Zircon-reidite relations in breccias from the Chesapeake Bay impact crater

Abstract

Reidite, a high pressure polymorph of zircon, was first described from ejecta of the CBIS (Glass and Liu, 2002) and has since been described in the ICDP-USGS Eyreville-B drill cores (Horton et al. 2009; Wittmann et al. 2009). This allows the rare opportunity to study the martensitic transformation of zircon to reidite in natural samples. The aim of this study is to resolve differences between nature and shock experiments (Leroux et al. 1999) and discuss these findings in the context of hypervelocity impact. A zircon-reidite grain hosted in a shock stage II or III lithic clast that underwent selective mineral melting at shock pressures of 45-60 GPa and post-shock temperatures of -900-1500°C (Stöffler and Grieve, 2007) exhibits three texturally distinct domains. Microscopic and SEM imaging suggests the outer, -5-25 μm thick rim of this grain shows a typical decomposition texture with vesicles and an excess of Zr02 (Wittmann et al. 2009). This decomposition front apparently produced a poorly crystallized domain towards the core with low mass contrast, beyond which is a core of well crystallized coexisting zircon and reidite. These textures may suggest that either decomposition in zircon-reidite occurs at temperatures <1676°C suggested by Butterman and Foster (1967) for zircon, or that much higher temperatures than the maximum -1500 °C assumed for stage III components are briefly reached in stage II components (Stöffler 1971; Stöffler & Grieve 2007). This study also presents Chroma-SEMCL images of zircon-reidite, which shows that reidite has a reduced CL signature compared to zircon.