Evolution of Mantle Magmatism and Formation of the Ultrabasic-Basic Rock Series: Importance of Peritectic Reactions of the Rock-Forming Minerals

Abstract

Peritectic mechanisms for fractional evolution of the upper mantle, transition zone and lower mantle magmatism and petrogenesis of the ultrabasic-basic rock series are justified in theory and experiments. Compositions of the ultrabasic upper mantle peridotite and pyroxenite rocks belong to the multicomponent olivine–orthopyroxene–(jadeite–poor clinopyroxene)–garnet system. On experimental evidence at 4 GPa, evolution of the primary olivine-normative magma, generated at the system, must be accompanied by a disappearance of the orthopyroxene through the invariant peritectic reaction with melt and formation of the univariant cotectic assembly olivine + clinopyroxene + garnet + melt. The problem of a termination of the olivine formation arises in view of the fact that compositions of the basic eclogite and grospydite rocks belong to the silica-oversaturated (jadeite-rich omphacite)–garnet–corundum–coesite system. A further ultrabasic-basic evolution of the system has been made possible with fractional crystallization of the evolved melts which compositions become progressively enriched with jadeitic component. Experimental study of melting relations on the olivine–diopside–jadeite–garnet system at 6 GPa demonstrates a disappearance of the olivine through the invariant peritectic reaction with jadeite-rich melt and formation of the basic univariant cotectic omphacite + garnet + melt. Hence the ultrabasic-basic evolution of the primary upper mantle ultrabasic magma is effected principally by the peritectic reactions of orthopyroxene and olivine when coupled with the regime of fractional crystallization. In the case of the transition zone, compositions of the ultrabasic ringwoodite-bearing and basic stishovite-bearing rocks are rated to the ringwoodite–(majoritic garnet)–magnesiowustite–stishovite system. It has been found experimentally at 20 GPa that the ultrabasic-basic evolution of the primary ringwoodite-normative magma in the polythermal section ringwoodite—(2FeO + SiO2) of the boundary MgO–FeO–SiO2 system is attended with a disappearance of ringwoodite in consequence of its invariant peritectic reaction with melt and formation of the univariant cotectic assembly stishovite + magnesiowustite + melt. For the lower mantle conditions, compositions of the ultrabasic bridgmanite-bearing and basic stishovite-bearing rocks are concerned with the bridgmanite–Ca–perovskite–(periclase ↔ wustite)ss–stishovite system. Experimentally at 24 GPa we find that ultrabasic-basic evolution of the primary bridgmanite-normative magma in the polythermal section bridgmanite—(FeO + SiO2) of the boundary MgO–FeO–SiO2 system has been marked by a disappearance of bridgmanite through its invariant peritectic reaction with melt and formation of the univariant cotectic assembly stishovite + magnesiowustite + melt (effect of “stishovite paradox”). The ultrabasic-basic evolution of the primary transition zone and lower mantle ultrabasic magmas, effected by the peritectic reactions of ringwoodite and bridgmanite, respectively, can result exclusively in the regime of fractional crystallization. The peritectic reactions of the upper mantle orthopyroxene and olivine, transition zone ringwoodite and lower mantle bridgmanite are also operable for genesis of the diamonds and associated minerals in the relevant silicate–(±oxide)–carbonate–carbon systems subjected to the ultrabasic-basic fractional evolution.