Did Natural Fission of 235U in the Earth Lead to Formation of The

GEOSCIENCE FRONTIERS 3(4) (2012) 369e374

available at www.sciencedirect.com China University of Geosciences (Beijing) GEOSCIENCE FRONTIERS

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GSF FOCUS Did natural ﬁssion of 235U in the earth lead to formation of the supercontinent Columbia?

John J.W. Rogers*

Department of Geological Sciences, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3315, USA

Received 28 February 2012; received in revised form 21 March 2012; accepted 23 March 2012 Available online 2 April 2012

KEYWORDS Abstract Steady decline in the percentage of 235U in terrestrial uranium made natural fission impos- Natural fission; sible after about 1.8 Ga. Fission before 1.8 Ga disturbed the lead isotope system at various places world- Archean; wide, such as Oklo, Gabon, and may have caused the first lead isotope paradox. Fission in areas of high Paleoproterozoic; uranium concentration may also have generated enough heat to localize sparse Archean and Paleoproter- Columbia supercontinent; ozoic UHT belts. The oldest widespread orogenic systems formed at approximately 2.0e1.8 Ga after Tectonics fission stopped contributing to the earth’s heat flow. These early orogenic systems partly created the supercontinent Columbia. ª 2012, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.

1. Introduction reactors that are not designed with neutron moderators (DOE, 1993). At 1.8 Ga pitchblende veins near Oklo, Gabon, went critical and The oldest worldwide system of orogenic belts also formed at underwent natural ﬁssion that destroyed most of the 235U in the about 1.8 0.1 Ga (Zhao et al., 2002). Rogers (2000), Rogers and pitchblende. Fission occurred at 1.8 Ga because 235U was about Santosh (2002, 2009), and Zhao et al. (2002, 2004) proposed that 3.2% of natural uranium, the lower limit for criticality in nuclear these belts led to the accretion of the supercontinent Columbia, whose nomenclature was discussed by Meert (2012). The distribution in Columbia of the old cratons proposed by Rogers (1996) þ þ * Tel.: 1 9193842435; fax: 1 9199664519. is controversial. Regardless of their distribution, proliferation of E-mail address: [email protected]. these orogenic belts shows that the earth’s lithosphere had cooled ª 1674-9871 2012, China University of Geosciences (Beijing) and Peking enough by 1.8 Ga to sustain lateral stresses. University. Production and hosting by Elsevier B.V. All rights reserved. The synchrony of the oldest orogenic belts and the Oklo reactor Peer-review under responsibility of China University of Geosciences suggests that the cooling of the earth reached a threshold at 1.8 Ga. (Beijing). In this paper I propose that this threshold represents a time when doi:10.1016/j.gsf.2012.03.005 widespread nuclear ﬁssion stopped contributing to heat production in the earth. This proposal requires answers to four questions:

Has natural fission occurred in the earth? Production and hosting by Elsevier Can natural fission explain anomalies in terrestrial lead isotopes, including the “first lead isotope paradox”? 370 J.J.W. Rogers / Geoscience Frontiers 3(4) (2012) 369e374

Is natural fission in the Archean and Paleoproterozoic consis- referred to as the “first lead isotope paradox.” Stacey and Kramers tent with our knowledge of the earth’s heat balance? (1975) recognized that modern lead isotopes could not have Was natural fission responsible for cooling of the earth by evolved in one stage from an earth with ratios starting on the 2e1.8 Ga to permit widespread formation of orogenic belts, geochron at 4.55 Ga, and they proposed a two-stage model in partly leading to the supercontinent Columbia? which the U/Pb ratio in the “accessible earth” increased at about 3.7 Ga. This increase requires creation of a reservoir of less- 2. Natural fission of uranium radiogenic Pb that is now in an inaccessible part of the earth. More recently, different investigators have explained the Credible evidence for natural fission of uranium was first pub- paradox in different ways, many of which center around the lished in the 1950s. Levine and Seaborg (1951) found microscopic location of the inaccessible reservoir. Allegre et al. (1982) concentrations of plutonium in large deposits of pitchblende. proposed that the hidden reservoir is the core. Chauvel et al. Wetherill (1953) reported high levels of fission-produced xenon (1995) suggested that the U/Pb ratio in the continental crust is and krypton in uranium minerals from Madagascar and Congo lower than in the mantle because lead is preferentially added to the (Kinshasa). Kuroda and Edwards (1957) reported 140Ba concen- continental crust during accretion by subduction. Halliday et al. trations in pitchblende ore from Great Bear Lake, Canada, were so (1996) proposed that rapid formation of the core could partially high that they must have been formed by fission of uranium. Both explain the paradox. Murphy et al. (2003) suggested that the Wetherill (1953) and Kuroda and Edwards (1957) also noted that depleted reservoir is subducted oceanic lithosphere and associated all known pitchblende ores are younger than about 2.1 Ga. Natural sediment transformed to garnetite and stored in the mantle fission in pitchblende is now widely recognized as the generating transition zone. Lee et al. (2007) proposed that dense, non- mechanism for Kr and Xe in the pitchblende (Eikenberg and radiogenic continental crust would delaminate and be stored in Koppel, 1995). Natural fission is also probably responsible for the mantle. the absence of massive large pitchblende deposits older than about I propose that an alternative explanation for the paradox that is 2 Ga. the depleted reservoir was formed by natural fission. The amount The possibility of natural fission within the earth was of depletion does not need to be large. The value of 207/204Pb for confirmed by Bodu et al. (1972), Naudet (1978) and De Laeter the present accessible earth is 15.8, and the value on the geochron et al. (1980) in studies of uranium veins near Oklo, Gabon. One is 16.1 (Fig. 1). This difference of 0.3 suggests that about 20% of of the principal observations at Oklo is that 235U constitutes only the 235U in the earth was destroyed by nuclear fission. 0.29% of the uranium, although whole-earth uranium should have been 3.2% 235U at 1.8 Ga. This low concentration shows that 4. Location of a depleted reservoir approximately 90% of the original 235U at Oklo was destroyed by fission. The preceding information leads to the following inferences about Fission of uranium in pitchblende on the earth’s surface has the depleted reservoir if it was formed by natural fission: been discussed by several investigators. Adam (2007) described possible effects of radiation from various sources on the origin of The 90% destruction of 235U in the veins at Oklo compared life on the early earth. Coogan and Cullen (2009) discussed the with only 20% reduction in the whole-earth suggests that possibility of fission in pitchblende trapped in the sticky mats of depletion occurred only in scattered places of high uranium stromatolites. concentration. Veins and other places of high uranium concentration could 3. Effect of natural fission on lead isotopes develop only in continental crust and not in the mantle. The reservoir must have been accessible to the earth’s surface The position of modern lead from the “accessible” earth does not at some time before 1.8 Ga. lie on the presumed whole-earth meteorite isochron (geochron) on Surface heat flow in areas underlain by depleted crust should a 207/204Pb vs. a 206/204Pb diagram (Fig. 1). This position is be lower than in other continental areas. Any search for this depleted reservoir must be based on the thermal history of the earth, including its present heat production and surface heat flow.

5. Effect of natural ﬁssion on thermal history of the earth

Variations in heat production and accompanying differences in continental thermal gradients have been attributed to numerous processes by different investigators (I list primarily only the most recent articles that discuss each process). Processes can either increase or decrease thermal gradients. Processes that cause increase in thermal gradients include rising Figure 1 207/204Pb vs. 206/204Pb diagram. The meteorite isochron mantle plumes that transfer heat to the continental lithosphere (geochron) extends from the value of the primitive earth. Departure of (Kuzmin et al., 2010; Taylor et al., 2010); lithosphere delamina- the current accessible earth from the geochron is the first lead isotope tion that brings the new base of the lithosphere into contact with paradox. The depleted lower Archean crust is approximately located. hot mantle (Lee et al., 2007); and construction of a younger upper J.J.W. Rogers / Geoscience Frontiers 3(4) (2012) 369e374 371 crust on top of a crust enriched in heat-producing elements (Fraser et al., 2010). The major process that decreases thermal gradients is “intracrustal differentiation” (Taylor and McLennan, 1995). This process lowers heat production by magmatic or metasomatic movement of heat-producing and other LIL elements upward in the crust and their subsequent erosion. Assignment of amounts of heat production to different parts of the continental lithosphere has been made difficult by continuing discovery of vertical and horizontal complexity in the structure of cratons. Hieronymous and Goes (2010) showed that vertical and horizontal transition zones only a few kilometers wide separate blocks with different seismic and thermal characteristics. Peltonen et al. (2006) showed that the lower crust of the Karelian craton developed over nearly 2 Ga, from 3.5 Ga to 1.7 Ga. Despite these abrupt variations, some generalizations can be made about continental cratons. Perry et al. (2010) discovered that heat flow from the mantle beneath Canada made only a small, and constant, contribution to variability of surface heat flow in different crustal provinces. The principal variations are caused by differences in the vertical distribution of radioactive elements, with concentrations decreasing more rapidly downward in Archean areas than in Proterozoic terranes. Heat flow data for Gondwana summarized by Gupta (1993) are similar to those reported for Canada by Perry et al. (2010). Areas of Archean outcrop in Gondwana have an average heat flow of about 40 mW m2 (formerly designated as 1 heat flow unit, 1 HFU), and Proterozoic areas about 60 mW m2. The lower heat flow and more rapid decrease of heat production downward in present Archean areas than in Proterozoic areas indicate that average modern Archean lower crust is more depleted in heat-producing elements than Proterozoic lower crust. The only way to check this conclusion directly is by analysis of xenoliths whose equilibration pressures indicate formation at lower crustal depths of 20e30 km. Studies summarized by Villaseca et al. (1999), Sachs and Hansteen (2000), Bolhar et al. (2007) and Ying et al. (2010) demonstrate that the lower conti- Figure 2 Locations of Archean and Paleoproterozoic UHT belts. nental crust consists of a wide variety of felsic to mafic rocks at The base map is Pangea before accretion of southern Asia and granulite grade. Vila et al. (2010) confirm the low heat production southern Europe. Information for North China from Santosh et al. in Archean felsic rocks. The low heat production may be related to (2007a,b). Information for Voronezh from Fonarev et al. (2006). e e the more mafic composition of tonalite trondhjemite granodiorite Information for Eastern Ghats from Bhui et al. (2007). Information for suites in the Archean than in younger rocks (Martin and Moyen, Napier province from Choi et al. (2006) and Hokada et al. (2008). 2002). Information for Kaapvaal from Schmitz and Bowring (2003) and None of the Archean xenoliths shows evidence of natural fis- Taylor et al. (2010). sioning in the whole rocks. I regard this absence as the result of a sampling problem. The extreme heterogeneity of the lower crust concentration of U in crustal rock in the Archean could be very plus the small number of xenoliths reduces the likelihood that different from the present approximately 4 ppm. In the late xenoliths sample crust that displays the effects of natural fission. Archean (approx. 2.5 Ga), 235U would have constituted at least Metamorphic suites show that most Archean thermal gradients 50% of total uranium. Thus one cubic meter of rock with the were similar to Phanerozoic gradients (Brown, 2007), and only crustal average of 4 ppm uranium, would have contained about 6 g a few localized ultrahigh-temperature (UHT) belts developed of 235U. (Fig. 2; Schmitz and Bowring, 2003; Fonarev et al., 2006; Bhui Fissioning of 20% of that 6 g of 235U, as suggested by Fig. 1, et al., 2007; Santosh et al, 2007a,b, 2008; Santosh, 2010; Taylor would have yielded about 1014 J (104 kW s) of energy because et al., 2010; Tsunogae et al., 2011; Zhang et al., 2012). The only 0.01% of the mass of 235U is destroyed during fissioning; the cause of this UHT metamorphism is commonly ascribed to remainder of the 235U would have created one nuclide with a mass intrusion of hot magma from the mantle, but I would like to of about 95, one nuclide with a mass of about 137, and 3 neutrons. suggest the possibility that the cause was nuclear fission in The actual amount of energy released, however, would have been localized regions. higher because some of the 238U would also have fissioned in Suggesting that fission contributes to UHT metamorphism a natural reactor (as it did at Oklo; De Laeter et al., 1980). requires an estimate of the amount of heat generated by fission. The effect of natural fission on heat production and thermal The contribution of fission to heat production at any time can only gradients clearly depends on the period of time over which fis- be estimated because the proportion of total U that was 235U sioning occurred. If the 235U in one cubic meter of rock underwent decreases toward the present value of 0.007% and the fissioning for 1 Ma (3 1013 s), then the rate of release of energy 372 J.J.W. Rogers / Geoscience Frontiers 3(4) (2012) 369e374

Figure 3 Columbia atw1.8 Ga, including the positions of the three old continentalassemblies (Ur, Arctica, and Atlantica) proposed by Rogers (1996). Atlantica was stabilized at about 2.0 Ga during an orogeny named Eburnian in Africa and Trans-Amazonian in South America. The Rio NegroeJuruena (rnj) continental-margineorogen became active at about 1.8 Ga and progressed toward younger deformation as the continent grew toward the west. The image shown for Ur is “expanded Ur,” with the following cratonic fragments added at w2.0 Ga to the w3.0 Ga Ur: the Zimbabwe craton to the Kaapvaal craton along the Limpopo belt (li); the Yilgarn craton (Yi) to the Pilbara craton along the Capricorn belt (ca) and severalsmall cratonic fragments now on the coast of East Antarctica. The Central IndianTectonic Zone (citz), which trends approximately EeWacross India,separates old cratons of Ur fromyounger onestothe north;the old cratons include western Dharwar (WD). Bhandara (Bastar, BH), and Singhbum (SI). The southern and eastern edges of expanded Ur are rimmed by the Bunger Hills (bu) and Windmill Islands (Wi) orogenic belts in Antarctica and the Albany (al) and Fraser (fr) belt in Australia. Most of Arctica is now in North America, with smaller parts in Siberia and Greenland. Orogenic belts within Arctica in North America are mostly intracratonic (“confined,” using the terminology of Aspler and Chiarenzelli, 1998,andPedrosa-Soares et al., 2001). These belts include the Trans-Hudson (th) and Taltson-Thelon (ta). The Akitkan (ak) may also be intracratonic at 1.8 Ga, and the Angara fold belt (ag) was formed along the Siberian continental margin. The Nagssuqtoqidian orogen (nq) of Greenland apparently sutured Archean terrains in northern Greenland with terrains farther south that have ages of about 1.9e1.8 Ga. Numerous orogenic belts with ages of 1.9 Ga and slightly younger occur in North America and Northern Asia. Most of the southeastern margin of Arctica (present orientation) is covered by Paleozoic sediments, but sediments of w1.9 Ga age are exposed in the Yavapai (Yv) and Penokean (Pe) suites. J.J.W. Rogers / Geoscience Frontiers 3(4) (2012) 369e374 373 would have been 2 1012 W. This rate of heat production is would have interrupted extension of individual subduction several orders of magnitude lower than the 2 106 mWm3 zones to networks of orogenic belts. reported for present lower crustal rocks by Vila et al. (2010). This 2. Shifting of local areas of fission throughout the Archean would low rate of heat production is consistent with the scarcity of ultimately have developed a lower crust that was generally Archean UHT belts. Perhaps the UHT belts developed only in depleted in 235U. This depletion causes modern Archean crust areas of the continental crust where uranium concentration was to have a lower heat flow than Proterozoic areas. high. 3. This localization and shift in places from time to time before about 1.8 Ga is consistent with the absence of 235U defi- 6. Columbia and 1.8 Ga orogenic belts ciencies in xenoliths thus far discovered because xenoliths have not yet been obtained from the small areas of the crust where fissioning occurred. The worldwide network of approx. 1.8 Ga orogenic belts (Zhao 4. It caused the “first lead isotope paradox” by storage of et al., 2002) was largely responsible for accretion of the Paleo- depleted material in a lower continental crust now in an proterozoic supercontinent Columbia (Fig. 3). Fig. 3 distinguishes inaccessible part of the earth. two different kinds of belts, including extracratonic ones formed by destruction of oceanic lithosphere; and intracratonic belts formed by compression within continental crust without destruction of an ocean basin (these belts are referred to as “interior” by Acknowledgments Aspler and Chiarenzelli (1998) and “confined” by Pedrosa-Soares et al. (2001)). I benefitted greatly from discussions with Sarah Mazza, formerly Columbia developed from three old continental nuclei identi- of the University of North Carolina at Chapel Hill and now at fied by Rogers (1996). Ur consisted of 5 cratons stabilized at Virginia Tech. 3.0 Ga, and it expanded before incorporation in Columbia. This expansion occurred by attachment of the Zimbabwe craton along References the 2.0 Ga Limpopo belt and the Yilgarn craton along the 2.0 Ga Capricorn belt. This “extended Ur” contained some fragments of Adam, Z., 2007. Actinides and life’s origins. Astrobiology 7 (6), 852e872. old continental crust and was mostly surrounded by orogenic belts Allegre, C.J., Dupre, B., Richard, P., Rousseau, D., Brooks, C., 1982. e with ages of 1.9 1.8 Ga. Subcontinental vs. suboceanic mantle: II. NdeSrePb comparison of Original Atlantica consisted of a large area stabilized by the continental tholeiites with ocean ridge tholeiites and the structure of the Eburnean orogeny at 2.0 Ga in northwestern Africa and the continental lithosphere. Earth and Planetary Science Letters 57, 25e34. equivalent Trans-Amazonian terrane in South America. Starting Aspler, L.B., Chiarenzelli, J.R., 1998. Two Neoarchean supercontinents? at 1.8 Ga, Atlantica grew westward to form much of South Evidence from the Paleoproterozoic. Sedimentary Geology 120, 75e104. America and grew southward to create much of Africa by merging Bhui, U.K., Sengupta, P., Sengupta, P., 2007. Phase relations in mafic with Ur. dykes and their host rocks from Kondapalle, Andhra Pradesh, India; e The cluster of 2.5 Ga terranes that defined much of Arctica implications for the time depth trajectory of the Palaeoproterozoic (late Archaean?) granulites from southern Eastern Ghats Belt. also grew before incorporation in Columbia. Much of the Precambrian Research 156, 153e174. southern (present orientation) margin in North America is Bodu, R., Bouzigues, H., Morin, N., Pfiffelmann, J.P., 1972. Existence of covered by Paleozoic sediments, but the Yavapai and Peno- isotopic peculiarities encountered in uranium from Gabon. Comptes kean suites of about 1.8 Ga formed on the margin of Rendus des Seances de l’Academie des Sciences, Serie D: Sciences a substantial continent that was already growing southward. Naturelles 275, 1731e1732. Similarly, growth to the northwestward began with the Bolhar, R., Kamber, B.S., Collerson, K.D., 2007. UeThePb fractionation in Wopmay orogen. Southward growth in Greenland started with Archaean lower continental crust: implications for terrestrial Pb isotope the Nagssuqtoqidian orogen. Ages of 2.0e1.8 Ga are systematics. Earth and Planetary Science Letters 254, 127e145. common in Northern Europe and Siberia but directions of Brown, M., 2007. Metamorphic conditions in orogenic belts; a record of e growth are not clear. secular change. International Geology Review 49, 193 204. Chauvel, C., Goldstein, S.L., Hofmann, A.W., 1995. Hydration and dehydration of oceanic crust controls Pb evolution in the mantle. 7. Conclusions Chemical Geology 126, 65e75. Choi, S.H., Mukasa, S.B., Andronikov, L., Kelly, N.M., 2006. LueHf Natural fission before 1.8 Ga had several effects: systematics of the ultra-high temperature Napier metamorphic complex in Antarctica: evidence for the early Archean differentiation of Earth’s mantle. Earth and Planetary Science Letters 246, 305e316. 1. It could not have formed a worldwide continental crust that Coogan, L.A., Cullen, J.T., 2009. Did natural reactors form as a conse- was so hot that subduction could not occur. Fission could, quence of the emergence of oxygenic photosynthesis during the however, have developed local areas of high temperatures that Archean? GSA Today 19, 4e10.

In addition, a large number of orogens of 1.8-Ga and slightly younger occur is northeastern North America but are not mapped separately in this ﬁgure. Arctica grew northwestward along the Wopmay orogen (Bear province) between about 1.9 Ga and 1.8 Ga. The Trans-North China belt (tnc) with an age of approximately 1.8 Ga, sutured the eastern and western halves of the craton. Although the location of the craton in Columbia is controversial Santosh et al. (2010) it seems likely that the craton was part of Columbia. The Baltic shield and Ukrainian shield (east European platform) are not shown in this ﬁgure but contain both known and inferred Paleoproterozoic belts. Much of the Baltic shield seems to have grown from the w2.0 Ga Svecofennian gneiss terrain in southern Sweden (Welin, 2012). 374 J.J.W. Rogers / Geoscience Frontiers 3(4) (2012) 369e374

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