- Research article
- Open Access
Melting phase relations in the MgSiO3–CaSiO3 system at 24 GPa
© The Author(s). 2017
Received: 13 July 2017
Accepted: 14 November 2017
Published: 28 November 2017
The Earth’s lower mantle is composed of bridgmanite, ferropericlase, and CaSiO3-rich perovskite. The melting phase relations between each component are key to understanding the melting of the Earth’s lower mantle and the crystallization of the deep magma ocean. In this study, melting phase relations in the MgSiO3–CaSiO3 system were investigated at 24 GPa using a multi-anvil apparatus. The eutectic composition is (Mg,Ca)SiO3 with 81–86 mol% MgSiO3. The solidus temperature is 2600–2620 K. The solubility of CaSiO3 component into bridgmanite increases with temperature, reaching a maximum of 3–6 mol% at the solidus, and then decreases with temperature. The same trend was observed for the solubility of MgSiO3 component into CaSiO3-rich perovskite, with a maximum of 14–16 mol% at the solidus. The asymmetric regular solutions between bridgmanite and CaSiO3-rich perovskite and between MgSiO3 and CaSiO3 liquid components well reproduce the melting phase relations constrained experimentally.
Determining melting phase relations in natural rock-forming, multi-component systems at high pressures is essential to understand the origin and consequences of the deep mantle melting (e.g., Williams and Garnero 1996) and the crystallization of the deep magma ocean of the early Earth (e.g., Ito et al. 2004). Although numerous melting experiments have been performed for these purposes, the range of parameter space (pressure (P), temperature (T), and compositions (X)) studied is limited for tracing the crystallization of the magma ocean and for a comprehensive investigation into the origin of the deep mantle melting. Indeed, it is not practical to cover the vast P–T–X space relevant to deep Earth events based on experiments alone. In contrast, experiments in simpler systems combined with thermodynamic models provide a powerful tool to understand the melting chemistry in the multi-component system. Such comprehensive datasets and thermodynamic models have been constructed for pressures up to those of the mantle transition zone (Gasparik 2003).
In addition, the occurrence of a certain degree of temperature gradient is inevitable in melting experiments using laser-heated diamond anvil cells or multi-anvil apparatuses, especially under the high pressures of the lower mantle. The temperature gradient obscures the melting chemistry in multi-component systems because phase segregation changes the bulk compositions of interest. In these cases, only limited information, such as phase equilibria and element partitioning between the phases in contact, can be extracted by assuming local chemical equilibrium. This problem could be overcome by experiments in simple binary systems.
The lower mantle is composed of bridgmanite, ferropericlase, and CaSiO3-rich perovskite (Irifune 1994). Normal mid-ocean ridge basalt has a mineral assemblage of bridgmanite, CaSiO3-rich perovskite, a SiO2 phase, and an aluminous phase in the lower mantle (Hirose et al. 1999). Therefore, the binary melting phase relations between each mineral-constituting component are important in understanding the melting behaviors of the lower mantle. Previously, the MgO–MgSiO3 system was investigated at the lower-mantle pressures using a multi-anvil press (Liebske and Frost 2012), but little is known about the other systems.
This paper is aimed at establishing the melting phase relations in the MgSiO3–CaSiO3 system at 24 GPa, avoiding ambiguous implications for Earth science that require other melting phase relations such as MgO–CaSiO3. The eutectic composition, solidus temperature, and solubility between the two endmembers were investigated experimentally at 24 GPa. Although Gasparik (1996) has performed a melting experiment on the same system at 22.4 GPa, it is difficult to confirm melting from the backscattered electron image presented in his paper. In the present work, we re-investigate the system to find the interaction parameters between the MgSiO3 and CaSiO3 components by thermodynamic analysis in the framework for the endmembers (de Koker and Stixrude 2009; Stixrude and Lithgow-Bertelloni 2011; Bajgain et al. 2015) consistent with a previous work in the MgO-MgSiO3 system (Liebske and Frost 2012).
Chemical compositions of starting materials
Mg/(Mg + Ca)
High-pressure melting experiments
P = 24 GPa
Bdg + CaPv
Bdg + CaPv
[HT] Melt + Bdg, [LT] Bdg + CaPv
[HT] CaPv, [LT] Bdg + CaPv
Melt + CaPv
Melt + CaPv
Phase identification and chemical analysis
The recovered samples were embedded in epoxy resin and finely polished for subsequent analyses. Phase identification was conducted with a microfocus X-ray diffraction (XRD) apparatus (Rigaku MicroMax-007HF) using Cu Kα radiation. The sample surface was coated with carbon and analyzed using a field emission-type scanning electron microscope (FE-SEM; JEOL JSM7000F). Chemical compositions were measured using an energy-dispersive X-ray spectrometer with a silicon-drift detector (EDS; Oxford Instruments X-MaxN) attached to the FE-SEM at 15 kV and 1 nA with collection times of ~ 30 s. Mg2SiO4 forsterite was used as the compositional standard for Mg and Si, while CaSiO3 wollastonite was used for Ca. The grains of bridgmanite, CaSiO3-rich perovskite were analyzed in a focused mode with a beam size of less than 1 μm. The quenched melt was analyzed in a scanning mode larger than the spatial scale of heterogeneity.
Among many available thermodynamic approaches that can describe the melting behavior in a multi-component system, we followed that of Liebske and Frost (2012) to maintain consistency with the results for the MgO–MgSiO3 system, which has been studied extensively at high pressures, including lower-mantle pressures. Liebske and Frost (2012) used the chemical potentials given by Lithgow-Bertelloni and Stixrude (2005) and Stixrude and Lithgow-Bertelloni (2011) for the solid phases and by de Koker and Stixrude (2009) for the liquid phases; these papers constructed an internally consistent thermodynamic model, although the equations, parameterization, and parameter set differ slightly from those of Liebske and Frost (2012). For solids, we used the thermodynamic descriptions and parameter sets of Lithgow-Bertelloni and Stixrude (2005) and Stixrude and Lithgow-Bertelloni (2011) based on usability and consistency with other components. For liquids, we followed Liebske and Frost (2012) to maintain consistency with the MgO–MgSiO3 system. We describe all of these equations below for clarification.
Once the chemical potentials of the liquid and solid endmembers are evaluated, we can determine the interaction parameter W by fitting the experimental data to the above equations.
For liquids, we followed Liebske and Frost (2012) to maintain consistency. The expressions of the solid compression terms F cmp and P cmp were also applied to liquids, F cmp and P cmp . The thermal part is expressed as follows:
Thermodynamic properties of liquids
T 0 (K)
V 0 (cm3/mol)
K T0 (GPa)
C v (J/K/mol)
S 0 (J/K/mol)
F 0 (kJ/mol)
Thermodynamic properties of solids
T 0 (K)
V 0 (cm3/mol)
K T0 (GPa)
θ 0 (K)
F 0 (kJ/mol)
Results and discussion
X-ray diffraction and phase identification
Additional file 1: Figure S5 shows the XRD patterns of the run products. Intense peaks from Re were unavoidably observed in all the patterns due to the small sample sizes in this study. Peaks from bridgmanite were evidently observed in OS3097 and OS3099, indicating that the experiments were performed outside of the garnet stability region, which is consistent with a previous study that reported the phase transformation of a MgSiO3-rich composition from garnet to bridgmanite at 22.4 GPa (Gasparik 1996). Some weak peaks, presumably indexed to bridgmanite, were observed in OS3095 and OS3150 (e.g., Additional file 1: Figure S5e). The bridgmanite is interpreted as quench crystals from melt in the corresponding SEM images as shown later. CaSiO3-rich perovskite was not identified in all the samples since it should be amorphized at the ambient pressure.
Electron microscopic observation and compositional analysis
Chemical compositions of phases in run products
Mg/(Mg + Ca)
Melting phase relations in the MgSiO3–CaSiO3 system
The parameters were estimated using a linear least-squares method weighted by the compositional uncertainty. Therefore, the standard error of the fitting parameter, shown in parenthesis, does not include temperature uncertainty since the dataset from the literature did not describe how the temperature uncertainty was determined. The experimental data suggest that the interaction parameters approach to zero at high temperatures, suggesting an ideal behavior. These empirical (linear) fitting equations should not be extrapolated out from the experimental range. Figure 7 shows the resulting phase diagram from the thermodynamic calculations together with the experimental data. In prohibiting the extrapolation, the uncertainty of the thermodynamic model is described by the deviations from the experimental data (Figs. 1 and 7, Additional file 1: Figure S4).
Melting phase relations in the system MgSiO3–CaSiO3 were determined experimentally at 24 GPa. A thermodynamic model was constructed to reproduce the experimental data. The experimental dataset and the thermodynamic parameters derived in this study serve as the basis for understanding the complicated melting behaviors of the Earth’s lower mantle.
The advantage of establishing a thermodynamic model is that the degree of melting can be calculated at a certain P-T in a multi-component system. The seismic observations suggested a degree of melting of < 5–30% in ultra-low velocity zones at the base of the lower mantle (Williams and Garnero 1996). The estimation of the degree of melting depends on the wetting property and the elastic properties of the melt; conversely, these properties depend on the compositions of the melt; thus, they depend on the degree of melting. The degree of melting is also an important parameter in understanding the chemical differentiation during magma ocean crystallization. The melt-solid separation by rheological transition occurs at a crystal fraction of approximately 60% (e.g., Abe 1995). Therefore, it is of great importance to understand the phase relations together with the knowledge of the degree of melting at relevant pressures and temperatures. However, there are some difficulties in obtaining the degree of melting by high-pressure melting experiments under thermal gradients because the observed degree of melting arises from the thermal gradients, not from the phase equilibrium.
We thank Takashi Taniuchi (Ehime Univ.) for discussions and Dr. Toru Shinmei (Ehime Univ.), Dr. Takeshi Arimoto (Ehime Univ.), Dr. Wei Sun (Ehime Univ.), and Dr. Steeve Gréaux (Ehime Univ.) for their assistance in our experiments. We thank the editor and the anonymous reviewers for their constructive comments and suggestions. We thank Dr. Hideharu Kuwahara (Ehime Univ.) and Dr. Chie Kato (Tokyo Tech.) for their assistance in the compositional analyses.
This work was supported by JSPS KAKENHI Grant Numbers JP15H05470, JP15H05830, and JP25220712.
RN and YZ designed the project. YZ and TI designed the experiments. YZ carried out the melting experiments and the chemical analysis. RN analyzed the data. RN and YZ wrote the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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