T169 Microscopic Impact Records

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

REVISITING THE IMPACT CHRONOLOGY OF THE MOON: A TERRESTRIAL PERSPECTIVE ON USING EX SITU SHOCKED ZIRCONS TO DATE METEORITE IMPACTS

Paper number: T169-7
Program: Technical Sessions
Time: 3:05 PM – 3:20 PM

Authors

Abstract

Deformed lunar zircons yielding U-Pb ages from 4333 to 1407 Ma have been interpreted as dating discrete impacts on the Moon [1]. However, the cause of age resetting in lunar zircons is equivocal; as ex situ grains in breccias, they lack lithologic context and most do not contain diagnostic shock microstructures found in terrestrial zircons. Detrital shocked zircons provide a terrestrial analog to ex situ lunar grains, for both identifying diagnostic shock evidence and also evaluating the feasibility of dating impacts with ex situ zircons. Electron backscatter diffraction (EBSD) and SHRIMP U-Pb analysis of zircons eroded from the ~2020 Ma Vredefort impact structure show that complete impact-age resetting did not occur in microstructural domains characterized by microtwins, planar fractures, and low-angle boundaries, which record ages from 2890 to 2645 Ma. An impact age of 1975 ± 39 Ma was detected in neoblasts within a granular zircon that also contains shock microtwins, which link neoblast formation to the impact. However, we show that granular texture can also form during regional metamorphism, and thus is not unique to impact environments. These results demonstrate that dating an impact with ex situ shocked zircon requires identifying diagnostic shock evidence, such as reidite or microtwins to establish impact provenance, and then targeting specific age-reset microstructures. With the recognition that zircon can deform plastically in both impact [2] and magmatic [3] environments, age-resetting in lunar zircons that lack diagnostic shock deformation may record magmatic processes, rather than impact events. EBSD characterization is crucial for identifying age-reset shock microstructures in order to construct accurate zircon-based impact chronologies for the Moon, Earth, and other planetary bodies [4].

Support was provided by NSF (EAR-1145118) and the NASA Astrobiology Program.

[1] Grange et al. 2013a,b. [2] Moser et al. 2011. [3] Reddy et al. 2009. [4] Cavosie et al., 2015.