Sublithospheric diamonds extend Paleoproterozoic record of cold deep subduction into the lower mantle

Cold deep subduction – a hallmark of modern-style plate tectonics – is the dominant mechanism for recycling crustal materials into Earth's deep mantle, potentially changing its composition, heat budget and dynamics. The onset of cold deep subduction remains contentious and is largely constrained by crustal rocks, despite subduction being driven by mantle processes. Here we use the chemical and chronological information from sublithospheric mantle-derived minerals entrapped in diamonds from the DO-27 kimberlite, Slave Craton, Canada, to better constrain the timing and products of cold deep subduction reaching Earth's lower mantle. DO-27 sublithospheric diamonds preserve a complete lower mantle mineral assemblage: Ca-silicates (retrogressed CaSi-perovskite) ± enstatite (retrogressed bridgmanite) ± ferropericlase. The high Mg#s (molar Mg/(Mg+Fe)) of retrogressed bridgmanite (median Mg#=95) and ferropericlase (median Mg#=86) implicate a harzburgitic host rock for the diamonds. This strongly melt-depleted signature contrasts with the high abundance of Ca-silicate inclusions, some of which are variably enriched in incompatible elements and have enriched Sr-Nd-Pb isotopic signatures that collectively indicate the host rock was metasomatized by carbonatitic melts released from a relatively cold subducting slab in the lower mantle. The U-Pb systematics of the Ca-silicates define two ages of diamond crystallisation: 998 ± 18 Ma and 1679 ± 13 Ma. This is the first direct evidence of cold deep subduction in Earth's lower mantle at ∼1.7 Ga (Paleoproterozoic), in agreement with the oldest crustal low-temperature, high-pressure eclogites and is much earlier than previous Neoproterozoic estimates.