William
D.
Ehmann
University of Kentucky Lexington
Recent Improvement in Our Knowledge of Cosmic Abundances
A
knowledge of t.he relative ahundances of t,he chcmiral clement,^ on a coxmic scale is prerequisite to the dwclopment of a theory for the synt,hesis of t,he elements in the stars. Since our sun is considered to be a relat,ively "average" star, we can as a first approximat,ion equate the t,erms "cosmic abundances" and "solar syst,em abundances." However, determination of these solar ssst,em abundances has st,ill been a formidahle problem. Much of the early abundance data was derived from geochemical investigations of the crust, oceans, and at,mosohere of the earth. More recentlv. " , si~nificant, contributions to t-his field have been made by spectrographic investigat,ions of t,he light emit,t,ed by the sun and t,hc stars (1) and by chemical analyses of the met,eorites. Some informat,ion on the distribution of hydrogen in the universe has been obtained recently t,hrough t.he use of radiotelescopes. I t was only natural to look t,o t,he meteorit,es for cosmic ahundance data, since they represent t,he only extra-t,errtxtrial specimens available for lahnratory analysis. I t has been suggested by Urey and Craig ( f ) that a cert.ain class of stony met,eoritcs (aerolibes) called chondrites might represent t,he best, average sample obtainable for the determination of t,he relat,ive abundances of the non-volat,ile elements in our solar system (Fig. 1). In general, meteorit.ic matter appears to be fractionated into three major phases: the metal phase, t,he sulfidc phase, and the silicate phase. The per cent by weight of each of t,hese phases in meteorit,ic
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Presented as part of the Symposium, "Geochemistry: Analysis and Synthesis," before tho Divisions of Inorganic Chemistry, Phvsieal Chemistrv. and C h e m d Erloention at the 13ith ~ G t i n gof the ACS; Cleveland, Ohio, April 1Ni0 This work was supported in part hy the U. S. Atomic Energy Commission.
Figure 1 . Cra.5-rectionr of four chondritic meteoriter i11urtroting their unique i i r ~ t u r e ; from Nininger ( 4 4 ) .
mat,ter is estimated to be appn~simatelylo%, 5%, and 85%, respectively. Certain of t.he rlcments are k n o ~ ~ m to concentrate predon~iiiantlyin one or the other of these phases. Therefore, it is possible to analyze a. single meteorite phase in which we are quite certain the element in question is concentrated, and using the relative ratios of the various phases as stated ahove, cornput,e an elemental ahundance for all meteoritic matter. This has been done particularly for the det,ermination of the ahundances of some of the morc nohle metals that are presumed to be r.oncentrat,ed in thp met,allic phase. Analyses of the irnn met,eorites (siderites) for these elements have been used in some cases to compute cosmic abundance data. The difficulty with this procedure is that. there is still a measure of uncertaint,~as to t,he true valucs of these phase ratios and the true distribution of the elements bet,ween them. Analysis of the chondrites vhicb contain all three meteoritic phases appears to be the most reliable approach to thc determinatioii of elemental abundance data a t present,, and will he st,ressed in this paper. Largely on the basis of t.hcir st,rueture, Urey postulat,ed that the chondrites are made up of rubble-like mat,erial, h t , h metallic and non-metallic, which resulted from collisions bct,ween asteroid-sized bodies early in the history of t.he solar system. This rubble aggregated into layers on the larger asteroidal bodies of approximately lunar size. Finally, a t some later time, a collision bet,~vecntwo of t,hc larger bodies occurred, resulting in the compression of thc layers of rubble into the objects known as chondrites. More recently, Urey (3) has suggest,ed a slight,ly modified collision t.heory in which the moon may have had a role in chondrite formation. Andem and co-workers in this symposium (4,5) d o part sharply from the suggestion that a collision mechanism is necessary to explain chondrite structure. They have proposed that a unique type of volcanic action on asteroid-sized bodies may explain many aspects of chondritic struct,ure. Regardless of which of t,hese t,heories proves to be correct,, Urey's original suggest,iou that the chondrit,es are representat,ive of t,he non-volatilc clcmeiital ahundances in the solar system is a useful start,ing point for t,he determination of cosmic abunda~~rcs unt,il t,hat time when bet,ter samples become available through extrat,errestrial explorat,ion. Suess and Urey (6) in 1956 compiled a table of cosmic abundances based on various types of met,eoritic, solar, and terrestrial elemental abundance data. In addition, the accurat,e relative isotopic abundances available from mass spectrographic analyses mere used t,o derive isotopic abundances in the Suess and Urey table. .4 plot of the data from these tables shons a significant Volume 38, Number 2, February 196 1
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53
1ist.s only the chondrite data. The numbers in parentheses are tentative determinations based on work now in progress. These numbers may be suhject to considerable revision upon completion of the experimental work, but are included for completeness. A small number of analyses limit,ed to a single meteorite, or to the troilite phase of meteorit,es, were not included in t,histabulation. Table 2 lists analytical data on iron and stony-iron met,eorites that have appeared since t,he Suess and Urey paper. Due to the previously discussed difficulties in the interpretat,ion of data from a single meteoritic phase, these ahundances are here listed merely in units of ppm by weight for these specific meteorites. In analyses for several of the elements listed in Tables 1 and 2, a wide range of values was obtained for different met,eorite specimens. In these cases, where also the number of specimens analyzed was small, listing an average value might be misleading: Therefore, only the limb for the rauge of these experimental values are entered. It, is of interest to note that the majority of the results list,ed in Tables 1 and 2 were obtained bv use of neutron act,ivat,ion analysis. The great sensitivit,~ of this technique coupled with it.s inherent freedom from laborat,ory contamination makes it highly useful for determiuat,ion of certain t,race element ahundances. ~
~
~
Table 2.
Recent Anolyses o f Iron and Stony-Iron Meteorites
Nunber Z
Element
Ref
of the data undoubtedly represent the best available for a given element. Until additional analytical data are available and t,he compositional differences between "falls" and "finds" further investigated, more definitive assignment of cosmic abundances from some of these dat,a would be premature. Li (Z = 3 ) to Cu (Z = 2 9 )
With the exception of the new analysis for lithium which is based on only three chondrites, the new determinations listed in Table 1 are in reasonably good agreement with the Suess and Urey values in this region. The investigation of the chondritic abundances of scandium and chromium was prompted mainly by rather large discrepancies between the Suess-Urey values based on older chondrite analyses and the solar spectra data of Aller (1). The spectral ahundances for scandium and chromium relative to silicon = 10"re 40 aud 13,000 respectively. I t is seen in Table 1 that, the new analytical data of Bate, et al. (14) tend to confirm the Suess and Urey values. Due to considerable uncertainties in the det,ermination of the solar abundance for scandium, it nwuld appear that its cosmic abundance is best approximated by the new chondritic data. For chromium the solar value has been more precisely determined, and may represent a truly high abundance in the sun's atmosphere. Alterations of relative concentrations of element,^ in the sun's atmosphere by mixing with the interior are possible and lead one again to rely more heavily on the chondrite analyses for the chromium cosmic abundance. It is to be noted that the newly determined abundances of titanium and manganese (15) are much more nearly the same for chondrite "falls" and "finds," than was the case for barium. This may mean that compositional differences between "falls" and "finds" are small for certain elements; hence, much existing dat,a are adequate for comput,ing cosmic ahundances. Ge (Z = 3 2 ) to Lu (Z = 7 1 )
Discussion
The response to the plea of Suess and Urey (6) for more and bett,er analyses of chondrites is quite remarkable, as indicated by the lengfh of Table 1. I t is necessary, however, to he quite cautious about substituting some of t,he new data for t,he Suess and Urey values. It is to be not,ed that most of the results in Table 1 are based on the analyses of ten or less meteorite specimens, while in some cases the older data resnlted from a much larger number of analyses. In addition, t,he recent work of Moore and Brown (58) indicates t.hat compositional diierences may exist beheen chondrite "falls" and chondrite "finds." These differences for barium are approximately an order of magnitude. Therefore, in this paper no attempt will be made to reconst,mct t,he Suess and Urey cosmic abundance curve, using the new analyt,ieal data, although certain
In this region some serious discrepancies are noted bet,ween cert,ain of the new data and the Suess and Urey values. It has been suggested that neut'ron capture on a fast. time scale produces abundance peaks at approximately Z = 35 (Br), Z = 54 (Xe), and Z = 78 (Pt). Since abundance data for bromine and xenon in chondrites would be unreliable due to their volatility, accurate abundance dat,a on neighboring relatively non-volatile elements, such as Z = 34 (Se) and Z = 52 (Te) would be valuable in assessing the magnitude of these abundance peaks in this weight region. Little analytical data existed for these elements before 1956. The abundance value for tellurium used by Suess and Urey was an interpolated value. The new analytical data for these elements and other neighboring elements, such as iodine, rubidium, and cesium, all appear to be lower than the Suess and Urey abundances. A wide range of abundances were ohtained for iodine and t,ellurium, and the simple average would perhaps not be representative. Goles (24) has noted that the "ordinary" chondrites, such as used in most abundance determinations, have iodine and tellurium ahundances near the low end of the range of values listed in Tahle 1. Higher abundances for these elements are found in both carbonaceous chondrites and enstat.ite chondrites. Therefore, assignment, Volume 38, Number 2, February 1961
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of cosmic ahnndance values ;or these elements must depend on fut,ure reseawh to determine if certain of these suhtypes of chondrit,cs might be more rrpreseutat,ive s>zmplcsfnr cosmic abundance determinations t,hau the "ordinary" rhondrites. Due to t,he large difference between the barium abundances in chondrite "falls" and "finds" (15, 58), it is difficult to assign a cosmic ahundanre value to barium a t present. However, in contrast to the other new det,erminations in the xcnon peak region, all the uew data point to a barium abundance higher than t,hat of Suess and Urey. I t appears that a final evaluation of the magnit.ude of these fast, time scale neutron captaure peaks will require new determinations of ot,her elements in these regions, such as arsenic, st,rontium,and antimony. The ahundance of iodine is of int,erest for an additional reason. Reynolds (39)has recently fouud nnomalous amounts of Xe'ZY in the Richardton rhondrite. This prcsumahly results from t,he decay of ILZ9 (H = 2.5 X 10' years) which was t,rapped in t,he original parent body of the met,enrit.e. These results imply t,hat a period of approximately 3.5 X 10%years int,ervened between t,he end of nudensynthesis and t,he condensat.ion of t,he parent. meteorite maderial to a point at which rare gases were retained. Wasserhurg, Fowler, and Hoylc (40)interpret this number as a maximum value for t,he time iut,erval. Assuming nucleogenesis occurred a t a u~liformrate over a t,ime large as compared to the mean life of IIZ9,they calculat,e an interval of 2 X 10" years, using the data of Reynolds. These calculations involve the chondrit,ic abundance of I I 2 ' , which Reynolds est,imat,ed to be approximately 1 ppm. Alt,hough this t,ime interval is not extremely sensitive to the I I n ahundance, t,he new iodine dat,a for "ordinary" rhondrit,es list,cd in Table 1 indicate that. ~orrect~ions in the calculat,io~is will undoubtedly be necessary. The new value for t,he indium abundance (21) is approximately 100 times lower t,han the Suess and Urey selected ahundanre. It. appears impossible to resolve this extremely low abundance and the low ahu~idances of cert,ain heavy elements such as mercury, thallium, lead, and bismuth, with the existing t,heories of nucleosynthesis. I t may he that these extremely low abundance values in chondrit,es are due to some as yet not understood fractionation process that occurred prior to or during t,he formation of the solar system, including the parent meteoritic bodies. Fish, et al. (5) suggest that t,heir quasi-volcanic recycle mechanism for the formation of rhondrites would result, in the depletion of such chalcophilic elements as indium, mercury, t,halhum, lead, and hismuth in t,he chondrites. Addit.ional analytical dat,a on various met,eorit,icphases are needed t,oexplain t,hese discordant ahundances. The new data on t,he rare earths, although based on only two chnndrites, indicate slightly lower values than given hy Suess and Grey. A detailed discussion of the signifiraiice of these dat,a will not be attempted here. I t is of interest t.o nobe, hovever, t,hat the new values for the ahundance of europium (approximately 0.08, Si = lo6) are much closer t,o the Suess and Urey value of 0.187 than t o the solar spectra value of 0.6 (1). As in the case of scandium, the solar value is subject to considerable expcriment,al unrert,aint,ies and the best value for t,he cosmic ahundanre of europium is cert,ainly dcrived from t,he new data of Table 1. . 56
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Journal of Chemkol Education
As mentioned in the prrviims section, the element,.; mercury, thallium, lead, m d bismuth are all in much lower abundanrcs in the chondrites than can hc explained by the current theories for element synthesis. Since t,hese are all chala~philicelements, the theory of Fish, et al., (5) offers thc best explanat,ion of these Inw abundances to dat,e. The ahundances of lead, thorium, and uranium have considcrahle significance in the field of cosmochronology. Details of these applications are discussed most, recently by Kohman (dl), in this symposium, and by Fowler and Hoyle (48). Of great importmce t,o these calculations is the present. number ratio, Th23Z/U?"in the chondrites. Rate, et al., (29) list,ed a value of 3.6 for this ratio in chondrites based on their experiment,al results for thorium in fivc rhondrit,es and t,he uranium r r s u l t of Hamaguchi, el al., (22) ill four rhondrit,es. Recent,lysome new data for uranium in six chondrites has appeared (50, 51). Using these dat,a for r~ranium and the Bate, el al., data for thorium, the ratio Th232/U238 becomes approximately 3.3. I t mould appear that the best approximatioil of this ratio that, can he made a t present would he close to t,he average of the previously mentioned two values, or 3.45. Slightly different values may he obtained, depending on how t,his average is taken. Additional experiment.al determinations of this ratio vould he extremely valuable. I t has been pointed out by Hamaguchi, Reed, aud Turkevich (82) and nfarshall and Hess (28) that the amount of uranium and thorium found in chondrites is not consistent with t,he puhlished isotopic composition of t,he lead in chondrites. In fact, the new abundance data for uranium and thorium are approximately 1.5 t,o 3 times too lov. Here again, t,he explanation could me11 be loss of these elemmts h) chemical fractirm at'ion processes that are not fully underst,ood at prcsent. Summary
The analysis of meteorites has without questioil provided a great deal of information that is useful in the formulation of theories for nucleosynthesis. .\It,hough murh important meteoritic ahundanee data have been obtained since the ueed of such clat,a v a s point,ed out, by Suess and Urey ( 6 ) , precise analyses for many elements are still lacking. Among t,he most i m p ~ r t ~ aareas n t for future vork are t,he following: (1) I t would he desirable t,o obtain additional new experimental data for the ahundances of t,he element,s in of the magnitude of the contribution of t,his process in the regions of the bromine, xenon, and plat,inum ahundance peaks. Experimental work is undervay a t the University of Kentucky to determine the abn~idanresof tantalum, tungsten, iridium, and mercury in rhe region of t,he platinum ahundance peak by use of neut,ro~iactivation analysis. As noted in Table 1,new experimeutal work on osmium is underway at Argonne Xational Lahoratory, using this same t.echnique. I n all these cases previous experiment,al data are meager or lacking. (2) As discussed in this t,ext, the rat,io Th?"/U2" is highly import,ant t,o t,he aalrulat.ions involved in the field of cosmochronology. Additional abundance dat,a
on t,hese two element,^ would serve to pin down more closely the value of t,hisratio. (3) Data on the ahnndances of the chalcophilic elements in each of the t,hree principal meteoritic phases is needed in order to evaluat,ethe t,heory of Fish, et al. (5). (4) Kew data o~ the ahundance of iodine from experimental work now underway (24) will enable more accurate calculations of the time interval between the end of nucleosynthesis and t,he condensation of t,he parent. n~eteorit~ic hodies t,o the point of entrapment of the rare gases. I t is e\.ident t,hat cosmic abundance data obtained from t,hc meteoritex are based on the thesis that the chondritcs are represent.ative samples of the non-volatile matter in the solar system. The large discrepancies hetmeen some of the new met,eoritic dat,a and the ahundances reasonably predicted on, the basis of current theories of nucleosynthesis suggest that the "ordinary" c h o n d r i t , ~may ~ not be represent,ative samples for determining abundauces of certain elements. The possihi1it.y of chemiral fract,ionation processes operating prior to, or during, the condensation of the parent, rnet,eorit,ic bodies m w t be considered as a possibilit,y in the explanation of cert,ain of these data. As the tiifficulties in t,he inberpretation of solar and stellar spect,ra are resolved, some of t.he problems of translating chondrit,ic into cosmir ahm~dancesmay be eliminated. il recent paper hg Urey (43) discusses again the significance of the met,eoritic elemental abundances. In t.his paper is included t,he statement,, "We should not take meteoritic ahundnnces as more t,han an approximation to t,rue primitive mlues." In view of some of the llem analytical data, this stntement is well justified. Acknowledgment
The assist,ance of all t,hose who furnished informat,ion for this paper, especially in advance of pnhlicat,ion, is grat,efully acknowledged. Note Added in Proof
Kew analytical dat,a for selenium and t,ellurium (47) and for hurium, mercury, t,hallium, lead, bismuth, and u r a n i ~ m(49) have appeared in print, since t,he compilation of Table 1. In general, these data are in reasonably good agreement, with the values listed in this paper. I t is noted by Reed, et al. (49) that the ahundanccs of the rhalrophilir elements are much higher in the carhonarcous chondrites and enstat,it.e chondrit,es then in the "ordinary" chondrites, which are mmmonly uscd for cosmic abundance work. Indeed, these higher abundances are in quit,e good agreement wit,h values predi~t~edon the basis of theories of nucleosynthcsis. I t is suggested t,hat the carhonaceous chontlrites may he more representat,ive samples for the det.enninat,ion of msmic abundances, a t least for certain of t,he e l r m m t , ~t,han , t,he "ordinary" rhondrit,es previously uwd. Literature Cited (1) ALLER,L. H., " H m d h u ~ hder Physik," 51, 324 (1958). ( 2 ) UREY,H. C., A N D CRAIG,H., Geoch,im. et Cosrnochim. A d a , 4, 3(i-82 (10531). (33) UREI, H. C., J. Geoph?,~./2enmwh, 6 4 , 1721-1737 (1959).
38, 58 (1961). (4) A n u ~ n s ,E., A N D GOLES,6.6.. THIS JCCRNAI., (5) FISH, R . A,, GOLES,G. G., A N D ANDERS,E., Preprint (1959). ( H ) Srmss, H. E., ANI) UREY, H. C., I h v . Modern Phys., 28, 53-74 (1956). E. M., BuRsroGE, G. R., FOWLER,W. A,, A X D (7) HURBIDGE, HOYLE,F., R e w Modern Ph!/s., 29, 547-650 (1957). 181 C ~ N E R K A.~C.;. IV.. Chalk River Reoort CRL-41 i l 9 S i i .
rhim. Ada, 11, 252-262 (11157) Arla, 19, 1-4 (1960). (12) GAS?., 1'. W., Geoebim. el Coan~ochin~, H., A N D XOUIO,H., Z. ~ V I L t , ~ ~ f o r14a, ~ r h860 . , (l!)5!)). (13) \T'~YKII, (14) BATE,O. L., POTRATZ, H. A., -4SD HUIZENGA, J. R., Geoehinr. el Cosmoehini. A&, 18, 101-107 (1960). (15) ? d 0 0 R q C. B., Private communication (l!J60). A. A,, MAPPER,I)., .mn Wuoo, A. J., Anal@, 82, (18) SMALES, 75-88 (1957). (17) WARD AS^, 9. I . , Geochim. et ('asnmchinz. Acla, 10,321 (1956). (18) S C H ~ N ~ E ~ U., O L Geochim. F, et Cosrnochirn. Ada, 19, 13-1I :38 ( 1 MO). (I!!) WEBSTER,R. X., MORGAN,J. W.,A N D SYALES,A. .4., Geochim. el Cosmoehim. Aeln, 15, 150-152 11!158). 1201 (:. I,.., Private ~omrn~miclttiorl 11900). , ~,R,\TE. ,~ . . (21) SCHIXDEWULF, U., A N D T~.HLGRES, hf., Geochbri. el C0snl0chin^. A d a , 18, 36-41 (l!l(i0). A N D TURKEVICH, A,, Ge0(22) HaMaoucH~,H., REED, c;. cltim. el Cosmoehim. 4ela.. 1 2 .. 3 - 3 4 7 (195i). ( U ) STHMIP, It. A,, MUSEX,A. IT., S~FFREJIXI, C. 9 , LASCH, J. E., SIIARP,K. A,, A N D OLEHY,U. A., General Atomirs Rcport GA-1288 ( 1960). (24) GOLES,G. G., Private commllnicat~ion(I$lliO). (25) VISCEXT,E. A,, A W D CROCKE+J. H.,Geoeh,inz. el Comoc h i t . .lelo., 18.. 14:1-1-18 (19(iO). . . (21;) EHNANS,W. D., A N D HUISETIGA, J. R., Geochhn. el C o s m ehkn. Aetn, 17, 325-135 (1959). TVRKEYICH. 1271 , . REED. G. W.. KIGOSHI. I
