The absence of low-thermal gradients in old metamorphic rocks (<350 °C GPa⁻¹) has been used to argue for a fundamental change in the style of plate tectonics during the Neoproterozoic Era. Here, we report data from an eclogite xenolith in Paleoproterozoic carbonatite in the North China craton that argues for cold subduction as early as 1.8 Ga. The carbonatite has a sediment-derived C isotope signature and enriched initial Sr–Nd isotope composition, indicative of ocean-crust components in the source. The eclogite records peak metamorphic pressures of 2.5–2.8 GPa at 650–670 °C, indicating a cold thermal gradient, 250(±15) °C GPa⁻¹. Our data, combined with old low-temperature events in the West African and North American cratons, reveal a global pattern that modern-style subduction may have been established during the Paleoproterozoic Era. Paleoproterozoic carbonatites are closely associated with granulites and eclogites in orogens worldwide, playing a critical role in the Columbia supercontinent amalgamation and deep carbon cycle through time.

The stability field of siderite has been determined up to 10 GPa. Decarbonation of siderite occurs at pressures below 6 GPa with a Clapeyron slope of about 0.0082 GPa/K. At higher pressure, we observed direct melting of siderite without decarbonation. The melting temperature is about 1550 °C at 10 GPa. Our experimental results, compared with previous studies on the decomposition curve of magnesite, indicate that Fe has a significant effect on the stability of magnesite-siderite solid solutions under upper mantle conditions. The reaction products are strongly dependent on the oxygen fugacity of the system. The disproportionation reaction during decomposition of siderite might be an important mechanism to explain the stability of carbon as graphite (diamond) in the Earth’s mantle.

Novel discovered majorite from a depth of at least 380 km provides unique window into the redox state and dynamics of the mantle.