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

(Table 1) Compositions of magnetites and ferric hydroxides from sediments from DSDP Holes 31-301 and 31-296

Twenty-eight specimens of clastogenic magnetites and ferric hydroxides recovered by the “Glomar Challenger”. from Cenozoic deposits in the deep-sea basins of the Japan and Philippine seas were examined using an electron probe. Paragenesis of trace elements in these minerals suggests their source to be tholeiitic basalts of continental type and serpentinites. The latter may be an indication of the presence of an ophiolite complex within the deep-sea basins of marginal seas