ISSN 1028�334X, Doklady Earth Sciences, 2011, Vol. 441, Part 1, pp. 1579–1582. © Pleiades Publishing, Ltd., 2011.Original Russian Text © Yu.A. Shukolyukov, O.V. Yakubovich, B.M. Gorokhovskii, S.I. Korneev, I.A. Cherkashin, 2011, published in Doklady Akademii Nauk, 2011, Vol. 441, No. 3,pp. 372–375.

1579

The purpose of this work is to propose a newmethod of isotope geochronology of native platinumthat has been neither described in the published worksand nor used in the studies of deposits of platinumgroup elements (PGEs).

Among the native platinum isotopes, there are twothat are α�radioactive: 190Pt and 192Pt (Table 1). Theisotope 190Pt transforms by α emission to isotopes 4Heand 186Os. The rate of this process was determined as isshown in survey publications [1, 2] by three principallydifferent methods: nuclear–physical (direct calcula�tion of emitted α particles using ionization chambers,nuclear emulsions, and tracking detectors), a semi�empirical calculation model of penetration of α�parti�cles through the energy barrier of the 190Pt atomicnucleus, and a geochemical method. The thirdmethod consists in the calculation of the rate constantof 190Pt decay by isochrone parameters in the coordi�nate system 186Os/188Os–190Pt/188Os for objects withthe age known a priori. To construct these isochrones,we used both terrestrial rocks and iron–nickel IIABand IIIAB meteorites. The scatter of weighted�averagevalues of the period of 190Pt half decay, which wasobtained by different methods, is quite large, rangingfrom (3.5 ± 0.3) × 1011 to (4.78 ± 0.05) × 1011 years.

The analysis of all data allowed us to conclude thatthe most substantiated value of the period of half decay

and the rate constant of isotope 190Pt decay is obtainedby the geochemical method. At the relative occurrenceof isotope 190Pt = 0.01296% (the most accurate mea�surement), its half�decay period T = 4.69 × 1011 yearsand the decay rate constant λ190 = 1.477 × 10–12 year–1.Precisely these values were used in this work.

Except for 190Pt there is another native platinumisotope 192Pt that is capable of α emission (Table 1).But the period of half�decay of this isotope is37 000 times more than that of 190Pt [2], and the rela�tive occurrence is only 58 times greater than that of190Pt. Therefore, we can neglect accumulation of 4Hein platinum due to radioactive decay of 192Pt. For thisreason we do not take into account α decay of radio�genic 186Os in the generation of radiogenic 4He in plat�inum.

The earlier research [3] showed that native metals,for example, gold, silver, or copper, preserve radio�genic helium almost completely during heating to themelting point and even to higher temperatures becauseradiogenic 4He exists in their crystal lattice in a specialform, as helium clusters–bubbles. This makes heliumlosses at typical lower temperatures under real geolog�ical conditions highly unlikely. Helium thermal des�orption from native platinum requires heating at veryhigh temperatures to 1700–1800°С. The history ofnative elements from a platinum group is unlikely torecord the cases of heating to such temperatures, andpreservation of radiogenic helium in crystal latticesmust be close to 100%.

Therefore, in addition to the very well�developedPt–Os method [4], there is the possibility in principle

A New 190Pt–4He Method of Native Platinum Age Determination

Yu. A. Shukolyukov, O. V. Yakubovich, B. M. Gorokhovskii, S. I. Korneev, and I. A. Cherkashin

Presented by Academician D.Yu. Pushcharovskii June 28, 2011

Received July 15, 2011

Abstract—A new method of determining the age of native platinum based on alpha�radioactivity of one of itsnatural isotopes 190Pt is proposed. Due to a special form of occurrence of radiogenic helium in the crystal lat�tice of native metals as a helium cluster–bubbles, the stability of 190Pt–4He of the isotope system is extremelyhigh in the natural environment. To check the efficiency of the proposed 190Pt–4He method of isotope geo�chronology, six independent mineral aggregates of native platinum from chromite�bearing dunites of thesouthern part of platinum�bearing zonal Galmoenan dunite–cllinopyroxenite–gabbro plutonic complex(Koryak�Kamchatka belt, Russia) were analyzed. The age calculated by the tangent of the 190Pt–4He isoch�rone slope angle equals 69.5 ± 4.9 mln years. The obtained age value coincides with the results of isotope dat�ings, which were made previously by different methods of isotope geochronology.

DOI: 10.1134/S1028334X11110304

Department of Geology, Chair of Geology of Mineral Deposits, Saint Petersburg State University, Saint Petersburg, Russiae�mail: xekrarne@gmail.com

GEOCHEMISTRY

1580

DOKLADY EARTH SCIENCES Vol. 441 Part 1 2011

SHUKOLYUKOV et al.

for creation of a new effective Pt–He�geochronome�ter based on the natural radioactivity of platinum,which is simpler technically and requires finances anorder of magnitude smaller.

In accordance with the law of radioactive decay, theconcentrations of radiogenic 4He and radioactive 190Ptare mutually related:

(1)

(2)

Since the value is small, the approximate

equation is fair

(3)

(4)

λ= −PtHe Pt( )1904 190 1 ,

te

⎛ ⎞= +⎜ ⎟λ ⎝ ⎠

Pt

190

He1

Pt

4

190ln 1 .t

⎛ ⎞⎜ ⎟⎝ ⎠

PtHe

Pt

4

190

⎛ ⎞ ⎛ ⎞+ ≈⎜ ⎟ ⎜ ⎟

⎝ ⎠ ⎝ ⎠

Pt PtHe He

Pt Pt

4 4

190 190ln 1 ,

=λ

Pt

190

He1

Pt

4

190.t

Thus, the procedure of isotope dating by the 190Pt–4He–isotope system is principally quite easy. It simplyreduces to determination of the concentration ofradiogenic 4He in the dated sample of native platinum.

To measure the concentration of radiogenic 4He innative platinum, we used the MSU�G�01�M mass�spectrum complex made by ZAO Spectron. The cal�culations show that the native platinum accumulatesthe amount of radiogenic 4He that is sufficient for reli�able measurements: in 1 year, 100 mln years, and 1 blnyears, 1.8 × 10–12; 1.8 × 10–4; 1.8 × 10–3 cm3 of 4He/g isformed in 1 g of platinum, respectively. The responseof the specialized helium mass spectrometer used inour work was 5 × 10–13 cm3 4He/pulse. The measuringcomplex and the procedure of determining the radio�genic 4He are described in [3]. The major part ofradiogenic helium, as mentioned above, must emitfrom the crystal lattice of native platinum in the samemanner as that of other native metals only within therange of temperatures close to its melting point(1780°С) under destruction of helium clusters [3].This would complicate the process of necessary com�plete extraction of helium and would require using a

Fig. 1. Reduction in temperature of total desorption (the star on x�coordinate) of radiogenic 4He almost by 300оС during forma�tion of the melt from native platinum and metal copper: 1 is Pt + Cu + Ta; 2 is Cu + Ta; 3 is Pt.

4He × 10−7, cm3/g

3

2

1

0

123

400 600 800 1000 1200 1400 1600 1800Temperature, °С

Table 1. Isotopic characteristics of native platinum (relative occurrence, %)

0.01296 0.78267 32.9672 33.8318 25.2419 7.1636

190Pt 192Pt 194Pt 195Pt 196Pt 198Pt

T1/2 = 4.69 × 1011 years T1/2 ≈ 1 × 1015 years Stable Stable Stable Stable

4He 186Os 4He 188Os

T1/2 ≈ 1 × 1015 years

Stable

4He 182WСтабильный

DOKLADY EARTH SCIENCES Vol. 441 Part 1 2011

A NEW 190Pt–4He METHOD OF NATIVE PLATINUM AGE DETERMINATION 1581

high�temperature extractor. In this study we developedand used a new method of helium thermal desorptionstimulation at a low temperature: native platinum ismelted with another lower melting metal, which istechnical especially pure copper (Тmelt = 1083°С).Figure 1 shows the displacement range of sudden,burstlike emission of helium (burst�effect) for theplatinum–copper melt towards low temperatures ascompared with the range for pure native platinum.

The native platinum may contain uranium andthorium captured during crystallization. They canproduce 4He. But uranogenic and thorogenic 4He shouldbe taken into account only if the U� and Th�concentra�tion in platinum is high, not less than ~10–5 g/g.

The dating procedure for native platinum is some�what complicated also by the occurrence of noticeableconcentrations of base metals in it, for example, ofiron with formation of ferroplatinum. In these cases,the concentrations of iron and platinum can be deter�mined by methods of electron microscopy.

To check the efficiency of the proposed 190Pt–4He–method of isotope geochronology, we studied sixindependent mineral aggregates of native platinumfrom chromite�bearing dunites in the southern part ofthe platinum�bearing zonal Galmoenan plutoniccomplex (Koryak�Kamchatka belt, Russia). This big�gest regional massif (~48 km2) belongs to the intraoce�anic paleoarc system and consists of dunites, clinopy�roxenites, and some gabbro. It has been studied indetail due to available platinum deposits [5]. As isshown in [6], the main mineral phases of intrusiverocks in the Galmoenan massif (olivine, clinopyrox�ene, and chrome spinel) are typical of a complex ofliquidus rocks in primitive island–arc magmas ofultramafic volcanic rocks from mid�oceanic regions.

According to the data measured by a JSM�6510LAscanning electron microscope with JED 2200 EMF

adaptor, the studied samples from the Galmoenanpyroxenite–dunite plutonic complex represent izofer�roplatinum of the following composition: Pt ≈ 90.8%,Fe ≈ 8.3%, Rh ≈ 0.8%.

Table 2 presents the data on concentrations ofradiogenic 4He and radioactive 190Pt in the studiedsamples of izoferroplatinum. They were used to plot aplatinum–helium isochrone (Fig. 2). It indicates theabsence of captured 4He and total preservation of

Table 2. The examination results obtained for primary izoferroplatinum from chromite bearing dunites in the Galmoenanmassif using the 190Pt–4He method

Samples Mass,mg

Pt,at. %

190Pt,at./g *

4Не,pulses

4Не,cm3/g *

4Не,at./at. 190Pt

Izoferroplatinum, Galmoenan 71 + 2.18 mgof native Cu (Lake Superior, United States)**

0.41 90.8 3.63 × 1017 365 1.43 × 10–6 0.000106

Izoferroplatinum, Galmoenan 72 + 0.92 mgof technical Cu

0.61 90.8 3.63 × 1017 552 1.46 × 10–6 0.000108

Izoferroplatinum, Galmoenan 76 + 12.05 mgof technical Cu

3.21 90.8 3.63 × 1017 2889 1.45 × 10–6 0.000107

Izoferroplatinum, Galmoenan 87 + 19.4 mgof technical Cu

1.21 90.8 3.63 × 1017 1094 1.46 × 10–6 0.000108

Izoferroplatinum, Galmoenan 89 + 43.5 mg of technical Cu

8.66 90.8 3.63 × 1017 6898 1.28 × 10–6 0.0000950

Izoferroplatinum, Galmoenan 86 + 13.1 mgof technical Cu

4.88 90.8 3.63 × 1017 3763 1.24 × 10–6 0.0000920

* – calculated for 1 g of the sample.** – the concentration of helium in native copper from Lake Superior <10–9 cm3/g.

Fig. 2. Pt–He–isochrone: the age value calculated by thetangent of the isochrone slope angle t = 69.5 ± 4.9 mln years.

t = · = · , where = 0.0000933.1λ190

�������� He4

Pt190��������� 1

λ190

�������� αtan αtan

3.0

0 4190Pt × 1015, atoms

4He × 1011, atoms

2.5

2.0

1.5

1.0

0.5

321

α

1582

DOKLADY EARTH SCIENCES Vol. 441 Part 1 2011

SHUKOLYUKOV et al.

radiogenic 4He in platinum. The age calculated by thetangent of the isochrone slope angle equals 69.5 ±

4.9 mln years. The obtained age value is comparablewith the results of isotope datings, which were madepreviously by different methods of isotope geochro�nology (Fig. 3).

Thus, a new method proposed for determining theisotope age of native platinum using the 190Pt�4He�iso�tope system is promising for the study of primary andplacer platinum group elements in a wide age rangefrom about ten million years to dating of Precambrianplatinum�bearing objects.

In this study we used data on platinum isotopecomposition from [9].

ACKNOWLEDGMENTS

This work was supported by the Russian Founda�tion for Basic Research, project nos. 10�05�00321�aand 11�05�12046�ofi�m�2011, and program no. 4 bythe Earth Science Division, Russian Academy of Sci�ences.

We are grateful to O.L. Galankina (Institute of Pre�cambrian Geology and Geochronology, Russian Acad�emy of Sciences) for platinum examination under theJSM�6510LA scanning electron microscope.

REFERENCES

1. F. Begemann, K. R. Ludwig, G. W. Lugmair, et al.,Geochim. Cosmochim. Acta 65 (1), 111–121 (2001).

2. O. A. P. Tavares, M. L. Terranova, and E. L. Medeiros,Nucl. Instrum. and Meth. Phys. Res. Section B: BeamInteractions with Materials and Atoms 243 (1), 256–260 (2006).

3. Yu. A. Shukolyukov, O. V. Yakubovich, and E. Yu. Rytsk,Dokl. Akad. Nauk 430 (2), 243–245 (2010).

4. R. J. Walker, J. W. Morgan, E. S. Beary, et al., Geochim.Cosmochim. Acta 61, 4799–4807 (1997).

5. E. Yu. Vil’danova, V. P. Zaitsev, L. I. Kravchenko, et al.,The Koryak�Kamchatka Region Is a New Platinum–Bearing Province of Russia (SPb Kartfabrika VSEGEI,St. Petersburg, 2002) [in Russian].

6. V. G. Batanova, A. N. Pertsev, V. S. Kamenetsky, et al.,J. Petrol. 46 (7), 1345–1366 (2005).

7. E. A. Landa, B. A. Markovskii, B. V. Belyatskii, et al.,Dokl. Akad. Nauk DAN 385 (6), 812–815 (2002).

8. E. G. Sidorov, Platinum Content in Basite—Ultraba�site Complexes of the Koryak�Kamchatka Region,Extended Abstract of Doctoral Dissertation in Geol.�Min�eral. Sci. (Petropavlovsk�Kamchatskii: Far�East Geol.Inst., FEB RAS, 2009).

9. K. J. R. Rosman and P. D. P. Taylor, J. Phys. Chem.Ref. Data 27 (6), 1275–1280 (1998).

Fig. 3. The comparison of the data obtained by the Pt–Hemethod for the Galmoenan massif with other isotope char�acteristics obtained for this intrusive in [7, 8] by the meth�ods mentioned: 1 is Pb–U (SHRIMP), 2 is K–Ar, 3 isRb–Sr, 4 is Sm–Nd, 5 is 39Ar–40Ar.

Formations

Dunite Harzburgite

Dunite Clinopyroxenite

Norite Kortlandite

1

2

3

4

5

80 70 60 50Ma

190Pt−4He