The Equation of State of a Molten Komatiite. 2. Application to Komatiite Petrogenesis and the Hadean Mantle

Abstract

New experimental data for the equation of state of a komatiitic liquid were used to model adiabatic melting in a peridotitic mantle. If komatiites are formed by >30% partial melting of a peridotitic mantle, then komatiites generated by adiabatic melting come from source regions that began their unmelted ascent in the lower transition zone (≈500–670 km) or the lower mantle (>670 km). The great depth of incipient melting implied by this model suggests that komatiitic liquids may form in a pressure regime where they are denser than their coexisting crystals, possibly the bulk mantle. Although komatiitic magmas are thought to separate from residual crystals in the mantle at a temperature ≈200°C greater than modem mid-ocean ridge basalts (MORBs), their ultimate sources are predicted to be diapirs that, if adiabatically decompressed from initially solid mantle, were more than 700°C hotter than the sources of MORBs and were derived from great depth. We also studied the evolution of an initially molten mantle, i.e., a magma ocean. Our model considers the thermal structure of the magma ocean, density constraints on crystal segregation, and approximate phase relationships for a nominally chondritic mantle. Crystallization will begin at the core-mantle boundary. Perovskite buoyancy at >70 GPa may lead to a compositionally stratified lower mantle with iron-enriched magnesiowüstite content increasing with depth. Large convective velocities in the magma ocean would prohibit crystal-liquid fractionation by settling or flotation until quiescent boundary layers form. Such boundary layers could form when the crystal content of the magma reaches a critical value (near 44 vol %) and, in the late stages of crystallization, around unmelted blocks of foundered protocrust. Matrix compaction and diapirism could also lead to fractionation effects. The upper mantle could be depleted or enriched in perovskite components relative to the bulk mantle. Olivine neutral buoyancy may lead to the formation of a dunite septum in the upper mantle, partitioning the ocean into upper and lower reservoirs, but this septum must be permeable.

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