Search for detrital shocked zircon at the Santa Fe impact structure, New Mexico, USA

Abstract

The Santa Fe impact structure, located in northern New Mexico, is considered obscured because it is highly tectonized, deeply eroded and is located within a complex geologic setting. Shatter cones and shocked quartz have been previously documented to confirm the impact origin, however, both impact age (350 – 1200 Ma) and crater diameter (6 – 13 km) are poorly constrained. The study of detrital shocked minerals has the potential to identify obscured impact structures, since the impact record of Earth is not fully known. Preliminary studies have previously documented detrital shocked minerals in the Santa Fe impact structure, which include detrital zircon grains with planar fractures, shocked quartz, shocked apatite, and kinked muscovite. This study comprises a detailed search for shocked zircon from both sediment and rock samples located within a ~5 km radius from the Santa Fe impact structure to evaluate the extent of shocked rocks within the structure. This study shows that detrital shocked zircon, and other tentative shocked minerals, are present in both sediments and rock samples and they can be used as tools for better understanding obscured impact structures. Field work, scanning electron microscope (SEM) imaging, electron backscatter diffraction (EBSD) mapping, and sensitive high-resolution ion microprobe (SHRIMP) U-Pb geochronology were used in this study to confirm the provenance of shocked zircon, thus confirming the extent of shocked rocks within the Santa Fe impact structure. A total of six detrital shocked zircon grains were documented from ~6600 detrital zircon grains from seventeen sediment samples and one shocked zircon grain from shatter cone-bearing granite. EBSD analysis revealed the presence of {112} shock twins in five zircon grains, and SHRIMP U-Pb geochronology yielded 207Pb/206Pb crystallization ages from 1715±22 to 1472±35 Ma. These results indicate that exposed rocks at the Santa Fe impact structure record shock pressures of at least 20 GPa, and by applying scaling laws, establish a minimum final crater diameter of 9 – 14 km. U-Pb geochronology of the shocked-twinned zircon grain provides the first reliable maximum age constraint of 1472±35 Ma, which expands the impact age range to ca. 350 – 1473 Ma. The occurrence of shocked zircon at the Santa Fe impact structure confirms that the study of detrital zircon is an efficient way to identify diagnostic shock deformation features and provide age constraints of deeply eroded or obscured impact structures, and thus aids in the reconstruction of the impact record of Earth.