Computer Science
Scientific paper
Apr 1986
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986e%26psl..77..399k&link_type=abstract
Earth and Planetary Science Letters (ISSN 0012-821X), vol. 77, no. 3-4, April 1986, p. 399-408.
Computer Science
6
Earth Mantle, Garnets, Phase Diagrams, Planetary Composition, Silicates, Cubic Lattices, Iron Compounds, Lattice Parameters, Magnesium Compounds, Melting, Phase Transformations, Solid Phases
Scientific paper
High-pressure and high-temperature experiments at 20 GPa on (Mg,Fe)SiO3 have revealed stability fields of two types of aluminium-free ferromagnesian garnets; noncubic garnet and cubic garnet (majorite). Majorite garnet is stable only within a limited compositional variation, Fe/(Mg + Fe) ranging from 0.2 to 0.4 and in the narrow temperature interval of about 200 C to 2000 C, while the stability of noncubic garnet with more iron-deficient compositions persist up to higher temperatures. These two garnets show fractional melting into iron-deficient garnet and iron-rich liquid, and the crystallization field of cubic garnet extends over Fe/(Mg + Fe) = 0.5. The assemblage of silicate spinel and stishovite is a low-temperature phase, which also occurs in the iron-rich portion of the MgSiO3-FeSiO system. The sequence as given by the Fe/(Mg + Fe) value for the coexisting phases with the two garnets at 2000 C and 20 GPa is: silicate modified spinel approximately equal to aluminium-free garnets, which are much less than silicate spinel. Natural majorite in shock-metamorphosed chondrites is clarified to be produced at pressures above 20 GPa and temperatures around 2000 C. Similar shock events may cause the occurrence of noncubic garnet in iron-deficient meteorites. Non-cubic garnet could be a stable phase in the earth's mantle if a sufficiently low concentration of aluminium is present in the layer corresponding to the stable pressure range of noncubic garnet. The chemical differentiation by melting in the deep mantle is also discussed on the basis of the present experimental results and the observed coexistence of majorite garnet with magnesiowuestite in chondrites.
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