IN SITU STUDY OF SHOCKED ZIRCONS FROM THE CHICXULUB AND POPIGAI IMPACT STRUCTURES

106 zircons from all the suevite-type impactite units of drill core Yaxcopoil-1 (Yax-1), Chicxulub impact structure, Mexico [1] and 39 zircons from impact melt rocks of the Popigai crater, Siberia [2] were analyzed in thin section. The full range of progressive shock metamorphic features are developed in the Yax-1 zircons: 57 grains appear unshocked (P < 20 GPa); possible planar deformation features (PDFs) were found in 17 grains, indicating shock P > 20 GPa [3]; 29 show granular textures that indicate shock P > 20 GPa. These textures may represent (re)crystallized diaplectic zircon glass [4]; 3 grains exhibit distinct intergrowths of ZrO₂ with zircon, thus indicating T > 1770°C [5] and / or P > 94 GPa [6]. 14 Popigai zircons exhibit structures reminiscent of decorated or etched PDFs [7], indicating P > 20 GPa and thermal annealing, and 5 grains show granular textures. ZrO₂ is missing in the Popigai zircons, suggesting strong thermal annealing under the presence of SiO₂-rich fluids [8]. Paragenetic relationships with melt rock and lithic components were evaluated to constrain P and T of the shock features. Micro Raman spectroscopy was employed to characterize shocked zircon [9], possible (re)crystallized zircon glass and metamict states [10], to confirm ZrO₂ intergrowths with zircon, and to check for the presence of the high P polymorph reidite [9, 11]. Generally, reidite is missing, which indicates post shock T > 1200°C [6] in the samples studied. The implications for the stability of natural shock features of zircon in suevites compared to impact melt rocks is expected to make zircon applicable as an in situ shock indicator.

References: [1] Dressler, B. A. et al. 2003. EOS 84 / 14, 125+130. [2] Vishnevsky, S. & Montanari, A. 1999. GSA spec. paper 339, 19-59. [3] Leroux, H. et al. 1999. EPSL 169, 291-301. [4] Kamo, S. L. et al. 1996. EPSL 144, 369-387. [5] El Goresy, A. 1968. In Shock metamorphism of natural materials, eds. French, B. W. and Short, N. M., 531-553. [6] Kusaba, K. et al. 1985. EPSL 72, 433-439. [7] Gratz, A. J. et al. 1996. EPSL 142, 513-521. [8] Lumpkin, G. R. 1999. J. Nucl. Mat. 274, 206-217. [9] Gucsik et al. 2004. Min. Mag. In press. [10] Geisler, T. & Pidgeon, R. T. 2002. Cont. Min. Petrol. 143, 750-755. [11] Glass, B. P. & Liu, S. 2001. Geology 29, 371-373.