EFFECTS OF SHOCK METAMORPHISM ON XRD PEAK BROADENING IN CARBONATE TARGET ROCKS, LATE DEVONIAN ALAMO IMPACT

Marine carbonate rocks comprise the main target constituents of the early Late Devonian (mid-Frasnian, ~382 Ma) Alamo impact event, which struck an oceanic, off-platform site presently in south-central Nevada. X-Ray powder diffraction (XRD) conducted on nineteen samples taken from within and adjacent to the impact-emplaced Alamo Breccia show variable but significant shock effects when compared with pure calcite and dolomite control samples. Increased XRD peak broadening was observed in the Alamo samples when the full width at half maximum (FWHM) measurement for diagnostic calcite and dolomite peaks was compared to FWHM values of corresponding calcite and dolomite peaks in the unshocked control samples.

The analyzed carbonate rock samples were collected from lithic breccia clasts, lithic ejecta clasts, carbonate accretionary lapilli, and matrix within the Alamo Breccia; from impact-related sub-Breccia clastic dikes; and from adjacent layers in the Guilmette Formation and Devils Gate Limestone located stratigraphically up to 0.4 m beneath and 3.3 m above the Breccia. The Breccia samples were collected from localities both proximal and distal to the presumed impact site.

Carbonate samples for analysis were removed using a Dremel Tool, as some of the sampled clasts and lapilli were too small to isolate using other techniques. All other Alamo samples and control samples were prepared similarly, to eliminate preparation bias. Unshocked calcite and dolomite control samples show a normal peak FWHM of 0.149 and 0.183, respectively. Peak patterns collected from the nineteen Alamo-related samples display FWHM values of 0.126 to 0.242 for unshocked samples and significantly broadened FWHM values of 0.285 to 0.407 for shocked samples. FWHM variations within the shocked samples indicate the expected variable levels of shock within the target material. The highest level of shock (0.407) was recorded in isolated patches of very fine-grained, light-colored carbonate matrix associated with Breccia containing accretionary lapilli and rimmed carbonate ejecta clasts. This light-colored matrix is interpreted as highly comminuted, shocked target carbonate derived as fallout from the ejecta cloud.