Microporphyritic and microspherulitic melt grains, Hiawatha crater, Northwest Greenland: Implications for post-impact cooling rates, hydration, and the cratering environment

Adam A Garde,Nynke Keulen,Tod Waight

doi:10.1130/b36058.1

Abstract

Abstract Sand-sized impactite melt grains hand-picked from a glaciofluvial sample proximal to the Hiawatha impact crater in Northwest Greenland contain new information about the crystallization and cooling history of this impact structure, which is concealed by the Greenland Ice Sheet. Of course, the original locations of the individual sand grains are unknown, but this is offset by the substantial number and wide variety of impactite grains available for study. A detailed investigation of 16 melt grains shows that post-cratering crystallization took place under very variable conditions of strong undercooling with temperatures that dropped rapidly from high above their solidus to far below. A distinct event of near-isochemical hydration at above or ~250 °C is recorded by intense perlitic fracturing and the growth of closely packed mordenite spherulites only 1–3 μm across in felsic melt grains, which was followed by lower temperature hydrothermal alteration along the pre-existing perlitic fractures. The formation of abundant mordenite microspherulites appears to be very rare or not previously recorded in impactite melts and suggests the rapid infilling of the Hiawatha crater by a hydrous source. The infilling did not occur immediately after the impact as in submarine impacts, but soon thereafter, and before the establishment of a low-temperature hydrothermal alteration system common to the waning stage of cooling in many impact structures. These observations and previous documentation of terrestrial organic matter in the impactites are consistent with an impact into a water-rich terrestrial environment, such as through the Greenland Ice Sheet or into a forested, lacustrine–fluvial region.

Highlights

Terrestrial impact cratering is characterized by a transition from initial shock metamorphism to more normal geological processes
Sand-sized impactite melt grains handpicked from a glaciofluvial sample proximal to the Hiawatha impact crater in Northwest Greenland contain new information about the crystallization and cooling history of this impact structure, which is concealed by the Greenland Ice Sheet
A distinct event of near-isochemical hydration at above or ∼250 °C is recorded by intense perlitic fracturing and the growth of closely packed mordenite spherulites only 1–3 μm across in felsic melt grains, which was followed by lower temperature hydrothermal alteration along the pre-existing perlitic fractures

Summary

Introduction

Terrestrial impact cratering is characterized by a transition from initial shock metamorphism to more normal geological processes. The recently discovered 31-kmdiameter Hiawatha meteorite crater in Northwest Greenland (Fig. 1) is one of the 25 largest on Earth (Kjær et al, 2018; Garde et al, 2020) It is the only known impact crater located under ice, and its apparent young age makes it conceivable that the impact bolide might have penetrated the Greenland Ice Sheet. To address this question and constrain the post-cratering environment during the cooling stage, we studied the microstructures, compositions, and mineralogy of sand-sized glaciofluvial impactite melt grains collected at the margin of the ice sheet just outside the crater (Fig. 1)

Methods

Findings

Discussion

Conclusion