Postspinel Phase of Forsterite and Evolution of the Earth's Mantle

Kumazawa, Mineo; Sawamoto, Hiroshi; Ohtani, Eiji; Masaki, Kazuaki

doi:10.1038/247356a0

Cited by 121 publications

(22 citation statements)

References 16 publications

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“…It should be noted here that the component oxide mixture phase assemblages for MgSi03 and Mg,Si04 reported by Ming & Bassett (1975) and by Kumazawa et al (1974), respectively, were not observed by the author. Hence, the mixed oxide assemblage will not be included in the discussion of this paper.…”

Section: Phase Transformations In Mgsi03 and Mgsi04contrasting

confidence: 55%

Mineralogy and chemistry of the Earth's mantle above 1000 km

Liu

1977

Geophysical Journal International

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The isothermal compression curves for 0-Mg2SiO4, 7-Mg2Si04 (spinel), and the ilmenite and perovskite phases of MgSi03 have been estimated by a graphical method on the basis of the available data for forsterite, enstatite, periclase and stishovite. A comparison of these compression curves with the recent density profiles of the Earth's mantle discloses the following conclusions regarding the mineral phases and Mg/Fe ratio of the Earth's mantle. If an olivine upper mantle is assumed, the density of the mantle above cu. 400 km is consistent with that of the olivine (Mg0.9,Fe,,03)LSi04 at room temperature. Temperature corrections at 400 km indicate that the molecular ratio of FeO/(MgOtFeO) for the mantle is hardly greater than 0.1 1. The densities between 400 and 500 km and between 500 and 650 km are in good agreement with those of the 0-phase and the spinel phase respectively. The density of the assemblage perovskite plus rocksalt phase is about 1 per cent smaller than that of the mantle below 650 km; however, this difference could be eliminated by a 3 per cent increase in the ratio FeO/(MgO t FeO) of the assemblage. If the upper mantle is assumed to be a mixture of 60 per cent olivine plus 40 per cent pyroxene, the density of the mantle above cu. 400 km can be accounted for by this mixture with the ratio of FeO/(MgO + FeO) = 0.03 at room temperature. The density between 400 and 650 km can be represented by the phase assemblages of &phase plus clinopyroxene, 0-phase plus stishovite, spinel phase plus stishovite, spinel plus ilmenite phases, and spinel plus perovsicite phases, in order of increasing depth. Below 650 km, the density for the assemblage perovskite plus rocksalt phases can essentially account for that of the mantle without necessitating an increase of iron content.

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Section: Phase Transformations In Mgsi03 and Mgsi04contrasting

confidence: 55%

Mineralogy and chemistry of the Earth's mantle above 1000 km

Liu

1977

Geophysical Journal International

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“…From both seismological (Anderson 1967;Johnson 1967;Archambeau, Flinn & Lambert 1969;Wiggins & Helmberger 1973) and high pressure mineralogical (Aki-mot0 & Fujisawa 1968;Ringwood 1970;Bassett & Ming 1972;Kumazawa et al 1974;Ming & Bassett 1975) evidence, it is generally agreed that there are at least two major phase transitions in the Earth's mantle. They are believed to be the changes of phase from olivine to spinel at about 350-400 km depth and from spinel to a post spinel structure at about 650 km depth, primarily as a result of the large increase in pressure with depth in the Earth's interior.…”

Section: Introductionmentioning

confidence: 99%

Role of Phase Transitions in a Dynamic Mantle

Schubert

Yuen

Turcotte

2007

Geophysical Journal of the Royal Astronomical Society

362

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The interaction of solid-solid phase transitions with convection in the Earth's mantle involves, for univariant systems: (1) effects of latent heat and advection of ambient temperature on the position of the phase boundary and on its associated body force, and (2) the coupling of latent heat with the ordinary thermal expansivity of the material. For divariant systems, an effective thermal expansion coefficient and a modified adiabatic temperature gradient may be defined for the phase transition zone. Linear stability theory for a fluid layer with a univariant phase change is reviewed and applied to the endothermic spinel-oxide transformation. The theory of the stability of a fluid layer with a divariant phase transformation is developed and critical Rayleigh numbers are given for a model of the olivine-spinel transition. Of special interest is the case where the Earth's temperature gradient exceeds the adiabatic temperature gradient outside the phase transition zone but is smaller than the increased adiabatic temperature gradient in the two-phase olivine-spinel region. The thermal structure of the descending lithosphere is calculated, including the effects of frictional heating on the slip zone and of the olivine-spinel and spineloxide transitions; temperature contrasts of 700 O K can exist between the slab and adjacent mantle at 800 km depth. The net body force on the descending slab due to thermal contraction and the major mineralogical phase changes is downward. The olivine-spinel transition may be responsible for the tensional focal mechanisms of intermediate depth earthquakes while the spinel-oxide transformation may be related to the compressional focal mechanisms of deep earthquakes.

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“…The slope of the Mgo + SiO 2 = γMgSiO 3 equilibrium line in these diagrams is in essence arbitrary, although on this de pends the answer to the question as to whether a mixture of oxides or the perovskite modifica tion of MgSiO 3 is stable in the mantle (in accordance with Table 2). One should also not forget the experimental evidence for the existence of decomposition fields for γMgSiO 3 and γMg 2 SiO 4 to give MgO and SiO 2 above 200 kbar at high temperatures [15,[18][19][20].…”

Section: Discussionmentioning

confidence: 99%