MICROSTRUCTURAL AND GEOCHEMICAL CHARACTERIZATION OF TITANITE DEFORMED IN TORSION: POTENTIAL IMPLICATIONS FOR PETROCHRONOLOGY OF DEFORMED ROCKS

Titanite (CaTiSiO₅) has received attention as a promising deformation petrochronometer: The mineral is common in igneous and metamorphic lithologies, contains enough U for U-Pb geo/thermochronology, and also incorporates all rare earth elements and numerous trace elements. Of particular benefit is titanite’s temperature-dependent inclusion of Zr and its ability to recrystallize in response to deformation and fluids. In fact, the relative ease of titanite recrystallization has been called on to explain the occurrence of young U-Pb titanite dates (i.e., younger than U-Pb dates from co-occurring zircon) despite the apparently sluggish diffusion of Pb in titanite. Considering these useful attributes, it has nevertheless been difficult to determine the circumstances under which titanite may be used to reliably date deformation rather than the timing of passage through T_c for Pb diffusion. Therefore, an important task when interpreting titanite petrochronology data is determining whether the formation and movement of dislocations acts to expel Pb during deformation, or if microstructures act primarily as fast diffusion pathways, thereby effectively lowering T_c for Pb diffusion.

To explore deformation’s impact on titanite geochemistry, we deformed natural titanite in torsion, and then characterized microstructure and geochemistry of pre- and post-torsion samples using EBSD and EPMA, respectively. The sample set-up consists of mm-scale titanite single crystals in a matrix of powdered titanite, and post-torsion EBSD mapping indicates that both single crystals and matrix grains were deformed. Single crystals in the post-torsion sample contain misorientation axes indicating slip on (b) in the direction of <a>, consistent with the starting orientation of the crystals such that the b-axis is parallel to the axis of rotation. Kernel Average Misorientation (KAM) maps were used to guide spot placement for geochemical characterization. Notably, EPMA analyses of matrix grains with higher KAM values in the post-torsion sample yielded lower [Sr] (a proxy for Pb) than matrix grains with lower KAM values. Although preliminary, results so far suggest that deformation may play a role in directly modifying Sr content beyond lowering the T_c for Sr (and potentially Pb) diffusion.