The constancy of upper mantle fO 2 through time inferred from V/Sc ratios in basalts

Abstract

The geochemical behaviors of V and Sc during partial melting are more similar to each other than to most other elements but, in detail, the partitioning of V is redox-sensitive whereas that of Sc is not. The V/Sc ratio in basalts is shown here to be a sensitive and robust recorder of the oxygen fugacity (fO 2) of their mantle source regions. The use of V/Sc ratios differs from the use of V and/or Cr elemental systematics in basalts because the latter, strictly speaking, provide information on the fO 2 of the erupted magma and not necessarily that of the mantle source. V/Sc ratios on the other hand can “see through” magmatic differentiation processes, such as olivine crystallization, and therefore, provide a better memory of the fO 2 of the mantle. The V/Sc ratios of mid-ocean ridge basalts (MORBs) are shown here to average 6.74±1.11 (1σ), which constrains the fO 2 of the modern mantle to roughly 0.3±0.5 (1σ) log units below the fayalite–magnetite–quartz (FMQ) buffer. Archean basalts (up to 3.5 Ga) are shown to have identical V/Sc ratios (6.34±0.62, 1σ) to within error indicating that the fO 2 of the mantle source to Archean basalts differ from that of modern convecting mantle by no more than 0.3 log unit. The constraints based on V/Sc systematics represent an improvement on studies using V or Cr elemental systematics, which constrain the fO 2 of Archean and modern mantles to be identical, but only to within ±0.5 to 1 log unit. These observations show that there has been little or no secular evolution of the fO 2 of the upper mantle, at least since the early Archean.