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Fe-Ti oxide minerals from regionally metamorphosed quartzites of western New Hampshire

Fe-Ti oxide minerals from regionally metamorphosed quartzites of western New Hampshire,10.1007/BF00371584,Contributions To Mineralogy and Petrology,Do

Fe-Ti oxide minerals from regionally metamorphosed quartzites of western New Hampshire   (Citations: 20)
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The assemblages rutile-hematite, hematite, hematite-magnetite, hematite-ilmenite-magnetite, and ilmenite-magnetite occur in sillimanite- and kyanite-grade quartzites exposed in western New Hampshire. Different assemblages are found in interlayered sedimentary beds of single outcrops. Magnetites are nearly pure Fe3O4 and contain trace amounts of Al, Si, Ti, V, Cr, Mn, and Ni. Magnetites in contact with hematites contain up to 0.4 weight % MnO, but magnetites in contact with ilmenites containing up to 2.3 weight % MnO have no detectable Mn. Ilmenite is enriched in Mn relative to coexisting hematite, and hematite is so enriched with respect to magnetite. Systematic partitioning of elements between oxide minerals and absence of crossing tie lines suggest that the minerals attained chemical equilibrium during regional metamorphism. None of the assemblages are divariant because of the presence of components in addition to FeO, Fe2O3, and TiO2; therefore, none of them constitute oxygen buffers. Nevertheless, gradients in $$\mu _{O_2 } $$ between adjacent sedimentary beds can be measured using variations of oxide mineral composition in trivariant and quadrivariant phase assemblages. Oxygen behaved as an “initial value” component or “inert” component during regional metamorphism. It is likely that the $$\mu _{O_2 } $$ gradients are due to differences in bulk composition of sedimentary beds at the time of deposition.
Journal: Contributions To Mineralogy and Petrology - CONTRIB MINERAL PETROL , vol. 42, no. 3, pp. 181-195, 1973
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    • ... in entropy associated with the development of magnetic order only affects the FeüTi oxide geothermometer significantly at equilibration temperatures below500 C. As the effect is to destabilize the solid solution below its Nÿel temperature, perhaps the magnetic ordering is what ultimately accounts for the presence of nearly pure magnetite coexisting with hematite-ilmenite solid solutions under low grade metamorphic conditions (e.g., ...

    M. S. Ghiorso. Thermodynamic analysis of the effect of magnetic ordering on miscibili...

    • ...Rumble (1973) recorded magnetite + rutile in the garnet and staurolite zones of western New Hamsphire and deduced that the assemblage was formed from ilmeno-hematite by cooling...
    • ...Rumble 1973). In the New Caledonia metamorphic terrain, variations in oxygen activity are indicated by interbedding of pyrrhotite-bearing pelites and basites with hematite-bearing metacherts in a small outcrop...

    T. Itayaet al. Sulfides, oxides and sphene in high-pressure schists from New Caledoni...

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