81 Kr Radiation Ages of Stone Meteorites Impact Craters and the Relative Ages of Earth and Moon Marti1 and Eugster Et Al

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368 NATURE VOL. 225 JANUARY 24 1970 81 Kr Radiation Ages of Stone Meteorites Impact Craters and the Relative Ages of Earth and Moon Marti1 and Eugster et al. (ref. 2 and unpublished work) recently discovered 81Kr, the first cosmic ray produced RECENT aerial photographic and ground studies of islands radionuclide heavier than the Ni-Fe group, in stone in the Lake of the Woods area of the Canadian Shield meteorites. By measuring only the isotopic composition have revealed a series of arcuate ridges which together of Kr, they evaluated the radiation ages of the meteorites make up a conspicuous circular feature I6 km in diameter as follows (Fig. I). The appearance of these ridges suggests that they are igneous intrusions which have been constrained to I P(81Ir) MKr conform to a circular pattern with predominantly outward Tr= A (PMKr) 8-1K_r_s_p_a_ll (l-e-J.st-T,) dipping layers by distortions in the Earth's crust produced 81 by an ancien~ meteorite impact. The ridges are composed almost entirely of gneiss or granite gneiss similar to rocks whore Tr is the radiation age, ). is the decay constant of 81 immediately outside the circle and in many other areas 81Kr= 3·25 x 10-• yr-1 (refs. 3 and 4) and P(MKr) is tho of the Canadian Shield. A "volcanic" phenomenon of this production cross-section of the nuclide in question. The character could reasonably be explained by supposing exponential term becomes negligible if Tr ~2 m.y. that the magma feeding the intrusions was at a relatively shallow distance beneath the surface. This in turn Table 1. "Kr-"Kr RADIATION AGES OF SEVEN STONE l!ETEORITES ASSUMING (i) P("Kr)=l(P("Kr)+P("Kr)] AND (ii) P(81 Kr)=P(80Kr) COMPARED WITH suggests that the feature may have been formed at a very THE 'He AGES early period in the history of the Earth's crust before "Kr-"Kr age (10' yr) 3 He age solidification had proceeded to any great depth. .!lleteorite P("Kr) = l(P("Kr) + P("Kr)] IP("Kr) = P("Kr) (10' yr) Macibini 48 ± 4 (ref. 1) 38 26* (ref. 8) Stannem 41·9± 1·6 (ref. 2) 33·5 32 (refs. 7 and 8) Khor Tcmiki 50 ± 16 (ref. 2) 40 41 (ref. 9) 52 ±17t 42 Mots 16·2± 3·5 (ref. 2) 12·0 12 (ref. 10) 16·7± 3·5t 13·3 Ochan•k 8·1± 3·5 (ref. 2) 6·5 5·7 (ref. 10) 7·8± 3·5t 6·2 Bholghati 17 ± 2t 13·6 11-7 (ref. 11) Weston 30 ± St 24 29·5 (ref. 12) • The "Ar spallation age of Maeibini is 51 x 10' yr. t Unpublished work of 0. Eugstcr, P. Eberhardt and J. Geiss.

It is notable that the radiation age as estimated by 81Kr-78Kr seems systematically greater than the pub lished 3He radiation ages (refs. I and 2 and unpublished work of Eugster et al.). The purpose of this report is to suggest that the relative production rate, P(81Kr)/P( 78Kr) may have been overestimated by Marti and Eugster et al. They assumed that P(•1Kr)=! [P(80Kr)+P(82Kr)], an assumption supported by the Kr spallation yields from 540 MeVprotonsonAg (ref. 5). Funk et al.•, however, recently measured tho Kr spallation yields from 730 MeV protons on Sr and found that P(81Kr) :;6t[P(80Kr) + P(S2Kr)] but 81 80 rather P( Kr) = P( Kr). Assuming this is true, tho Fig. 1. Circular feature centred on Painted Rock Island, Lake of 81Kr-78Kr ages are recalculated from the data of Marti the Woods. Latitude, 49° 10', longitude 94• 29'. Scale: diameter of and Eugster et al. and shown in Table 1. The meteoritic circle equals 16 km. spallation Kr yields of Marti et al. 7 are adopted here. It can be seen that with tho exception of Macibini, the 81Kr-78Kr ages are brought into excellent agreement While the interpretation of this feature as a secondary with the 3He ages when P(81Kr) is assumed to be equal consequence of an impact event is admittedly speculative. P(••Kr). the picture of a thin crust in the early stages of its forma tion has independent and interesting implications in M. w. HOWl•] connoxion with the problem presented by the rarity, Department of Chemistry, relative to the lunar surface, of meteorite impact scars Texas A and M University, in the Canadian Shield. This question has been discussed College Station, by Beals and Halliday1•2, who attribute the observed low Texas 77843. frequency to obliteration due to erosion, deposition and various kinds of tectonic action including mountain Received November 24, 1969. building processes. A promising alternative explanation 1 ?IIarti, K., Phys. Rev. Lett .• 18, 264 (1967). of the small number of impact remains may be found in '.J£ugster, 0., Eberhardt, 1' .• and Geiss. J., Earth Planet. Sci. Lett., 2, 77 the state of the Earth's crust at the time that the most (1967). intense (or most prolonged) bombardment of the Earth 'lteynolds, J. II., Phys. Rev .. 79, 886 (1950). • Eastwood, T. A., llrown, F., and Crocker, J. H., Nucl. Phys., 58, 328 and Moon by large meteorites took place. If at that time (1964). tho Earth's surface was molten or partly molten, or even ·' Bieri, lL H .• and Rutseh, W., Tlelv. Phys. Acta, 35, 553 (1962). if there was a very thin solid crust, the conditions would 'Funk, H., Podosek, F., am! Rowe, M. W., Earth Planet. Sci. Lett. 3 193 (1967). ' ' be unfavourable for retaining permanent imprints of large 'Marti, K., Eberhardt, P., and Geiss, .T., Z. Natwjorsch., 21a, 398 (1966). meteorite impacts. (Similar considerations would apply 'Megruc, G., .J. Geophys. Res., 71, 4021 (1966). to other crater-forming processes.) 'Ebtrhardt, P., Eugstcr, 0., and Geiss, J., J. Geophys, Res., 70, 4427 (1965). A possible means of confirmation or otherwise of these 111 Eberhardt, P., Eugstcr, 0., Geiss, J- .• and Marti, K., Z. Naturjorsch., 21a, 414 (1066). ideas may be found in the Apollo series of Moon landings, " Zahringer. J., Jlieteoritica, 27, 25 (1966). two ofwhieh have already taken place. Preliminary reports " GanapathY. R., and Anders, E., Geochim. Cosmochim. Acta, 33, 775 (1969). on age studies of the rocks brought back by the Apollo