IMPACT MELT ROCK HETEROGENEITY AT THE HAUGHTON IMPACT STRUCTURE, CANADA: AN ANALOG FOR PLANETARY CRATERS
The impact melt rock is difficult to assess through traditional geologic field methods because it is too heterogeneous to collect a single sample representative of the outcrop (i.e., contains every phase present in the outcrop in the exact proportions). Instead, we use imaging spectroscopy to map the mineralogy of clast-rich impact melt rock outcrops in the field at the 23-km diameter Haughton impact structure, Nunavut, Canada, to determine the compositions of the melt rock and compare outcrops around the structure.
Results suggest that the impact melt rock is heterogeneous. For example, some outcrops have significant gypsum clasts, while others have little to no gypsum. Outcrops with little gypsum have more hydrated silica (in gneiss or sandstone) than high gypsum outcrops. The presence or absence of lithologies is consistent between outcrops and samples collected from each outcrop, but exact proportions vary, validating our outcrop-scale approach. Since silica-rich lithologies are generally from lower in the pre-impact stratigraphy while gypsum is from higher, our results suggest that the impact melt rock is derived from different depths spatially and the impact melt does not fully mix during the impact process, likely due to rapid quenching, at least for moderate-sized structures such as Haughton. These results are consistent with previous work finding heterogeneities qualitatively at the outcrop scale and quantitatively on a microscopic scale at several terrestrial impact structures but is the first to quantify the heterogeneities at the outcrop scale.
The dominantly sedimentary, volatile-bearing target rocks of the Haughton structure are an analog for solar system bodies such as Mars and Ceres. Similar heterogeneities would be expected for impact melt-bearing units on those bodies, and the scales over which the heterogeneities occur would be observable with imaging spectrometers from orbit and rovers on the ground.