Edited by DAN KALLUS Midland Senior High School 906 W. Illinois Midland. TX 79705
revi~i ted
RUSSELLD. LARSEN Texas Tech University Lubbock, TX 79409
Age of the Elements M. W. Rowe Texas A&M University, College Station, TX 77843 The annlication of lone-lived radioactivities to date eeological samples was discussed recently in THIS J O U R N A ~ ( I ) . There are a varietv of lone-lived isotones used for this Durpose, and their accuracy and reliabilitihave been repeatkdly demonstrated. In an even more recent article. I also described the various methods used to date archaeological items of interest (2). Some of these are based on decay of shorter-lived radioactive isotopes, but others are based on chemical and/or physical changes with time. Thus these two papers discuss themethods that scientists use to date archaeological and geological events of interest which cover a range of time from the near historical past back to the most ancient objects available for direct laboratory study, the meteorites, with ages which approach 4.6 billion years. We will consider here attempts to date the age of the elements. That means we must consider nrocesses which occurred greater than 4.5 billion years ago,kven before the formation of the planetarv bodies of our solar system. This makes the problem a difficult one, and the solution is strongly dependent upon how well the record of the particular event under question has been retained in the samples available for study. Thus samples must be sought in which the record of some of the physical and chemical events occurring prior to and during the formation of the solar system have been retained. The evidence suggests that neither terrestrial nor lunar samples are satisfactory; the oldest earth and lunar rocks seem to have suffered severe heating -3.5-4.0 billion years ago thus destroying the record of interest here. Fortunately, however, adequate samples are periodically provided. All of you have seen afalling star-astreak of light flashes across a clear night sky only to disappear without a trace. Perhaps some of you may have even seen a fireball that lights the entire sky, perhaps even rivaling the sun for a brief instant. But it is unlikely that many of you have seen an actual meteorite fall, with the accompanying hissing, clattering. and thunderine noises that are heard alone with the f i d explosion-like Feport. Meteorite falls are rare--and not even all of those are useful in our quest to determine the age of the elements. Some meteorites, however, have provided scientists with samples which have recorded the early events of the history of the solar system astonishingly well. Since theseimportant samples are the objects of study here, a brief discussi& of them isin order. There are three main categories of meteorites: irons, stones. and stonv-irons. Irons contain nrincioallv . nickeliron, the stones are principally stony material, and the stonv-irons contain rouehlv - .eaual . amounts of iron and stonv materials. Each of these groups are further subdivided in&
-
300
Journal of Chemical Education
many groups. Sinre our main interest here will be the stones. only these will be discussed further. The stones, which comprise about 900) hs numbers of the meteorites seen to fall. are suhdi\,ided into two main subgroups: rhondrites and achondrites. Both of these groups are further subdivided on the basis of chemical and petrological properties into a number of subgroups. Chondrites contain small silicate spheres called chondrules. Members of this group generally show evidence of lZ9I, the first extinct radioactive isotope found in nature (described later). The achondrites are rare, comprising only about 10%of the known stony meteorites. These are far more heterogeneous chemically than are the chondrites and in general show more evidence of chemical metamorphism than the chondrites. A member of this last class is important since one of the extinct radioactive isotopes, 2 4 4 Pwas ~ , first found in one of these. Historical Development
The door to nuclear chronoloev. so imnortant in all areas of dating of ancient objects, was opkneddith the discovery of radioactivity by H. Becquerel (31, hut i t was Lord Rutherford who was first to appreciate the implications of radioactive decay with regard to the origin of the earth and solar system (4). He suggested and experimented with the U-He method and the U-Pb methods for the determination of the ages of terrestrial materials. He also attempted the first measurement of the age of the elements (5) by assuming that 23SU/23sUwould not have been greater than unity when the elements were synthesized; the age of the elements was calculated knowing the half-life of the two uranium nuclides. Aten ( 6 )and ~ u r b i d g eet al. (7)later modified Rutherford's method bv taking into account the abundances of the heavier radioactive precursors of uranium and thorium which would have been produced in nucleosynthesis. Ages ranging between 5.9-7.7 billion years were estimated. We will examine now the ages derived from meteorite studies. Figure 1shows a schematic diagram of the general history of meteorites. The interval of interest here is that labelled t. In 1947 an imaginative proposal was made (8) suggesting that radioactive substances should have been made in nucleosynthesis which had half-lives too short to have survived the more than 4.5 billion years to the present day but lone enoueh to have been nresent when the solid bodies of thesolar iystem formed. ~ ; c hnuclides are termed extinct radioactivities. Thus the solid bodies would have retained a record of extinct radioactivities by virtue of the presence of their daughter products. This model, undoubt-
-4.8 x
4 . r X lb year$ PRESENT
STABLE lSOTOPES LONO LIVED 150TOPLS
ELEMENT TORMATION
INUCLEOSYNTHESISI
EQ'%
I
C
\
-- ---. . LlMlT OF
END FOEMATON OF OF SOL10 ELEMENT
SYNTHESIS
-K
PLANETARY MAiERiAi
DAYC.HIER TRAPPED IN METEORITES
DETECTION
EXTlNCT RADIOACTIVE
ISOTOPES E p "I '..PY
Figure 1. Schematicdiagram of same major chronologicai evenis in history of solar system formation and me meteorites. edly grossly oversimplified, will he called the "classical" model. One very interesting possibility suggested was lZ9I which decays by beta particle emission to 129Xe. The next year Aten (9)noted that the abundance of a t m o ~ p h e r i c ' ~ ~ X e is anomalously high compared to lZ8Xeand l3OXe, attrihuting the large excess to the decay of extinct '291. But Suess (10) argued that the '29Xe "excess" in the earth's atmosphere was only apparent, being due to abnormally low '28Xe and 130Xe, not an enrichment of '29Xe. Nonetheless, he stressed the importance of measuring the half-life of I29I, which was soon done by Katcoff et al. (11). Soon after Brown's suggestion of extinct radioactivities (a), scientists began to search for excess lZ9Xein meteorites, but initially with negative results (12). lodlne-Xenon Datlng-Classical
~
- -
'271(n,u) Iz8I lz8Xe+ 8
(1)
On subsequent heating, it was found that, except at lowest temperatures, the excess '29Xe was released in strict proportion to excess '2SXe from the iodine sites. That is to say, the iodine and '29Xe were strongly correlated with one another. Excess '"Xe, formed by neutron capture by tellurium, had a different release pattern. Fish and Goles (15) then pointed out that data from such an experiment can he readily examined for correlated release of '28Xe and '29Xe by plotting the ratio '28XePXe versus '29Xe/'32Xe. Suppose we have a sample in which radiogenic 129Xefrom '291 decay (which we shall designate as '29'Xe) resides only a t iodine sites in fixed proportion to the artificially implanted '2sXe from reaction 1, which we shall designate '28*Xe. This proportionality is expressed by '29'Xe/'2S*Xe, a constant for the specimen. Suppose that otherwise isotopes l28Xe,'29Xe, and the reference '32Xe are present only in a single trapped component of uniform isotopic composition (the subscript T will refer to this trapped component). Then one can easily show that in any fraction of xenon released from the sample,
I
Figure 2. The xenon hom the Richardlan meteorite. The lines through the peaks show atmosphericxenon, normalized to 131Xe.Notice me large excess at '2sXe. indimtino- me .oresence of '2s1 durino ihe eariv historv of the solar system.
Model
However, Reynolds (13) was successful only a few years later in finding meteoritic xenon which was isotopically distinct from atmospheric xenon. The outstanding feature found was the large - excess of 129Xein the Richardton meteorite. This now well-known xenon mass spectrum is shown schematically in Figure 2. The lz9Xe anomaly was at that time postulated to be the result of the insitudecay of extinct lZ9Iin the Richardton meteorite. Thisview still persists. The 129Xe excess in meteoritic xenon has now been confirmed - - ~ beyond doubt in many laboratories in xenon extracted from almost evervclassof meteorites. In fact, a n o r n a l ~ u s ' ~ ~isXae general feature of meteoritic xenon. An early experiment hy Jeffrey and Reynolds (14) provided strong evidence that the decay of '"I in meteorites took place in situ. They irradiated a sample of the Ahee meteorite with slow neutrons, thereby implanting excess '28Xe a t iodine sites of the meteorite by the nuclear reaction ~
" 10 Xenon Mars Numben
laxe= laxeT+ 1 2 q e and normalizing to '32Xe yields = ('%e/'32Xe)T ('29xe/L32Xe)
+ (129XePS2Xe)
or = ('29xe/'32Xe)T+ K('"*XeP3'Xe) (129XeP32Xe)
where K
=
hut llarxe= 1
-
2 8 ~ 1~28xeT
therefore, ('28*Xe/132Xe) = ('aXe/'32Xe)
- ('28Xe1'32Xe),
Substitution of eq 7 into eq 4 gives ('aXe/132Xe)= ('29xe/L32Xe)T + ('%e1132Xe) [(128Xe1'32Xe) - (128*Xe/'3%e)] rewriting yields ('%eP3%e) = [(1%eP"Xe)T]K('28XeP%e)T] +K('aXel'32Xe) (9)
so that the graph of '29Xe/'32Xe versus '28Xe/'32Xe for various fractions released is a straight line of slope K, passing through the composition (128Xe/'32Xe)~,(129Xe/132Xe)~. The quantity given by K, K = (12*XePnI)('"IP28*Xe) = (1291/'"I),('271/1"*Xe)
(10)
is seen t o depend through the second factor on the magnitude of the neutron irradiation which can be determined accurawly by assaying the 'jR'Xe in an iodine monitor which recei\.ed the same irradiation. With this factor determined. the slope of the graph gives (i9"ltr1)n, which is thr quantity Volume 63
Number 4
A ~ r i 1986 l
301
Comparison of Meteoritic Xenon Flsslon Ylelds with 244PuYlelds
Meteorite
Reference
P ~ ~ . l m o m e 2 2 from data in 2Ob Pasamome 22d st.. saverin ..-. ... Whitlockite 22f 22) Kapoeta 23 Z44Pu
- -
Figure 3. Tracing of xenon iaotopes hm the Pasamome melearlte showing the large excesses at '*'Xe, 's2Xe. '24Xe,and '*Xe duelo2"Pu, fission decay (20. No significant I39(e is formed in Z U P ~fission. A '2QXeexcess is also indicated.
of cosmological interest. The subscript zero refers to the ratio when the meteorite first beean to retain radioeenic xenon a t the iodine sites, and n o t t o the end of nucleosynthesis. The ratio (1"I/1271)n vields information concern in^ the relatiue ages oi the meteorites, that is, of this formation interval in different meteorites There is indication that the '271 and 1291were well mixed in the earlv solar system, so that this ratio was constant in all parent bodies a i a &en time in the epoch preceding 4.5 billion years ago. If we assume that a t a time, t, after the end of nucleosynthesis, the iodine bearing minerals began abruptly to retain radiogenic xenon, then this interval is given by (11)
The age equation requires calculation of the (1"I/1271)i,i~, that is, the ratio a t the end of nucleosynthesis in order to determine t. In 1967 Hohenberg, e t al. (16) showed that relative iodinexenon aees for the eieht chondrites demonstrated " s h a r ~ " isochronism" in the formation of chondrites. The average deviation in age from the mean of this group was only 2 million years. Podosek (17) refined and extended iodinexenon age determinations to a much more varied group of meteorites. His new samples include such diverse samples as an olivine-pigeonite chondrite (Karoonda); an ordinary amphoteric chondrite, St. Sbverin; an unequilibrated amphoterite Chainpur (where he was able to measure chondrules and matrix material senaratelv). - .senarated chondrules from an ordinary chondrite, Allegan; two enstatite achondrites, Pefia Blanca S.~ r i "n and e Bishonville:' and a silicate inclusion from an iron meteorite, El ~ a c i . One can remark that (1) the Podosek studv confirms a compact grouping of iodine-xenon ages, hut-extends the ranee of aces found as more diverse meteorites are included; (2) it is noteworthy that achondrites and the silicate phase from an iron meteorite are members of the same general, more or less, isochronous population of meteorites as the chondrites; (3) significant age differences can be detected between individual meteorites, although there is presently no interpretation to he made about these differences;and (4) significant age differences can he found between chondrules and matrix material from the same stone. The entire age
-
.
302
Journal of Chemlcal Education
+
33 3 25 i 3
Fission Yield '"Xe '34Xe
93i8 88.5 i 3
91 i 2.5 94 i 5
'=Xe
1100 1100
93*1 1100 31i8 97i8 26i3 88 i 4 91 i 5 =I00 25.1 i 2.2 87.6 3.1 92.1 i 2.7 =lo0
range to date is less than 30 million years, from samples as distinctly different as troilite (FeS) from the Pitw and Mundrahilla iron meteorite3, chondrules and matrix from chondrites, separates and whole pieces of carbonaceous chondrites, etc. Iodine-xenon dating is a powerful tool which has continued t o he used during the past quarter of a century since Reynold's initial discovery (13), and i t will undoubtedly continue to yield useful information regarding the formation of the solar system.
Xenon Mass Numben
t = x~,~~~(~"IP~X~)('~~I~I),,,
'='Xe
Model Plutonlum-Xenon Datlng-Classical Although Brown (8)had not listed 2 4 4 Pas ~ a possible candidate for an extinct radioactivity, Kohman (18)did and the role of 244Pu hegan to receive more serious consideration when Kuroda (19) argued that the excess of atmospheric h e a w xenon isotooes over those in Richardton (13) was due to fission of " + ~ u . ' ~ i n c2e 4 4 Pdecays ~ by fission as well as by n-decay, an enrichment of 13'Xe to 136Xeis expected. During the years immediately following 1960 much attention was directed toward finding such a fissiogenic xenon component in meteorites. Such a discovery would open the possibility of another datine method complementarv to the iodine-xenon method. clear cut evidenc' for fissiogenic xenon was first demonstrated by Rowe et al. (20) in the Pasamonte achondritic meteorite as shown in Figure 3. Independently a t about the same time Fleischer et al. (21) reported the existence of excess fossil fission tracks produced hy 2 4 4 Pin~ the meteorites Moore County and Toluca. Much supporting evidence for the existence of 2 4 4 Pin~the early solar system was rapidly accumulated from a number of laboratories (22). Conclusive evidence that the fission xenon found in the meteorites was in fact due to spontaneous fission of 2 4 ' P ~ was r e ~ o r t e dof Alexander e t al. (23) in 1971. They obtained 13mg bf"pure"244P~ and measured the relative yields of the 131-136Xe i s o t o ~ e sand compared them with those derived from meteorites. The table; taken from their paper, shows that the results agree almost perfectly with the mass yields deduced from th'meteoritic data. Evidence for 2 M Pdecay ~ in lunar material was alsb found soon after the return of the Apollo missions. Both fissiogenic xenon and fission tracks due to the fission decay of extinct 2 4 4 Phave ~ been found (24). The age equation first used t o calculate the interval t discussed above, but using 2 4 4 Pand ~ excess 136Xein the Pasamonte meteorite, was as follows (20)
where A,,, = ('SXePNXe)p,,,,
- (l"Xel'~e)t,,,,~
(13)
and 23SU and 'NXe are the numbers of atoms of 238Uand '30Xe in the sample, Xa23s and inas are the a-decay and spontaneous fission decay constants for 238U,X.244, and in44
are the a-decay and spontaneous fission decay constants for ?%Pu,y l j ~is the fission yield of ' W e from 22sU and j 4 ~ P u fission. T is the aee of the solar system, t is as defined earlier, p refers to passmonte, and c.;. refers to trapped components, and finally B is the calculated 244P~P38U ratio a t the end of nucleosynthesis. An inherent assumption in eq 13 is ~ virtually identical in their cosmothat WJ and 2 4 4 Pare chemical behavior. The usefulness of 2 4 4 Pas~a chronometer has been limited, mainly because its abundance has been related to a long-lived isotope of the nearest actinide, uranium, because no stable or very long-lived isotope of P u exists. Therefore, i t is necessary to have an accurate knowledge of the 244P~/238U ratios a t the time of initial condensation of the solar svstem in order t o define the time interval sought, i.e., the time between the condensation of solar system material and the cessation or nucleosynthesis. Since the chemical properties of P u are not exactly the same as those of U, substantial differences in the 2'4PuP38U ratio have been found in a number of samples (25). Recently, Lugmair and Marti (26) suggested that the 244P~/Nd ratio, as opposed to the 2 4 4 P ~ / ratio, U may he constant from sample t o sample and thus renewed the hope that the extinct nuclide 244Pu mav vet be used as an accurate cosmochronometer. All things considered, i t would appear that the last major production of 2 4 4 Ptook ~ place perhaps 100-200 million years prior to solar system firmati&. Certainly, taking a larger view, there has now been a full acceptance of the 2 4 4 Phypothesis ~ which has demonstrated that a rapid neutron capture nucleosynthetic process (273 was occu;ring early in our solar system nehula and the temporal history is recorded. I t simply remains for scientists to "play hack"the recording to get accurate ages. Extlnct 2eAland Io7Pd With only the '291 and ~ P decay u products the classical model seemed capable of explaining the data a t least to a first approximation. More reient de;elopments (28), mostly from the Laboratorv of Wasserburg at the California Institute of ~echnolog$, have confirmed the existence of two additional extinct radioisotopes, 26A1 (t1/2 = 0.73 X lo6) Y and '07Pd (t112 = 6.5 X lo6 y). Both of these are shorter-lived than either lZ9Ior ~ P u The . abundances of all of these radioisotopes and the time scales thus indicated lead t o the conclusion that 2W1, '07Pd, and 1291 were produced in a different nucleosynthetic event than the 2 4 4 P ~ The . matter is far from clear a t this point. As Wasserburg (29) stated, "No self-consistent theorv has vet been proposed that adequately explains the key i&topic, chemiial and mineralogic observations." Interested readers may get a more complete discussionof the details in the recent reviews by ~ a s s e r b u r g (29). The detection of 26A1 as a galactic component (30) argues that 26A1 came from the interstellar medium during the early history of the solar system rather than local shortterm production. If true, theZ6A1decay offers no potential as a dating tool for the time interval t. Since the 2"l and lo7Pd systems are less well understood than the lZ9I and 2 4 4 P ~ systems, much more experimentation will be required before these can be used to calculate decay intervals.
elements with the classical model. The situation is in some ways clearest for %IPu, which is produced only in the rprocess (7) and more nearly fits the classical model than the others. Though still not fullv developed. the 2"4P~-186Xe method holdspromise for the Future. ~ s s u m i n gthe classical model for 1291-129Xeyields precise information indicating that the total spread in the formation time of many diverse meteorites is less than 30 million years. Again the method is expected to continue t o develop in the coming years. The potential of the 107Pd-107Ag and 26A1-26Mgmethods is less clear. 26Al may in fact he produced in the galactic interstellar medium and hence not yield information about local solar system nucleosynthesis.Much more research on all these systems is needed. Acknowledgment I appreciate the comments and criticisms of J. H.Rernolds,~niversityof California, Berkeley, and the review by J. R. Watson, which were helpful to me. G. J. Wasserburg of the California Institute of Technology kindly sent me two reviews prior publication, for which I am also grateful.
(1) Rowe. M . W. J. Chem. Educ. 1985.62.580. (2) R o w , M. W., J. Chem. Edue. 1986.63.16. (3) Bequerel, H. Compf.rend.,Paris 1906. 122,420,501,689. (I) Rutherford. E. Pop. Sei. Monthly 1905. (May) "Radloaotiue Traneformatioru";Y&
U.Preaa: NslaHaven.CT, 1906. (5) Rutherford. E. Nature 1329,123,313. (6) Atcn. A. H. W., Jr. Phyaico 1957,23,1073. (7) Bu;b;dgedg., E. M.; Burhidge, G. R.: Fowler. W.A,:Hoyle, F.Reu.Mod.Phyr. l957.18, ~
,.,
,~a~ .
~~~~
- ~ ~ ~ ~ ~ .
~
(lor sue=,H. E. Zeifarhrl. PhyGh 1946. I&.386. (11) Katkoff, S.: Schaeffer.A. 0.;Haatings. J. M . Phya.Reu. 1951.82.688. (12) W-rbu~, G. J.; Hayden, R. Nature 1956, 176, 130: Reynolds. J. H.: L i p n , J. &whim.Cmmachim. Acts 1957.12.330. (13) Reynolds, J. H.Phya. Re". LOLL PI^ 1960.4.8. (14) Jeffrey, P. M.: Reyno1ds.J. H. J . Geophya. Res. IWI.66.3582.
(19) Kurda. P. K. Nature 1960.181.36. (20) aIRowe, M . W.;Kuods,P.K.J.Geaphya.Rea.1965,70,7W;hlRors,M.W.;Bqd, D.D. J. Geaphys.Ros.1966.71.888. (21) Fleiaeher, R. L.; Price, P. 6.: Walker. R. M . J. Oeophya. Rea. I%6,70,210a. (22) a) K u r d s . P. K.; R m s , M. W.; Clsrk. R. S.: Ganapethy, R Nctve 1966,ZIZ; b) Pppin,R.O.J.Geophys.Ros.1966.7I,28lS:e) Msrti.K.;Eberhardt.P.;Gsiss.J.Z. Nolur/onrh. 1966,210,398:d) Hohenhrg, C. M.:Munh,M. N.;Raynoldl, J. H. J. Gaophys.Rea. 1967.72,3139:el Cafs1auh.Y.; Maurette, M.: Pcllaa, P."Fadioretive Dating and Methoda of Low Level Counting"; Intern. A m . Ener. k e n w Vienna, 1967,p 215; fl Waaserburg, 0. J.; Huneke, J. 0.;Burnett, D. S. Phya. Re". Left. 1969.22.1198:J. Ceoohva.Rea.1969.74.4221:al Hohenberg, C.M. Geochim.
"" 2 1 , 1.ur.m S a w & Pmlirninery Eiamulat8cn &am. Science 197a. 172.681; I 11. V r ~ nP. R Saenre 1972.176.%JY:C,maz.G. llmad. R L M hlonnm M ; w a n D . k l r m n . C.. M.. s'rhtrc*. J W s l w r . R M . Z,mnwrmsn.J fior T n r r d L u ~ ~ SDtnl ~ c .Supp. J.Georh,m Caarnrrhm Arto ,a,, ",-,. 9 ,A,, "-". (25) J.; Huneke, J. C.: Burnett, D. S. J 74, 4221: Podasek, F. Earth Lett. Cormachim
...
.
Leiu
Huuhson. .Glsl.H.;Hohsnkn.
.
wesserburg, 0. Geophya. Rer. L(b9, A. Pionof.Sci. 1970,8,183,Gmehim. Act. 1972.36.155: Padwk, F. A,: Lowis, R. S. Earth Phnet. Sci Lall. 197% 16, 101; Lewis. R S. Georhim. Cosmochim. Aete 1975.39.417. (261 Lugrnair, G. W:. Marti, K. EorthPlamt.Sci. Loft. 1977,35.273. (27) Burbidge, E. M.;Burbidge,G. R.; Fowler, W . A,: Hoy1e.F. Reu.Mod.Phya.1957,29, K*S 1181 Grav. C. M.: Camostoo. W . Nature IW4.251.49S: h s . T.: P a m a s w i a u ; W w a r -
Summary There are now four well-established cases of extinct radioisotopes present during the early history of the solar system. All of these hold some degree of potential as dating tools for various events of nucleosynthesis, if not for the dating of the
Volume 63
Number 4
April 1986
303
