OXYGEN FUGACITY CONTROLS IN THE EARTHS UPPER MANTLE

DESPITE significant progress in recent years1-3, there is still considerable debate surrounding the oxygen fugacity (fo2) of the Earth's upper mantle and the chemical reactions that buffer it. Electrochemical measurements1,3 give heterogeneous oxidation states, with one group near the fayalite-magnetite-quartz (FMQ), and the other near the iron-wüstite buffer (IW). Thermobarometric calculations based on Fe3+/Fe2+ ratios in basaltic glasses4, mantle-derived spinels5,6, ilmenites7 and garnets8 support uniform oxidation states bounded by wustite-magnetite (WM) and FMQ. Here we compare the behaviour of four 'oxygen barometers'5,6,9,10 based on the compositions of coexisting minerals, including a new empirical olivine-orthopyroxene-spinel barometer10 that has been calibrated experimentally in spinel Iherzolite at pressures, temperatures and compositions appropriate to the upper mantle. Application of this barometer to mantle-derived rocks and basaltic melts of different tectonic settings suggests that the upper mantle is only weakly buffered by Fe3+/Fe2+ equilibria. Large-scale heterogeneity in fo2 structure is governed by recycling processes and injection of oxidized crustal material into a moderately reduced, poorly buffered asthenosphere. © 1990 Nature Publishing Group.