Elemental and isotopic abundances of molybdenum in some meteorites

&ochimitx etcoSmochlmlca Acta,1968, Vol.27,pp.1171 to1178.Pergamon PIWELtd. Printed InNorthern Ireland

Elemental and isotopic abundances of molybdenum in some meteorites V. RAMA MURTHY School of Science and Engineering, University of California, San Diego La Jolla, California (Received 11 March 1963;

in retiedform

7 May

1963)

AM&---The isotopic composition of molybdenum in eight iron meteorites and one stone meteorite, aa well as the isotopic composition of zirconiumin one stone and one iron meteorite, are reported. Molybdenum and zirconium in some meteorites are similar in their isotopic abundances to terrestrial samples. The anomalies previously reported in the other meteorites are considereddoubtful becauseof some observedmass fractionationduring analysis, and because of the fact that the anomalous MO in Aroos meteorite was not found in another sample of ‘Aroos taken about 6 cm from the first sample. The elemental abundanceof molybdenum in seven iron meteoriteswas determinedby isotope dilution method. The concentration of molybdenum is found to range from 6.2 to 7.4 ppm in all types of iron meteorites investigated. INTRODUCTION INVESTIOATIONS of the isotopic abundances of elements in meteorites supply importtlnt clues to our understanding of the history of the solar system in mstters related to the chronology of various events and the nature and details of some nucleosynthetic processes. At present our information on the subject of isotope abundances of elements in meteorites is somewhat meagre. Particularly for the steble isotopes of heavy elements our knowledge is confined, with a few exceptions to those elements which customarily have been used for age determination purposes. It is often assumed that the isotope abundances of hesvy elements in meteorites are similar to those in the earth except where a distinct process, such as irradiation by cosmic rays or radioactive decay, has sltered suoh abundances. Recent studies in the field of isotopic investigation in both iron and stone meteorites have indicated that some elements rareanomalous in their isotope abundances in meteorites oompared to the es&h. The discovery by REYNOLDS (1960 a,b) of exoess XelBo as well as the general anomalous nature of the other isotopes of xenon in meteorites stimulated determinations of the isotopic compositions of other elements. MURTHY (l.960, 1962ct)reported an excess Ag107in three iron meteorites and general anomalies in barium isotope abundances, similar to those of xenon isotopes, were reported by UYEMOTO (1962). The presence of excess Xe’a and and Ag107have been ascribed by their investigators to the decay of extinct shortlived radioactivities, Ilag and Pdls7 respectively. This information was used to construct model-dependent chronologies of events in the early history of the solar system under varying assumptions as to the mode of generation and content of extinct radioactivities in primitive solid bodies (FOWLER, GREENSTEIN and HOYLE, 1962) or in the interstellar medium out of which the solar system formed (CAMERON, 1962; MURTHY and UREY, 1962). The existence of nuclear proaesses invoked in

7

1171

the above models can conceivably by tested by a search for anomalies in other elements, and therefore a general program of isotopic investigation of some elements in meteorites was initiated in this laboratory. The implications of the isotopic compositions of some rare earth elements 3n meteorites and the earth to the model of the early solar system proposed by FOWLEB;et ak. (l962) have been discussed elsewhere (MURTB[Yand SCHBZITT, 1963). In an earlier short note, MURTHY(1962bf reported some preliminary investigations on molybdenum isotopes in some iron meteorites in which it was noted that some meteorites contained anomalous MO while others did not. This paper describes and discusses further detailed work on the isotopic composition of molybdenum in eight iron meteorites, one stone meteorite and terrestrial samples> and the elemental abundance of MO in some iron meteorites. The choice of molybdenum for this investjgation stemmed from the following reasons: (1) it has a large number of isotopes and is close to Ag in its mass range; (2) it is markedly aiderophile whereas the other elements for which the anomalies are reported are mostly lithophile; and (3) the isotopes Moe5 and MoQ6have some resonance peaks in absorption cross section for neutrons of about 100 eV range, so that variations in the ratio Moe6/MoQsmay be expected, if the meteorites were samples of material subjected to strong neutron irradiation during the early history of the sofar system. E~~~~~~~~T~~ Microgram quantiti8s of molybdenum were extracted from 3-7 g samples of iron meteorites by very simple ion-exchange techniques. The meteorite samples were pre-cleaned by etching their surfaces with warm 6 M HCl, and then dissolved in about 20 ml of 6 M HCI to which a few drops of cone. HNO, were added. The solution was taken to complete dryness and redissolved in about 20 ml of 6 M HCX. The insolubles were centrifuged and the supernate was transferred to a 100 ml ,TeBon beaker and taken to complete dryness. The dry residue was dissolved in O-5 M HCl-1-O M HF solution and was put an a Lli-20 cm long anion exchange column in a polyethylene tube. Before loading the sample, the anion exchange resin was equilibrated with @5 M HCl-1-O M HF acid by washing it with 2 column volumes of this solution. After the sample was loaded the column was washed with 3-4 column volumes of the acid solution, Under these conditions, MO along with W is absorbed while the majority of the other elements are not absorbed and pass through the column (&AuS, NELKIXand MUOR~~1955). When the effluent from the column is colorless, the column is stripped with 1-5 M HCl and about 30 ml of this effluent is collected in a Teflon beaker. This solution is evaporated to complete dryness and is processed through a second similar anion exchange column, but of 5-8 cm in length. The final effluent of I. M HCl is collected in a Teflon beaker and evaporated to dryness. The dry mass in the Teflon beaker was dissolved in a few drops of f N H&Q and transferred to 1 ml centrifuge tube. The centrifuge tub8 is warmed and Mo is ~~c~pita~ as MO& by passing H,S gas and allowing to stand under tight cover for at least 6 hr. The MoS, precipitate is centrifuged and transferred to the tantalum filament of the mass spectrometer by a transfer pipette,

Elemental and isotopio abundances of molybdenum in some meteorites

1173

The isotopic analysis was carried out on a Nuclide Analysis -Associates 12 in. radius 60” sector field single focusing solid source mass. spectrometer, using a single tantalum filament. By slow heating in vacuum, the MoS, on the filament was decomposed; then at much higher temperatures, stable and very slowly decreasing ion beams &MO+ were obtained. Ion currents of the order of lo-l4 A were detected with an electron multiplier and corrected for multiplier discrimination by applying a square root of mass ratio factor (INUHRAM, HAYDEN and HESS 1953). In general, about 15-40 sets of ratios were measured by repeated magnetic Because of the large number of isotopes, the mass scanning of the mass-spectrum. spectra were divided into two groups consisting of four isotopes each and then normalized to a common. isotope to give the total isotopic abundances. This procedure was followed to minimize the effects of instrumental drift, if any, during the time of measurement of the ratios. During each analysis, prior to the actual measurement of-the isotope ratios, a scan was made from about mass 40 to well over mass 150 to search for any interference by contamination. There were no visible peaks at the sensitivity used, which indicated thatimpurity contribution from background, if any, was less than about one part in 560. As this error is significantly much smaller than other errors in the measurement described later, it is considered unimportant. However, unfortunately no high sensitivity scans of background were made after the measurement of the ratios. If the background interference’has grown during the measurement, its extent remains undetermined. The error limits.for the isotopic analyses given in this paper are about &-O-S per cent of each ratio. This.is approximately equal to two standard deviations for any given ratio. An additional error is introduced in each analysis due to mass The consistency and extent of this error fractionation in the mass spectrometer. are discussed later. Elemental abundance of MO in iron meteorites was determined by the isotope dilution method using enriched MolOOspike. A known quantity of the spike was added to the weighed sample at the beginning of the dissolution step and procedures of extraction of MO exactly similar to those described earlier were employed. DATA AND DISCUSSION The isotopic composition of MO in Baker and Adamson analytical grade reagent MoCl, and a sample of molybdenite from the ore deposits at Butte, Montana, given in Table 1 were used as terrestrial standards. Molybdenum from these samples was extracted and subjected to the same chemical procedures as molybdenum from meteorite samples. These values are different from the isotopic abundance reported by WILLIAMS and YUSTER (1946) which were used in G. E. Chart of Nuclides 1961 edition. WILLIAMS and YUSTER’S determinations were made by a gas-source mass spectrometer using Mo-carbonyl vapour and electron bombardment for ion production, whereas the present experiments were made by thermionic emission of a sample applied to a tantalum filament. Although no effort was made to determine the absolute abundances of MO isotopes in the present study, the differences between the present data and that reported by WILLIAMSand YUSTER may be real and suggest the need for a new determination of the absolute

1174

V. RAWAkkmrai~ Table 1. Izotopia composition of molybdenum from te~&rial Da%

S&r@6

92/100

94/100

m&xi&

95~100

95/100

971100

S%/lOO

June 12,196l Sept. 24, 1961

Reagent MoC4 1.55 Reagent JkEoC& I.53

0*958 0.955

1.66 1‘66

I.74 I.74

O-993 1.74

2.52 2.52

Oct. 25, 1961

Molybdenite, Butte, Mont.

I.54

0,959

1-66

1.74

0.994

2.84

I.54

0,957

1.66

1.74

0.993

2.53

I.64

0.938

1.63

1.72

0.983

2.47

Average TerrestrialMO (%LLIABB and YW-R 1946)

Table 2. Isotopic composition of molybdenum in some meteorites* Date June 15,196l June 20, 1961 Aug. 14, 1961 Aug. 16, 1961 Aug. 17, 1961 Sept. 15, 1961 Sept. 18, 1961 Sept. 21, 1961 Sept. 24, 1961

Sept. 28, 1961 Sept. 30, 1961 Oct. 5, 1961 Oct. 10, 1961 Oct. 11, 1961 Oct. 15, 1961

S&mple

92/100

Canyon Diablo Toluoa Sikhote Alin Grant sant& Luzia-I Amos-I Odessa Santa Luzifb-II Amos--I (after purifkation) Aroos-xI Forksville Deep Springs-I Aroos-III (6 cm apart) Aroos-Iv (with inclusions) Deep Springs-II

94/100

95/100

QS/lOO 971100

98/100

1.54 1.55 l-54 1.54 I.50 1.44 l-51 I.50 1.42

0.959 0,966 0.971 0.957 0.944 0.912 0.943 0.941 0.901

1.66 1.65 1~65 I.66 l-63 1.58 I.64 1.63 1.57

1.73 1.74 1.73 1.74 f-71 1.67 1.72 1.72 1.66

0.993 0~991 0,988 0.992 0.989 O-967 0.985 0.985 O-961

2.52 2.52 2.52 2.53 2.50 2.48 2.52 2.52 2.47

l-45 I.53 I.48 1.53

@918 0.956 0.929 0.961

1.59 I,65 l-61 1.64

I.67 1.73 1.70 1.73

0,965 O*QQO 0.974 0.991

2.48 2.52 2.49 2.52

l-54

0,963

I.65

1.74

0.989

2~53

1.49

0.932

1.62

1.70

0.975

2.49

* The can&&al error for each of these ratios is t&en to be 10.6 percent of e&ohratio, a8 defined by the interval of twice the standard deviation.

relative abundances of MO isotopes in terrestrial materials. For purpose of oomparison between meteorites and terrestia;l materials, the v&es reported in this paper fox terrestrial molybdenum were used. In Table 2 are listed the isotopic compositions of molybdenum in eight iron meteorites and one stone meteorite. Of these samples, four iron meteorites, Tolucs, Grsnt, Canyon Diablo and Sikh&e Alin, contain entirely normal molybdenum and no differences exist between the isotopic ~orn~sit~o~E of molybdenum in these meteorites and the earth. In particular, no speak% or general anom&lies are observable in the isotopic ratios.

Elemental and isotopic abundeuctas of molybdenum in some metaoritie

1176

The anolamous isotopic composition of MO in Aroos and Deep Springs meteorites as well as possibly in Odessa and Santa Luzia meteorites has been reported in a previous short note (MURTHY,1962b). In these meteorites, the lighter isotopes of MO seem progressively depleted relative to the heavier ones. This is shown graphically in Fig. 1 where using the notation that Hn/H,,, is the abundance of an isotope of mass M normalized to the heaviest isotope Mol00, the ratio (Hn/ Hloo)met: (Hrd/H1oo)terris plotted against the isotope mass M.

RELATIVEAMMANGE

OF

uoLreoEllllMlsoTopEs OS-

97

98

Fig. 1. Relative abundances of MO isotopes

in four

92

94

95

96

M-

loo iron meteorites.

The trend of the curves in Fig. 1 suggests offhand that a mass fractionation prooess in the mass spectrometer may have been responsible for these MOanomalies. To verify this, two experiments were made to determine the extent of mass fractionation in the mass spectrometer during an analysis. In one a sample of terrestrial MO, and in the other a sample of meteoritic MO were loaded on the tantalum filament and the change in the ratio Mosa/MoloOwith time in the mass spectrometer was observed up to about 20 hr, leaving the sample at operating temperature. The variation of Mos~/MolOOwith time is shown graphically in Fig. 2. It can be seen from this that the total change in the Mosa/MolO’Jin 20 hr was about 1.5 per cent of the ratio and is approximately linear with time. As all the meteoritic samples were run in less than half of this time (about 6-8 hr), it was felt that fractionation during analysis may not be the cause of these anomalies. There is of course the possibility that irregular fractionation can occur in some way and although the meteorite runs were made in times short compared to the fractionation experiments, substantial fractionation of the meteorite samples before the “emission period” occurred. Such an irregular fractionation has now come to be well known in mass spectrometric studies of Sr, an element close to MO in its mass range. Because of the complex nature of processes during surface ionization which are not understood completely, it is hard to rule out the above possibility with any

V. RANA MURTEY

1176 I55

1

I

I

0

TERRESTRIAL

.

METEORITE

MO

SAMPLE

MO SAMPLE

I 54 -

I

d”

0

5

20

I5

Fig. 2. Variation of Moga/MolWwith time of baking in the

m&s8spectrometer.

Table 3. Isotopic composition of zirconium in some meteorites Sample 1. Terre&rid 2. Fork&& (Stone) 3. Aroos (Iron)

9Oi96

91196

92196

94/96

18-4 l&O 18.3

4.01 3.94 4.01

6-11 6.07 6.11

6.21 6.16 6.16

certainty, even though the two fractionation experiments seem to point to consistent time dependent fractionation. The two samples of Aroos that showed anomalous MO exhibited curious chemical behavior. In both of these samples, considerable amounts of alkraline and rare earth elements were detected during the extraction of MO, rendit was necessary to use further chemical steps than are normally used so as to avoid these elements, before the mass spectrometrio analysis of MO was done. Zirconium, which was one of the impurities in Aroos, was separated, purified and isotopically analysed. The isotopic composition of Zr in terrestrial material, the stone meteorite Forksville and the iron meteorite Aroos are given in Table 3. It can be seen from this table that although the MO in Aroos iron was anomalous, Zr from the same sample was similar in its isotopic composition to that of terrestrial materials. The isotopic composition of Zr in these two meteorites, one stone (~orks~lle) and one iron meteorite (Aroos) is similar to that in terrestrial materials. FEITICNECHT, HERR and HOFFMEISTER (1962) measured the isotopic composition of Ru in some iron meteorites and find it similar to terrestrial materials. Although three analyses of two independent samples of Aroos showed anomalous MO, we are puzzled by the fact that & third sample of Aroos meteorite, taken about 6 cm apart from the first two samples, contained normal molybdenum. Beoause of the fact that the Aroos sample with anomalous Xo also contained significtantamounts of strongly lithophile elements like the alkaline and rare earth elements, it was felt that perhaps the anomalous MO resides in Bornesilicrtte phase

Elemental and isotopic abundanoeaof molybdenum in Bornemeteorites

1177

which occurs as inclusions in the iron meteor&e. We have sub~quently chosen samples from Aroos which appeared to contain inclusions and analysed the Mo in them, but so far, these searches to find the anomalous MOin Aroos have not been successful. Thus, we have neither been able to locate the exact phase with which the anomalous Mo is associated, nor &tribute the anomltlies with any certainty to mass fractionation or contamination during analysis, Unsystematio fractionation during measurement or chemical extraction may be responsible for these anomalies, and for the present the author oonsiders them doubtful. The iuvestig&ion is being extended to other iron and stone meteorites. Table 4. Eiemental abnndances of molybdenum in iron meteorites Sample TOlUCS

Canyon Diablo cmnt Silkhote Alin Santa Luzia ChtWCrtS

Aroos

Concentrationof Mo (ppm) 64 6-3 7-4 6-9 6-2 79 7*1t

* Isotopic compo&ion assumed to be similar to tarree.trial. t Isotopic ratio of MaQ8iMo100 a~ given in Table 2 is used in the cali4ation of concentration.

If real, however, the ~stribu~on of these anomalies of MO in some meteorites reseinblea the situation of the occurrence of leads of dissimilar isotopic composition found in a single meteorite (STARIK, SOBOTOVITCH, LOVTSYWS,SHATSand LOVTSPUS, 1959, 1960; STARIK and SHATS, 1960). The nature of the anomalies of Mo, h&ever, belong to the class of “general anomalies” described for the isotopes of xenon and barium, for which there is at present no unique or satisfactory expla~tionThe elemental abundances of iXo in seven iron meteorites were determined by isotope dilution techniques, and is found to range from 6-Z to 7.4 ppm. The data is shown in Tabls 4. KURODAand SANDELL(1954) determined the abundance of Mo in some stone meteorites as well as in the metal and troilite phases of stone meteorites, using ctalorimetric techniques. Their average of 8.0 ppm for the metal phase of stone meteorites agrees well with the present values for iron meteorites. However, the present values for Canyon Diablo are distinctly less than their 17 ppm for the same meteorite. The abundance of Xo in iron meteorites seems to fall in a narrow range. Compared to the abundance of 1-2 ppm in stone meteorites (KURODAand SANDILL, 1954) the present abundance values of Mo in iron meteorites confirm the expectation that Mo is siderophile in meteorites. am grateful for the benefit of many disoussions with ProfessorerHc. C. G.G. GOLES. The JaboratoryassistanceO~&CHARD A.Coxisacknowledged. This work W&Bperformed under a contract with the Atomic Energy C?ommission. Aohowledgements-4

TJREY,J.R.ANRoLD~~~

1178

V. Rm

&&~&THY

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