PAIRED RECORDS OF CRYSTALLIZATION AND DEFORMATION IN AL2SIO5-BEARING ROCKS (Invited Presentation)

The Al2SiO5 polymorphs-andalusite, kyanite, and sillimanite-are index minerals that provide a first-order estimation of metamorphic conditions under which the crust deforms. Polymorphic reactions are energetically expensive: they may require significant overstepping (e.g, Spear et al., 2017) be facilitated by the presence of fluids, and/or be enhanced by deformation (e.g., Goergen et al., 2008). In this study, we investigate records of Al2SiO5 polymorph crystallization and associated fluid signatures in rocks composed primarily (>90%) of 2-3 coexisting aluminosilicate polymorphs (Al2SiO5 veins in Hamedan, Iran, Sepahi et al., 2004; kyanite-andalusite-sillimanite schist, Norway, Whitney & Samuelson, 2019). Polymorphs in these rocks range in size from 2mm to ~3cm. Variations in relative trace element abundances (e.g., Fe, Mg, Ti, Cr) were investigated using qualitative EPMA mapping, and fluid signatures of Al2SiO5 polymorphs and quartz were determined from in situ oxygen isotope analyses by SIMS. Crystallographic orientations and deformation microstructures of Al2SiO5 phases were measured using EBSD.

Small variations in polymorph δ¹⁸O values and textural observations suggest sequential crystallization of different polymorphs rather than equilibration along an invariant curve or at the aluminosilicate triple point. Polymorph growth likely involved polymorphic transformation with metastable persistance of some polymorphs outside of their stability field. The Al2SiO5 polymorphs record a range of deformation microstructures such as kink bands and twinning in kyanite, and subgrain development in andalusite. Trace element variations are correlated with some growth features identified by cathodoluminescence imaging, but show no correlation with variations in δ¹⁸O values. We suggest that fluid-assisted development of large Al2SiO5 porphyroblasts may have participated in local strengthening of the crust, with porphyroblasts accommodating strain via internal deformation, or mechanical twinning, rather than breakdown and nucleation of new crystals. These results suggest that fluid-assisted crystallization processes don’t systematically correlate with rheological weakening of the crust during metamorphism.