Carbonatite melt–peridotite interaction at 5.5–7.0 GPa: Implications for metasomatism in lithospheric mantle

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Interaction between carbonatite melt and peridotite is studied experimentally by melting samples of interlayered peridotite–carbonatite–peridotite in graphite containers at 1200–1350°C and 5.5–7.0GPa in a split-sphere multianvil apparatus. Starting compositions are lherzolite and harzburgite, as well as carbonatite which may form in the upper part of a slab or in a plume-related source. Most experimental runs were of 150h duration in order for equilibrium to be achieved.The interaction produced carbonatitic melts with low SiO2 (≤7wt.%) and high alkalis. At 1200°C, melt–peridotite interaction occurs through Mg–Ca exchange, resulting in elimination of orthopyroxene and crystallization of magnesite and clinopyroxene. At 1350°C hybridization of the carbonatite and magnesite-bearing peridotite melts occurred with consumption of clinopyroxene and magnesite, and crystallization of orthopyroxene at MgO/CaO≥4.3. The resulting peridotite-saturated melt has Ca# (37–50) depending on primary carbonatite composition. Compositions of silicate phases are similar to those of high-temperature peridotite but are different from megacrysts in kimberlites. CaO and Cr2O3 changes in garnet produced from the melt–harzburgite interaction at 1200 and 1350°C perfectly match the observed trend in garnet from metasomatized peridotite of the Siberian subcontinental lithospheric mantle. K-rich carbonatite melts equilibrated with peridotite at 5.5–7.0GPa and 1200–1350°C correspond to high-Mg inclusions in fibrous diamond. Carbonatite melt is a weak solvent of entrained xenoliths and therefore cannot produce kimberlitic magma if temperatures are ~1350°C on separation from the lithospheric peridotite source and ~1000°C on eruption.