Fig. 10: Schematic illustration of the formation of excess orthopyroxene in Archaean cratons.
From: The role of C-O-H-F-Cl fluids in the making of Earth’s continental roots
Hashed region illustrates where orthopyroxene-rich garnet harzburgites are most likely to form in relation to subduction zones. Devolatilisation of carbonated pelite, altered oceanic crust or serpentinite at subduction zones—via aqueous fluids, supercritical fluids or carbonated-silicate melts—provides the primary flux of C, H and halogens (F, Cl) into the convecting mantle73,105,106,107,108. Fluxing of the carbonated pelites by fluids from underlying basaltic crust (eclogites) and serpentinites may lead to the mobilisation, ascent and reactive infiltration of Si, Mg, C, F, H2O and Cl in the overlying lithospheric mantle to form excess orthopyroxene and volatile enrichment. Since the Archaean, some of this signature may have been overprinted by infiltrating small-fraction volatile-rich melts from the convecting mantle, especially at the base of the lithosphere. The diamond-graphite boundary is from ref. 109.
