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Search for Petrographic and Geochemical Evidence for the Late Heavy Bombardment on Earth in Early Archean Rocks from Isua, Greenland

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Search for Petrographic and Geochemical Evidence for the Late Heavy Bombardment on Earth in Early Archean Rocks from Isua, Greenland 4 Search for Petrographic and Geochemical Evidence for the Late Heavy Bombardment on Earth in Early Archean Rocks from Isua, Greenland Christian Koeberl ~, Wolf Uwe Reimold2, Iain McDonald 3, and Minik Rosing 4 qnstitute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria. (christian.koeberl @univie.ac.at) 2Impact Cratering Research Group, Department of Geology, University of the Witwatersrand, Johannesburg 2050, South Africa. 3School of Earth & Environmental Sciences, University of Greenwich, Chatham Maritime, Kent ME4 4AW, U.K. 4Geologisk Museum, Oster Voldgade 5-7, DK-1350 Kobenhavn K, Denmark. Abstract. The Moon was subjected to intense post-accretionary bombardment between about 4.5 and 3.9 billion years ago, and there is evidence for a short and intense late heavy bombardment period, around 3.85 _+ 0.05 Ga. If a late heavy bombardment occurred on the Moon, the Earth must have been subjected to an impact flux at least as intense. The consequences for the Earth must have been devastating. In an attempt to investigate if any record of such a late heavy bombardment period on the Earth has been preserved, we performed a petrographic and geochemical study of some of the oldest rocks on Earth, from Isua in Greenland. We attempted to identify any remnant evidence of shock metamorphism in these rocks by petrographic studies, and used geochemical methods to detect the possible presence of an extraterrestrial component in these rocks. For the shock metamorphic study, we studied zircon, a highly refractive mineral that is resistant to alteration and metamorphism. Zircon crystals from old and eroded impact structures were found earlier to contain a range of shock-induced features at the optical and electron microscope level. Many of the studied zircon grains from Isua are strongly fractured, and single planar fractures do occur, but never as part of sets; none of the crystals studied shows any evidence of optically visible shock deformation. Several samples of Isua rocks were analyzed for their chemical composition, including the platinum group element (PGE) abundances, by neutron activation analysis and ICP-MS. Three samples showed somewhat elevated Ir contents (up to 0.2 ppb) compared to the detection limit, which is similar to the present-day crustal background content 74 Koeberl et al. (<0.03 ppb), but the chondrite-normalized siderophile element abundance patterns are non-chondritic, which could be a sign of either a small extraterrestrial component (if an indigenous component is subtracted), or terrestrial (re)mobilization mechanisms. In absence of any evidence for shock metamorphism, and with ambiguous geochemical signals, no unequivocal conclusions regarding the presence of extraterrestrial matter (as a result of possible late heavy bombardment) in these Isua rocks can be reached. Introduction The most probable theory of the origin of the Earth and other planets explains their formation by accretion of smaller objects (planetesimals). Towards the end of the accretion of the Earth, at about 4.55 Ga, it may have been impacted by a Mars- sized body, which is the currently accepted hypothesis for the origin of the Moon (e.g., Cameron and Benz 1991; Taylor 1993). The consequences of such an impact event for the proto-Earth would have been severe; they would have included almost complete re-melting of the Earth, loss of any primary atmosphere (Vickery 1990), and admixture of material from the impactor. The material remaining in orbit after accretion of the Moon would have continued to impact onto the Earth for quite some time. Due to later geological activity, no record of this very early bombardment remains on the surface of the Earth. On the other hand, there is abundant evidence from Apollo rocks that indicates that the Moon was subjected to intense post-accretionary bombardment between about 4.5 and 3.9 billion years ago (e.g., Ryder 1989, 1990). In addition, the lunar highlands show much evidence for isotopic resetting, consistent with a short and intense late heavy bombardment period, around 3.85 -+ 0.05 Ga (e.g., Tera et al. 1974; Ryder 1990). However, these data were disputed by some workers (e.g., Baldwin 1974; Hartmann 1975), who interpreted the resetting as having been caused by the tail-end of the decaying late accretionary impact flux. There is also some evidence that meteorite parent bodies were subjected to a late cataclysmic bombardment (e.g., Bogard and Garrison 1991), indicating that this event may not have been restricted to the Moon (or the Earth-Moon system). During this period (and later in its history), the Earth would have been subjected to a significantly larger number of impact events, as it has a larger diameter and a much stronger gravitational attraction than the Moon. It seems realistic to assume that if a late heavy bombardment occurred on the Moon, the Earth (and other terrestrial planets - cf. Wetherilll 1975) must have been subjected to an impact flux at least as intense as that onto the Moon. The consequences for the Earth must have been devastating. Mantle temperatures Search for Petrographic and Geochemical Evidence of the LHB 75 shortly after the end of the accretion of the Earth were probably much higher than today. It has been estimated that about half of all heat produced by 235U decay to 2°Tpb was released before 3.9 Ga, which, together with thermal energy released during the impact of late accretionary bodies, added several hundred degrees to the internal temperature of the Earth (e.g., Smith 1981; Davies 1985; Taylor 1993, 1999). Atmospheres could have been generated (Matsui and Abe 1986), chemically altered (Fegley et al. 1986), and eroded (Vickery 1990) by large-scale impacts. There is some geochemical evidence (e.g., Nd isotope data) indicating that the Earth's upper mantle had already undergone some differentiation at the time of formation of the oldest rocks preserved on the Earth's surface (Greenland, Canada, Australia) (e.g., Harper and Jacobsen 1992; McCulloch and Bennett 1993; Bowring and Housh 1995; but see also Gruau et al. 1996, for a different viewpoint). The differentiated Nd isotope data (and the presence of some relict zircons with ages >3.8 Ga; e.g., Compston and Pidgeon 1986) could be interpreted to suggest that small amounts of crust had formed prior to 4 billion years ago, but had later been mixed back into the upper mantle. This view has been confirmed by Hf istope studies on single zircons (Amelin et al. 1999). However, it is unlikely that large amounts of crust were present at that time (although a few authors - e.g., Kr6ner and Layer 1992 - argue otherwise). It is evident that any large-scale early impacts had some influence on the development of the continental crust (Goodwin 1976), and it is conceivable that the late heavy bombardment has been responsible for the rehomogenization of the minor amounts of pre-3.85 Ga crust with the upper mantle. Such very early impact events must have occurred, no matter if there was a significant peak in the impact flux at about 3.8 to 3.9 Ga, or if this was just the time of the cessation of the enhanced post-accretionary impact flux. The search for any evidence of these impacts has important implications, as the presently documented terrestrial impact record basically covers less than half of the Earth's history. So far the oldest known terrestrial impact structures are the Proterozoic Vredefort and Sudbury Structures of 2023___4 and 1850_+3 Ma age, respectively (e.g., Deutsch and Sch~irer 1994; Gibson and Reimold, this volume). These structures also represent two of the three largest impact structures known on Earth, with respective original diameters of 250-300 and 200 kin, and they represent the complete, current, pre-l.85 Ga terrestrial impact crater record. It has been estimated (e.g., Grieve 1980; Frey 1980) that about 200 impact structures >1000 km in diameter formed on Earth between 4.6 and 3.8 billion years ago, which would have covered about 40% of the surface of the Earth. Using the minimum estimate for the cratering frequency, Grieve (1980) calculated that these impact events would have added a cumulative energy of about 1029 J to the Hadean Earth. 76 Koeberl et aL So far, no firm evidence for this "early bombardment" - or even of early Archean impact events - has been found, except for some unusual spherule horizons in South Africa and Australia, whose origin is controversial and which are discussed elsewhere (cf. Lowe et al. 1989; Koeberl and Reimold 1995; Koeberl et at. 1999; Reimold et al, this volume). The suggestion that terrestrial greenstone belts, which formed early in the geologic history, are equivalents of the lunar mare (Green 1972), has not been confirmed. Motivation and Background of Study The time period preceding the available rock record on the Earth (i.e., pre-3.85 Ga) is called the Hadean Eon (see, e.g., Taylor, 1999). So far the available evidence for the pre-2 Ga impact record of the Earth-Moon system comes from lunar data. Whereas the records for the Hadean Eon on the Earth are very sparse, the pre-Nectarian and Nectarian periods cover this time interval on the Moon (cf. Ryder et al. 1999). Soon after the formation of the Moon from a giant impact, a feldspathic crust formed the lunar highlands around 4.44 _+ 0.02 Ga (e.g., Carlson and Lugmair 1988; Premo and Tatsumoto 1992), which represents the oldest dated event in the Earth-Moon system. The currently observable morphology of the lunar highlands is almost exclusively the result of numerous impacts that occurred prior to the extrusion of the volcanic flows that form the visible mare plains (e.g., Wilhelms 1984, 1987).
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