Stability of spinelloid phases in the system Mg2SiO4-Fe2SiO4-Fe3O4 at 1100 °C and up to 10.5 GPa

Details Stability of spinelloid phases in the system Mg2SiO4-Fe2SiO4-Fe3O4 at 1100 °C and up to 10.5 GPa Stability of spinelloid phases in the system Mg2SiO4-Fe2SiO4-Fe3O4 at 1100 °C and up to 10.5 GPa

Jun 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004pepi..143..171k&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 143, p. 171-183.

Physics

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Scientific paper

Experiments in the system Mg2SiO4-Fe2SiO4-Fe3O4 have produced three intermediate phases that are stable between ~4.0 and 9.0GPa and 1100°C. These phases are different spinelloid polytypes and are structural analogues to spinelloids II, III and V in the Ni-aluminosilicate system [J. Solid State Chem. 44 (1982) 257]. The behaviour of spinelloid III is of particular interest since it is isostructural to (Mg,Fe)2SiO4-wadsleyite. Spinelloid II is the first intermediate phase to appear at low pressures in Mg-poor compositions. With increasing pressure spinelloid II is replaced by an isochemical spinelloid V polytype between 6.0 and 6.5GPa. Spinelloid III is relatively Si-rich and, depending on pressure, can coexist with either spinelloid II or V. At 1100°C, spinelloid III gives way to the assemblage olivine + spinelloid V between 7.5 and 8.0GPa and its maximum Mg content is limited to ~15mol% Mg2SiO4 at 7.5GPa. Thus, there is a very large gap in composition and pressure separating the stability fields of spinelloid III and wadsleyite in this system. In general, the addition of Mg or an increase in temperature acts to shift the phase stabilities of the spinelloids to higher pressures. At 1100°C and above 10.0GPa, no spinelloid phases are stable. We find no compelling evidence for the incorporation of significant Fe3+ in olivine, even at very high pressures. In Mg-rich compositions more relevant to the Earth's mantle, the assemblage oliviness + spinelss is stable. In such bulk compositions, the spinel is practically Si-free and contains a Mg-ferrite component. Thus there appears to be a mutual incompatibility between Mg and Si in high-pressure spinels. It is conceivable that the presence of Fe3+ in a mantle assemblage (e.g. through metasomatism) could stabilise small quantities of either spinelloid V or a magnesioferrite-bearing spinel in equilibrium with olivine. Unlike the spinel that is considered to be present in the lower parts of the transition zone, this phase will contain little or no Si.
Present address: Crystallography Laboratory, Department of Geological Sciences, Virginia Tech, Blacksburg, VA 24060, USA.

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