Slab melting as a barrier to deep carbon subduction

Interactions between crustal and mantle reservoirs dominate the surface inventory of volatile elements over geological time, moderating atmospheric composition and maintaining a lifesupporting planet1. While volcanoes expel volatile components into surface reservoirs, subduction of oceanic crust is responsible for replenishment of mantle reservoirs2,3. Many natural, ‘superdeep’ diamonds originating in the deep upper mantle and transition zone host mineral inclusions, indicating an affinity to subducted oceanic crust4–7. Here we show that the majority of slab geotherms will intersect a deep depression along the melting curve of carbonated oceanic crust at depths of approximately 300 to 700 kilometres, creating a barrier to direct carbonate recycling into the deep mantle. Low-degree partial melts are alkaline carbonatites that are highly reactive with reduced ambient mantle, producing diamond. Many inclusions in superdeep diamonds are best explained by carbonate melt–peridotite reaction. A deep carbon barrier may dominate the recycling of carbon in the mantle and contribute to chemical and isotopic heterogeneity of the mantle reservoir

Nitrogen speciation in upper mantle fluids and the origin of Earth's nitrogen-rich atmosphere

Volatile elements stored in the mantles of terrestrial planets escape through volcanic degassing, and thereby influence planetary atmospheric evolution and habitability. Compared with the atmospheres of Venus and Mars, Earth's atmosphere is nitrogen-rich relative to primordial noble gas concentrations1, 2, 3. The compatibility of volatile elements in mantle minerals versus melts and fluids controls how readily these elements are degassed. However, the speciation of nitrogen in mantle fluids is not well constrained4, 5, 6. Here we present thermodynamic calculations that establish the speciation of nitrogen in aqueous fluids under upper mantle conditions. We find that, under the relatively oxidized conditions of Earth's mantle wedges at convergent plate margins7, 8, 9, nitrogen is expected to exist predominantly as N2 in fluids and, therefore, be degassed easily. In contrast, under more reducing conditions elsewhere in the Earth's upper mantle and in the mantles of Venus and Mars, nitrogen is expected predominantly in the form of ammonium (NH4+) in aqueous fluids. Ammonium is moderately compatible in upper mantle minerals10, 11 and unconducive to nitrogen degassing. We conclude that Earth's oxidized mantle wedge conditions—a result of subduction and hence plate tectonics—favour the development of a nitrogen-enriched atmosphere, relative to the primordial noble gases, whereas the atmospheres of Venus and Mars have less nitrogen because they lack plate tectonics.</p

Research data supporting "Synthesis and stability of xenon oxides Xe2O5 and Xe3O2 under pressure"

Crystallographic information files of structures in the Xe-O binary system predicted to be stable at high pressures (83 GPa, 150 GPa and 200 GPa). Pressures and space groups given in file names. Structures found using Ab Initio Random Structure Searching (AIRSS) coupled with Density Functional Theory (DFT) methods.This work was supported by the EPSRC [research grant numbers EP/J017639/1 and EP/K014560/1], European Synchrotron Radiation Facility [grant numbers HS-4067 and HC-767] and Royal Society Wolfson Research Merit Award (for Chris J. Pickard)