Abstract
AbstractConnecting the surface expression of impact crater‐related lithologies to planetary or regional subsurface compositions requires an understanding of material transport during crater formation. Here, we use imaging spectroscopy of six clast‐rich impact melt rock outcrops within the well‐preserved 23.5‐Ma, 23‐km diameter Haughton impact structure, Canada, to determine melt rock composition and spatial heterogeneity. We compare results from outcrop to outcrop, using clasts, groundmass, and integrated clast‐groundmass compositions as tracers of transport during crater‐fill melt rock formation and cooling. Supporting laboratory imaging spectroscopy analyses of 91 melt‐bearing breccia and clast samples and microscopic X‐ray fluorescence elemental mapping of cut samples paired with spectroscopy of identical surfaces validate outcrop‐scale lithological determinations. Results show different clast‐rich impact melt rock compositions at three sites kilometers apart and an inverse correlation between silica‐rich (sandstone, gneiss, and phyllosilicate‐rich shales) and gypsum‐rich rocks that suggests differences in source depth with location. In the target stratigraphy, gypsum is primarily sourced from ~1‐km depth, while gneiss is from >1.8‐km depth, sandstone from >1.3 km, and shales from ~1.6–1.7 km. Observed heterogeneities likely result from different excavation depths coupled with rapid quenching of the melt due to high content of cool clasts. Results provide quantitative constraints for numerical models of impact structure formation and give new details on melt rock heterogeneity important in interpreting mission data and planning sample return of impactites, particularly for bodies with impacts into sedimentary and volatile‐bearing targets, e.g., Mars and Ceres.
Highlights
IntroductionHypervelocity impacts are a geological process affecting every solid solar system body
We find large differences in impact melt rock composition with location in the crater, recording rocks excavated from different depths
While data were acquired from both sensors, our analyses focus on shortwave infrared (SWIR) data, which were most effective for compositional discrimination of clasts of the particular lithologies present at this site
Summary
Hypervelocity impacts are a geological process affecting every solid solar system body. Impacts modify surfaces and excavate subsurface lithologies that can be studied by remote sensing and in situ measurements. Impact craters are the primary windows into subsurface lithologies of other. A study of fresh lunar highlands craters of different sizes found an upper crust dominated by norite, while the deeper crust excavated by larger craters contained less norite and more gabbro, anorthosite, and troctolite compositions (Pieters, 1986). Pan et al, 2015; L. Pan et al, 2017; Sun & Milliken, 2015). An understanding of how impactites represent and sample subsurface lithologies is critical to proper interpretation of crater‐ related compositional units
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