The Redox History of the Earth's Mantle: Evidence from Ultramafic Lavas

Abstract

In order to determine the evolution of the redox state of the mantle, the oxygen fugacities of sixteen mantle-derived komatiitic and picritic systems, ranging in age from 3.55 Ga to present day, were determined using the redox-sensitive partitioning of V between olivine and komatiitic/picritic magma, a method refined by this study. The oxygen fugacity data for the studied systems was determined to reflect that of their respective mantle source regions. The dataset defines a well-constrained trend indicating an increase in oxygen fugacity of the bulk convecting mantle of 1.33±0.43 FMQ log units from 3.48 to 1.87 Ga, and nearly constant oxygen fugacity from 1.87 Ga to the present. The oxygen fugacity data for the 3.55 Ga Schapenburg komatiites, the mantle source region of which was shown to have been isolated from mantle convection within the first 30 Ma of the Solar System history, plot well above the trend defined by the data for the contemporaneous lavas. This anamolous data point likely reflects preservation of early-formed magma ocean redox heterogeneities until at least the Paleoarchean.

The observed increase in the oxygen fugacity of the mantle requires admixture of a likely geochemically detectable amount of oxidized material. Three mechanisms were considered to account for the observed change in mantle redox state. The first two mechanisms: recycling of altered oceanic crust and venting of oxygen from the core due to inner core crystallization, were found to be unfeasible due to the large mass of recycled crust required, and the likely young age of the inner core, respectively. The third accessible mantle oxidation mechanism: convection-driven homogenization of an initially redox-heterogeneous mantle, is the most likely given available geochemical constraints. The new data presented here provide evidence for the mantle having triggered the Great Oxidation Event at ~2.4 Ga. We have additionally determined the Os isotopic and HSE systematics of 89 Ma komatiites from Gorgona Island, Colombia. The subset of these Gorgona samples that were also analyzed for oxygen fugacity shows BSE-like Os isotopes and HSE abundances in their mantle source, showing that their oxygen fugacity is likely representative of the mantle at 89 Ma.