The Role of C-O-H-F-Cl Fluids in the Making of Earth’s Continental Roots

The deep cratonic ‘roots’ of Earth's major continents extend over 160 km and have remained stable for more than 2.5 billion years. This longevity is primarily due to the presence of low-density, refractory peridotites (harzburgites) that formed by extensive mantle melting in the Archean. However, mantle harzburgites from some global cratons (e.g., Kaapvaal, Siberia, Slave, Rae and Tanzania) often show unusual enrichments in orthopyroxene and silica, alongside titanium depletion, which cannot be explained by simple melting processes. The proposed origins of the orthopyroxene-rich harzburgites are highly controversial and include high-pressure melting residues, komatiite interactions, or subduction-related silicic melts and fluids. The uncertainty primarily arises because of the inherent difficulties involved in reconstructing the tectonic settings responsible for the stabilisation of early Earth. To investigate further, we analysed volatile (H₂O, F, Cl) contents in Kaapvaal craton peridotites. Our new results show that harzburgites rich in orthopyroxene, including a diamond-bearing sample, also have higher volatile concentrations. This suggests infiltration by super-critical C-O-H fluids -- rich in silica, fluorine and chlorine fluxed from subducted oceanic lithosphere (carbonated pelites and serpentinites) -- was important in driving olivine-to-orthopyroxene transformation and diamond formation under reduced conditions during the Archean. These findings highlight the role of C-O-H-F-Cl bearing fluids in shaping cratonic lithosphere and offer a new framework for understanding craton evolution, mantle metasomatism and diamond genesis in early Earth.