Heavy Bombardment of the Earth at -3.85 Ga: The Search for Petrographic and Geochemical Evidence
Graham Ryder Lunar and Planetary Institute
Christian Koeberl U"iversity 0/Vienna
Stephen J. Mojzsis University o/California, Los Angeles
The Moon experienced an interval of intense bombardment peaking at -3.85 ± 0.05 Ga; subsequent mare plains as old as 3.7 or 3.8 Ga are preserved. It can be assumed that the early Earth must have been subjected to an even more intense impact flux resulting from its larger size and because of its proximity to the Moon. Siderophile-element analyses (e.g., Ir abun dance) of the oldest sediments on Earth could be used to indicate past escalated influxes of extraterrestrial material. In addition, shocked minerals may also be present in the oldest extant rocks of sedimentary origin as detrital minerals. and remnants of impact ejecta might exist in early Archean formations. Searches for impact signatures have been initiated in the oldest sedi ments on the Earth, from the early Archean (>3.7 Ga) terrane of West Greenland; some of these rocks have been interpreted to be at least 3.8 Ga in age. So far, unequivocal evidence of a late heavy bombardment on the early Earth remains elusive. We conclude that either the sedimen tation rate of the studied sediments was too fast and therefore too diluting to record an obvi ous signal, or the ancient bolide flux has been overestimated, or the bombardment declined so rapidly that the Greenland sediments, some even at -3.85 Ga in age, do not overlap in time with it.
1. INTRODUCTION Earth appears to have been completed about 50-100 m.y. after the initial collapse of the solar nebula (Lee and Halli Collisions between planetary bodies have been funda day, 1995, 1996; Halliday et al.. 1996; Podosek and Ozima, mental in the evolution of the solar system. Studies under 2000), in a timescale apparently more protracted than that taken over the last few decades have convinced most for smaller planetesimals and Mars. As a result of later workers that the planets formed by collision and hierarchi geological activity, no record of any primary accretion bom cal growth starting from small objects, i.e., from dust to bardment history remains on the surface of the Earth. planetesimals to planets (e.g., Wetherill. 1994; Taylor, Thc period on Earth between the end of accretion and I992a,b; see also chapters in section II of this volume), and the production of the oldest known crustal rocks is com not from condensation downward. Late during the accre monly referred to as the Hadean Eon (Cloud, 1976. 1988; tion of the Earth (some time after -4.5 Ga), when it had Harland et aI., 1989; Taylor, 2000), which is a chronostratic reached about 70% of its eventual mass, it was most prob division (Fig. I). Its terminal boundary is actually not de ably impacted by a Mars-sized or larger body (see Cameron, fined on the Earth; Harland et af. (1989) equate it with the 2000). The consequences of such an impact event for the Orientale impact on the Moon. Others do not even use the proto-Earth would have been severe and seminal, ranging teml Hadean, either distinguishing the chronometric divi from almost complete melting and formation of a magma sions of Archean Eon and (older) Priscoan time (Harland ocean, thermal loss of preexisting atmosphere, changes in et al., 1989), provisionally at 4.0 Ga. Here we use the term spin rate and spin-axis orientation, to accretion of material Hadean to represent the time period between the formation from the impactor directly, or through rapid fall-out from of the Moon at -4.5 Ga and the beginning of the continu orbital debris below the Roche limit. Much of the material ous terrestrial rock record at 3.8 Ga. blasted off in the impact eventually reimpacted the Earth; In contrast with the youthful age for the crust of the some of the ensuing Earth-orbiting debris would have rap Earth, the surface of the Moon displays abundant evidence idly coalesced to form most of the Moon and probably some of an intense bombardment at some time between its origi smaller moonlets. Some of this geocentrically orbiting nal crustal formation and the outpourings of lava that form material would have continued to impact both bodies for the dark mare plains. Even prior to the Apollo missions, perhaps tens of millions of years after the lunar forming these plains were calculated to be about 3.6 Ga in age based impact. The essential accretion and core formation of the on crater counts and realistic flux estimates. Hence, the
475
Origin ofthe Earth and Moon Eds.: Robin Canup and Kevin Righter University of Arizona Press, Tucson (2000) Fig. L. Comparative chronostratigraphies of the Earth and Moon, based on Harland el al. (1989) and Wilhelms (1987). The times of interest in this paper are the Isuan and Hadean Eras for the Earth and the Pre-Nectarian, Nectarian, and Imbrian Periods for the Moon. The Imbrian is divided into the two Epochs of Early Imbrian and Late Imbrian, which have greatly differing styles of geological activity (rock stratigraphic units, i.e, systems, are not used in this paper). Although the chronostratic divisions into these two Epochs (the Nectarian and the pre-Nectarian) are perfectly clear, the cotTeiation with absolute time is less established, although the age of the Fra Mauro Formation (Imbrium ejecta morphology) that defines the division of Early Imbtian and Nectarian is fairly well established at 3.84 or 3.85 Ga (e.g., Dalrymple and Ryder, 1993). Ryder et al.: Heavy Bombardmenl olthe Earth 477
heavy bombardment was inferred to be ancient (Hartmann, according to the recognition oflunar ferroan anorthosite of 1966). Lunar highland sample data show isotopic resetting that age. The present morphology of the highlands of the from thermal heating, for which there is abundant evidence Moon reflects, almost exclusively, a history of numerous for impact sources dominated by ages of around 3.8-3.9 Ga. subsequent impacts that occurred prior to the extrusion of The most ancient volcanic rocks from mare plains have ages the volcanic flows that form the visible mare plains (e.g., of about 3.8 Ga (see, e.g., Taylor, 1982; Wilhelms, 1987). Wilhelms, 1984, 1987). These ancient impact structures in The highland ages have been interpreted to either represent clude giant multiring basins and their debris (Spudis, 1993), a short and intense heavy bombardment period at 3.85 ± as well as a size-seriate range of smaller craters. Hartmann 0.05 Ga or so (e.g., Tera et al., 1974; Ryder, 1990), or the (1965, 1966) recognized that most ofthis cratering occurred tail end of a prolonged postaccretionaly bombardment (e.g., early in lunar history according to an estimate of the aver Baldwin, 1974; Hartmann, 1975), as discussed in Hartmann age age of mare plains of 3.6 Ga, which was calculated et af. (2000). In any case, the bulk of this bombardment, based on present-day cratering rates. He inferred a cratering which produced size-seriate scars up to multiring basins rate averaging roughly 200x higher for the first one-seventh many hundreds of kilometers across, preceded 3.8 Ga. We of lunar history than for the remainder. The general correct will use the term late heavy bombardment to refer specifi ness of Hartmann's conclusion was demonstrated by the cally to that bombardment of the Moon and the Earth from return ofApollo samples, and the dating ofthe oldest mare -3.90 to 3.80 Ga. plains at close to 3.8 Ga (Wilhelms, 1987). In any given time-span, the Earth must have been sub Geochronological studies of impact-brecciated highland jected to a significantly greater bombardment than was the samples show thermal events, most of them of impact ori Moon, as it has a larger diameter and a much larger gravi gin, concentrated at -3.8-3.9 Ga. These ages have been tational cross section, thus making it an easier target to hit taken to represent the tail end of a heavy but declining (e.g., Maher and Stevenson, 1988; Oberbeck and Fogleman, bombardment dating back to the accretion of the Moon 1989; Zahnle and Sleep, 1997). If a late heavy bombard (e.g., Shoemaker, 1972, 1977; Hartmann, 1975. 1980; ment occurred on the Moon, the Earth was subject to a flux Neukum et af., 1975; Baldwin, 1971, 1974. 1981, 1987; scaling because of the ratio of the impact cross sections Taylor, 1982; Wilhelms, 1987); alternatively, they may record (Sleep et al., 1989), which may have resulted in an impact a sharp or cataclysmic increase in bombardment for that rate ~20x greater than the lunar one, containing both more short interval (e.g., Tera et aI., 1974; Ryde/; 1990; Dalrymple and larger impact events. The consequences for the hydro and Ryder, 1993, 1996). There exists a sharp drop-off in sphere, atmosphere, and even the lithosphere of Earth at that estimates for the cratering rate from the youngest high time must have been devastating (Zahnle and Sleep, 1997; land surfaces, the Orientale and Imbrium ejecta blankets, Grieve, 1980; Frey. 1980). There is evidence that the Earth's to the oldest mare surfaces. This is according to crater upper mantle had already undergone some differentiation counts of those surfaces, which differ by a factor of -3-4 at the time of formation of the oldest igneous rocks, sug (e.g., Wetherill. 1977. 1981; B VSp, 1981). As a result of the gesting the prior existence of chemically evolved crust (e.g., difference in cratering rates, a flux at least lOOx higher can Harper and Jacobsen, 1992; McCulloch and Bennett, 1993; be calculated for this transition period, even if those young Bowring and Housh, 1995). It has been suggested that the est highland surfaces are as much as 100 m.y. older than absence of any rocks older than about 3.9-4.0 Ga is the the oldest mare plains, which have been collecting craters result of the ancient heavy bombardment, during which for 3.8 G.y. With new studies that have expanded the age impact-induced mixing recycled early crustal fragments ranges for the oldest known rocks on Earth, the time span back into the upper mantle (e.g., Grieve, 1980; Frey, 1980; for lunar bombardment now overlaps that of these oldest Koeberl et af.. 1998a,b). In the present contribution we rocks. Therefore, a more detailed look at the chronology and outline the evidence for the character and timing of the late intensity of the lunar bombardment can help to understand heavy bombardment on the Moon, and in light of this, de the conditions on Earth at the time of life's emergence scribe petrographical and geochemical attempts to investi (Mojzsis et aI., 1999). The reader is referred to the paper gate if any coeval record has been preserved on the Earth. by Hartmann et al. (2000) for a more complete discussion of lunar cratering history. 2. THE BOMBARDMENT HISTORY OF THE LUNAR HIGHLANDS 2.2. Relative and Absolute Ages of Highland Stratigraphy 2.1. General The stratigraphy of the lunar highlands has been divided Whereas there is almost no evidence for terrestrial wit on the basis of basin fonnation and ejecta into pre-Nectarian nesses to the Hadean Eon, the pre-Nectarian Period, Period, Nectarian Period, and Early Imbrian Epoch Nectarian Period, and the Early Imbrian Epoch cover this (Wilhelms, 1984, 1987) (Figs. I and 2). These are separated time interval on the Moon (e.g., Harland et af., 1989; by the time of production of the Nectaris Basin deposits, Wilhelms, 1987) (Figs. 1 and 2). The formation of a felds the Imbrium Basin deposits, and the debris blanket of the pathic crust was essentially complete by -4.44 ± 0.02 Ga, Orientale Basin respectively. Several basins were produced 478 Origin ofthe Earth and Moon
Ga 3.6 -,------r ** = age of oldest Akilia sediment
Apollo 11 mare lavas 3.7 Late Imbrian Apollo 17 mare lavas
!III!!l~!III!!lt:::::".~orientale 3.8 basin (3.80/3.84?) =: Schr6dinger basin ~~~.-.iL~ -"4__.lmbrium basin (3.85) " Serenitatis basin (3.89) 3.9 -+------'---1--.. Crisium basin (3.89) Nectaris basin (3.90/3.92?) .~-??--....- ...... ~ Numerous basins 4.0 South Pole-Aitken basin . 1# jCProcellarum basin?
4.1 / /'A-P-O-II-0-1-5-A-p-e-n-n-in-e--'V / / Bench KREEP basalts ??; . {/ (3.84) 4.2 / ~rustal '-m-a-g-m-a-t-is-m------' / and volcanism 4.3 /.?~ 1
4.4 Oldest ferroan anorthosites/ crustal formation
Magma ocean 4.5
Fig. 2. Stratigraphy and chronology of early lunar history, based on relative stratigraphy discussed in Wilhelms (1987) and absolute agc inferences as discussed in this paper. The basins with underlined names define the stratigraphic column. Some other significant events or features of early lunar history are shown. While significant impacting and contraction of the geologic column is obvious at 3.8-3.9 Ga, the event/time correlations within the pre-Nectarian, and even the age of the Nectaris Basin, are much more contentious. The age of the oldest Akilia sediments, discussed in this paper, are shown (**) for comparison with lunar stratigraphy.
during the Nectarian Period, including Serenitatis and Cri Jager (1977), and thus most are slightly younger than those sium, whose ejecta regions have been sampled (Apollo 17 given in some of the original publications. and Luna 20). The Schr6dinger Basin is Early Imbrian, as Although these divisions for lunar time were introduced are several large craters, including some that are almost above in normal stratigraphic sequence from oldest to 200 Ian in diameter. The oldest mare deposits were erupted youngest, it is more convenient to discuss the absolute dat in the Late Imbrian Epoch, the end of which is defined in ing of the boundaries from youngest to oldest, from the sim terms of crater degradation and crater counts in the absence plest interpretations based on the best preserved impact of any globally useful stratigraphic-datum horizons compa craters, to the more difficult. rable to basin ejecta. The dating of these boundaries, as well 2.2.1. The oldest mare surfaces. The Late Imbrian as of other basins within the stratigraphic units, defines the Epoch commenced with the formation of the Orientale chronology of lunar bombardment and the flux over the Basin, the final large multiring basin to have formed on the main period of interest here. Absolute ages quoted have been Moon. It was followed by few large (> IO kIn) cratering recalculated using the revised decay constants of Steiger and events. The end of the Late Imbrian Epoch is arbitrarily Ryder et al.: Heavy Bombardment ofthe Earth 479 defined, and includes the mare basalts at the Apollo IS land certainly no older than 3.870 ± 0.010 Ga, and probably no ing site that have been dated at -3.25 Ga. Lavas dating to older than 3.836 ± 0.016 Ga. Third, similar arguments ap the Late Imbrian compose roughly two-thirds of the mare plied to the contents of the Fra Mauro Formation and the surfaces. Most important for the discussion here are the Cayley Formation suggest a similar age constraint. For ex older mare units, including those from which mare basalt ample, impact-melt fragments in the white rock 14063 from samples were collected at the Apollo II and Apollo 17 land Cone Crater show a range from 3.87 to 3.95 Ga; other ing sites. The common Apollo II group B2 mare basalts and samples that are probably not from the Fra Mauro Forma the rare ApolIo II group D mare basalts are 3.80 Ga or tion, but represent later local events (such as the 14310 slightly older; some Ar-Ar age determinations are as old as group samples), are a little younger (3.82 ± 0.02 Ga). Thus, 3.85 Ga (Snyder et al., 1994, 1996). At the Apollo 17 land it is safe to bracket Imbrium as 3.85 ± 0.02 Ga. This is ing site, the oldest mare basalt so far identified formed at consistent with the the older Serenitatis Basin (below) hav 3.87 ± 0.10 Ga (Dasch et al., 1998), and other mare basalts ing formed at about 3.89 Ga. from there are almost 3.80 G.y. old (see summary in Wil 2.2.4. The age ofthe beginning ofthe Nectarian Period helms, 1987). Although younger basalts were also collected (the age of Nectaris). Stratigraphic and crater count data from these locations, it seems likely that the presence of show that the Nectaris Basin is older than the Crisium Ba such old basalts close enough to the surface to be in the sin, but melt-rock samples from it cannot be identified with sample collection suggests that for all but the smallest cra certainty. The Apollo 16 site was modified by Nectaris ters (those that are a few meters across) the crater counts ejecta, and subsequently by Imbrium ejecta. Fragments for these areas represent surfaces very close to 3.80 G.y. old, within the breccias collected on the Apollo 16 mission prob and perhaps slightly older. At a minimum, the crater counts ably include samples of melt created prior to Nectaris in for these sites represent surfaces that are at least 3.6, and several clearly recognizable large craters that underlie the probably more than 3.7, G.y. old. site. None ofthe melt samples dated so far is reliably older 2.2.2. The age of the beginning of the Late Imbrian than 3.92 Ga. The analysis of the rocks and ages by James Epoch (the age of Orientale). The Orientale and Schri:i (1981) strongly suggests an age for Nectaris of less than dinger Basins are far removed from any sites sampled so 3.92 Ga, and probably an age of -3.90 Ga is reasonable, far. Their ejecta have crater counts that are similar to each consistent with earlier derivations by Turner and Cadogan other and they are slightly less cratered than the Imbrium (1975) and Maurer et al. (1978). ejecta (Fra Mauro Formation, Cayley Formation). Schri:i The Nectalian Period also witnessed the formation ofthe dinger is older than Orientale, as it is superposed by Serenitatis and Crisium Basins, and samples were collected Orientale secondaries. However, their absolute ages cannot from their rims or ejecta. At the Apollo 17 landing site, the (yet) be independently dated; they are older than the oldest highland materials are dominated by coherent poikilitic melt affected mare plains, and thus are construed as older than rock, commonly in the form of boulders whose trails can -3.80 Ga. be seen to run high up the massifs. These samples are most 2.2.3. The age of the beginning of the Early Imbrian readily interpreted as melt formed in the Serenitatis Basin Epoch (the age ofImbrium). The best way to date an im event. If they are not, then they are probably older, as it is pact is by using the radiogenic isotopes in a clast-free or inconceivable that they are balIistic ejecta from the Imbrium clast-poor impact-melt rock (Ryde/; 1990; Deutsch and event. Most of these samples belong to one chemical group Scharer, 1994). Unfortunately, impact-melt rock that can be whose age, as determined on several samples, is now pre identified specificalIy as a product of the Imbrium impact cisely established as 3.893 ± 0.009 Ga (Dalrymple and is lacking, and those considered most likely (the Apollo 15 Ryder, 1996). This age is outside of the bracket for the dimict breccias; Ryder and Bower, 1977) have disturbed Ar Imbrium age described in the previous section. The Luna 20 Ar systems (e.g., Bogard et aI., 1991). Recently, Haskin et sample from Crisium ejecta includes impact-melt rock al. (1998) have argued that all Th-rich impact melt brec samples. From these, Swindle et al. (1991) suggested an age cias (low-K Fra Mauro) collected 011 the Apollo missions of-3.89 Ga for the Crisium Basin. These ages for Serenitatis are products of the Imbrium impact event. Despite the dat and Crisium are consistent with an age of the older Nectaris ing problems, there are ways to bracket the age of Imbrium. Basin of3.90 Ga. Several other basins, e.g., Herzsprung and First, the Apennine Bench Formation is a volcanic plateau Humorum, also formed after Nectaris. Thus, there was con inside (hence younger than) the Imbrium Basin. Remote siderable bombardment of the Moon in the 60 m.y. between sensing of its morphology and chemistry allow correlation 3.90 Ga and -3.84 Ga. with Apollo IS volcanic KREEP basalt samples, which have 2.2.5. Pre-Nectarian Period and events. The lack of been dated. This provides a lower age limit on the Imbrium impact-melt rocks in the sample collections that are older Basin of 3.84 ± 0.02 Ga (Ryder. 1994). Second, the than -3.92 Ga cannot be due to resetting of all older ages, Apennine Front has been little modified since the forma given the difficulties of such resetting (e.g., Ryder, 1990; tion of the Imbrium Basin. Thus crystalline impact melt Deutsch and Scharer, 1994). Most of the lunar upper crust collected there should be almost entirely Imbrium impact has not been converted into impact-melt rock, which would melt, or older impact melt. Dalrvmple and Ryder (1991, be subject to resetting. Thus the paucity of pre-3.92-Ga 1993) dated such melt rocks and suggested that Imbrium is impact melt can be taken as evidence that there was little 480 Origin of the Earth and Moon
impacting prior to that time, other than that expressed by 60C units/Ga. Therefore, the relative cratering rate of the the metamorphosed breccias of uncertain origin, the felds 3.85-3.80-Ga period compared with the average since pathic granulites, that may well date back to the very earli 3.80 Ga is -228. est postaccretionary bombardment at about 4.4 Ga. Further Furthermore, let us assume that Nectaris is 3.90 Ga and more, the Pb-isotopic data of Tera et al. (1974) indicate Imbrium is 3.85 Ga. There are -16C units of craters on Nec events at -3.85 Ga and events at >4.4 Ga, but not much taris ejecta. Thus the cratering rate during this period is evidence of events in between; continual resetting of Pb (l6C-4C) per 0.05 units and Ga, i.e., 240 units/Ga. This is clocks would show up as intersects in the 4.4-3.9-Ga Pb 912x the average rate since 3.80 Ga. isotope growth curve. There is also a lack of the comple The present rate, or the Phanerozoic rate, of cratering is ment of siderophile elements that would be expected to be probably a little lower than the average over the last 3.80 G.y., present in older upper crustal rocks if a heavy bombardment because there was a higher flux in the Late Imbrian Epoch between 4.4 and 3.9 Ga had occurred (Ryder, 1999), despite than in the succeeding Eratosthenian and Copernican (in claims to the contrary (e.g., Sleep et al., 1989; Chyba, 1991). deed, there is evidence that suggests a higher flux in the A more complete discussion of these features appears in the Eratosthenian than in the Copernican; Ryder et al., 1991; chapter by Hartmann et at. (2000). Culler et al., 1999). In round figures the cratering rate in the period 3.90 Ga to 3.85 Ga was probably at least 1000 2.3. Summary of the Significance of the Lunar 1500x that of the Phanerozoic, and in the period 3.85-3.80 Cratering Record from 3.90 to 3.80 Ga Ga was probably at least 250-400x that of the Phanero zoic. These are higher than the rates inferred by Hartmann During the period from 3.90 to 3.80 Ga, a substantial (1966), because he assumed that the observed cratering amount of the extant lunar highland features, including record stretched back almost to the origin of the Moon, Nectaris and many younger basins, formed on the Moon. whereas it is actually much more restricted in time. It is even Based on the above discussion, it is possible that this in possible that the later decline took place over only the first tense activity terminated at 3.85 Ga with the near-simulta 10 or 20 m.y. after -3.85 Ga, such that by 3.84 Ga or neous creation of the Imbrium, Schr6dinger, and Orientale 3.83 Ga the flux was approaching within a few factors of Basins. Bombardment may even have finished as early as that of the present day. This is the record that needs to be 3.87 Ga. Although the last two basins might have fOlmed compared with that of the oldest rocks on Earth. as late as 3.80 Ga, this seems unlikely given that their su perposed crater populations are almost as high as those on 3. STATE OF THE SURFACE OF the Imbrium ejecta, yet greater than those on the oldest mare THE EARTH FROM 4.5-3.8 GA plains, which themselves are -3.8 G.y. old. Many lunar basins, including South Pole-Aitken, formed prior to this 3.1. Earliest Crust period, but at present there is no direct or definitive way to date their formation; South Pole-Aitken might be as young Recognized extant terrestrial crustal rocks extend back as 3.95 Ga, or as old as 4.3 Ga. In principle, future missions to only about 89% of the history of the planet, to -4.0 Ga; can obtain samples from which the chronology of ancient the record is improved somewhat if we include the poten intense bombardment can be more reliably determined. In tial information gleaned from rare detrital zircon grains that particular, the age of Orientale could be precisely deter are up to 4.27 Ga in age, -94% of Earth history. As men mined, as its impact melt sheet is intact and accessible, and tioned above, is likely that the Moon-forming impact led would constrain the younger end of bombardment. South to a large-scale melting of the Earth and the existence of Pole-Aitken may be datable and could provide a constraint an early magma ocean (e.g., papers in this volume). Mantle at stratigraphically older times, because although it has been temperatures in the Hadean were probably much higher than battered, remnants of the impact-melt sheet should be col today. About half of all heat produced by 235U decay to lectable and recognizable. z07Pb was released during the Hadean, 40K was more abun The chronology outlined above suggests a massive de dant, as well as latent heat from accretion, all of which cline in the flux of bombardment on the Moon over a short added several hundred degrees to the internal temperature period of time following the Nectaris event. The cratering of the Earth. Any late accretionary bodies would have added on the Nectaris ejecta (3.90 Ga) is a factor of -4 higher than further thermal energy to the already elevated budget of heat that on Imbrium ejecta (3.85 Ga), which, in tum, is a fac flow in the early Earth (e.g., Smith. 1981; Davies, 1985; tor of -4x that recorded on the oldest mare plains (about Taylm; 1993. 2000). 3.80 Ga, or even slightly older). The nature of the earliest crust on Earth, and the amount As an exercise, let us define that there are C units of of crust present, has been the subject of intense debate. craters on 3.80-G.y.-old mare plains. Then the average Petrological melting concepts and comparisons with other cratering rate from 3.80 Ga to the present is C per 3.80 units planets suggests that the earliest crust on Earth was basal and Ga (i.e., 0.263C units/Ga). Imbrium ejecta has -4C units tic in composition (e.g., Taylor, 1989. 1992. 1993; Arndt and of craters. Thus, the cratering rate between Imbrium forma Chauvel. 1991). The existence of any substantial early feld tion and oldest mare plains is (4C-IC) per 0.05 units and spathic crust on the Earth seems precluded by the higher Ga (assuming 50-m.y. age differences), which is a rate of pressure at shallower depth on the Earth (in contrast to the Ryder et a/.: Heavy Bombardmenr of/he Earth 481
Moon, which does have an ancient feldspathic crust). Cal spherules. The projectile and some target rock are vapor cium and Al are sequestered in the deeper Earth in early ized, and a fraction of the projectile vapor can be incorpo high-pressure phases (particularly garnet), which delays the rated into melt and ejecta. Minerals of both the autoch concentration of those elements reaching the point of pla thonous target and the allochthonous ejecta could exhibit gioclase crystallization. In addition, plagioclase itself can shock effects (e.g., planar deformation features, high-pres not crystallize at significant pressure (hence depth), so any sure polymorphs, diaplectic glasses) from the interaction of plagioclase-bearing terrestrial crust would be thin. Neither the rocks and minerals with the shock wave. If the target would any plagioclase be likely to float in a water-bearing, includes water (e.g., ocean impact) then that water gets basaltic magma ocean on the Earth, so no concentration of vaporized. If the impactor contains typical chondrite-like plagioclase toward the surface would be realized. Finally, abundances of platinum-group metals (e.g., Ir), as all me there is no indication of any ancient reservoir of Eu or of teorites other than most differentiated stony meteorites do, primitive 87S r/86S r signatures that could have resided in an then these will be added to the impactite. Depending on the early high-Sr and low-Rb anorthositic crust (e.g., Taylor, density of the atmosphere, there is a lower size limit below 1989). Some pre-4.0-Ga differentiation of the mantle seems which small impactors do not penetrate the atmosphere and to have occurred, as indicated by isotopic evidence (e.g., therefore will not form craters. At very large impactor di Hmper and Jacobsen, 1992; Bowring and Housh, 1995; but ameters, excavation of mantle material is possible, as well see also Gruau et aI., 1996, for cautionary remarks). Detrital as large-scale vaporization (and possible loss) of atmosphere zircon crystals in an Archean quartz-pebble conglomerate and hydrosphere. Judging from the lunar record (see sec from the Narryer Gneiss Complex, Western Australia, are tion 2), very little of the Earth's surface in the period of the oldest known minerals on Earth, with ages up to 4.27 Ga -3.9-3.8 Ga should have escaped being the target of sig (e.g., Compston and Pidgeon. 1986). The morphological, nificant impacts at one time or another, and therefore es mineralogical, and geochemical characteristics, as well as caped being covered by ejecta from craters that are at least similarities with post-3.75-Ga zircons, indicate a compos a few kilometers in diameter. However, a more vigorous ite granitoid source of continental provenance for these zir rock cycle than at present continually resurfaced the early cons (Maas et al., 1992; Mojzsis, 1998). Thus there is evi Earth and erased (most?) evidence for such an impact en dence for at least minor amounts of felsic igneous rocks in vironment. the Hadean, which may have been present in small amounts Calculations that scale impact-melt production with in from remelting of basaltic crust that sank back into the creasing crater dimension (Melosh, 1989; Cintala and Grieve. mantle (e.g., Taylor, 1989, 2000). lt remains unlikely, though 1994, 1998) show a breakdown of this geometric relation unproven, that significant amounts of continental crust ex ship for very large impact structures. As the magnitude of isted on Earth during much of the Hadean Eon. The lack of the impact increases, the melt volume relative to the tran initial Hf-isotopic heterogeneity and the absence of nega sient crater size increases, with a larger proportion being tive cHf values in early Archean rocks provides evidence retained inside the crater, and the depth of melting for large against the presence of large amounts of continental crust on impact structures exceeding the depth of excavation. There the Hadean Earth (Vervoort and BUchert-Tojt, 1999). Gra fore, the thermal effects of an impact (i.e., the large-scale nitic crusts require multistep derivation from the primitive melting) will actually reduce the amount of shocked rocks mantle by recycling of subducted basaltic crust through a that are formed and preserved. In large-scale impact events, "wet" mantle, which will slowly lead to an increasing amount leading to the formation of craters larger than a few hun of granitic crust through time (Taylor and McLennan, 1995). dred kilometers in diameter, thermal metamorphism may be The lithosphere of the Hadean Earth was most probably more important than shock metamorphism. However, cra characterized by a basaltic crust, covered by an ocean, and ters smaller than a few hundred kilometers in diameter with little dry land and only minor amounts of felsic rocks would still largely have fragmental and shocked ejecta and (granitoids). Any sedimentological record, which would host basement. infOimation specific to surface environments such as the rate Most of the (considerable) speculation regarding the ef and violence of meteorite impact and the presence of life, fects of ancient impacts on the Earth has focused on large, has been almost completely lost from Hadean times, and potentially basin-forming, events. These models attempt to only appears at its conclusion, near 3.90 Ga (Mojzsis et al., understand the localization and extent of endogenic activ 1996. 1999; Mojzsis and Harrison, 2000; Nutman et al., ity, such as volcanism, proto-ocean basin formation, atmo 1996, I997). spheric disturbance, continental growth and assembly, and changes in sedimentation style and topography, rather than 3.2. Effects of Ancient Impacting: relying on direct impact evidence. Grieve (1980) and Frey From Basins to Dust (1980) discussed the effect of impact structures with diam eters exceeding 100 km on the ancient Earth, prior to about Individual impacts have considerable physical (morpho 3.8 Ga. By scaling the lunar impact record to the Earth these logical) and chemical effects on the target and on the at authors concluded that about 2500-3000 impact structures mosphere. A crater is excavated, fragmental ejecta are with diameters larger than about 100 km could have formed. strewn around and into the crater, and a melt unit can be Their simulation resulted in almost 1000 craters with diam created. Some of the ejecta might be in the form of molten eters exceeding 200 km, and possibly about 10 structures 482 Origin of the Earth and Moon with diameters larger than that of the Imbrium Basin on the Archean oceanic crust), ubiquitous banded iron formations Moon (about 1300 km diameter). This crater population (chemical sedimentary precipitate, dominated by quartz and would have covered about 40% of the surface of the Earth. magnetite), rare graywacke, and metapelites. The inferred Using the minimum estimate for the cratering frequency, environment of deposition for these volcanosedimentary Grieve (1980) derived a cumulative energy of about 1029 J successions is a sediment-poor arc or back-arc basin in rela added to the Hadean Earth from impact events, and con tively deep water (Nutman et aI., 1984). Studies of early cluded that the net effect of large impact events was to Archean sediments from the Isua Supracrustal belt (ISB), localize and accelerate a variety of endogenic geological which are -3.80 Ga, and rocks of the Akilia association in activity. the Godthabsfjord region (>3.80 Ga) of southern West Several studies have considered the effects of impact on Greenland, suggested that they are the oldest sediments yet the atmosphere and hydrosphere, again, particularly for very identified (Nutman et al., 1997). large events (Maher and Stevenson, 1988; Oberbeck and There are uncertainties concerning geological relation Fogleman, 1989; Sleep et al., 1989; Chyba, 1993; Zahnle ships on Akilia island and the nearby islets of the Godthabs and Sleep, 1997). These studies have largely been expressed fjord archipelago that host the oldest known sediments of in the context of the early evolution of life and impact-in marine origin, and also contain evidence for life (Mojzsis duced sterilization. An Imbrium-scale impact onto the early et al., 1996; Natman et aI., 1996, 1997). These derive from Earth would have the ultimate effect of boiling off about reconnaissance-scale geological mapping that reveals little 40 m of seawater, with a subsequent hot surface layer and about the structural relationship of the banded iron forma annihilation of any surface ecosystems (Zahnle and Sleep, tions to the polyphase, geochemically heterogeneous ortho 1997); expected events lOx as large as this would have gneisses that intrude them. The geochronological relation correspondingly larger and more devastating effects. It prob ships as they are currently inferred have been used to place ably requires the impact of an asteroid several hundred ki a minimum age of formation for some of the sediments in lometers in diameter to totally vaporize one present-day excess of 3.85 Ga (Nutman et aI., 1997). In contrast, Moor ocean mass of water. The scale of these events is probably bath and co-workers (e.g., Moorbath et aI., 1997; Kamber too great and destructive to allow preservation of evidence. et aI., 1998; Moorbath and Kamber, 1998; Kamber and It is the probability for these vaporizing impacts on the early Moorbath, 1998; see also Rosing, 1999) argued that these (; Earth that has led to the general impression that impact oldest ages represent only those of zircon inherited from as events were a negative forcing function for the development similated preexisting rocks older than the intruding grani and evolution of emergent life (e.g., Grieve, 1998). toid orthogneisses on Akilia and the surrounding islands. Along with the mega-impacts there would be numerous However, evidence for much Pb contamination from hypo smaller impacts, producing more recognizable ejecta blan thetical assimilated zirconiferous rocks is absent from the kets, shock features, and input of siderophile elements. Si orthogneisses of the Itsaq, so the zircons are probably not multaneously, there should be a correspondingly greater inherited. Furthermore, the intruding granitoids are low in abundance of input of interplanetary particles and continu Zr, granodioritic melts are strongly undersaturated with re ous rain of dust (that ultimately is incorporated into rocks spect to Zr, and the rocks they intrude are poor in zircon. with ongoing sedimentation) than there is at the present day. Age estimates of3.65 Ga derived for the intruding gneisses It is to these smaller-scale features that attention should be of southern West Greenland, which are based on whole-rock paid, to find evidence of impact in the oldest rocks. Pb-Pb, Sm-Nd, and Rb-Sr errorchrons, are susceptible to open-system REE-, Sr-, and Pb-diffusion behavior, in con 4. SEARCH FOR EVIDENCE OF A LATE trast to precise and concordant zircon geochronology (Mojz HEAVY BOMBARDMENT ON sis and Harrison, 2000). It is not possible to resolve the age THE EARLY EARTH issue here; however, this question has important implications for the search of traces of any late heavy bombardment on 4.1. Earliest Sedimentary Rocks on Earth the Earth, as these terranes presently provide the only quali fied samples to search for extraterrestrial components of a The critical sedimentary record of the earliest Archean late heavy bombardment on Earth. We recognize that the is preserved in the North Atlantic province, principally in evidence for a 3.85-Ga or older age for the sedimentary the Isua district and the Akilia association in southem West Akilia rocks under consideration here is stronger than that Greenland that are part of the Itsaq Gneiss complex (Nutman for a younger age. et aI., 1996). The Isua Supracrustal belt is in effect a giant version of the smaller enclaves of Akilia rocks with abun 4.2. Search Strategies and Their Rationales dant gneisses. The Itsaq Gneiss complex of West Greenland is a 3000-krn2 terrane dominated by orthogneisses of grani We discuss three strategies used to search for evidence toid compositions that intrude, in some locations. packages of a late heavy bombardment on the early Earth. First, it is of associated sediments and volcanic rocks. These supra possible to search for chemical evidence in sedimentary crustal rocks are composed of massive amphibolites and rocks that would indicate an enhanced flux of extraterres complex metasomatic carbonates (metamorphosed equiva trial materials, using different techniques and samples from lents of pillow basalts and other components of early both Isua and Akilia rocks from Greenland. Second, evi Ryder et al.: Heavy Bombardment ofthe Earth 483 dence of detrital shocked minerals that might have formed (low estimate) and -104 (high estimate) greater than at as a result of an inccssant early bombardment may be pre present. While this estimate is based on visible lunar cra served. Third, it may be possible to recognize remnants of ters, generally of the order of a few kilometers in diameter impact ejecta (albeit strongly altered and metamorphosed) and larger, it is inferably true of smaller craters and of in that might have been incorporated into early Archean rock terplanetary particles and dust as welL In a geochemical formations. sense, it does not matter whether a projectile makes a cra ter or bums up in the atmosphere; it wil\ be added to the 4.3. Meteoritic Siderophile-Element Signatures sediment as the dust settles. On the Moon the extralunar material also has high abundances of the siderophile ele 4.3.1. Siderophile elements on the early Earth. The Earth ments (for example, the Serenitatis impactor was almost is a highly differentiated body, with a core, a mantle, and certainly an EH chondrite, James, 1995). All lunar impact evolved crust. During planet formation, the highly sidero melt rocks from the late heavy bombardment contain Ir in phile elements (e.g., Ir, Pt, Au) partition strongly into me the 2-20-ppb range (Papike et al., 1998). On Earth, a sedi tallic cores. The formation of the Earth's core was completed mentary layer at -3.85 Ga might show evidence for an in early, well before the formation of the most ancient of pre flux of siderophile elements from an enhanced continuous servcd terrestrial crustal rocks, and certainly by the time of background fallout, or from a specific event comparable lunar formation. Thus, Earth's earliest mantle and crustal with the Cretaceous-Tertiary boundary layer, where such rocks were effectively stripped of their highly siderophile' events had a higher probability than at the present. element inventory early on. However, the present-day up 4.3.2. Terrestrial sources ofiridium in marine sediments. per mantle has abundances of highly siderophile elements Experiments have shown that -50% of Ir in sediments is much higher than expected from presently known silicate scavenged from seawater by Fe-Mn-O-OH particles (Anbar metal distribution coefficients and under the assumption of et aI., 1996) in oxic to suboxic environments. Anoxic envi core-mantle equilibrium (lr -3 ppb) (Chou, 1978; Chou et ronments, such as would be the case for much of the al., 1983; Newsom, 1990). The siderophile-element abun Archean hydrosphere (Holland, 1984), are not a major sink dances show chondritic relative proportions, which plot for Ir because ofthe redissolution ofparticulate hydroxides, subparallel to thc Clline. The addition of-0.75% chondritic except at rapid sedimentation and relatively shallow water material after tennination of the core-upper mantle equilib depths. Iridium is well mixed in the oceans: The residence rium under increasingly oxidizing upper mantle conditions time for it in the hydrosphere is 2000-20,000 yr. This im seems necessary to explain the abundances and chondritic plies that extraterrestriallr could persist in seawater and be relative proportions of the siderophile elements in the mantle incorporated into sediments by particulate scavenging be (e.g., Chou et al., 1983; Newsom, 1990; Holzheid and tween impacts of a frequency of Jess than 2000 yr. It was Palme, 1998). The emplacement timing of such a veneer is also found by Anbar et al. (1996) that Ir (and as) abun not constrained by direct evidence, but is often invoked to dances in present-day seawater are extremely low. Thus have been as early as 4.40 Ga, or as late as 3.80 Ga. weathering and hydrothermal alteration of ultramafic rocks, Siderophile elements are strongly fractionated during such as peridotite, which could supply Ir (and other plati partial melting; for example, basalts are strongly depleted num-group elements) to seawater, is insignificant in deter in Ir «0.05 ppb) relative to mantle peridotites (-3 ppb). In mining the abundances of these elements in present-day rare circumstances, siderophile elements can be concen seawater. trated in specific crustal reservoirs, e.g., platinum-rich lay Studies of REE distributions in banded iron formations ers in some basic intrusions; these have relative platinum demonstrate that hydrothermal activity had a strong influ group element abundances that are strongly fractionated ence on overall seawater chemistry in the Archean (Bau and from chondri tic values. More evolved rocks, such as Moller, 1993). The average concentrations of lr in pelagic pyroclastics, granites, and the sediments derived from them, clays with sedimentation rates of -0.001-0.003 mm a-I contain negligible siderophile-element abundances from range from 0.07 to 2.0 ppb (Barker and Anders, 1968; Kyte terrestrial sources. The source of significant abundances of and Wasson, 1986); in metalliferous sediments that scavenge siderophile elements in evolved crustal rocks, such as the Ir, concentrations are even higher (Anbar et aI., 1996). Some melt rock at East Clearwater Lake crater (Palme et al., of these higher abundances might result from organic mat 1979), or in the Cretaceous-Tertiary boundary clay layer ter scavenging, and therefore do not reflect extraterrestrial (Alvarez et al., 1980), can be reliably attributed to an ex input directly. Because these pelagic sediments are very slow traterrestrial source. Thus siderophile elements in sedimen to accumulate, they contain measurable lr even at the tary rocks, other than in the rarest of circumstances, can be present-day very low rates of meteoritic input. taken as an indication of an extraterrestrial flux at the time 4.3.3. Extraterrestrial sources or iridium to the hydro of formation of the sediments, particularly if they are in sphere. Estimates of the influx of extraterrestrial matter chondritic relative abundances. reaching Earth's surface during the past 100 m.y. have been Estimates of the flux of extraterrestrial material to the the subject of numerous studies aimed at quantifying the Earth, based on lunar stratigraphic-chronologic studies dis current rate of dust accretion and the composition and cussed above, suggest that during the peak of the late heavy source of the materiaL A number of methods have been used bombardment this flux was, or ranged, between -3 x 102 to determine this flux, using the collection of dust in the 484 Origin of the Earth and Moon atmosphere, glacial ice, and pelagic sediments (Love and siderophile-element signal from hydrogenous sources of Ir Brownlee, 1993). The current mass flux of extraterrestrial (Anbar et aI., 1996) and other metals, although this is not a Ir is based on measurements of sedimentary Ir in systems major concern. Banded iron formations from the Isua dis with calculable sedimentation rates and calculations of the trict of southern West Greenland, and also younger ones flux of infalling dusts by satellite, radar, and airborne ob from West Australia and southern Africa, contain no signifi servations. Love and Brownlee (1993) have estimated the cant clastic sediment components and no near-shore or amount of chondritic material raining into the Earth as dust, evaporitic facies. Therefore, when the oldest banded iron from measurements ofabundances and sizes of microcraters fornlations formed, they must have sampled for the most developed on the Long Duration Exposure Facility (LDEF) part the chemistry of the water column from which they experiment, as 40 (± 20) x 109 g a-I. Assuming chondritic precipitated, including any extraterrestrial component. relative proportions, this translates to 70 ± 35 mol Ir a-I to Unfortunately, the sedimentation rate for banded-iron the whole Earth. The uncertainties reflect counting and, formation deposition is poorly constrained. These rocks do more importantly, the inferred encounter velocities. An as not form in Phanerozoic environments because the p02 of sumption is that the six-year length of the LDEF experiment the atmosphere has been too high since the Proterozoic Era, is adequate to be representative of the current (i.e., last few and because Fe2+forming by rapid oxidation to Fe2+(Fe3+h million years) flux and its possible variations. The abun 0(OH)6 transforms to Fe2+(Fe3+)204 (magnetite), which has dances are consistent with those derived from Os isotopes low solubility in seawater. In general, detailed sedimento in deep-sea sediments (Esser and Turekian, 1988) and lr in logical interpretations of banded-iron-formation sequences both Antarctic ice (Ganapathy, 1983) and abyssal red clays have not been available (Klein and Beukes, 1990). There (Kyte and Wasson, 1986). Studies by Bonte et al. (1987) have been considerable differences of opinion about the have shown that almost all platinum-group elements present origin of banded iron formations and the particular envi as cosmic debris occur in grains «I0 ~m. These grains are ronments of their deposition (James, 1954; Trendall and quickly incorporated into sediments (Esser and Turekian, Blackley, 1970; Cloud, 1973; Holland, 1973; Klein and 1988). The finer dust grains are probably sensitive to sea Beukes, 1990), with general agreement that deposition took water oxidation and hydrolysis after burial and have prob place below wave-base. The individual bands of iron for ably always contributed to a small hydrogenous component mations are considered by many workers as being equiva of seawater Ir. lent to varves associated with seasonal changes in upwelling, 4.3.4. Ancient sediments and model extraterrestrial influx. productivity, and local 0 production and other factors (Hol The oldest terrestrial sediments might be expected to pre land, 1984, and references therein). Trendall and Blackley serve a signal of higher incident fluxes from interplanetary (1970) estimated the rate of deposition of the Hamersley dust particles, micrometeorites, local impacts, airburst, banded iron formation (-2.5 Ga). From counting chert + cometary showers, and ablation products of such phenom magnetite ± hematite microband couplets between volcanic ena. The amount of Ir from the background that would be rocks of known age that were interbedded with the iron expected to be sampled by the water column and thus a stones, these authors estimated a deposition rate of 0.65 sediment deposited or precipitated from the early Archean 1.3 mm a-I, which is much faster than even typical detrital ocean, [Ir]SED' can be estimated by sediments such as shale and siltstone. However, while band ing in the Hamersley Basin may be on a scale of - I mm, banding elsewhere is on much coarser (centimeters) and much finer (submillimeter) scales. Indeed, banding occurs where cl>m = estimated present extraterrestrial flux for all at various repetitions in any sequence, with laminae bundled incoming material, f = factor increase for ancient flux, into alternatively quartz-rich and magnetite-rich "beds" and [Ir]ET = concentration oflr in extraterrestrial material, ffiA= higher-order packages. Ifbanded iron formations are domi Earth surface area,
TABLE 2. Neutron activation analyses of rocks from southwest Greenland.
FeO· Na20 La lr Cr Co Ni (%) (%) (ppm) (ppb) (ppm) (ppm) (ppm) Akilia Island banded iron formations> 3.85 Ga ANU-92-197/I-A 5.8 0.027 0.52 <.4 I.I 4.8 49 ANU-92-197/1-B 6.4 0.034 0.51 <.27 1.5 4.5 33 ANU-92-197/2-A 7.4 0.051 1.36 <.5 1.6 5.1 26 ANU-92-197/2-B 7.4 0.044 1.97 <.3 1.6 5.0 33 ANU-92-197/3-AI 7.8 0.030 0.57 <.5 1.4 5.5 40 ANU-92-197/3-A2 5.2 0.025 0.52 <.4 1.1 3.9 29 ANU-92-197/3-B 9.0 0.030 0.66 <.5 4.1 5.8 42 ANU-92-197-X 20.1 0.D15 0.78 1.6 8.7 114
Innersuartuut banded iron formations >3.77 Ga SM/l55746-A 19.1 0.017 0.68 <.9 3.9 8.3 74 SM/l55746-B 23.8 0.018 0.66 <.6 4.7 10.2 81 SM/l55746-X 18.1 0.008 0.92 0.4 2.3 5.0 19 SM/155746-C 28.0 0.034 2.04 <.8 6.7 9.1 41 SM/155746-D 36.1 0.030 2.52 <.9 7.5 10.0 27 SM/171770-A 4.5 0.030 0.34 <.4 1.7 2.7 21 SM/171770-B 13.0 0.050 1.49 <.6 4.6 6.6 23 SM/l71770-X 14.8 0.031 1.48 4.4 8.7 55 SMlI7177I-A 70.2 0.129 1.40 <1.5 87.5 10.1 56 SM/17177I-B 54.3 0.242 1.70 <.6 50.0 11.9 58 SM/l7177I-X 51.9 0.301 2.56 63.9 11.7 66
{sua banded iron formations 3.77-3.80 Ga /3446-AI 54.5 0.002 1.05 <1.8 7.1 17.5 99 /3446-A2 53.7 0.002 0.63 <1.5 7.2 16.0 73 /3446-B I 52.1 0.004 0.62 <.7 5.8 16.1 64 /3446-B2 51.7 0.002 0.67 <1.1 6.5 16.1 76 /3446-CI 52.9 0.002 0.58 <.7 6.9 15.9 86 /3446-C2 51.7 0.002 0.70 <1.2 6.2 14.5 57 /3451-A 52.5 0.003 0.33 <2.1 6.3 13.7 53 /3451-B 51.5 0.002 0.45 <1.9 7.0 15.8 86
{sua Mt. - Isua banded iron formations SM178/248471 5.2 0.010 0.13 0.5 0.6 0 SMiGR/93/44 54.0 0.000 0.19 4.1 4.1
Isukasia - Isua banded iron formations SM/GR/96/8 69.5 0.002 2.07 152.9 38.1 80 SM/GR/96/9 48.9 0.006 1.90 198.7 20.7 68 SM/GR/96/1 55.5 0.006 7.43 8.9 27.8 166
Innersuartut Amitsoq orthogneiss >3.77 Go SM/l55742 10.7 2.746 8.70 0.0 4.1 0 SM/l71773-A 2.0 2.872 9.47 <.7 3.8 4.0 <17 SM/l71773-B 1.6 2.998 13.38 <.8 3.1 3.0 <18 SM/171773-X 2.8 2.504 12.89 3.6 6.2 0
GU'!flint Chert -2.1 Ga GF7-A 4.3 0.011 . 0.77 <.41 0.9 3.2 <10 GF7-B 3.4 0.012 0.56 <.21 0.5 2.6 A = saw-cut free, small pieces; B = saw-cut free, larger pieces; C = saw-cut enriched; D = mafic-enriched separate; X = remainder, fines. See text for analytical information. • Total Fe as FeO. Ryder et al.: Hea,y Bombardment ofthe Earth 487 time of their deposition was not of the order of thousands rock, one graywacke, one gravity flow sample, and one of ti mes the present flux. Clearly, at the present time we pelagic shale. Such variation could be the result of a terrig cannot provide a more definitive answer; both a better un enous detrital component. The elevated Ir content in the derstanding of banded-iron-formation deposition rates and banded-iron-formation and pelagic shale samples may in more precise methods of analysis for Ir are desirable. dicate a remnant meteoritic phase, but it is more likely they More precise analyses have been made for Ir and Pt in result from mafic contamination given the nonchondritic some banded-iron-formation samples from Akilia Island ratios of the other elements. If the Ir were demonstrably (Arnold et al., 1998; Anbar et aI., 2000). The analyses used extraterrestrial, it would indicate a flux of-I04x present for a NiS fire assay and isotope dilution ICP-MS. Detection lim a deposition rate of 0.05 mm a-I, or a flux of -I 03x the its were -0.003 ppb Ir and -0.030 ppb Pt for the samples present with more typical sedimentation rates. However, analyzed, which had abundances below those detection lim these samples are at least 50 m.y. younger than the Imbrium its. This is somewhat surprising as the crustal background event, and therefore most likely postdate the main episode value is about 0.020 ppb for Jr. With such precision, even of late heavy bombardment. for a flux of 2000x the present and a sedimentation rate as The chemical search for an enhanced amount of extra fast as 0.5 mm a-I, Ir should have been detected in these terrestrial matter in these Greenland samples was not samples (Table I). One can postulate even faster sedimen deemed successful. This could indicate that either the rocks tation rates, or nonrepresentative sampling (a nugget effect), investigated were deposited very rapidly, that they do not or postdepositional loss of siderophile elements to explain overlap in time with the late heavy bombardment, or that these data. However, literal reading of the data would sug the late heavy bombardment flux to the Earth was less in gest that at the time of deposition, the bombardment rate tense than commonly predicted. was less than 2000x the present rate, probably much less. Seventeen samples of Isua rocks, which could be up to 4.4. Search for Shocked Minerals 100 m.y. younger than the Akilia samples, were analyzed by Koeberl et al. (1998a,b, 2000) for their chemical com In normal terrestrial impact crater studies, the presence position, including siderophile-element abundances. These of shocked minerals is taken as confirming evidence for the authors also used Ni-sulfide + Te co-precipitation fire as impact origin of a purported astrobleme. The first petro say and ICP-MS. The samples included metamorphic graphic search for shock features in rocks from Isua was equivalents of turbidites, greywacke/felsic gneiss, conglom reported by Koeherl and Sharpton (1988). Their study con erate/felsic metasomatites, pelagic shale, gravity flow from centrated on the search for shocked quartz; however, none the Bouma sequence, phyllite, and banded iron formations. was found. This is understandable given the multiple upper Four of 17 samples analyzed yielded measurable amounts amphibolite-grade metamorphism that these rocks under of Ir (ranging from 0.06 to 0.18 ppb) above the detection went after their formation; such metamorphism would have limit (0.03 ppb), as well as Ru and Rh. The contents of the repeatedly annealed the quartz. On the other hand, a vari other siderophile elements (Pt, Pd, Au) are highly varied; ety of shocked minerals has been preserved in 2-Ga rocks chondrite-normalized abundance patterns show variations by from the Vredefort impact structure in South Africa. More a factor of 3-4 (Fig. 4). The elevated contents were observed recently, Koeherl et al. 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