EXAMINING RELATIONSHIPS BETWEEN O-ISOTOPES, TRACE ELEMENTS, AND CATHODOLUMINESCENCE IN ROCKS WITH COEXISTING AL2SIO5 POLYMORPHS

The study of the aluminosilicate (Al₂SiO₅) polymorphs andalusite, kyanite, and sillimanite is critical to our understanding of past metamorphic events. These polymorphs form under distinct pressure (P) and temperature (T) conditions and provide first-order information about metamorphic history. Rocks that contain two or more Al₂SiO₅ polymorphs either formed at equilibrium or crystallized sequentially along a P-T path that can be derived from their textural context. O-isotope measurements can help determine whether co-existing polymorphs formed at equilibrium, or along a P-T path during past metamorphic event(s). Furthermore, measurements of O-isotopes in Al2SiO₅ polymorphs and quartz can be used for O-isotope thermometry to quantify the P,T conditions of metamorphism. In situ O-isotope measurements preserve the textural context of the polymorphs, and allows us to assess these data in the context of the minerals' crystallization history. This study aims to investigate the significance of in situ O-isotope measurements in the context of mineral crystallization in two samples of Al₂SiO₅ rich quartz-bearing veins in metapelitic schists in the Sanandaj-Sirjan metamorphic belt, Hamadan, Iran.

We investigated two samples: ASVI, containing sillimanite and andalusite, and ASKV, containing all three polymorphs, as well as quartz. In situ O-isotope analyses were conducted at WiscSIMS, at the University of Madison, WI. Images of the SIMS analytical pits were captured on a Phenom ProX Tabletop SEM and examined for validity at the WMARC at William & Mary. Data from pits displaying cracks, inclusions, or grain boundaries were discarded. O-isotope data from pristine SIMS pits was plotted against trace element abundances determined by EPMA (Mg, Fe, Mn, Cr, Ti relative abundances) as well as CL textural domains (core vs. rim structure) and intensity of sillimanite and kyanite.

The goal of this study is to investigate potential correlations, or lack thereof, between O-isotopes and other indicators of crystallization history in co-existing Al₂SiO₅ polymorphs and assess O-isotope data application of O-isotope thermometry. Future work will apply the knowledge gained to rocks with different mineral assemblages and provide another tool to assess equilibrium and calculate P,T information in Al₂SiO₅-bearing rocks.