrial hydrothermal ore deposits, it would have important consequences for the interpretation of the redox condition existing at terrestrial magma-hydrothermal interfaces. Glassy I-type dacites extruded on the seafloor in the Manus Basin have ferric/ferrous ratios that are significantly lower than in I-type plutons, terrestrial volcanics. The ferric/ferrous ratios in the Manus Basin dacites yield oxygen fugacities from about one to more than two log units below that of nickel–nickel oxide. Oxidation of ferrous iron in these silicic magmas is predicted to generate an amount of H2 that is proportional to the amount of ferrous iron present, rather than the amount of H2O (as in the case of MORB) due to the high ratio of H2O to ferrous iron in silicic magmas. These relatively reduced dacitic magmas would yield significant quantities of H2 on crystallization, which would make hydrothermal fluids at the interface strongly reducing, thus affect the speciation in fluids at the magma-hydrothermal interface. |
| |
| Keywords: | Redox Auto-oxidation Oxygen fugacity Ferric/ferrous ratio |
| 本文献已被 ScienceDirect 等数据库收录！ |
|
