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

Interaction between carbonatite melt and peridotite is studied experimentally by melting samples of interlayered peridotite-carbonatite-peridotite in graphite containers at 1200-1350 degree C and 5.5-7.0 GPa 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 150 h duration in order for equilibrium to be achieved. The interaction produced carbonatitic melts with low SiO sub(2) ( less than or equal to 7 wt.%) and high alkalis. At 1200 degree C, melt-peridotite interaction occurs through Mg-Ca exchange, resulting in elimination of orthopyroxene and crystallization of magnesite and clinopyroxene. At 1350 degree C hybridization of the carbonatite and magnesite-bearing peridotite melts occurred with consumption of clinopyroxene and magnesite, and crystallization of orthopyroxene at MgO/CaO greater than or equal to 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 Cr sub(2)O sub(3) changes in garnet produced from the melt-harzburgite interaction at 1200 and 1350 degree 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 5.5-7.0 GPa and 1200-1350 degree 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 degree C on separation from the lithospheric peridotite source and ~ 1000 degree C on eruption.