OBSERVATIONS AND ANALYSIS OF SHOCK METAMORPHISM, SUEVITE MELT BODIES, MICROSPHERULES, AND NI-CR SPINELS IN SELECTED SAMPLES FROM THE ICDP-USGS CHESAPEAKE BAY IMPACT STRUCTURE EYREVILLE CORES

We report preliminary petrographic results from our examination of 52 samples from representative intervals of the ICDP-USGS Eyreville cores. We have not observed convincing examples of shock metamorphism in the succession above the granite megablock zone (<1095 rmcd). However, polymict clay-matrix clasts at the top of the Exmore beds may represent proximal lithic or altered melt ejecta washed back into the crater. Although we have not seen quartz with well-developed planar deformation features (PDFs) within the granite, we have documented pervasive mechanical twinning in microcline and prominent sheared kink bands in biotite near the top of the megablock.

Coarse sands underlying the granite (~1375 rmcd) consist of angular quartz grains displaying a variety of fluid inclusion trail patterns not observed in higher units. Some grains are toasted and exhibit strong mosaicism; fewer contain PDFs and diaplectic domains. This is the first level where we observe significant mixing of highly shocked and unshocked clasts. The bottom of this interval contains rip-up clasts from an underlying suevite which host what appear to be altered glass microspheres. They have an average normalized composition (anc) (wt%) of 62.8 SiO₂, 21.9 Al₂O₃, 3.3 MgO, 8.5 FeO, 1.7 CaO, 0.5 K₂O, and 0.2 Na₂O and have radially fibrous chamosite rims.

Lower suevites contain relatively large quartzose clasts with abundant PDFs dominated by {10-13} with subordinate {21-31}and {51-61}orientations. Portions of the clasts are extensively recrystallized. Within their cores, grains with decorated PDFs occur alongside those with fresh PDFs and possible ballen. This may reflect the original distribution of water within the clasts.

These suevites also contain abundant mm to cm-size bodies of white and amber-brown altered glass. The former have anc (wt%) of 88.7 SiO₂, 6.9 Al₂O₃, 0.9 MgO, 1.6 FeO, 0.6 CaO, 0.2 K₂O, and 0.5 Na₂O. They contain spindle-shaped greenish glasses with anc (wt%) of 65.9 SiO₂, 20.0 Al₂O₃, 2.8 MgO, 9.9 FeO, 0.9 CaO, 0.1 K₂O, and 0.0 Na₂O. The amber glass has anc (wt%) of 83.2 SiO₂, 10.0 Al₂O₃, 0.8 MgO, 4.2 FeO, 0.7 CaO and contain Ti, Si-rich lamellae likely derived from rutilated quartz. We have observed tiny fragments of Ni-Cr spinel within these glasses. We are studying those grains to determine if they have an endogenic or extraterrestrial source.