T69 Impact Cratering on the Earth, Moon, and Planets: Remote, Field, and Lab Studies

Sunday, 9 October 2011
1:30 PM - 5:30 PM
Room L100H-J

CRYSTALLOGRAPHIC MODELING AND OBSERVATIONS OF {112} PLANAR FRACTURES IN NATURALLY SHOCKED ZIRCON

Paper number: T69-12
Program: Technical Sessions
Time: 4:30 PM – 4:45 PM

Authors

Abstract

Detrital shocked zircons eroded from the 2.02 Ga Vredefort Dome in South Africa have been reported with up to 5 orientations of planar fractures (PFs), however only (00Ι), (0Ι0), and (Ι00) orientations have been indexed. Here we present results of a modeling study that indexes 5 additional orientations, bringing the total to 8, and compare these results with microstructures from a detrital shocked zircon transported 700 km from the impact basin by the Vaal River (Erickson et al., 2011). Interpenetrating {Ι00} and {ΙΙ0} tetragonal prisms based on zircon unit cell parameters were constructed with the program SketchUp. Multiple orientations of PFs were modeled, including four orientations of {ΙΙ2}: (ΙΙ2), (ΙĪ2), (ĪΙ2), (ĪĪ2). Attention was focused upon lineations on the exterior of the crystal that result from the intersection of PFs on prism faces. Model results were then compared with SEM images of PF intersections with multiple surfaces of detrital shocked zircon crystals. Zircons mounted parallel to c-axes were imaged, rotated 90˚ about the c-axis and re-imaged until all four prism faces were documented.

Modeling four {ΙΙ2} orientations produces variable lineation patterns on {Ι00} and {ΙΙ0} forms. Two lineations form on {Ι00} that create an acute angle of ~49˚. Only two form because on (Ι00), for example, the (ΙΙ2) and (ĪΙ2) set and the (ΙĪ2) and (ĪĪ2) set are parallel. Three lineations form on {ΙΙ0}. On (ΙΙ0) for example, the (ΙΙ2) and (ĪĪ2) sets, which are parallel, form one lineation 90˚ to the c-axis. The other two PFs, (ĪΙ2) and (ΙĪ2), form lineations with an acute angle of ~65˚. Along all other c-axis parallel crystallographic orientations (e.g., {Ι20}) four lineations will be formed by the {ΙΙ2} PFs.

Imaging a subhedral detrital zircon with {Ι00} and {ΙΙ0} prisms confirms the presence of {ΙΙ2} PFs in 4 orientations; additional PFs on (Ι00) and (0ΙΙ) result in 6 PF orientations. PFs and deformation micro-twins in {ΙΙ2} orientation have been reported in zircon shocked experimentally to 60 GPa (TEM, Leroux et al. 1999) and in bedrock samples from Vredefort shocked to >20 GPa (EBSD, Moser et al. 2011). Factors that favor formation of specific PF orientations in zircon are not well understood, but likely include shock pressure, grain orientation relative to the shock wave, and rheological properties of surrounding phases.