The role of C-O-H-F-Cl fluids in the making of Earth’s continental roots

The cratonic ‘roots’ of Earth's major continents extend to depths of over 160 km and have remained stable for more than 2.5 billion years due to buoyant, refractory harzburgites formed by Archean mantle melting. However, kimberlite-hosted harzburgite mantle xenoliths from some global cratons (e.g., Kaapvaal, Siberia, Slave, Rae and Tanzania) show unusual orthopyroxene and silica enrichment, alongside titanium depletion, which cannot be explained by simple melting processes (Boyd, 1989). These are abundant in the xenolith suites and were typically entrained at depths shallower than ~150 km. Many have escaped the pervasive carbonated silica undersaturated melt metasomatism that dominates the base of global cratonic lithosphere. The origins of the orthopyroxene-rich harzburgites have long been debated: hypotheses include high-pressure melting residues (Aulbach et al., 2011), komatiite melt interaction (Tomlinson & Kamber, 2021), or subduction-related silicic melts and fluids (Bell et al., 2005).

To further investigate the origin of the widespread excess silica in ancient mantle we analysed volatile (H₂O, F, Cl) contents by Secondary Ion Mass Spectrometry in well-characterised peridotites from the Kaapvaal craton. The orthopyroxene-rich harzburgites, including a diamond-bearing sample, show elevated volatile contents and depletions in Ti. The results of mass balance calculations suggest that the orthopyroxene-rich harzburgites formed by reactive infiltration of supercritical C-O-H fluids -- rich in silica, potassium, fluorine and chlorine but depleted in Ti -- fluxed from subducted oceanic lithosphere (carbonated pelites, eclogites and serpentinites). These findings highlight the role of C-O-H-F-Cl bearing fluids in shaping cratonic lithosphere globally and offer a new framework for understanding craton evolution, mantle metasomatism and diamond genesis in early Earth (Gibson et al., 2025).

 

Figure 1. Schematic illustration of the formation of excess orthopyroxene in Archean cratons. 

Aulbach, S., Stachel, T., Heaman, L. M., Creaser, R. A. & Shirey, S. B. (2011). Formation of cratonic subcontinental lithospheric mantle and complementary komatiite from hybrid plume sources. Contributions to Mineralogy and Petrology 161, 947–960.

Bell, D. R. et al. (2005). Silica and volatile-element metasomatism of Archean mantle: a xenolith-scale example from the Kaapvaal Craton. Contributions to Mineralogy and Petrology 150, 251–267.

Boyd, F. R. (1989). Compositional distinction between oceanic and cratonic lithosphere. Earth and Planetary Science Letters 96, 15–26.

Gibson, S.A., Jackson, C.J., Crosby, J.C. & Day, J.A.F. (2025). The role of C-O-H-F-Cl fluids in the making of Earth’s continental roots. Nat Commun doi:10.1038/s41467-025-62888-3

Tomlinson, E. L. & Kamber, B. S. (2021). Depth-dependent peridotite-melt interaction and the origin of variable silica in the cratonic mantle. Nat Commun 12, 1082.