The absence of any identified impact structures from the early Earth remains a conundrum. The record of meteorite impacts on Earth becomes more fragmentary further back in geologic time. Several useful ‘impact benchmarks’ appear to have wide acceptance. The oldest terrestrial impact deposits are distal impact ejecta horizons in South Africa and Australia that were deposited ca. 3.45 to 3.47 Gyr ago [1, 2]. The oldest shocked mineral is a single grain of quartz with PDF in a 2.63 Ga ejecta horizon in Australia [3]. No source craters for any Archean-age impact structures have been reported. The oldest intact confirmed and accurately dated impact structure is the 2.23 Ga Yarrabubba structure in Western Australia [4]. The lack of Archean-age impact structures is difficult to reconcile, given the higher impact flux on the early Earth, the number of cratons, and the preservation of Hadean age detrital zircons from numerous sites [5]. Attempts to identify evidence of shock deformation in detrital populations containing 1000s of early Archean zircons have thus far not been successful [6,7]. The successful identification of an Archean terrestrial impact structure requires documentation of the same types of diagnostic evidence found at terrestrial craters of any age - evidence of shock deformation at macro- and/or micro-scale, or identification of extra-terrestrial geochemical signatures of the projectile [8]. An example of a de-bunked Archean ‘crater’ is Maniitsoq, a site in Greenland interpreted to represent a 100 km-diameter impact crater formed at ca. 3.0 Ga [9]. Despite a complete absence of diagnostic impact evidence [8], multiple papers proclaiming an impact origin have been published. Rocks across the Maniitsoq region are broadly gneissic granitoids, and thus contain ideal minerals (quartz and zircon) for recording shock deformation. A survey of 5,600 zircons from 18 bedrock samples and 14 fluvial samples across the Maniitsoq region failed to identify evidence of shock deformation; the absence of any evidence for impact processes in Maniitsoq zircons precludes an impact origin [10]. The resilience of shocked zircon in bedrock [4], as well as detrital systems [11-14] is well-established, which highlights detrital zircon surveys as an effective tool to detect shocked grains in ancient sedimentary rocks. Until such grains are found, the hunt for an Archean impact structure continues.

Reference(s)

[1] Byerly et al. (2002) Science. 

[2] Glikson et al. (2016) Precam. Res. 

[3] Rasmussen and Koeberl (2004) Geology. 

[4] Erickson et al. (2020) Nature Comm. 

[5] Valley et al. (2014) Nature Geosci. 

[6] Montalvo et al. (2014) LPSC. 

[7] Cox et al. (2017) LPSC. 

[8] French and Koeberl (2010) Earth-Sci. Rev. 

[9] Garde et al. (2012) EPSL. 

[10] Yakymchuk et al. (2021) Earth Planet. Sci. Lett. 

[11] Cavosie et al. (2010) GSA Bull. 

[12] Erickson et al. (2013) Geochim. Cosmochim. Acta. 

[13] Thomson et al. (2014) GSA Bull. 

[14] Montalvo et al. (2017) Am. Mineral.