1988 Fiscal Year Final Research Report Summary
Eperimental studies on the structure of the lower mantle and themantle fractionation
| Project/Area Number |
62540621
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| Research Category |
Grant-in-Aid for General Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Research Field |
鉱物学(含岩石・鉱床学)
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| Research Institution | Okayama University |
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Principal Investigator |
ITO Eiji Institute for Study of the Earth's Interior, Okayama University, 地球内部研究センター, 助教授 (00033259)
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| Co-Investigator(Kenkyū-buntansha) |
TAKAHASHI Eiichi Institute for Study of the Earth's Interior, Okayama University, 地球内部研究センター, 助手 (40144779)
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| Project Period (FY) |
1988 – 1989
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| Keywords | The 670 km discontituity / The lower mantle / Perovskite / Geotherm / 弾性定数 |
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| Research Abstract |
The olivine( )-modified spinel( )-spinel(y)-postspinel phase equilibria in the system Mg_2SiO_4-Fe_2SiO_4 were determined up to 26 GPA and 1600 C. It was demonstrated that the magenesian spinel (with Fe/Mg+Fe<0.2) dissociates into perovskite and magnesiowustite within an extremely narrow pressure interval (<0,15 GPa at 1600 C). The dissociation pressure is almost independent of iron content and coincide to that at 650km. These results indicate that the sharpness of the 650km discontinuity may indeed be due to this dissociation in peridotitic mantle. Assigning the phase boundaries of the spinel dissociation to the sharp discontinuity implied by study of reflected waves, the temperature at the base of the transition zone (655km depth) is estimated to be about 1600 C. Taking the dissociation of the majorite dissociation confirmed in the system CaSiO_3-MgSiO_3-Al_2O_3 and mantle peridotites into consideration, the mineralogical model was constructed down to the lower mantle.
Melting experim
… More
ents on a fertile peridotite was performed up to 2.5 GPa and at temperatures 2000-3000 C. The MgSiO_3-rich perovskite was found to be the liquidus phase at the lower mantle conditions. Using the Mg/Si, Al/Si, Ca/Si, and Ca/Al ratio as constraints, it was shown that primitive upper mantle compositions can be derived from a chondritic source through a small degree of MgSiO_3-perovskite fractionation (10-20wt%).
Crystal structure of MgSiO_3perovskite was determined at high pressures up to 9.6GPa as well as at atmospheric pressure, using single crystals. The structure was characterized by the three dimensional framework of SiO_6 octahedra with Mg in the eight-fold coordinated cuboctahedron and the unit cell compression is controlled dominantly by the tilting of SiO_6 octahedra. The singly-crystal elastic module of MgSiO_3 perovskite was determined for the first time under atmospheric pressure and room temperature using Brillouin spectroscopy. Comparison of our elasticity data with seismic models appropriste to the lower mantle reveals that petrological models having either pyrolite or pyroxene stoichiometries are compatible with the experimental data only if the shear modulus of MgSiO_3 perovskite has a very strong negative temperature derivative. Less
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Research Products
(11 results)
