ISSN:
1432-0967
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
Abstract 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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00371584
