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-()cean ridge basalts (MORBs), their ultimate sources are predicted to be diapirs that, if adiabatically decompressed from initially solid mantle, were more than 70Cf'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 magnesiowiistite 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.

Nonfractional crystallization of a terrestrial magma ocean

“…Together with a small difference in slope between the adiabats and the phase boundaries at high pressures, this explains why the partially molten region extends over a large depth interval, which is even larger than the interval predicted by Miller et al [1991].…”

High-pressure radiative conductivity of dense silicate glasses with potential implications for dark magmas

“…Knowledge of the pressure dependence of optical absorption bands of iron-bearing dense silicate glasses is therefore essential to understand changes in high-pressure electronic configurations of the glasses and thereby to infer the radiative part of the thermal conductivity. Two types of silicate glasses were used as analogues for dense silicate melts at the CMB: one with (Mg 0.8 Fe 0.2 )SiO 3 composition (E-glass) to isolate the effect of iron 22 , and the other with a multicomponent basaltic composition (M-glass) (Table 1) to simulate a more realistic compositional system at the CMB 6,9,24,25 . In the optical absorption measurements, both samples visibly became optically darker with pressure, as shown in Fig.…”

Physical volcanology and geochemistry of Palaeoarchaean komatiite lava flows from the western Dharwar craton, southern India: implications for Archaean mantle evolution and crustal growth

“…The strong negative Hf, Zr, and Y anomalies on primitive mantle-normalized spider diagrams in the Al-depleted komatiites of Barberton, Dharwar, and Superior Province have been attributed to komatiite magma generation at great depths with majorite garnet in the residue (Lahaye et al 1995;Polat et al 1999;Jayananda et al 2008). Majorite garnet stability in mantle peridotite indicates pressures in the range 14-24 Gpa, corresponding to depths of magma generation below 400 km (Miller et al 1991;Herzberg 1992;Xie et al 1993;Fan and Kerrich 1997). Consequently, majorite retention in the residue results in strongly negative Hf, Zr, and Y anomalies.…”