Effects of low-pressure igneous processes and subduction on Fe3+/ΣFe and redox state of mantle eclogites from Lace (Kaapvaal craton)

Oxygen fugacites (fO2) relative to the fayalite-magnetite-quartz buffer (FMQ) range from Δlog⁡fO2 = FMQ-1.3 to FMQ-4.6. At those reducing conditions, the solubility of carbon in the fluids released by dehydration is higher than in fluids closer to FMQ. The implication is that Archean processes of C transport and deposition would have differed from those known in modern-style subduction zones, and diamond would have formed from methane-rich fluids. In addition, such reducing material could drive redox melting or freezing upon deep recycling and migration of CH4-bearing fluids into the ambient mantle.