Atmospheric oxygenation caused by a change in volcanic degassing pressure

International audienceThe Precambrian history of our planet is marked by two major events: a pulse of continental crust formation at the end of the Archaean eon and a weak oxygenation of the atmosphere (the Great Oxidation Event) that followed, at 2.45 billion years ago. This oxygenation has been linked to the emergence of oxygenic cyanobacteria1,2 and to changes in the compositions of volcanic gases3,4, but not to the composition of erupting lavas--geochemical constraints indicate that the oxidation state of basalts and their mantle sources has remained constant since 3.5 billion years ago5,6. Here we propose that a decrease in the average pressure of volcanic degassing changed the oxidation state of sulphur in volcanic gases, initiating themodern biogeochemical sulphur cycle and triggering atmospheric oxygenation. Using thermodynamic calculations simulating gas-melt equilibria in erupting magmas, we suggest that mostly submarine Archaean volcanoes produced gases with SO2/H2S,1 and low sulphur content. Emergence of the continents due to a global decrease in sea level and growth of the continental crust in the late Archaean then led to widespread subaerial volcanism, which in turn yielded gases much richer in sulphur and dominated bySO2. Dissolution of sulphur in sea water and the onset of sulphate reduction processes could then oxidize the atmosphere

Diffusion and partition coefficients of minor and trace elements in magnetite as a function of oxygen fugacity at 1150 oC

Lattice diffusion coefficients and partition coefficients have been determined for Li, Mg, Al, Sc, Ti, Cr, V, Mn, Co, Ni, Cu, Zn, Ga, Y, Zr, Nb, Mo, In, Lu, Hf, Ta and U in single crystals of natural magnetite as a function of oxygen fugacity (fO2) at 1150 °C and 1 bar by equilibration with a synthetic silicate melt reservoir. Most experiments were run for twelve hours, which was sufficient to generate diffusion profiles from 25 to > 1000 µm in length. The results were checked at one condition with two additional experiments at 66.9 and 161 h. The profiles were analysed using scanning laser-ablation inductively-coupled-plasma mass-spectrometry. Diffusion coefficients (D) were calculated by fitting data from individual element diffusion profiles to the conventional diffusion equation for one-dimensional diffusion into a semi−infinite slab with constant composition maintained in the melt at the interface. Equilibrium magnetite/melt partition coefficients are given by the ratio of the interface concentrations to those in the melt. Plots of log D as a function of log fO2 produce V-shaped trends for all the investigated elements, representing two different mechanisms of diffusion that depend on (fO2)−2/3 and (fO2)2/3. Diffusion coefficients at a given fO2 generally increase in the order: Cr < Mo ≈ Ta < V < Ti < Al < Hf ≈ Nb < Sc ≈ Zr ≈ Ga < In < Lu ≈ Y < Ni < U ≈ Zn < Mn ≈ Mg < Co < Li < Cu. Thus, Cu contents of magnetites are most susceptible to diffusive reequilibration, whereas the original content of Cr should be best preserved

A nickel for your planet's thoughts

International audienceVariability of iron isotopes among planetary bodies may reflect their accretion or differentiation histories. Experiments suggest nickel may be the ingredient controlling iron isotope signatures, supporting fractionation during core formation

Accretion of the Earth and segregation of its core.

The Earth took 30-40 million years to accrete from smaller 'planetesimals'. Many of these planetesimals had metallic iron cores and during growth of the Earth this metal re-equilibrated with the Earth's silicate mantle, extracting siderophile ('iron-loving') elements into the Earth's iron-rich core. The current composition of the mantle indicates that much of the re-equilibration took place in a deep (&gt; 400 km) molten silicate layer, or 'magma ocean', and that conditions became more oxidizing with time as the Earth grew. The high-pressure nature of the core-forming process led to the Earth's core being richer in low-atomic-number elements, notably silicon and possibly oxygen, than the cores of the smaller planetesimal building blocks