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[SCG47-P08] Evaluation of Enstatite chondrite model based on melting relations in the system MgSiO3-SiO2
キーワード：エンスタタイトコンドライトモデル、MgSiO3–SiO2系、高圧、マグマオーシャン、マントル、コア中のSi含有量
Enstatite chondrite (E-chondrite) has been recommended as the source material of bulk Earth [e.g. 1] because the isotope compositions of the Earth, Moon and E-chondrite are indistinguishable over O, N, Mo, Ru, Os, Cr, and Ti. On the other hand, the melting relations of the system MgO-SiO2 as a representative of the mantle have been extensively studied since a pioneering work by Bowen and Anderson [2]. However, almost all of these works have been carried out on the compositions ranging from MgO to MgSiO3 because the bulk mantle composition has been assumed to be peridotitic or close to that derived from CI chondrite. Their molar ratios of SiO2/(SiO2+MgO) (which is denoted by XSi hereafter) are 0.43 to 0.48, which are depleted in Si compared with that in E-chondrite (XSi = 0.55). In order to understand the mantle differentiation in the E-chondrite model, it is indispensable to clarify the melting relations in the system MgSiO3-SiO2 at high pressures. Nevertheless, there have been limited works on the melting behavior of the system MgSiO3-SiO2 under high pressures. Therefore, recently we determined the melting relations in the system MgSiO3-SiO2 at 13.5 GPa which displays eutectic melting with the eutectic point located at XSi = 0.61 and at 2350 ± 50 ℃ [3].
The available eutectic compositions in the system MgSiO3-SiO2 experimentally determined from 1 to 128 GPa indicate that the XSi of the melts produced from E-chondrite source materials are around 0.6, significantly higher than the current upper mantle, 0.43. To evaluate the E-chondrite model, taking into account the incorporation of Si into the core during core formation in a magma ocean, we estimated the range of Si content in the core assuming an E-chondrite model. Our results showed that Si content in the core would be between 2.7 to 8.6 wt.%, which is within the range of 2 to 9 wt.% Si in the core as predicted by metal-silicate element partitioning [7-9]. On the other hand, through determining the P wave velocity of liquid Fe–Si at the core–mantle boundary conditions based on inelastic X-ray scattering measurements in a laser-heated DAC, the estimated upper limit of silicon concentration in the outer core to be <1.9 wt.% [10]. Considering with the core density deficit and higher Si content in the core predicted by the metal-silicate element partitioning, Nakajima et al. [10] suggested that the present-day liquid outer core was depleted in silicon after crystallizing SiO2 (and MgSiO3) through the history of the Earth. Thus, our results indicate that the E-chondrite model could explain the bulk Earth composition if the Si depletion in the core has operated through Earth’s history.
References
[1] Javoy et al. 2010 Earth Planet. Sci. Lett. 293, 259–268. [2] Bowen and Anderson 1914 Am. J. Sci. 4th ser. 37, 487–500. [3] Dalton and Presnall 1997 Geochim. Cosmochim. Acta 61, 2367–2373. [4] Hudon et al. 2005 J. Petrol. 46, 1859–1880. [5] Moriguti et al. in press Am. Min. [6] Ozawa et al. 2018 Geophys. Res. Lett. DOI: 10.1029/2018GL079313. [7] Wood et al. 2009 Geochim. Cosmochim. Acta, 72, 1415–1426. [8] Rubie et al. 2011 Earth Planet. Sci. Lett. 301, 31–42. [9] Siebert et al. 2013 Science, 339, 1194–1197. [10] Nakajima et al. 2020 J. Geophys. Res. Solid Earth 125(6). DOI: 10.1029/2020JB019399.
The available eutectic compositions in the system MgSiO3-SiO2 experimentally determined from 1 to 128 GPa indicate that the XSi of the melts produced from E-chondrite source materials are around 0.6, significantly higher than the current upper mantle, 0.43. To evaluate the E-chondrite model, taking into account the incorporation of Si into the core during core formation in a magma ocean, we estimated the range of Si content in the core assuming an E-chondrite model. Our results showed that Si content in the core would be between 2.7 to 8.6 wt.%, which is within the range of 2 to 9 wt.% Si in the core as predicted by metal-silicate element partitioning [7-9]. On the other hand, through determining the P wave velocity of liquid Fe–Si at the core–mantle boundary conditions based on inelastic X-ray scattering measurements in a laser-heated DAC, the estimated upper limit of silicon concentration in the outer core to be <1.9 wt.% [10]. Considering with the core density deficit and higher Si content in the core predicted by the metal-silicate element partitioning, Nakajima et al. [10] suggested that the present-day liquid outer core was depleted in silicon after crystallizing SiO2 (and MgSiO3) through the history of the Earth. Thus, our results indicate that the E-chondrite model could explain the bulk Earth composition if the Si depletion in the core has operated through Earth’s history.
References
[1] Javoy et al. 2010 Earth Planet. Sci. Lett. 293, 259–268. [2] Bowen and Anderson 1914 Am. J. Sci. 4th ser. 37, 487–500. [3] Dalton and Presnall 1997 Geochim. Cosmochim. Acta 61, 2367–2373. [4] Hudon et al. 2005 J. Petrol. 46, 1859–1880. [5] Moriguti et al. in press Am. Min. [6] Ozawa et al. 2018 Geophys. Res. Lett. DOI: 10.1029/2018GL079313. [7] Wood et al. 2009 Geochim. Cosmochim. Acta, 72, 1415–1426. [8] Rubie et al. 2011 Earth Planet. Sci. Lett. 301, 31–42. [9] Siebert et al. 2013 Science, 339, 1194–1197. [10] Nakajima et al. 2020 J. Geophys. Res. Solid Earth 125(6). DOI: 10.1029/2020JB019399.