Carbonatitic Melts and Their Role in Diamond Formation in the Deep Earth

Abstract

Carbonatitic high-density fluids and carbonate mineral inclusions in lithospheric and sub-lithospheric diamonds reveal comparable compositions to crustal carbonatites and, thus, support the presence of carbon-atitic melts to depths of at least the mantle transition zone (~410–660 km depth). Diamonds and high pressure–high temperature (HP–HT) experiments confirm the stability of lower mantle carbonates. Experiments also show that carbonate melts have extremely low viscosity in the upper mantle. Hence, carbonatitic melts may participate in the deep (mantle) carbon cycle and be highly effective metasomatic agents. Deep carbon in the upper mantle can be mobilized by metasomatic carbonatitic melts, which may have become increasingly volumetrically significant since the onset of carbonate subduction (~3 Ga) to the present day.

Evolution of Carbonatite Magmas in the Upper Mantle and Crust

“…Such integrated studies are providing new insights into the nature and origin of the diverse spectrum of carbonatites, the role of carbonate in the mantle, and the sources of carbon in carbonatites. Similarly, Ca isotopic compositions of carbonatites (Amsellem et al 2020) and chemical and isotopic compositions of inclusions and their diamond host (e.g., using O isotopes) (Timmerman et al 2021 this issue) are providing increasing evidence of recycling of crustal carbonates during subduction into the deep, convecting mantle as a major source of carbon in carbonatite rocks.…”

Mineral Assemblage of Olivine-Hosted Melt Inclusions in a Mantle Xenolith from the V. Grib Kimberlite Pipe: Direct Evidence for the Presence of an Alkali-Rich Carbonate Melt in the Mantle Beneath the Baltic Super-Craton

“…Numerous experimental works have shown that at P-T parameters of the lithospheric mantle, carbonate/alkaline-carbonate melts can be in equilibrium or in partial equilibrium with various mantle mineral assemblages (e.g., [47][48][49][50][51][52][53][54][55][56]). It is currently accepted that these melts can be generated by low degrees of partial melting of mantle rocks (e.g., [24,[57][58][59][60][61][62][63]).…”

“…Carbonate melts display unique physical properties, such as the lowest viscosities among known terrestrial melts, high mobility and wettability (e.g., [63]). Hence, carbonatitic melts may be highly effective metasomatic agents in the lithospheric mantle, as their low viscosity and high mobility permit percolation along grain boundaries of silicates over a large distance (e.g., [64]).…”