EXPERIMENTAL EVIDENCE FOR LIQUID IMMISCIBILITY IN THE MODEL SYSTEM CaCO3 - PYROPE - PYRRHOTITE AT 7.0 GPa: THE ROLE OF CARBONATITE AND SULFIDE MELTS IN DIAMOND GENESIS

Abstract

We studied the melting relations in the ternary system CaCO 3 (aragonite) – Mg 3 Al 2 Si 3 O 12 (pyrope) – FeS (pyrrhotite) at a pressure of 7.0 GPa and over the temperature interval 1130–1925°C along the boundary joins Mg 3 Al 2 Si 3 O 12 – CaCO 3 , Mg 3 Al 2 Si 3 O 12 – FeS, and CaCO 3 – FeS, as well as the binary polythermal section (CaCO 3 ) 50 (Mg 3 Al 2 Si 3 O 12 ) 50 – FeS. Some additional runs were performed with a mixture of aragonite and magnesite of dolomite stoichiometry. The melting curve in these phase diagrams was constructed. Melting in the silicate – carbonate – sulfide systems is characterized by the effects of complete miscibility of silicate and carbonate melts and immiscibility of the homogeneous carbonate–silicate melt with a sulfide melt. The solubility of silicate and carbonate components in sulfide melts is negligible. Both carbonate–silicate and sulfide melts with dissolved oversaturated carbon are highly effective for diamond nucleation. Based on the experimental evidence, carbonatite (carbonate–silicate) melts are most capable to form mantle-derived diamond, with silicate and carbonatite primary inclusions. But the immiscible sulfide melts may be also involved in diamond formation without silicate and carbonate inclusions and with specific physical features. Such functions of carbonatite and sulfide parental media in the formation of the mantle-derived diamond satisfy the “syngenesis” experimental criterion and the mineralogical data.