ALUMINUM SILICATE POLYMORPH TRANSFORMATIONS AND THE SIGNIFICANCE OF FIBROLITE VERSES PRISMATIC SILLIMANITE. A STUDY FROM THE 'TRIPLE-POINT ISOBAR', NEW HAMPSHIRE

Factors controlling the polymorphic transformation between the aluminum silicates (AS) kyanite, andalusite and sillimanite, such as the amount of 'overstepping' of the univariant curves required to nucleate a new AS, or the importance of net transfer reactions to the transformation, have proved difficult to constrain. In addition, the petrologic significance of fibrolite verses prismatic sillimanite remains ambiguous. These problems are related to the 1) low free energy differences between the AS polymorphs and 2) lack of appreciable solid solution between the AS polymorphs required to determine coexisting equilibrium.

In this study, temperatures of formation of the AS polymorphs are calculated using quartz-AS oxygen isotope thermometry. Samples were collected along the AS 'triple-point isobar' in New Hampshire. These rocks preserve textural evidence of prismatic sillimanite replacing andalusite. Fibrolite is present as large mats associated with biotite or as inclusions in sillimanite. Rocks near the 'triple-point isobar' have texturally unstable staurolite. Farther west and up-temperature of the 'triple-point isobar' samples have coarse-grained prismatic sillimanite, are free of staurolite and have minor amounts of fibrolite. None of the samples studied have potassium feldspar.

Near Mt. Mooselauke, NH, temperatures of formation using oxygen isotope thermometry (D d¹⁸O qtz-AS) for coexisting garnet, fibrolite and prismatic sillimanite are 610ºC, 620ºC and 656ºC respectively. Quartz-andalusite temperatures from eastern New Hampshire are 580ºC and quartz-kyanite temperatures from Black Mt. are 590°C.

These results illustrate that the polymorphic transformation from andalusite to fibrolite involves either a significant amount of overstepping (~ 40ºC), or more likely, net transfer reactions that consume staurolite. These results appear true for rocks that have not reached the second sillimanite reaction. If the bulk composition is such that no AS forming reactions are crossed until the 2nd sillimanite reaction, andalusite may persist metastably to higher temperatures (i.e. northern New Mexico). Secondly, in contrast to laboratory experiments, these results illustrate that fibrolite forms at temperatures 50ºC to 80ºC lower than prismatic sillimanite.