EQUILIBRIUM AT THE TRIPLE-POINT OR CRYSTALLIZATION ALONG A P-T PATH? INSIGHTS FROM THE FIRST IN SITU O-ISOTOPE MEASUREMENTS OF QUARTZ AND COEXISTING AL2SIO5 POLYMORPHS

Al₂SiO₅ polymorphs (kyanite, andalusite, sillimanite) are powerful index minerals in metamorphic rocks: their presence informs the pressure (P) and temperature (T) conditions of equilibration of crustal rocks. Rocks containing two and three Al₂SiO₅ polymorphs have been interpreted as: (1) equilibrated along invariant curves or at the Al₂SiO₅ triple point, (2) formed sequentially along a single P–T path, or (3) formed along different P-T paths (polymetamorphic). Some polymorphs persisted metastably outside of their stability field in (2) and (3), and polymorphs formed in (3) are not temporally or tectonically related. Distinguishing among these is essential to parse tectonic scenarios resulting in polymorph growth. O-isotopes can be leveraged to assess equilibrium between co-existing polymorphs and equilibration temperatures between Al₂SiO₅ polymorphs and other phases. Such studies have been carried out on bulk mineral separates using laser fluorination methods. However, the loss of textural context undermines the interpretation of the significance of these isotopic signatures.

In this study, we present the first in situ O-isotope analyses of co-existing kyanite, sillimanite, andalusite and quartz (WiscSIMS). O-isotope analyses were carried out on crustal rocks from Norway and Iran that contain 2-3 Al₂SiO₅ polymorphs. In samples containing all three, we find δ¹⁸O differences of 0.2–0.3‰ between at least two of the Al₂SiO₅ polymorphs, as well as grain-scale variations in δ¹⁸O that we investigate in the context of relative trace element abundances (Cr, Ti, Mg, V, Mn, Fe), growth or recrystallization textures (e.g., oscillatory and complex patchy zoning) visible using cathodoluminescence (CL), and mineral orientations. These results exclude equilibration at the triple point, and other petrochronological tools are required to distinguish between single events or polymetamorphism. When choosing mineral pair fractionations to calculate oxygen isotope thermometry, core to rim δ¹⁸O zonation in quartz (up to 1.7‰ VSMOW) yields temperatures differing by up to 300ºC. These results suggest that preserving the textural context is critical for the application of O-isotope thermometry on Al₂SiO₅-bearing rocks and for the interpretation of tectonic settings in which co-existing polymorphs are found.