Abstract
The MgO – FeO – SiO2 – CaO system is representative for the dominant lower-mantle ultramafic assemblage ferropericlase (MgO∙FeO)ss + bridgmanite (Mg,Fe)SiO3 + Ca-perovskite CaSiO3. Melting relations of the system were studied over its polythermal section (MgO)70(FeO)30 – (SiO2)70(FeO)30 at 24 GPa. A peritectic reaction of bridgmanite with melt was revealed to form the mafic assemblage stishovite SiO2 + periclase-wustite solid solution phases (MgO∙FeO)ss (effect of “stishovite paradox”). Experimental phase relations demonstrate the possibility of ultramafic-mafic fractional evolution of the lower-mantle magma and in situ formation of stishovite. Moreover, origin of ultradeep diamonds and associated phases in melts of the MgO – FeO – SiO2 – CaO – (Mg-Fe-Na-K-carbonate) – carbon system have been examined at 26 GPa. A key role of the peritectic reaction of bridgmanite in genesis of mafic inclusions in lower-mantle diamonds is verified.
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