The Olivine--Orthopyroxene--Spinel Oxygen Geobarometer, the Nickel Precipitation Curve, and the Oxygen Fugacity of the Earth's Upper Mantle |
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| Authors: | O'NEILL HStC; WALL V J |
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| Institution: | 1Research School of Earth Sciences Australian National University Canberra A.C.T. 2600
2Department of Earth Sciences, Monash University, Clayton Victoria 3168 |
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| Abstract: | The common upper mantle assemblage olivine-orthopyroxene-spinelmay be used to calculate the oxygen fugacity at which mantle-derivedperidotites have equilibrated. The equilibrium has been calibratedusing the large amount of existing data on the thermodynamicproperties of each phase in this assemblage. A by-product ofthis procedure is a new calibration of the olivine-spinel Mg-Fe2+exchange geothermometer. Application of the equilibrium to avariety of peridotite xenoliths indicates that the oxygen fugacityof the upper mantle lies between the quartz-fayalite-magnetite(QFM) and w?stite-magnetite (WM) oxygen buffers; the few apparentexceptions to this rule may be due to analytical error, particularlyin the Fe3+ content of the spinet phase. In fact, the determinationof Fe3+ in spinet is at present the limiting factor in the accurateapplication of the method: within this limitation, the presentlyavailable evidence suggests that the oxygen fugacity of themantle may be laterally homogeneous over wide regions, but mayalso show small differences between these regions. The fluidspecies in the system C-H-O at such oxygen fugacities are predominantlyCO2 and/or H2O, and not CH4/H2 The minimum possible oxygen fugacity of the mantle is givenby the nickel content of olivine in equilibrium with orthopyroxene;for typical mantle compositions this minimum curve is virtuallycoincident with the iron-w?stite (IW) oxygen buffer. |
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