Redox states of lithospheric and asthenospheric upper mantle

Abstract

The oxidation state of lithospheric upper mantle is heterogeneous on a scale of at least four log units. Oxygen fugacities ($f_{O_2 }$) relative to the FMQ buffer using the olivine-orthopyroxene-spinel equilibrium range from about FMQ-3 to FMQ+1. Isolated samples from cratonic Archaean lithosphere may plot as low as FMQ-5. In shallow Proterozoic and Phanerozoic lithosphere, the relative$f_{O_2 }$ is predominantly controlled by sliding Fe³⁺-Fe²⁺ equilibria. Spinel peridotite xenoliths in continental basalts follow a trend of increasing$f_{O_2 }$ with increasing refractoriness, to a relative$f_{O_2 }$ well above graphite stability. This suggests that any relative reduction in lithospheric upper mantle that may occur as a result of stripping lithosphere of its basaltic component is overprinted by later metasomatism and relative oxidation. With increasing pressure and depth in lithosphere, elemental carbon becomes progressively refractory and carbon-bearing equilibria more important for$f_{O_2 }$ control. The solubility of carbon in H₂O-rich fluid (and presumably in H₂O-rich small-degree melts) under the P,T conditions of Archaean lithosphere is about an order of magnitude lower than in shallow modern lithosphere, indicating that high-pressure metasomatism may take place under carbon-saturated conditions. The maximum$f_{O_2 }$ in deep Archaen lithosphere must be constrained by equilibria such as EMOG/D. If the marked chemical depletion and the orthopyroxene-rich nature of Archaean lithospheric xenoliths is caused by carbonatite (as opposed to komatiite) melt segregation, as suggested here, then a realistic lower$f_{O_2 }$ limit may be given by the H₂O +C=CH₄+O₂ (C-H₂O) equilibrium. Below C −H₂O a fluid becomes CH₄ rather than CO₂-bearing and carbonatitic melt presumably unstable. The actual$f_{O_2 }$ in deep Archaean lithosphere is then a function of the activities of CO₂ and MgCO₃. Basaltic melts are more oxidized than samples from lithospheric upper mantle. Mid-ocean ridge (MORB) and ocean-island basalts (OIB) range between FMQ-1 (N-MORB) and about FMQ +2 (OIB). The most oxidized basaltic melts are primitive island-arc basalts (IAB) that may fall above FMQ+3. If basalts are accurate$f_{O_2 }$ probes of their mantle sources, then asthenospheric upper mantle is more oxidized than lithosphere. However, there is a wide range of processes that may alter melt$f_{O_2 }$ relative to that of the mantle source. These include partial melting, melt segregation, shifts in Fe³⁺/Fe²⁺ melt ratios upon decompression, oxygen exchange with ambient mantle during ascent, and low-pressure volatile degassing. Degassing is not very effective in causing large-scale and uniform$f_{O_2 }$ shifts, while the elimination of buffering equilibria during partial melting is. Upwelling graphite-bearing asthenosphere will decompress along$f_{O_2 }$-pressure paths approximately parallel to the graphite saturation surface, involving reduction relative to FMQ. The relative$f_{O_2 }$ will be constrained to below the CCO equilibrium and will be a function of$a_{CO_2 }$. Upwelling asthenosphere whose graphite content has been exhausted by partial melting, or melts that have segregated and chemically decoupled from a graphite-bearing residuum will decompress along$f_{O_2 }$-decompression paths controlled by continuous Fe³⁺-Fe²⁺ solid-melt equilibria. These equilibria will involve increases in$f_{O_2 }$ relative to the graphite saturation surface and relative to FMQ. Melts that finally segregate from that source and erupt on the earth's surface may then be significantly more oxidized than their mantle sources at depth prior to partial melting. The extent of melt oxidation relative to the mantle source may be directly proportional to the depth of graphite exhaustion in the mantle source.