Investigating the Response of Zircon to Shock Metamorphism

Abstract

The Haughton Impact Structure is a complex crater located on Devon Island in the Arctic Archipelago. The 23-km structure formed 39 Mya in mixed target rock. The remote location of Haughton renders samples difficult to come by. A granitic hand-specimen (8 cm x 6 cm) cut by veinlets of impact melt was obtained through Western University, from which a polished thin-section was prepared. Shock effects manifest in the thin-section's zircons were studied by optical petrography, scanning electron microscopy and Raman spectroscopy. Zircon is a refractory mineral, resistant to physical / chemical weathering and is stable to ultrahigh temperatures. The documentation of shock effects in zircon therefore provides the opportunity to study shock effects not probed by most minerals. We observe planar fractures, planar deformation features and the transformation of zircon to reidite, a high-pressure polymorph. Reidite is stable at 30-60 Gpa, but at temperatures <1473 K. At temperatures >1963 K, zircon decomposed to ZrO2 + SiO2, observed as a light-dark vermicular texture in BSE images. Our results represent the first detailed study of zircons at Haughton, reveal a previously unrecognized phase transformation at the crater and constrain the pressure-temperature-time history experienced by granitic basement rocks during the impact event.

Faculty Mentor: Erin Walton

Department: Earth and Planetary Sciences