5. D. S. Coombs, A. J. Ellis, W. S. Fyfe, A.M. Taylor, Geochim. Cosmochim. Acta 17,53 (1959); E. E. Senderov, Geokhim. Issled.Obi. Povysh. Davlenii Temp. 1965, 173 (1965).

6. C. W. Correns, Einfdihrung in die Mineralo-gie (Springer-Verlag, Berlin, 1949); G. B.Alexander, W. M. Heston, R. K. Iler, J. Phys.Chem. 58, 453 (1954); G. Okanmoto, T.Okura, K. Goto, Geochim. Cosmochim. Acta12, 1923 (1957).

7. G. Lagerstrom, Acta. Chem. Scand. 13, 722(1959).

8. B. F. Jones, S. L. Rettig, H. P. Eugster,Science 158, 1310 (1967).

9. C. Waldon, personal communication.10. Glass was prepared according to the method

outlined by J. F. Schairer and N. L. Bowen[Amer. J. Sci. 245, 193 (1947)].

11. Silica concentrations were determined by useof the molybdate blue method [U.S. Geol.Surv. Water Supply Pap. 1454 (1960), p.260], and aluminum concentrations were de-termined by use of the aluminum colorimetricmethod [Standard Methods of the Examina-tion of Water and Waste Water (AmericanPublic Health Association, New York, ed. 12,1965), p. 53].

12. A similar relationship can be demonstratedfor natural rhyolitic glass [E. A. Gloor,thesis, University of Wyoming (1969)].

13. Ratios of Si to Al in the phillipsite crystalswere determined with an electron-probemicroanalyzer (Hitachi model XM-AS). Na-tural phillipsite samples were used asstandards.

Most of the carbon in carbonaceouschondrites exists in reduced form, large-ly as an aromatic polymer and, to alesser extent, as extractable organiccompounds (1). A minor part (~ 3 to5 percent) exists in oxidized form, as thecarbonates breunnerite [(Mg,Fe)CO8]and dolomite [MgCa(CO3)2] (2).

These two types of reduced carbondiffer greatly in their isotopic com-position. The polymeric carbon is light:8C18 =-15 to -17 per mil in bothtype I and type II carbonaceous chon-drites (3). The carbonate carbon ismuch heavier: SC13 = + 60 to + 70per mil in type I and + 40 to + 50per mil in type II carbonaceous chon-drites (3, 4), for a total difference of 83to 87 and 57 to 67 per mil. For compar-ison, terrestrial organic matter andcarbonates (including that formed co-genetically, for example, by shellfish)generally differ by only about 25 permil, typical values being - 25 to - 30per mil for organic carbon and ~ 0per mil for carbonates. The meteoriticcarbonates fall well outside the rangeof terrestrial carbon, - 80 to + 25 permil (5).

Clayton (4) has considered four980

14. The rims of the glass fragments also aredepleted with respect to sodium.

15. J. D. Hem and C. E. Roberson, U.S. Geol.Surv. Water Supply Pap. 1827-A (1967), p.55; A. L. Reesman, E. E. Pickett, W. D.Keller. Amer. J. Sct. 267, 99 (1969).

16. The only inorganic precipitation of silica gelin a modem sedimentary environment knownto us was reported by M. N. A. Peterson andC. C. von der Borch [Science 149, 1501(1965)]. This gel precipitated when the pH ofan alkaline lake (pH 9.5 to 10.2) drasticallydecreased (pH 7.0 to 6.5).

17. H. P. Eugster, Contrib. Mineral. Petrol. 22,1 (1969).

18. T. P. Rooney, B. F. Jones, J. T. Neal,Amer. Mineral. 54, 1034 (1969).

19. H. P. Eugster and B. F. Jones, Science 161,160 (1968).

20. L. L. Ames [Amer. Mineral. 48, 1374 (1963)]has shown experimentally that mordenite anda clinoptilolite-like phase are stable overa pH range from 7.1 to 8.7.

21. R. A. Sheppard and A. J. Gude, III, U.S.Geol. Surv. Prof. Pap. 597 (1968).

22. We thank R. A. Sheppard for supplyingsamples of natural phillipsite. Laboratorywork supported by grant 3809-A2 from thePetroleum Research Fund of the AmericanChemical Society. We thank H. P. Eugsterfor his instructive discussion during the pro-gress of this work and for his comments onthe manuscript.

22 June 1970; revised 19 August 1970 0

possible origins of the meteoritic carbonisotope fractionation. The first of thesepossibilities, single-stage equilibriumfractionation, can, in principle, give aneffect of the right sign and magnitude.Data for organic molecules of appro-priate complexity are unavailable, butif calculations for CH4 and CO2 (6)are used as a first-order approximation,temperatures of 2650 to 3350K wouldseem to be required for the observedfractionations in type I and type II car-bonaceous chondrites. However, nomechanisms on earth are known thatwould establish equilibrium at the lowerof these temperatures. Kinetic isotopeeffects are a second alternative, but inthe reactions studied thus far, the frac-tionation was too small (20 to 40 permil) and in the wrong direction, theoxidized carbon becoming lighter ratherthan heavier (7). The third possibility,biological activity, produces fractiona-tions of the right sign, but usually ofinsufficient magnitude [algae, 10 to 12per mil; vascular plants, 20 to 30 permil (8)]. Finally, it is conceivable thatthe differences reflect nuclear processes,the two types of carbon having had dif-ferent histories of nucleosynthesis. This

alternative was favored by Urey (9).However, all work to date has failed tobring to light any isotopic differencesthat might be attributed to incompletemixing in the solar nebula.The origin of the organic compounds

is, of course, pertinent to this problem.Studier et al. (10) have shown thatnearly all compounds identified inmeteorites can be produced by aFischer-Tropsch type of reaction be-tween CO, H2, and NH3 in the pres-ence of a nickel-iron or magnetitecatalyst. They proposed that the or-ganic compounds in meteorites weremade by such reactions, catalyzed bydust grains in the solar nebula.The Fischer-Tropsch reaction pro-

duces both oxidized and reduced car-bon. Much of the oxygen from COusually is converted into H20 (Eq. 1),but a variable proportion is convertedto CO2 (Eq. 2):

n CO + (2n + 1) H2-e Cs 92+ n H2O(1)

2nCO+ (n+ 1) Ha- CnH2.+2+nCO2(2)

Nothing is known about carbon isotopefractionation in this reaction, however.We, therefore, decide to investigateit.A difficulty encountered in our ex-

periments was the tendency of any CO2formed to disappear by secondary reac-tions. [Hydrogenation of CO2 to hydro-carbons is thermodynamically feasible(11).] We circumvented this problem bycollecting CO2 as fast as it was formed,using a Ba(OH)2 absorbent.

For our syntheses, we heated 0.8 literof an equimolar mixture of CO andH2, initially at 1 atm, to 4000 ± 10°Kin a Vycor flask in the presence of 1.5g of cobalt catalyst (12). (Cobalt waschosen because it is effective at lowertemperatures than iron catalysts; other-wise there is little difference betweenthe two.) As the reaction progressedCO2 was removed continuously by ab-sorption in 5 ml of a saturated Ba(OH)2solution in a 35-ml sampling tube, con-nected to the reaction vessel by a sidearm. The sampling tube was replaCedfrom time to time. In order to ensurecomplete recovery of all CO2 formedduring the sampling interval, the tubewas cooled to -196°C just before re-moval. All condensable gases werethus frozen out. After closing the stop-cock between the sampling tube and thereaction flask, we warmed the tube toroom temperature and shook it to pro-mote absorption of CO2. The stopcockwas opened briefly to equalize pressure,

SCIENCE, VOL. 170

Carbon Isotope Fractionation in theFischer-Tropsch Synthesis and in Meteorites

Abstract. Carbon dioxide and organic compounds made by a Fischer-Tropschreaction at 400°K show a kinetic isotope fractionation of 50 to 100 per mil,similar to that observed in carbonaceous chondrites. This result supportsthe view that organic compounds in meteorites were produced by catalytic re-actions between carbon monoxide and hydrogen in the solar nebula.

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and the sampling tube was replaced bya new one.The gas phase was separated into

three fractions: CO, CH4, and heavierhydrocarbons (C2+). Those componentsthat remained gaseous at -196°C (CO,H2, and CH4) were passed over CuOat 150°C. This reaction converted COto C02 and H2 to H20, which werefrozen out. The CH4 remaining wasoxidized to C02 by passage over CuOat 8500C. Trial runs showed that thisprocedure separated CO from CH4 with99.1 to 99.5 percent efficiency.The fraction condensed at -1960C

(the "C2+ fraction"), which consistedof hydrocarbons heavier than CH4, wasoxidized to CO2 by passage over CuOat 8500C. Finally, the original C02fraction was recovered from theBaCO3-Ba(OH)2 suspension by the ad-dition of 85 percent H3PO4 to thesampling tube. The volume of eachfraction was determined on a mercurymanometer.

Because of the nature of our sam-pling procedure, the C02 fraction wasnot quite comparable to the other threefractions. It was removed continuouslyfrom the entire reaction volume, andthus represents the incremental yieldbetween successive samplings. Theother three fractions were isolated fromsmall portions of the gas phase and thusrepresent cumulative yields. We cannotcompletely rule out the possibility thatthe C02 was slightly fractionated as aresult of incomplete freezing out andabsorption. This result would tend tomake the recovered C02 too light. Acontrol experiment showed, however,that C02 recovery was : 89 percentcomplete in 4 hours, our shortest sam-pling interval. Moreover, the CO-C02 fractionation remained constantthroughout the experiment as samplingtimes increased.

Isotopic compositions were measuredon a 15-cm, 600-sector double-collect-ing mass spectrometer (13). All resultsare given relative to the Pee Dee belem-nite (PDB) standard. The isotopicmeasurement itself is usually accurateto + 0.1 per mil. However, samplessmaller than 40 ,tmole had to be dilutedwith standard C02 before measurement,and, since our initial procedure formeasuring small gas volumes was notaccurate enough, uncertainties in the di-lution factor led to rather large errorsin isotopic composition.The results reported here are based

on two experiments under nominallyidentical conditions, comprising a totalof eight fractions (Table 1). The27 NOVEMBER 1970

Table 1. Carbon isotope fractionation in Fischer-Tropsch synthesis; equimolar ratio of CO toH2; temperature = 400°K; pressure = 1 atm; cobalt catalyst.

Time Percentage of total* aCis (%o) t(hr) CO CH4 C2+ CO. CO CH4 C2+ C°0

Experiment 17 90.2 8.2 0.5 0.05 -37.6 -81.2 -30 + 30 0 + 9

31 30.5 68.6 0 0.05 -46.8 -38.5 -8 + 658 15.8 80.7 1.5 0.10 -47.7 -35.1 -61 + 3 -11.5 -- 3

320 8.1 87.9 3.3 0.68 -48.0 -35.3 -62.2 -11.1Experiment 2

4 95.6 1.9 1.5 0.05 -37.3 -100 ± 4 -62 ± 3 -1 + 515 93.8 3.5 1.7 0.05 -37.4 -94 ± 3 -55 ± 230 64.7 29.4 2.3 0.18 -33.1 -50.2 -61.2 -5.3

217 24.1 69.0 3.0 0.20 -47.2 -37.1 -63.1 -11.1* Values for CO, CH4, and C2+ were determined on a 1/20th aliquot and reflect the compositionof the gas phase at the time of sampling. Carbon dioxide, on the other hand, was removed con-tinuously from the entire reaction mixture. The amounts given in the table are 1/20th of the amountactually collected and refer to the CO2 formed between consecutive sampling intervals. t InitialCO: -38.6 per mil. Errors are ± 0.1 per mil, unless otherwise indicated. A "wax" fraction wasrecovered from the catalyst in experiment 1 at the end of the reaction. It comprised 1.5 percentof the total carbon and had a 6C13 of -72.2 per mil.

catalyst in experiment 2 seems to havebeen slightly less active, judging from theslower rate of reaction. In combiningthe results on a single graph, we havetherefore used degree of conversionrather than time as the abscissa (Fig. 1).A strikingly large fractionation

(- 100 per mil) between CH4 and C02appears in the early stages of the reac-tion. The equilibrium fractionation at400°K is only 45 per mil (6), and sothis is clearly a kinetic isotope effect.As the reaction progresses, the meth-

ane becomes isotopically heavier. This isdue in part to material balance: as CH4becomes the dominant product, it must,

0 20 40 60 80Percentage completion

Fig. 1. Carbon isotope fractionation in thePischer-Tropsch reaction at 4000K. Solidsymbols, experiment 1; open symbols, ex-periment 2. In the early stages of the re-action CH4 shows a fractionation as largeas 100 per mil relative to CO2. Hydro-carbons of higher molecular weight (C2+,"wax") show smaller fractionations (50to 60 per mil) which, however, persistthroughout the reaction.

of necessity, approach the isotopic com-position of the starting material. Thistrend seems to be further enhanced byisotopic equilibrium. The CH4-CO frac-tionation approaches the equilibriumvalue of 15 per mil (14) in the latterstages of the reaction. Apparently theCH4-CO exchange is fairly rapid underour experimental conditions.The C2+ fraction is perhaps more

pertinent to the meteorite problem, be-cause the bulk of the reduced carbonin meteorites consists of polymeric ma-terial and molecules with ten or morecarbon atoms. This fraction remains- 50 per mil lighter than the C02throughout the reaction, as its amountincreases from - 0.5 to 3 percent.Apparently, its rate of exchange withCO is much slower than that of CH4.

Because of the nature of our experi-ment, the C2+ fraction was not trulyrepresentative of all heavy hydrocar-bons produced. Only those that werevolatile at 4000K found their way intothe sampling tube. Compounds of highmolecular weight remained on thecatalyst in the reaction vessel and wererecovered only after the end of theexperiment. The fractionation betweenthis "wax" fraction and C02 was 61per mil in the first experiment, some 10per mil greater than the C2+-CO2fractionation. Unfortunately the waxfraction in experiment 2 was too smallfor isotopic analysis.

It thus appears that the Fischer-Tropsch reaction gives a carbon isotopefractionation of the same sign andmagnitude as that observed in meteo-rites. Although our results thusstrengthen the case for catalytic synthe-sis of the organic compounds in meteo-rites, they also place additional con-straints on the model.

981

On the basis of the observed frac-tionation between "wax" and CO2 at400°K, the organic compounds in typeII carbonaceous chondrites may haveformed at - 400°K and those in typeI carbonaceous chondrites at a slightlylower or higher temperature, with theexact temperature dependent on thetemperature coefficient of the kineticisotope effect. This value is in goodagreement with previous estimates (IS),based on trace element content and theformation temperatures of certainminerals in carbonaceous chondrites(Fe3O4, 400°K; hydrated silicates,

3500K). However, in a cooling solarnebula it may be difficult to keep mostof the carbon in the form of CO downto such low temperatures. AlthoughCO is the stable form of carbon at hightemperatures, it becomes unstable withrespect to hydrogenation (Fischer-Tropsch) reactions at lower tempera-tures. Thermodynamically, the mostfavored product is CH4; the next mostfavored product is ethane, then ben-zene, and other hydrocarbons. Thus, ifequilibrium were maintained, most oft-he CO would be converted to CH4 be-fore formation of heavier hydrocarbonsbecame possible. (Their formation tem-peratures lie some 1000 to 1500 be-low those of methane.) At total pres-sures of 10-4 and 10-5 atm, only 1percent of the carbon would remain inthe form of CO by the time the tem-perature had dropped to 5700 and520°K. Essentially no CO would beleft at 4000K.

This dilemma does not seem to beinsoluble. Hydrogenation of CO isknown to be very slow in the absenceof a suitable catalyst. The fact thatcarbon compounds are abundant onlyin meteorites containing magnetite andhydrated silicates may mean that thesecompounds themselves were the re-quired catalysts. Above the formationthresholds of these minerals (400°Kand 3500K) lack of a catalyst mayhave permitted the survival of CO. Insupport of this suggestion, there is someevidence that the solar nebula cooledrapidly from high temperatures downto the accretion range of meteorites,and then much more gradually (16).For the ordinary chondrites, the accre-tion range was a 100-degree intervalcentered on 510+800K (17). For thecarbonaceous chondrites, this rangeseems to have been ~ 300° to

4000K (15). No carbonaceous chon-drites without magnetite are known,which suggests that accretion in thesource region of carbonaceous chon-

982

drites commenced only below 400°K.Presumably the range above 400°K wastraversed too rapidly for any significantaccretion to take place.

Formation of hydrated silicates mayalso have provided the cations neededfor the carbonate. These silicates, al-though mineralogically ill-defined (18),have a distinctly lower cation-to-siliconratio than the olivine from which theyseem to be derived (2, 15). In a gas ofcosmic composition at a total pressureof 10-4 atm, formation of hydrated sili-cates is expected to commence belowabout 350°K (15). The following equa-tions schematically represent the proc-ess:

12(Mg,Fe)2SiO4 + 14H202Fe304 + 2H2 + 3 (Mg,FeX)(OH) 8Si4010

(3)4(Mg,Fe)2SiO4 + 41H20+20222

2(Mg,Fe)COs + (Mg,Fe)6(OH)sSi4O1o(4)

Just as in terrestrial serpentinizationreactions (19), some basic cations areleft over. The excess cations can appeareither as magnetite (Eq. 3) or carbonate(Eq. 4). Basic oxides react rapidly withCO2 and would thus remove it fromthe gas phase fast enough to preventsecondary reactions or isotopic equili-bration.

Alternatively, the organic polymerin meteorites may have a formationtemperature close to that of methane.Thermodynamic calculations (20) haveshown that aromatic polymers may be-come the favored product under avariety of conditions. Moreover, an es-sential first step in the Fischer-Tropschsynthesis is adsorption of CO on thesurface of the catalyst. At the low par-tial pressures of CO expected in thesolar nebula (10-7 to 10-8 atm), it isconceivable that effective adsorptionmay not commence until temperatureshave fallen low enough to permit theformation of products other thanmethane.

Our experiments also provide a verytentative clue to the initial isotopiccomposition of the carbon in thenebula. Carbonaceous chondrites con-tain only about 6 percent of their cos-mic complement of carbon; the re-mainder was presumably left behind inthe nebula as compounds of low molec-ular weight. Assuming that CO was thesole starting compound, and comparingthe C02-CO-wax fractionation in ourexperiments with the polymer-carbonatefractionation in type II carbonaceouschondrites, we would estimate an initialcarbon composition of +15 per mil on

the PDB scale. This is remarkably closeto the isotopic composition of carbonin lunar soil (21), which is a mixtureof indigenous, meteoritic, and solar-wind carbon.

MICHAEL S. LANCETEDWARD ANDERS

Enrico Fermi Institute and Departmentof Chemistry, University of Chicago,Chicago, Illinois 60637

References and Notes1. J. M. Hayes, Geochim. Cosmochim. Acta

31, 1395 (1967).2. E. R. DuFresne and E. Anders, ibid. 26,

1085 (1962).3. J. W. Smith and I. R. Kaplan, Science 167,

1376 (1970). For the extractable organic mat-ter, values from -5 to -27 per mil havebeen reported [see also H. R. Krouse andV. E. Modzeleski, Geochim. Cosmochim. Acta34, 459 (1970) and unpublished work by T.C. Hoering and E. Anders], but at least someof these results may have been affected bycontamination. All 5C13 values are given rela-tive to the PDB standard [H. Craig, Geo-chim. Cosmochim. Acta 12, 133 (1957)].

4. R. N. Clayton, Science 140, 192 (1963).5. P. Deines, Geochim. Cosmochim. Acta 32,

613 (1968).6. Y. Bottinga, ibid. 33, 49 (1969).7. P. E. Yankwich and R. L. Bedford, J. Amer.

Chem. Soc. 75, 4178 (1953); M. Zielinski,J. Chem. Phys. 47, 3686 (1967).

8. P. H. Abelson and T. C. Hoering, Proc.Nat. Acad. Sci. U.S. 47, 623 (1961); R.Park and S. Epstein, Plant Physiol. 36, 133(1961). A larger fractionation (90 per mil) hasbeen seen in the bacterial degradation ofmethanol in the laboratory [W. D. Rosenfeldand S. R. Silverman, Science 130, 1658 (1959)]but seems to have few, if any, parallels innature.

9. H. C. Urey, Quart. J. Roy. Astron. Soc. 8,23 (1967).

10. M. H. Studier, R. Hayatsu, E. Anders, Geo-chim. Cosmochim. Acta 32, 151 (1968); R.Hayatsu, M. H. Studier, A. Oda, K. Fuse,E. Anders, ibid., p. 175.

11. P. H. Emmett, Ed., Catalysis (Reinhold,New York, 1956), vol. 4, p. 9.

12. A Fischer-Tropsch catalyst containing 38 per-cent Co was prepared by precipitating cobaltcarbonate on Filter-Cel diatomaceous earth(Johns-Manville), according to the procedureof Emmett (11, p. 46). The catalyst wasreduced in 11.2 at 300'C and at a pressure of1 atm before reaction. The carbon monoxideused in our reactions was purified fromtraces of C02 by overnight exposure to acold trap at -196C.

13. A. 0. C. Nier, Rev. Sci. Instrum. 18, 398(1947); C. R. McKinney, J. M. McCrea, S.Epstein, H. A. Allen, H. C. Urey, ibid. 21,724 (1950).

14. H. Craig, Geochim. Cosmochim. Acia 3, 53(1953).

15. J. W. Larimer and E. Anders, ibid. 31, 1239(1967); E. Anders, Accounts Chem. Res. 1,289 (1968).

16. A. G. W. Cameron, Icarus 1, 13 (1962);, in Meteorite Research, P. M. Mill-

man, Ed. (Reidel, Dordrecht, 1969), pp. 7-15; S. P. Clark, K. K. Turekian, L. Gross-man, in preparation.

17. R. R. Keays, R. Ganapathy, E. Anders,Geochim. Cosmochim. Acta, in press.

18. J. F. Kerridge, in Mantles of the Earth andTerrestrial Planets, S. K. Runcorn, Ed. (In-terscience, London, 1967), pp. 35-47.

19. I. Barnes, V. C. LaMarche, Jr., G. Himmel-berg, Science 156, 830 (1967).

20. M. 0. Dayhoff, E. R. Lippincott, R. V. Eck,ibid. 146, 1461 (1964); R. V. Eck, E. R.Lippincott, M. 0. Dayhoff, Y. T. Pratt,ibid. 153, 628 (1966).

21. S. Epstein and H. P. Taylor, Jr., ibid.167, 533 (1970); I. R. Kaplan and J. W.Smith, ibid., p. 541.

22. We are indebted to Prof. R. N. Clayton forthe use of his mass spectrometer and for valu-able criticism. Mrs. T. Mayeda kindly provid-ed advice and assistance. Work supported inpart by NASA grant NGL 14-001-010.

29 June 1970; revised 30 September 1970

SCIENCE, VOL. 170