Phase Relations and Thermodynamic Properties of Spinelloid Phases in the System Mg2SiO4–Fe2SiO4–Fe3O4 at high Temperatures and Pressures

Experiments in the system Mg2SiO4–Fe2SiO4–Fe3O4 produced three spinelloid polytypes that are stable between 4 and 9 GPa and 1100°C. 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 and 6.5 GPa. Spinelloid III is relatively Si-rich and, depending on pressure, can coexist with either spinelloid II or V. Spinelloid III gives way to the assemblage olivine + spinelloid V between 7.5 and 8.0 GPa and its maximum Mg content is limited to ~0.15 mol % Mg2SiO4 at 7.5 GPa. Thus there is a very large gap in composition and pressure separating the stability fields of spinelloid III and (Mg,Fe)2SiO4-wadsleyite. At 12 GPa, olivine is replaced by wadsleyite with a rather narrow compositional range, that coexists with a spinel that contains a significant MgFe2O4 component. Above 16 GPa, ringwoodite replaces wadsleyite as the stable Si-rich phase. It also can coexist with a MgFe2O4-bearing spinel. It appears that the presence of Fe3+ has very little influence on the occurrence of wadsleyite and ringwoodite. Also no compelling evidence was found for the incorporation of significant amounts of Fe3+ in olivine, even at very high pressures. In Mg-rich compositions more relevant to the Earth's mantle, no spinelloid phase was found except for (Mg,Fe)2SiO4-wadsleyite, and the assemblage oliviness + spinelss is stable over a wide range of pressure. In such bulk compositions, the Fe3+-rich spinel is practically Si-free and contains a Mg-ferrite component. Thus there appears to be a mutual incompatibility between the Mg2SiO4 and Fe3+-rich components in high-pressure spinels. The interphase partitioning of Fe2+ and Mg for different mineral pairs yields consistent results with respect to bulk composition and shows no noticeable pressure dependence. Pyroxene, which appeared in some experiments due to a minor amount of oxidation, exhibits Mg-Fe2+ partitioning with olivine in agreement with literature data. This indicates that despite minor oxidation in some experiments, Mg-Fe2+ equilibrium was achieved, at least on a local scale. The molar volumes decrease systematically with decreasing magnetite content as well as increasing Mg-content in all three spinelloid polytypes and spinel.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Heidelberg

Year

2003

Author dc:creator

• Koch, Mario

Contributors dc:contributor

• Woodland, Alan B.

Repository record source_url

http://www.ub.uni-heidelberg.de/archiv/3567

OAI identifier oai:identifier

oai:archiv.ub.uni-heidelberg.de:3567