The oxidation state of the Earth's sub-oceanic mantle from oxygen thermobarometry of abyssal spinel peridotites

Abstract

THE oxygen fugacity (fO2), or redox state, of the Earth's mantle is an important parameter in the evolution of mantle-derived magmatic rocks. Most estimates of the upper-mantle redox state have been based on samples of the sub-continental mantle and terrestrial lavas, and estimates of fO2 obtained by thermo-barometric methods tend to cluster within ±1.5 log units of the FMQ (fayalite–magnetite–quartz) reference oxygen fugacity buffer1–3. Much less is known about the redox state of the sub-oceanic mantle, the ultimate source of mid-ocean-ridge basalts (MORBs). Here we use oxygen thermobarometry of abyssal spinel peridotites, representative of most of the Earth's mid-ocean-ridge systems, to show that the redox state of the sub-oceanic mantle lies at values of up to 4 log units (average of 1.35 log units) more reduced than FMQ. Our results are in excellent agreement with oxygen fugacities of MORBs obtained from Fe3+/Fe2+ ratios of quenched glass in pillow basalts4. This agreement confirms the redox state of the MORB source region and suggests that MORB glasses (as opposed to the cores of pillow basalts) have not undergone significant oxidation (hydrogen degassing) during their ascent.