T169 Microscopic Impact Records

Monday, 2 November 2015
1:30 PM - 5:30 PM
Room 344

ORIGIN OF VESICULAR RIMS ON DETRITAL ZIRCONS FROM THE CHELMSFORD FORMATION, SUDBURY IMPACT STRUCTURE, CANADA

Paper number: T169-9
Program: Technical Sessions
Time: 3:30 PM – 3:45 PM

Authors

Abstract

Detrital zircons from the Chelmsford Formation, a greywacke deposited within the Sudbury impact structure during the Paleoproterozoic, were surveyed to search for shock microstructures to study the erosional history of the impact basin. A population of 1,000 zircons was surveyed by SEM. No definitive shock microstructures, such as planar microstructures or granular texture, were identified. However, an unusual vesicular surface texture was observed on 4% of the grains surveyed (~40 grains). Backscattered electron (BSE) images of exterior grain surfaces show that the vesicles are dominantly round, and occur as both open and closed voids that range from ~1 to ~10 um in diameter. Variations in BSE intensity of vesicles indicate variable compositions. On interior surfaces, the vesicles are dark in CL, and range from 1-10 µm thick. Energy dispersive spectroscopy analysis of vesicles yields spectra consistent with both mineral (zircon, quartz, albite, alkali feldspar) and melt (zircon + aluminosilicate melt) compositions.

The roundness of vesicles is suggestive of volatile release during high temperature boiling. Similar textures were reported in shocked zircons from K-Pg ejecta [Bohor et al., 1993]. The location of the Chelmsford Fm. within the Sudbury basin is consistent with an impact origin for the vesicles. However, the mineral separation method, electric pulse disaggregation (EPD), was also evaluated as a potential cause, as it involves high voltage discharges through a water bath to disaggregate a rock. The dominant minerals that melted during vesicle formation are anhydrous; an externally sourced volatile phase is thus needed. Both the EPD water bath and surface water during the Sudbury impact are possible sources. However, the Chelmsford Fm. zircons are detrital; it is difficult to envision the preservation of hollow glass vesicles on grain surfaces during erosion, deposition, lithification, and greenschist facies metamorphism. The origin of vesicular texture in the Chelmsford Fm. detrital zircon suite thus remains speculative; ongoing studies are focused on distinguishing the two possible formation mechanisms (impact vs. EPD) described above.

Support was provided by NSF and the NASA Astrobiology Program.