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Early Archean Asteroid Impacts on Earth dpg15 v.2007/06/04 Prn:6/06/2007; 11:09 F:dpg15041.tex; VTEX/JOL p. 1 aid: 15085 pii: S0166-2635(07)15085-4 docsubty: REV Earth’s Oldest Rocks Edited by Martin J. van Kranendonk, R. Hugh Smithies and Vickie C. Bennett Developments in Precambrian Geology, Vol. 15 (K.C. Condie, Series Editor) 1 © 2007 Elsevier B.V. All rights reserved. DOI: 10.1016/S0166-2635(07)15085-4 1 Chapter 8.5 1 2 2 3 3 4 EARLY ARCHEAN ASTEROID IMPACTS ON EARTH: 4 5 STRATIGRAPHIC AND ISOTOPIC AGE CORRELATIONS 5 6 6 7 AND POSSIBLE GEODYNAMIC CONSEQUENCES 7 8 8 9 ANDREW GLIKSON 9 10 Department of Earth and Marine Science and Planetary Science Institute, Australian 10 11 National University, Canberra, ACT 0200, Australia 11 12 12 13 13 14 14 15 8.5-1. INTRODUCTION 15 16 16 17 The heavily cratered surfaces of the terrestrial planets and moons testify to their long- 17 18 term reshaping by asteroid and comet impacts and related structural and melting processes. 18 19 Following accretion of Earth from the solar disc at ca. 4.56 Ga (see Taylor, this volume), 19 20 the Earth–Moon system is believed to have originated by a collision between a Mars- 20 21 size planet and Earth, followed by episodic bombardment by asteroids and comets, with a 21 22 peak documented at ca. 3.95–3.85 Ga – the Late Heavy Bombardment (LHB) – recorded 22 23 on the Moon, but not on Earth due to the paucity and high grade metamorphic state of 23 24 terrestrial rocks of this age (Wilhelm, 1987; Ryder, 1990, 1991, 1997; see Iizuka et al., 24 25 this volume). Major impact episodes on Earth are recorded from rocks dated at 3.46, 25 26 3.26–3.24, 2.63, 2.56, 2.48, 2.02 Ga (Vredefort), 1.85 Ga (Sudbury), 0.58 Ga (Acraman), 26 27 0.368–0.359 Ga (late Devonian), 0.214 Ga (late Triassic), 0.145–0.142 Ga (late Jurassic), 27 28 0.065 Ga (K–T boundary), and 0.0357 Ga (late-Eocene) (World Impact List GSC/UNB; 28 29 Grieve and Shoemaker, 1994; Grieve and Pesonen, 1996; Grieve and Pilkington, 1996; 29 30 French, 1998; Simonson and Glass, 2004). Some of these impact events coincided with, 30 31 and are referred to as the direct cause of, mass extinctions and/or the radiation of species 31 32 (Alvarez, 1986; Keller, 2005; Glikson, 2005a). These impacts represent a small part of 32 33 the extraterrestrial impact record, as the retention of any impact-generated structure is a 33 34 direct function of the preservation and/or destruction of crust. By analogy, the continu- 34 35 ous re-accretion of sulphur on Io and ice on Europa obliterates their impact records, as 35 36 does volcanic re-surfacing on Venus and plate tectonics on Earth. Estimates of the impact 36 37 incidence on Earth is inferred from the lunar cratering record, and the current asteroid 37 38 and comet flux, which suggests the formation of some 100–200 craters of diameter (D) 38 39 D 100 km, and 5–10 craters of D 300 km since ca. 3.8 Ga (Neukum et al., 2001; 39 40 Ivanov and Melosh, 2003; Glikson et al., 2004). Most of these craters would have impacted 40 41 on hitherto subducted oceanic crust on Earth. The larger continental impacts are less likely 41 42 to be destroyed by subduction, and resulted in deeper excavation of the continental crust 42 43 and thicker ejecta layers, thereby being generally better preserved than their counterparts 43 dpg15 v.2007/06/04 Prn:6/06/2007; 11:09 F:dpg15041.tex; VTEX/JOL p. 2 aid: 15085 pii: S0166-2635(07)15085-4 docsubty: REV 2 Chapter 8.5: Early Archean Asteroid Impacts on Earth 1 that impacted oceanic crust. Glikson (2001) estimated that only between 8–17% of the 1 2 larger impacts (D 100 km) is recorded as impact structures and/or ejecta. However, as- 2 3 suming a relatively smaller proportion of large projectiles, over 50% of the larger impacts 3 4 have been recorded at this stage (Ivanov and Melosh, in Glikson et al., 2004). 4 5 Models suggest impact triggering of igneous, tectonic, or plate tectonic events (Green, 5 6 1972, 1981; Grieve, 1980; Alt et al., 1988; Jones et al., 2002; Elkins-Tanton et al., 2005; 6 7 Glikson and Vickers, 2005), but this correlation has not been directly demonstrated on 7 8 Earth. Ivanov and Melosh (2003) questioned impact-induced volcanic activity, whereas 8 9 Glikson et al. (2004) state: ‘Indeed, a few giant impact events certainly occurred on 9 10 Earth in post-LHB history, and a few might have encountered the fortuitous circumstances 10 11 necessary for them to enhance magma production. This is an important topic for future 11 12 study.’ Lowe et al. (1986, 1989) remarked on the potential tectonic significance of the co- 12 13 incidence between 3.26–3.24 Ga impact horizons recorded in the Barberton Greenstone 13 14 Belt (Kaapvaal Craton) and the abrupt the break between the underlying mafic–ultramafic 14 15 volcanic crust (3.55–3.26 Ga Onverwacht Group) and the overlying turbidite, felsic vol- 15 16 canic, banded iron formation and conglomerate-dominated succession (3.26–3.225 Ga Fig 16 17 Tree and Moodies Groups), which includes the earliest observed granite-derived detritus 17 18 (Fig. 8.5-1). This age period correlates broadly with a similar break in the Pilbara Craton, 18 19 Western Australia, where the mafic–ultramafic volcanic rocks of the ca. 3.255–3.235 Ga 19 20 Sulphur Springs Group are unconformably overlain by Soanesville Group clastic sedimen- 20 21 tary rocks, including a local unit of olistostrome breccia (Glikson and Vickers, 2005; Van 21 22 Kranendonk et al., 2006a). Widespread 3.27–3.24 Ga plutonic igneous rocks (dominantly 22 23 granitic) occur in both the Kaapvaal Craton and Pilbara Craton (see Poujol, this volume, 23 24 and Van Kranendonk et al., 2007a, this volume). 24 25 25 26 26 27 8.5-2. 3.47 GA IMPACT EVENTS 27 28 28 29 Microkrystite spherule-bearing lenses in chert and in the matrix of diamictite have been 29 30 reported from the 3.47 Ga Antarctic Creek Member (ACM) of the Mount Ada Basalt, in 30 31 the North Pole Dome area of the East Pilbara Terrane, Pilbara Craton, Western Australia 31 32 (Lowe et al., 1989; Byerly et al., 2002; Van Kranendonk et al., 2006a). The diamictite, 32 33 defined as ACM-S2, consists of a 0.6–0.8 m thick unit of spherule-bearing, chert-intraclast 33 34 conglomerate, which is separated from the main ACM-S3 unit by dolerite and felsic vol- 34 35 canics/hypabyssals. Zircon dating from the spherule-bearing sandstone unit yielded an 35 36 age of 3470 ± 2 Ma (Byerly et al., 2002). The microkrystite spherules are discrimi- 36 37 nated from angular to subangular detrital volcanic fragments by their high sphericities, 37 38 inward-radiating fans of sericite, relic quench textures, and Ni–Cr–Co relations. SEM- 38 39 EDS (scanning electron microscope coupled with electron-dispersive spectrometer) and 39 40 LA-ICP-MS (laser induced coupled plasma mass spectrometer) analysis indicate high Ni 40 41 and Cr in sericite-dominated spherules, suggesting a mafic composition of source crust. 41 42 Ni/Cr and Ni/Co ratios of the spherules are higher than in associated Archaean tholeiitic 42 43 basalts and high-Mg basalts, rendering possible contamination by high Ni/Cr and Ni/Co 43 dpg15 v.2007/06/04 Prn:6/06/2007; 11:09 F:dpg15041.tex; VTEX/JOL p. 3 aid: 15085 pii: S0166-2635(07)15085-4 docsubty: REV 8.5-2. 3.47 Ga Impact Events 3 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10 11 11 12 12 13 13 14 14 15 15 16 16 17 17 18 18 19 19 20 20 21 21 22 22 23 23 24 24 25 25 26 26 27 27 28 28 29 29 30 30 31 31 32 32 33 33 34 34 35 35 36 36 37 37 38 38 39 39 40 40 41 Fig. 8.5-1. Composite columnar section of the Barberton greenstone belt stratigraphy, showing the 41 42 principal groups and formations and U-Pb zircon isotopic age determinations (after Byerly and Lowe, 42 43 1994; Byerly et al., 1999; Lowe et al., 2003). 43 dpg15 v.2007/06/04 Prn:6/06/2007; 11:09 F:dpg15041.tex; VTEX/JOL p. 4 aid: 15085 pii: S0166-2635(07)15085-4 docsubty: REV 4 Chapter 8.5: Early Archean Asteroid Impacts on Earth 1 chondritic components. The presence of multiple bands and lenses of spherules within 1 2 chert, and scattered spherules in arenite bands within ACM-S3, may signify redeposition 2 3 of a single impact fallout unit or, alternatively, multiple impacts. Controlling parameters 3 4 include: (1) spherule atmospheric residence time; (2) precipitation rates of colloidal silica; 4 5 (3) solidification rates of colloidal silica; (4) arenite and spherule redeposition rates; and (5) 5 6 arrival of an impact-generated tsunami. The presence of spherule-bearing chert fragments 6 7 in ACM-S3 may hint at an older spherule-bearing chert (?S1). Only a minor proportion of 7 8 spherules is broken and the near-perfect sphericities of chert-hosted spherules and arenite- 8 9 hosted spherules constrain the extent of shallow water winnowing of the originally delicate 9 10 glass spherules.
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