Journal of Nuclear and Radiochemical Sciences, Vol. 6, No. 3, pp. 249-252, 2005

Technetium in Cosmo- and Geochemical Fields
Hiroshi Hidaka*
Department of Earth and Planetary Systems Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan
Received: July 31, 2005; In Final Form: September 30, 2005
Since 98Tc and 99Tc decay to 98Ru and 99Ru, respectively, via the β− transition, the property of Tc in cosmo- andgeochemical fields can be deduced from precise isotopic studies of Ru. Through the approaches on isotopic analyses of Ru in cosmo- and geochemical samples, mass spectrometric techniques with thermal ionization massspectrometry (TIMS), inductively coupled plasma mass spectrometry (ICP-MS) and secondary ion mass spectrometry (SIMS) have been effectively used. I report herewith the following three topics about the Ru isotopic
studies related with the behavior of Tc in cosmo- and geochemical fields: (1) Initial abundance of Tc in the solar
system deduced from the Ru isotopic compositions in meteoritic samples, (2) geochemical behavior of fissiogenic
Tc in natural fission reactor zones at the Oklo uranium deposit, and (3) a possibility of cosmic-ray produced Tc in
terrestrial molybdenite ore bodies.

Mo-100Ru. Through the detection of double-beta-decay products, a possibility of Tc production by cosmic-ray irradiation on
Mo matrices would be discussed from the Ru isotopic data.
100Introduction
Physicochemical behavior of extinct nuclide Tc in naturalenvironments at planetary surface is one of major concerns forgeochemists. Although the properties of metallic Tc under
reducing conditions and at high temperatures and pressures arepoorly constrained, Tc has the potential to escape to the environment owing to volatility (HTcO 4 and Tc2O7) and aqueous
mobility in soil systems (TcO4 −). Tc has no stable isotopes, but
two isotopes, 98Tc and 99Tc, have relatively long half-life, 4.2 ×
106 and 2.1 × 105 years, respectively. Since 98Tc and 99Tc decay
to 98Ru and 99Ru, respectively, via the β− transition, the behavior of Tc in cosmo- and geochemical fields can be deduced
from precise isotopic studies of Ru. Elemental abundances ofRu in cosmo- and geochemical samples are generally lower
than a few ppb, because Ru is a highly siderophile element
which is incompatible with silicate materials. Therefore, preconcentration and purification of Ru are required for the mass
spectrometric works. Recent development of mass spectrometric instrumentation and technical advance of separation chemistry make it possible to more sensitively and precisely
determine Ru isotopic composition than before.
I report herewith current three topics of Ru isotopic studies
related with the behavior of Tc in cosmo- and geochemical
fields. First, the Ru isotopic measurements in meteoritic samples were performed for the purpose of estimation of the initial
Tc abundance in the solar system. The development of 98Tc-98Ru
and/or 99Tc-99Ru chronological systems may provide important
information on the chemical evolution process of the solar system. Next, the geochemical behavior of Tc was deduced fromthe isotopic variation of fissiogenic Ru components of natural
fission reactors at the Oklo uranium deposits, the Republic ofGabon. Since 99Tc is highly produced by fission reactions anddecays to 99Ru, geochemical differentiation between Tc and Rucauses the isotopic variations of 99Ru. The Oklo natural fission
reactor at the Republic of Gabon is the best sample to study thelong-termed behavior of 99Tc in and around the uraninite ore.
The knowledge from the Oklo study is usefully applicable toradioactive waste disposal in geosphere. The third approach tomeasuring the Ru isotopic compositions in old terrestrial molybdenites is based on the cumulative effects of double beta decay*Corresponding author. E-mail: hidaka@hiroshima-u.ac.jp.
FAX: +81-824-24-0735.

Elemental Abundance of Tc in the Early Solar System
In order to estimate the initial abundance of Tc in the solar
system, precise isotopic analyses of Ru in several meteorite
samples have been performed.1−3 Quantitative isotopic treatment of 98Tc-98Ru and 99Tc-99Ru decay systems may provide
chronological information on the chemical evolution processesof planetary materials in the early solar system. However, theRu isotopic compositions of primitive materials in the solar
system have been affected probably by extra addition of r- ands-process nucleosynthetic components,4 and therefore Ru isotopic deviations due to Tc decay have not been clearly found inany meteorite samples yet. The lack of Ru isotopic deviations
due to Tc decay in meteorites (iron meteorites and chondriticmeteorites) suggests low abundance of Tc (98Tc/ 96Ru < 2 × 10−5
as upper limit) in the solar system.3
Here I focus on the Ru isotopic compositions of metallic
grains in carbonaceous chondritic meteorites. Considering therefractory and highly siderophile properties of Tc, metallic fractions in carbonaceous meteorites may have a great advantage tosearch for isotopic evidence of initial Tc in the early solar system. Four kinds of carbonaceous chondrites, Murchison (typeCM2), Isuna (CO3), Maralinga (CK4) and Karoonda (CK5)
were used in this study. One gram of each sample was crushed
in an agate mortar. Metallic fraction was magnetically removed
from the powdered sample. Each sample was divided into twoportions for the determination of Ru content by isotope dilution
mass spectrometry (IDMS) and for the Ru isotopic measurement. The metallic fraction (0.03–0.10 g) was weighed and dissolved by 5 mL of aqua regia. The Ru in each sample waschemically separated by a distillation method. For IDMS, a99
Ru spike solution was added into each sample solution prior tothe distillation process. The isotopic analyses were performedby TIMS. The details of separation procedures and mass spectrometric techniques are described in the previous paper.5
Measurements via IDMS resulted in Ru concentrations of24.31 ppm for Isuna, 14.83 ppm for Maralinga, and 44 ppm for
Murchison. The IDMS analysis for Karoonda was failed. It
should be noted that the Ru concentrations of individual magnetic fractions are more than 10 times higher than those of the© 2005 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/28/2005

250 J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005
98

Ru/ 101Ru

99

Hidaka99

Ru/ 101Ru

Murchison
Isuna
Maralinga
Karoonda
STD (this study)
STD (ref (1))
STD (ref (4))
STD (ref (6))
STD (ref (7))
STD (ref (8))
STD (ref (3))
0.1093 0.1094 0.1095 0.1096 0.1097

0.74700.74750.7480Figure 1. The 98Ru/101Ru and 99Ru/101Ru isotopic data for metallic
fractions of individual meteorites and standard materials. Shadedzones in the figure represent standard data from this study: 98Ru/101Ru= 0.109529 ± 0.000014 and 98Ru/101Ru = 0.747552 ± 0.000044. The
analytical uncertainties are 2σ of the mean.

carbonaceous bulk components.
The isotopic results are shown in Figure 1. Judging from highsensitivity of TIMS analysis, more precise analytical results
were expected. However, the analytical precision in this study
was not good enough to make a critical discussion about thepresence of isotopic excesses on 98Ru and 99Ru in the individualsamples, because the quantities of Ru treated in this study wereextremely low. As shown in Figure 1, all of the data from meteorite samples overlap with those of standard materials1, 3, 4, 6−8
within analytical precisions indicated by 2σ of the mean.
Further analysis is required to discuss about the initial abundance of Tc in the early solar system from the Ru isotopic data.
Fissiogenic Tc in the Oklo Natural Reactors
Technetium-99 is highly produced by fission and turned toRu via β− decay. Kenna and Kuroda found natural 99Tc produced by 238U spontaneous fission in a pitchblende sample.9
One rare example that Tc behavior can be studied in terrestrial
field is natural fission reactors at the Oklo and Bangombe uranium deposits at the Republic of Gabon. Large-scaled fission
chain reactions spontaneously occurred, and significant
amounts of fissiogenic isotopes were produced in the natural
reactors around 2.0 billion years ago.10 Isotopic studies of thenatural reactors provide long-termed behavior of fission products in and out of uranium matrices.11 All of fissiogenic isotopes produced in the reactors completely decayed to stable
isotopes at present. Isotopic analyses of Ru for the Oklo samples have been also carried out in order to study geochemical
behavior of fissiogenic Tc.12−15
Four of seven Ru stable isotopes, 99Ru, 101Ru, 102Ru and 104Ru,
are produced by fission. In the natural reactor samples, only
four fissiogenic Ru isotopes and 100Ru produced by neutroncaptured 99Ru can be detected because of high fission productyields of Ru and extremely low abundance (in general, less thanppb) of non-fissiogenic Ru in terrestrial crustal samples. TheRu isotopic compositions of natural reactor samples areexpected from the fission product yields of 99Ru, 101Ru, 102Ru
and 104Ru. However, significant variations of 99Ru isotopic
abundances have been often observed in the natural reactor
samples.12−15 In particular, micro-metallic grains consisting
mainly of Pb, S, Ru, Rh and As found in the reactor samples
show a large excess of 99Ru.15 This fact suggests the chemical
fractionation between Tc and Ru during and after fission reactions. Since 99Ru experienced long-lived 99Tc as a radioactive
precursor, differentiation between Tc and Ru can be presently
seen as enrichment or depletion of the 99Ru isotopic abundance.
Isotopic excesses of fissiogenic 99Ru is defined as the following parameter ∆99Ru:
99143(99Ru/101Ru) measured− 1 × 100,(99Ru/101Ru) calculated

143∆99Ru(%) =101

99101

where ( Ru/ Ru) measured is the Ru/ Ru isotopic ratio of eachsample, and (99Ru/101Ru) calculated is the 99Ru/101Ru isotopic ratio
calculated from fission product yields of 238U, 239Pu and 235U.
Table 1 shows ∆99Ru values resulted from the Ru isotopic
measurements of several kinds of the Oklo samples. The positive ∆99Ru value means the enrichment of Tc relative to Ru in
the sample, while the negative one means the depletion. The
difference of physico-chemical properties between Tc and Ru
finally causes such isotopic variation of 99Ru. It is consideredthat both Ru and Tc are easily mobile when they form oxide
compounds such as RuO4 and Tc2O7. As shown in Table 1, the∆99Ru values vary in uraninite matrices. Uraninite is generally
stable under reducing condition, because U(IV) is much more
insoluble than U(VI). Therefore, the uraninite ore body might
have kept stable in reducing condition. However, in a rare case,some minerals which form under a strongly oxidizing condition,such as minium (Pb3O 4) coexist in uraninite marices.16 The
presence of a minium shows evidence for a local oxidizing condition even in uraninite. As a reason of making the local oxidizing condition in uraninite, radiolysis of water is reasonablyconsidered. The oxygen arising from radiolysis of water waslocally accumulated, and played a role in changing the oxidizing
conditions in uraninite. In this situation, fissiogenic Tc and Rumight have been locally mobile and have differentiated witheach other. The variation of ∆99Ru in several uraninite samples
also supports the presence of local oxidizing condition due towater radiolysis in uraninite.
TABLE 1: ∆99Ru Values of Various Kinds of the Oklo
Samples
Sample∆99Ru

Whole rockGL13-3477
GL13-3492
GL13-3522
GL13-3535
SF84-1469
SF84-1480
SF84-1485
SF84-1492
SD37-S2–15.3+4.7+103.6–41.2+4.41+2.82
–5.15
–1.21
+11.6

Metallic aggregate
SD37-S2/CD.2
SD37-S2/CD.3
SD37-S2/CD.5+85.9+49.9+118

Data are from Hidaka et al.14, 15

Figure 2 illustrates the comparison of Ru isotopic abundance
patterns between metallic aggregates and the whole rock takenfrom one of the Oklo reactor zones. The Ru isotopic compositions of all of the metallic aggregates show positive ∆99Ru(+49.9 to +118) much larger than that of the whole rock (+11.6).
This is caused by the enrichment of 99Tc in the metallic aggregate because of differentiation between Ru and Tc, suggestingthat Tc has a stronger chalcophile property (chemical affinitywith sulfur) than Ru.
Cosmic-ray-induced Tc from Mo in Old Terrestrial Molybdenite (MoS2)
Double beta decay is known as the rarest nuclear phenomenon havi ng ext remely long ha l f-l ife over 10 18 yea rs.
Geochemical analysis of isotopic excesses in geological materials caused by accumulation of the decay daughter isotope is a
useful method for detection of the double beta decay products.

J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005 251

Technetium in Cosmo- and Geochemical Fields
2.5

Isotopic abundance (atom.%)

Isotopic ratio

2.0

whole rockinclusion-1
inclusion-2
inclusion-3

1.5

1.0

0.5

04030
20
10096/101

98/101

99/101

100/101 102/101 104/100

Figure 2. The Ru isotopic patterns of metallic aggregates and wholerock samples taken from the Oklo reactor zone 13 SD37-S2.

The half-life of 100Mo-100Ru double beta decay has recently
been determined from a 100Ru isotopic excess in geologicallyold molybdenum minerals.17 However, the half life of 100Mo
estimated from the present geochemical analysis, 2.1 × 1018
years, is a factor of 4 lower than that from direct counting measurements.
Geologically old molybdenite is a suitable mineral for thisstudy. The samples were collected from the molybdenite ores
at Mt. Mulgine (Western Australia) and Osbourne (Queensland,Australia). The crystallization ages of the two minerals datedby 187Re-187Os method were (2.903 ± 0.004) × 109 years for Mt.
Mulgine and (1.051 ± 0.001) × 109 years for Osbourne. In addition, an extremely low abundance of primordial Ru content in
the molybdenite (50 ppt for Mt. Mulgine, 22 ppt for Osbourne)
is a great advantage for isotopic detection of 100Ru produced by
the double beta decay.
There are some possible sub-reactions to provide Ru isotopic
anomalies in Mo matrices through the Tc production by muoninduced and proton-capture reactions of Mo and spontaneous
fission of 238U. Fissiogenic Tc from spontaneous 238U can be
estimated to be negligible because of low U contents (<10 ppm)
in the molybdenites. In previous work by Cowan and Haxton18,
the amount of muon-induced Tc in a molybdenite ore was estimated. On the basis of their concept, the amount of muoninduced 100Tc, and therefore 100Ru, is simply calculated to be lessthan 90 pg for a 3 billion-years old sample with 6.7 m3 volume.
This result shows that the amount of muon-induced products isnot enough to produce the isotopic excess of 100Ru. Of moreconcern are the proton capture of 100Mo(p,n)100Tc and the subsequent decay of 100Tc to 100Ru. It should be noted that proton capture reactions on Mo would also involve the 98Mo isotope andproduce 98Tc by 98Mo(p,n) 98Tc. The production rates can bedefined by ψσN; where ψ is the proton fluence from cosmicrays, σ is the cross section for proton capture reaction, N is thenumber of target isotopes. Considering the proton capture crosssections19 and isotopic abundances for 98Mo and 100Mo, the production rate of 98Tc is estimated to be 2.8 times higher than thatof 100Tc. However, the Ru isotopic compositions of two terrestrial molybdenites show significant isotopic excesses only in100
Ru. Figure 3 shows the Ru isotopic abundance patterns oftwo molybdenite, Mt. Mulgine and Osbourne, used in this study.
If the differentiation between Tc and Ru occurred in the molybdenite ore bodies, it may be possible to produce an isotopic
excess of only 100Ru. The half-life of 98Tc (4.2 × 106 years) ismuch longer than that of 100Tc (15.8 s). The half-life of 98Tc islong enough for occurrence of chemical differentiation between
Tc and Ru by geological processes. However, the molybdenite
samples used in this study do not show visible evidence forSTDMt. Mulgine
Osbourne

100969899100101102104

Mass number
Figure 3. The Ru isotopic patterns of two molybidenite, Mt. Mulgineand Osbourne. Error bars in the figure are 2σ of means. The data arenormalized at 96Ru and 101Ru for correction of instrumental mass fractionation.

alteration and weathering. In conclusion, it is implausible toproduce isotopic excess of only 100Ru by interaction of cosmicray with molybdenite. Therefore, the production rates of Tc in
Mo matrices under natural circumstances are too small to detectby isotopic measurements.
Summary
Isotopic measurements of Ru by mass spectrometric approaches were made to understand the long-termed behavior ofTc in cosmo- and geochemical fields. In particular, the samples
containing enough amount of Tc, like natural reactors, showedsignificant isotopic variations of Ru due to Tc-Ru differentiationin association with geological activities. The existence of micrometallic inclusion containing a large amount of fissiogenic Ru
and significantly positive ∆99Ru reveals that siderophile propertyof Tc is higher than that of Ru. More precise analysis is requiredfor determination of the initial Tc abundance in the solar system,
because current isotopic data from meteorite samples provide no
information of initial Tc abundance in the solar system. Tc-Ru
chronological system may put a temporal constraint on evolution
processes of early planetary bodies such as condensation andevaporation of dust grains, metal-silicate segregation, core crystallization, and aqueous alteration. Ruthenium isotopic datafrom old molybdenite ores showed isotopic excesses of 100Ru
due to double beta decay 100Mo-100Ru. There is no evidence forcosmogenic Tc in the samples.
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