Slab–mantle interaction, carbon transport, and kimberlite generation in the deep upper mantle

Abstract

Low-degree partial melts from deeply subducted, carbonated ocean crust are carbonatite liquids with ∼35–47 wt% CO2. Their reactions with the overlying mantle regulate the slab–mantle interaction and carbon transport in the deep upper mantle but have not been investigated systematically. Here we present new multi-anvil experiments and parameterized phase relation models to constrain the fate of slab-derived carbonatite melts in the upper mantle. The experiments were conducted at 7 GPa/1400°C and 10 GPa/1450°C, and used starting compositions mimicking the ambient mantle infiltrated by variable carbonatite fluxes (0–45 wt%) from the slab surface. Kimberlitic melts (CO2 = 14–32 wt%, SiO2 = 15–33 wt%, and MgO = 20–29 wt%) were produced from experiments with 5.8–25.6 wt% carbonatite influxes. Experimental phase relations demonstrate a reactive melting process in which the carbonatite influx increases in proportion by dissolution of olivine, orthopyroxene, garnet and precipitation of clinopyroxene. This manifests a feasible mechanism for slab-derived carbonatite melts to efficiently transport in the ambient mantle through high-porosity channels. The melt and mineral fractions from this study and previous phase equilibria experiments in peridotite + CO2±H2O systems were empirically parameterized as functions of temperature (900–2000°C), pressure (3–20 GPa), and bulk compositions (e.g., CO2 = 0.9–17.1 wt% and Na2O + K2O = 0.27–2.51 wt%). Applications of the phase relation models to prescribed melting processes indicate that reactive melting of a carbonatite-fluxed mantle source could produce kimberlitic melts with diverse residual lithologies under various melting conditions. However, reactive melting at the slab–mantle interface can only commence when the slab-released carbonatite melt conquers the carbonation freezing front, i.e., the peridotite solidi suppressed by infiltration of CO2-rich melts in an open system. Depending on temperatures and local influxes, reactive melting and carbonation/redox freezing can occur simultaneously above the slab–mantle interface, yielding heterogeneous lithologies and redox conditions as well as various time-scales of carbon transport in Earth's mantle.