Volcanic activity worldwide and carbonaceous phases in mantle xenoliths demonstrate that carbon is present in trace amounts in the modern mantle. We have determined that carbon solubility in olivine at 0.1 GPa and 1200 °C is below our detection limit (30 ppm by weight) but dissolves in excess of 100 ppm wt at 3.0 GPa, 1180–1530 °C. The significant solubility in mantle silicates may explain the rarity of carbonate-bearing xenoliths and the extreme age of diamonds. We propose a model in which low-pressure melting during mantle convection scavenges hydrogen and carbon from the primitive mantle and thereby reduces the content of these volatiles to below the silicate saturation limit and restricts carbonaceous and hydrous phases to the roots of continents isolated from convection.

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