CO₂-Undersaturated Melt Inclusions From the South West Indian Ridge Record Surprisingly Uniform Redox Conditions

The behavior of Fe3+ during mantle partial melting strongly influences the oxidation state of the resulting magmas, with implications for the evolution of the atmosphere's oxidation state. Here, we challenge a prevailing view that low-degree partial melts are more oxidized due to the incompatible behavior of Fe3+. Our study is based on measurements of Fe3+/& sum;Fe along with major, minor, trace and volatile elements in olivine- and plagioclase-hosted melt inclusions of CO2 undersaturated mantle melts in South West Indian Ridge lava. These inclusions record minimum entrapment pressures equivalent to depths up to 10 km below the seafloor, record magma ascent rates of 0.03-0.19 m/s, and display exceptionally high CO2/Ba, CO2/Rb, and CO2/Nb ratios, indicative of a CO2-rich mantle source. Accounting for fractional crystallization, we find a uniform melt oxidation state (with an Fe3+/Sigma Fe at 0.140 +/- 0.005 at MgO = 10 wt.%) that displays no systematic variation with major, minor, volatile or trace element contents, thus providing no evidence for a relationship between the degree of partial melting and Fe3+/Sigma Fe. This can be explained by efficient buffering of Fe3+/& sum;Fe and fO(2) of mid-ocean ridge basalt melts by their surrounding mantle and/or a decrease in the bulk peridotite-melt Fe2O3 partition coefficient with increasing partial melting. We conclude that changes in the Earth's upper mantle temperature over geological time need not have affected the oxidation state of volcanic products or of the atmosphere.