- Akkretion <Geologie> (1) (remove)
- Mantle-Melting at High Pressure: Experimental Constraints on Magma Ocean Differentiation (2005)
- In this study geochemical processes and a geophysical parameter were investigated that are relevant to the crystallisation of a deep magma ocean, that likely existed during Earth’s accretion. The melting relations of potential magma ocean compositions, such as peridotitic and chondritic bulk compositions, were investigated using multianvil apparatus at pressures of 25-26 GPa and temperatures up to 2400°C. Compositional effects on the melting relations were investigated by varying bulk Mg/Si and Mg/(Mg+Fe) ratios (the latter is denoted as Mg-number, Mg#). At 26 GPa, peridotite liquids show a crystallisation sequence of ferropericlase (Fp) followed down temperature by Mg-silicate perovskite (MgPv) + Fp, which is in contrast to the sequence of MgPv followed by MgPv + Fp in chondritic composition. The melting relations along the different compositions depend primarily on the bulk Mg/Si ratio and not on the Mg#. Melting relations and eutectic compositions were studied in the simple binary MgO-SiO2 system between 10 and 26 GPa. Combining the new results with previously published data shows that the eutectic composition between Mg2SiO4 and MgSiO3, up to 20 GPa, moves towards MgO with increasing pressure. Between 20 and 23 GPa the direction in which the eutectic is moving with pressure reverses. At higher pressures, this trend is again reversed and the eutectic composition moves towards MgO. The multiple changes in the direction in which the eutectic is moving as a function of pressure explains qualitatively the differences in liquidus phase relations in the more complex peridotite and chondrite compositions. The effect of bulk chemical composition on the partitioning of major, minor and trace elements between MgPv and coexisting silicate melts was investigated using micro-beam techniques. MgPv/melt partition coefficients for Mg (DMg) and Si (DSi) are related to the melt Mg/Si ratio, such that DSi becomes smaller than DMg at chondritic Mg/Si melt ratios. This shows that the Earth’s upper mantle Mg/Si ratio is unlikely to be derived from chondrites by MgPv fractionation. Partition coefficients of tri- and tetravalent elements increase with increasing Al concentration of MgPv. A crystal chemical model indicates that Al3+ substitutes predominantly onto the Si-site in MgPv, but most other elements substitute onto the Mg-site. This is consistent with a charge-compensating substitution mechanism. A crystal fractionation model, based on refractory lithophile element ratios, is developed to constrain the amount of MgPv and Ca-silicate perovskite (CaPv) that could have fractionated in a magma ocean and could still be present as a chemical heterogeneity in the lower mantle today. It is shown that a fractionated crystal pile composed of 96% MgPv and 4% CaPv could comprise up to 13 wt% of the entire mantle. Fe3+/Fetotal ratios have been determined for MgPv, crystallised at temperatures below and above the peridotite solidus, using Mössbauer and electron-energy-loss spectroscopy. The amount of Fe3+ in MgPv is positively correlated to the Al content of this phase. In recovered samples, homogeneously distributed Fe-rich metal on the sub-micron scale was observed on grain boundaries, although the MgPv has Fe3+/Fetotal ratios between 0.2 and 0.5. This suggests that the amount of Fe3+ in MgPv is independent of oxygen fugacity and that the presence of Fe-rich metal in the samples is the result of disproportionation of FeO to Fe3+ and Fe-metal. This has potentially implications for the mantle oxidation state and the mantle geochemistry during magma ocean solidification. The viscosity of peridotite liquid, as an analogue for a magma ocean composition, was investigated at high pressure using in-situ falling sphere viscometry. Experiments were performed between 2.5 and 13 GPa at temperatures between 2043 to 2523 K. Measured viscosities range from 0.018 (±0.003) to 0.13 (±0.01) Pa s. Up to 9 GPa the data indicate an isothermal increase in viscosity with increasing pressure but viscosity decreases between 9 and 13 GPa at constant temperature. The observed change in the pressure dependence of the viscosity is likely associated with structural changes in the liquid upon compression. The new high pressure data are combined with 1 bar viscosities for peridotite liquid (Dingwell et al. 2004), and a non-Arrhenian Vogel-Fulcher-Tamman equation, to which an empirical pressure-dependent term has been added, is presented to parameterize all experimental data. This approach reproduces measured viscosities to within 0.08 log10-units on average. The model can be used to calculate magma ocean viscosities to depths of 400 km. At likely magma ocean temperatures, viscosities down to transition zone pressures are extremely low and comparable to water at room temperature. The results of the different aspects of this study were used to investigate magma ocean crystallisation and its effect on the geochemistry and the evolution of the Earth’s mantle.