geothermochronology based on noble gases: iii. migration of radiogenic he in the crystal structure...

ISSN 0869�5911, Petrology, 2012, Vol. 20, No. 1, pp. 1–20. © Pleiades Publishing, Ltd., 2012.Original Russian Text © Yu.A. Shukolyukov, O.V. Yakubovich, S.Z. Yakovleva, E.B. Sal’nikova, A.B. Kotov, E.Yu. Rytsk, 2012, published in Petrologiya, 2012, Vol. 20, No. 1,pp. 3–24.

1

1 INTRODUCTION

The group of native elements includes the mineralsthat consist of atoms of a single element; they occurrarely and account for only 0.05% of the Earth’s crust.Nonetheless, some of them are very important. Amongthem are diamond, graphite, sulfur, arsenic, antimony,selenium, tellurium, and some other elements, as wellas native metals discovered in nature: magnesium, alu�minum, copper, zinc, mercury, iron, silver, lead, tin,bismuth, gold, platinum, palladium, rhodium,osmium, iridium, indium, chromium, cadmium, tung�

1 This paper continues a series of contributions, includingYu.A. Shukolyukov, M.M. Fugzan, I.P. Paderin, S.A. Sergeev,and D.P. Krylov, “Geothermochronology Based on NobleGases: I. Stability of the U–Xe Isotopic System in Nonmetam�ict Zircons,” Petrology 17 (1), 1–24 (2009); and O.V. Yakubov�ich, Yu.A. Shukolyukov, A.B. Kotov, S.Z. Yakovleva, andE.B. Sal’nikova, “Geothermochronology Based on NobleGases: II. Stability of the (U–Th)/He Isotope System in Zir�con,” Petrology 18 (6), 555–570 (2010).

sten, cobalt, and some solid solutions (natural alloys),for instance, Au–Ag, Pt–Fe, Cu–Zn, Pb–Sn, Fe–Cr,Cu–Pb–Zn, and Cu–Zn–Sn–Pb.

Although modern isotopic geochronology makesuse of a variety of minerals for the application of vari�ous dating techniques, native metals have remainedbeyond the scope of these investigations. Only the pio�neering studies of Eugster et al. (1995) illustrated theprospects of the use of the (U–Th)/He isotopic systemfor the dating of gold in archeological objects and goldmineral deposits.

Gold is only one of the native metals. In our opin�ion, the development of (U–Th)/He isotopic geo�chronology for other metals, primarily, platinum�group elements, silver, copper, and iron, is an impor�tant scientific problem.

Although the (U–Th)/He system proved to beunsuitable for dating mineral formation processes formost of mineral geochronometers (silicates, oxides,

Geothermochronology Based on Noble Gases: III. Migrationof Radiogenic He in the Crystal Structure of Native Metals

with Applications to Their Isotopic Dating1

Yu. A. Shukolyukova,b, O. V. Yakubovichb, S. Z. Yakovlevaa, E. B. Sal’nikovaa, A. B. Kotova,and E. Yu. Rytska

aInstitute of Precambrian Geology and Geochronology, Russian Academy of Sciences, nab. Makarova 2,St. Petersburg, 199034 Russiae�mail: [email protected]

bFaculty of Geology, St. Petersburg State University, Universitetskaya nab. 7/9, 199034 RussiaReceived April 14, 2011; in final form June 28, 2011

Abstract—It was shown that the behavior of 4He in native and technical metals is very similar owing to the sym�metric and stable electron shells of its atoms, which cannot gain electrons from other atoms or donate their ownelectrons to metal atoms in a crystal lattice. Therefore, they rapidly migrate toward grain boundaries and dislo�cations, where they are released as vesicles or He clusters. It was found that the thermal desorption of radiogenicHe occurring in the crystal lattice of native metals as gas clusters requires activation energies of 100 and even180 kcal/mol up to the attainment of the melting temperature of the metal. The frequency factor is several ordersof magnitude higher than the limiting value k0 ~ 1013 s–1 for the migration of single atoms in the crystal lattice.Near the melting temperature and tens–hundreds degrees above it, the character of the thermal desorption ofradiogenic 4He changes fundamentally. The migration is strongly accelerated, and sharp narrow peaks appearon the kinetic curves of thermal desorption. A similar phenomenon was observed during the annealing of tech�nical metals and is known as the burst�effect. The destruction of the crystal structure results in the disappearanceof helium clusters (vesicles). At the very high temperature, He migrates as individual atoms relatively rapidlyfrom the melt. The activation energy for He thermal desorption and the pre�exponential frequency factoracquire values characteristic of ordinary migration. Such peculiarities of radiogenic He provide unique oppor�tunities for its preservation in the structure of gold and other native metals below their melting temperatures. Therapid advances of (U–Th)/He geochronology is still hampered by the experimentally established extremely het�erogeneous distribution of U, He, and, probably, Th in the structure of gold and other natural metals. This dif�ficulty can be circumvented by the development of a method for the determination of the contents of all thementioned chemical elements in a single aliquot from each sample.

DOI: 10.1134/S0869591112010043

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PETROLOGY Vol. 20 No. 1 2012

SHUKOLYUKOV et al.

phosphates, tantaloniobates, etc.), because of the easymigration and loss of radiogenic He, it may appearmuch more stable in native metals. There are strongarguments for this suggestion.

First of all, the density of native metals is signifi�cantly higher than that of other minerals, many ofwhich were previously used in attempts of (U–Th)/Heisotopic dating (e.g., Gerling, 1961). Their densitiesare 3–7 times higher than those of typical hypogeneminerals (Fig. 1a). It is reasonable to suggest that themigration rate of radiogenic He in a very dense metalmaterial is significantly lower than in the minerals thatare commonly used as isotopic clocks. A more correctand convincing inference can be derived by consider�ing the density of atom packing in mineral structures.

A direct correlation between the retention of radio�genic He and the packing density of minerals wasestablished by Gerling (1939a): the smaller the freespace in the crystal structure not occupied by atoms,the lower the migration rate of He in it and, corre�spondingly, the higher He retentivity in the mineral.Most of native metals and their solid solutions have asimple crystal structure of identical atoms arranged incubic or hexagonal closest packing. For instance, thecrystal structures of natural gold, copper, silver, andaluminum are cubic, and those of zinc, ruthenium,and osmium are hexagonal. With respect to the densityof atom packing, these metals are in general muchsuperior to other minerals, which can be seen from acomparison of the free space in the crystal lattices ofhypogene minerals and native metals (Fig. 1b). Thestructure of natural Pd, Ru, Os, Cr, Co, Rh, Au, Ir, W,Ag, and Cu contains 10–25% of unoccupied space,whereas 50–70% of interatomic space is free in thecrystal lattices of such minerals as muscovite, diop�side, jadeite, ringwoodite, ilmenite, corundum, for�sterite, olivine, zircon, and pyrope. It is reasonable toexpect that the availability of free space is favorable forthe migration and escape of He atoms from the crystalstructure of more complex minerals.

One property of the crystal structures of metals pre�vents the penetration and migration of free He atomsin them. The investigation of He behavior in construc�tion metals used in the nuclear power industryrevealed its specific feature (e.g., Evans, 1977). Theatoms of chemically active gases (hydrogen, nitrogen,and oxygen) can be dissolved in metals (an exceptionis the negligible solubility of nitrogen in copper).Complex molecules of gaseous Н2О, CO2, and hydro�carbons (CnHm) are too large to be incorporated inmetals, but their fragments (atoms of hydrogen, car�bon, and oxygen) can be dissolved separately.

In contrast to the above chemically active gases, theatoms of inert gases, including He, have symmetricaland stable electron shells and cannot therefore gainelectrons from other atoms or easily donate their ownelectrons. When He is incorporated in the crystal lat�tice of metal, the distribution of valence charge in it is

strongly changed. The theoretical calculations of theelectron structure of pure metals and the metal–helium system (e.g., Koroteev et al., 2009) indicated apositive energy for He dissolution. Theoretically, Hecannot be dissolved in metals at all. There is ampleexperimental evidence confirming very low He solu�bility in metals. It can be incorporated in the volumeof a metal only under special conditions, for instance,by implantation (irradiation by α�particles).

When the atoms of poorly soluble He are forcedinto a technical metal, for instance, through the for�mation of 4He by the α�decay of a parent radioactiveisotope, 3He generation by the β�decay of tritium(3H), or metal irradiation by a flux of energetic 4He+

particles, they inevitably migrate to sinks (grainboundaries, zones of structures occupied by a secondphase, dislocations, etc.) and are released in them asvesicles (He clusters). Such a form of He occurrencein the structure of technical metals is energeticallymore favorable than the uniform distribution of singleatoms (Loshmanov, 1983; Garner et al., 2001, 2010).The size of primary He vesicles–clusters varies from~1 to ~n × 10 nm and increases upon metal heating toseveral tens of nanometers and even to 0.5–1.0 μm.The behavior of vesicles of He and other inert gasesduring metal heating is fundamentally different fromthe behavior of normal gas inclusions. The latter dis�appear owing to gas dissolution in the metal, whereasthe microscopic vesicles–clusters of inert gas migrateas a whole, similar to the movement of gas bubbles in aheated viscous glass, simultaneously increasing in size(Brovko et al., 1979; Ruedl and Schiller, 1979; Gho�niem et al., 1983; Jäger et al., 1983; Horwitz et al.,1992; Garner et al., 2001, 2010; Zaluzhnyi andSuvorov, 2001; Klyavin et al., 2002, 2008; Neklyudovand Tolstolutskaya, 2003; Tolstolutskaya et al., 2004;Kopanets et al., 2008; Kompaniets, 2009). The move�ment of He vesicles–clusters in construction metals isalways regular: they migrate toward those microscopicareas in metals in which the distribution of atoms isless ordered, i.e., the potential energy of atoms is high.If a microscopic vesicle of inert gas is located in ametal at the boundary of domains with ordered andstrongly distorted crystal lattices, the heating of themetal, owing to the self�diffusion of its atoms tendingtoward positions with the lowest possible potentialenergy, will result in the movement of metal atomsalong the surface of the microscopic vesicle from theregion with a disturbed crystal lattice into the regionwith an ordered structure. With a certain probability,these atoms will join the undistorted crystal lattice andoccupy positions with low potential energies. As aresult, one wall of the microscopic vesicle will beetched, and the opposite wall will be overgrown; thus,the microscopic vesicle–cluster will move in a certaindirection as a whole inclusion. During heating of tech�nical metals saturated to a varying degree in 4He,larger He vesicles–clusters of different geometry arealso formed in the structure. These He or Ar inclusions


GEOTHERMOCHRONOLOGY BASED ON NOBLE GASES 3

Fig. 1. Comparison of the properties of native metals and some other minerals. (a) Density. (1) Hypogene minerals (ringwoodite,phlogopite, diamond, diopside, apatite, jadeite, perovskite, pyrope, olivine, stishovite, zircon, ilmenite, and troilite), (2) area ofless dense native minerals and (3) Pt�group metals, gold, and tungsten. (b) Packing density (fraction of volume occupied byatoms). Regions: (1) supergene minerals, (opal, quartz, sulfur, kaolinite, calcite, hydrogeothite, gypsum, orpiment, realgar,cuprite, sulfur, arsenolite, and gibbsite), (2) hypogene minerals (diamond, stishovite, zircon, olivine, jadeite, ilmenite, diopside,phlogopite, pyrope, apatite, troilite, periclase, perovskite, and ringwoodite), and (3) native metals (Sanderson, 1962; Sutton,1965; Porterfield, 1984; Huheey et al., 1993; James and Lord, 1992).

25242322212019181716151413121110

987654321

0 1 2 3 4 5 6 7 8 9 1110 12 13 14 15 16 17 18 19 2423222120

ZnInSb

CdCoCu

Cr(Te) Fe Bi Ag Pb

Rh

Mg

Al Au W

Pt

Ir Os

Density, g/cm3

(а)

(b)

25

20

15

10

5

0 0.2 0.4 0.6 0.8 1.0

1 2 3

Ru, Pt

Pd, Os Au Ir, Rh

Re

Relative density of crystal lattice packing(fraction ofvolume occupied by atoms)

1 2 3

Pb

4


SHUKOLYUKOV et al.

preliminarily implanted into, for instance, steel or Tiand V alloys strongly bond the noble gases in the crys�tal structure and appear to be efficient traps for atomsof other gases (for instance, hydrogen) at temperaturesof 500–1000°С (Cherdantsev, 2005; Chernov et al.,1996; Nagata and Takahiro, 2001; Neklyudov and Tol�stolutskaya, 2003; Tolstolutskaya et al., 2004; Kopa�nets et al., 2008). Such He clusters–vesicles have longbeen imaged by various methods in constructionmaterials and technical metals (Van Siclen et al., 1992;Neklyudov and Tolstolutskaya, 2003; Kalin et al.,2008; Tolstolutskaya et al., 2004; Binyukova et al.,2005). In extreme cases, bubbles clearly visible underan optical microscope are formed. The appearance ofsuch bubbles is known as blistering (Das and Kamin�sky, 1976; Evans, 1977; Astrelin et al., 2005). Duringthe irradiation of metals, for instance, Cu, Ni, and Al,by sufficiently large doses of Н+ and Не+, He accumu�lates in near�surface clearly discernible bubbles, partsof a micrometer in size. Further ion beam irradiationresults in their spontaneous opening.

The kinetics of He migration in the structure oftechnical metals provides compelling evidence for theexistence of He microscopic vesicles (clusters). A veryimportant feature is intense He release only at hightemperatures approaching the melting point of metalsand explosion�like He release near the melting tem�perature (burst�effect) (Kompaniets, 2009; Iwakiriet al., 2000; Klyavin et al., 2008; Tolstolutskaya et al.,2004). This moment corresponds to the burst�likedestruction of all microscopic He vesicles–clusters ator slightly above the melting temperature of the metal.

Thus, the experimental data for technical metalsand their interpretation lead to the conclusion on thevery high stability of He in the metal structure up to themelting temperature. Based on this evidence, it wassupposed that natural native metals must also showsimilar properties: retain radiogenic 4He and be good(U–Th)/He isotope geochronometers (Shukolyukovet al., 2010). In this connection, investigations werecarried out for the elucidation of the kinetics of radio�genic 4He release from native gold, silver, and copper,determination of the migration characteristics of Hein native metals (activation energy and frequency fac�tor), and calculation of the retention of radiogenic 4Heas a function of temperature and heating time. Inorder to achieve these goals, more than 30 samples ofgold and other native metals were investigated; thesesamples are briefly characterized in Table 1.

METHODS

The kinetics of radiogenic 4He release from nativemetals was investigated using a mass spectrometricsetup designed and manufactured in accordance toour specifications by the SPEKTRON–ANALITcompany (St. Petersburg). Its main advantage is thesmall volume of the analytical unit and, correspond�ingly, the very high sensitivity of He measurement. The

device allows reliable registration of 105 He atoms, andthe vacuum system provides a total residual pressure inthe working volume no higher than 10–9 Torr and anHe partial pressure of 10–17 Torr. Figures 2 and 3 showthe schematic design of two variants of the mass spec�trometric setup.

Vacuum system. High vacuum is provided by athree�level pumping technique. The system is pree�vacuated by an oil rotary pump, and an oil diffusionpump is used to produce high vacuum. In order tomaintain vacuum conditions in an extractor and ananalyzer (see below) at a necessary level during analy�sis, two getter sorption pumps were installed in theextractor and analyzer. The whole high�vacuum line ismanufactured from austenitic stainless steel.

Extractor. Two variants of extractor systems wereused in our investigations. The first variant (Fig. 2) wasdesigned for the investigation of extremely small sam�ples with very low bulk amounts of He: the mass of anative metal sample is from hundreds of micrograms to10 mg. The sample is dropped from a sluice device intoa heating container, a thin�walled cylinder of a refrac�tory metal (W, Ta, or Re) crimped in its central part. Itis heated by electric current up to a temperature of1450°С within 30 min. The possible residual pressure inthe extractor is n × 10–9 Torr. The dependence of thesample temperature on the heating current was prelim�inarily estimated using an optical pyrometer. During theheating (melting) of a sample and extractor connectionto the mass spectrometric analyzer (see below), theextractor is disconnected from the external pumpingsystem, and the vacuum is maintained by the gettersorption pump, which is inert to He.

The second variant is based on another method ofsample loading for He extraction from native metals(Fig. 3). A sample in a Ta container is inserted througha sluice device into a cylindrical extractor providingsample heating up to 1600°С without breaking thehigh vacuum. After He extraction, the sample isremoved from the extractor through the sluice devicefor its subsequent chemical treatment and determina�tion of U and Th contents.

Analyzer. The mass spectrometer is equipped witha sector analyzer (120°С) with an external permanentmagnet and pole pieces installed in the analyticalchamber. The 200�cm3 chamber is equipped with anion source with a hot�cathode electron emitter.Helium atoms are ionized by electron bombardment.Focused ion beams are directed into an ion collectorand registered using a secondary electron multiplier. Amass spectrum is obtained by varying the acceleratingvoltage. The resolution of the mass spectrometer isapproximately 10 at 50% peak height.

The heated analytical chamber can be degassed byeither an external pump or a getter sorption pump in aquasi�static vacuum mode. The residual pressure inthe analytical chamber is 10–9 Torr. The blank is nohigher than 5 × 104 4He atoms after heating. The sen�



Table 1. Brief description of the samples

Sample no. Deposit Mineral Region Collection

1

Nesterovskoe

Native gold

Polar Urals S.V. Petrov, St. Petersburg State University

2

3

4

Chudnoe5

6

7

8

Kitoyskiy uzel Eastern Sayan E.Yu. Rytsk, FROG, St. Petersburg9

10

11

12 Bogo�molovskoe Southern Urals Yu.L. Ronkin, Institute of Geology and Geochemistry, Ural Branch,

Russian Academy of Sciences

13

KaralonBuryatia

E.Yu. Rytsk, FROG, St. Petersburg

14

15

16

17 Quartz Buryatia

18 Drazhnyi

Native gold

Yakutia

L.A. Ostapenko, Central Institute of Geological Exploration for Base and Precious Metals, Moscow

19 Dzhugadzhag Far East

20 Sorevnovanie Yakutia

21 Degdekan Kolyma

22 Lazurnyi Yakutia

23 Iourirn Morocco

24

Tinserin Morocco

25

26

27

28

29

30

31

32 Maiskoe Karelia S.A. Bushmin, Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, St. Petersburg

33 Witwatersrand South Africa Yu.G. Safonov, Institute of Geology of Ore Deposits, Petrography, Miner�alogy, and Geochemistry, Russian Academy of Sciences, Moscow

34

Unknown Native copper

Chili

Mineralogy Department, St. Petersburg State University35 North America

36 Olonets

37Unknown Native

silver Urals S.V. Petrov, St. Petersburg State University38

6


SHUKOLYUKOV et al.

sitivity of the device was determined for the recalcula�tion of the number of detected ions to the initial num�ber of atoms:

S = V/n, (1)

where n is the number of counts per unit time from thevolume of helium (V) entering the mass spectrometerchamber. Samples of stony meteorites with well�known initial He concentrations (Schultz and Franke,

2002) were used for calibration. The sensitivity wasestimated as 5.7 ×10–13 cm3/count for the first variantof the extractor and is higher for the second variant,approximately 10–12 cm3/count per 100 ms at an emis�sion current of a cathode ion source of ~1 μA (Fig. 4).The loss of sensitivity at the expense of a larger volumeis compensated by the possibility of sample recoveryafter He extraction for chemical analysis. The workingvacuum remained stable at a level of (3–7) × 10–8 Torr

(а)

LVM1

HV1

LVM2HV2

He in

RP

HV3 HV4

HV5 HV6

HVMDP

MS

SP2

EXTR SD

HV4SP1

SD

SD

EXTR

EXTR

LD

HVOV

Sample loadinginto sluice

device

Sample loading into extractor

LD HV

HV

HV

(b)

Fig. 2. Mass spectrometric setup used in this study, first variant. (a) Sketch of the vacuum system of the mass spectrometric setup:MS, analyzer of the mass spectrometer; EXTR, extractor for the thermal desorption of radiogenic He from samples; SD, sluicedevice for sample loading into the extractor; He in, He inlet for sensitivity calibration; SP, sorption vacuum pump; RP, rotarypump; LVM, low�vacuum manometer; HVM, high�vacuum manometer; HV, high�vacuum valve; and DP, oil diffusion pump.(b) Scheme of sample loading without breaking high vacuum: LD, loading device; SD, sluice device; OV, oblique valve for closingthe sluice device; EXTR, extractor for the thermal desorption of radiogenic He; and HV, high�vacuum valve.

OV



(а)

LVM1

HV1

LVM2HV2

He in

LVM

HV3

HV8

HV5

HV6

HVMDP

MS

SP2

EXTR2 SD2

HV4SP1

SD

SD

EXTR

EXTR

LD

HV

OV

Sample loadinginto sluice

device

Sample loadinginto extractor

LD

HV

HV

HV

OV

(b)

EXTR1 SD1

HV9

HV11

HV7

HV10

MSU�G�01�M

Fig. 3. Mass spectrometric setup used in this study, second variant. (a) Sketch of the vacuum system of the mass spectrometricsetup: MS, analyzer of the mass spectrometer; EXTR1 and EXTR2, extractors for the thermal desorption of radiogenic He fromsamples; SD1 and SD2, sluice devices used alternatively for sample loading into the extractor; He in, He inlet for sensitivity cal�ibration; SP, sorption vacuum pump; RP, rotary pump; LVM, low�vacuum manometer; HVM, high�vacuum manometer; andHV, high�vacuum valve. The line HV9–SD2–HV10–EXTR2–HV1 assembled in this variant allows the introduction of a sampleinto the extractor and its removal after 4He extraction for subsequent U and Th analysis. DP is the oil diffusion pump. (b) Schemeof sample loading without breaking high vacuum: LD, loading device; SD, sluice device; OV, oblique valve for closing the sluicedevice; EXTR, extractor for the thermal desorption of radiogenic He; and HV, high�vacuum valve.

8


SHUKOLYUKOV et al.

during the experiment. The conditions of the evacua�tion of the analytical chamber were such that even theamounts of 4He corresponding to ion currents of tensof thousand count/ms were removed and a back�ground level of 1–2 count/ms was restored in 1–2min, which allowed us to begin a new experimentalmost immediately. Such metrological parameters are

appropriate for the investigation of the kinetics of Herelease from micrograms of Au.

The kinetics of radiogenic He release was exploredby a step�heating method. A sample from parts of amilligram to tens of micrograms in weight was heatedto a certain temperature, and the amount of 4Hereleased at this temperature was measured. The gaswas pumped out, and temperature increased to adesired level. The exposure time at each step was con�stant in a particular experiment.

The concentration of U in gold was determined byisotope dilution using a 235U–208Pb tracer, which wasadded to the sample directly before decomposition. Inorder to remove surface contaminants, the selected Aufragments (800–250 μg) were washed with 1 M HNO3

and, then, pure water. The chemical decomposition ofAu was performed in a Parr�4782 bomb in an aquaregia solution at a temperature of 220°С, and U wasseparated using the methods of Krogh (1973) andHorowitz (1992). The results of parallel experimentswith solution aliquots containing U extracted fromgold are reproducible within 0.5%, which indicatescomplete U extraction from the Au sample. The blankwas no more than 1 pg U during our measurements.The isotopic composition of U was determined using aFinnigan MAT 261 mass spectrometer operating ineither a static (using Faraday collectors) or dynamic(using a secondary electron multiplier) mode. Theexperimental data were processed by the PbDAT pro�gram (Ludwig, 1993). The experiments showed thatthe two variants of analytical techniques and twomethods of ion current measurement (Faraday collec�tors and a secondary electron multiplier) provide suf�ficiently high sensitivity and precision of U analysis(Table 2).

Sen

siti

vity

, cm

3 /cou

nt

9E–12

8E–12

7E–12

6E–12

5E–12

4E–12

3E–12

2E–12

1E–12

0 0.2 0.4 0.6 0.8 1.0

4He

3He

Emission current, mA

Fig. 4. Calibration curve based on experimental data onthe contents of He isotopes in the Knyahinya L/LL5 mete�orite (Schultz and Franke, 2002).

Table 2. Precision of the isotope dilution analysis of U in native gold

Sample Solutionaliquot no.

Outline of chemical procedures

Method of ion current registration

270MU/267 U, ppb***

Solution obtained after the decompo�sition of 840 µg of native gold from the Chudnoe deposit in aqua regia in a Parr�4782 bomb at 130°C for 24 h

U4�1TRU.Spec resin

SEM 0.018303 674.6

U4�2 SEM 0.018276 673.6

U4�3

Method of Krogh (1973)

SEM 0.01840 673.2

U4�4 FC 0.018416 672.5

U4�4 SEM 0.018152 669.0

U5 SEM 0.006171 652.5

U5 FC 0.006250 654.5

U2 FC 0.00908 897

Notes: * SEM is the secondary electron multiplier, and FC is the Faraday collector. ** MU is the combined U + O2 ion. *** Parts per billion, 10–9 g/g.

MU**



EXPERIMENTAL RESULTSAND INTERPRETATION

Investigation of the Kinetics of He Release

The samples can be divided into two groups differ�ing in the shape of the kinetic curve of radiogenic Herelease. Figure 5a shows the results on the kinetics ofradiogenic He release from the crystal lattice of thefirst group of native gold samples. It can be seen thatthe major portion of He is released from the nativegold forming a single sharp peak at a temperature closeto the melting point of Au (1064°С). A significantfraction of He (from 30 to 90%) escapes from the Austructure only after metal melting. A similar behavioris characteristic of native copper, which was also stud�ied by us (Fig. 5b). Such a shape of the kinetic curvesof He release from native metals resembles the burst�effect, an explosion�like He escape from technicalmetals approaching their melting temperatures, and isrelated to the presence of He gas vesicles. This is illus�trated in Fig. 6 showing examples of the burst�effect insome technical metals. Helium is incorporated inthem either through the irradiation by high�energy ionbeams or by the absorption of tritium, which is subse�quently transformed into 3He by β�decay. It can beseen that the maximum rate of He release from tech�nical metals is observed during their melting or nearthe melting points. This similarity in the character ofHe migration during thermal treatment suggests thatHe probably occurs as gas clusters (vesicles) both innatural and technical metals and indicates a uniquepossibility of the complete retention of radiogenic Hein native metals during geologic history.

The kinetic curves of the thermal desorption ofnative gold samples of the second group are shown in

Fig. 7. In addition to the main peak above the meltingtemperature of Au, there are much smaller peaks atlower temperatures (700–900°С) (Table 3). The heat�ing of native gold at temperatures of 600–1000°Сresults in the release from ~2 to 23% of 4He. This couldhave been attributed to the heterogeneity of the realstructure of gold. However, the same patterns of ther�mal desorption with a main peak at or even above themelting temperature and peaks of much lower inten�sity were also obtained for native copper and silver(Fig. 7).

Eugster et al. (1995) supposed that quartz inclu�sions in native gold may affect the kinetics of He ther�mal desorption. In order to test this hypothesis, anexperiment was made on the stepwise thermal releaseof He from gold�bearing quartz from the country rocksof the Karalon Au deposit. The temperature of Herelease from quartz appeared to be close to 600°С.However, the He content in the quartz is several ordersof magnitude lower that that at the region of the low�temperature peak of native gold. Hence, quartz cannotbe considered as a source of low�temperature He(Table 3).

In addition, a significant role of He thermal des�orption from gas–liquid inclusions in native gold wasproposed (Pettke et al., 1997; Eugster et al., 1995).This suggestion seems to be supported by the observedincrease in the content of hydrogen in the analyticalchamber of the mass spectrometer simultaneouslywith the release of He from native gold in the region ofthe low�temperature peak (600–800°С). According toPosukhova (2001), gold�hosted gas–liquid inclusionscontaining water, carbon dioxide, methane, etc. aredecrepitated approximately within this temperature

Table 3. Concentration of radiogenic He in the samples

Sample no. Weight, mg 4He, 10–7 cm3/g Sample no. Weight, mg 4He, 10–7 cm3/g

1 0.38 350 20 0.33 1.32 4.86 120 21 0.47 0.673 0.14 340 22 0.55 0.134 1.50 2.6 23 0.69 175 0.45 3800 24 0.30 0.746 0.10 11000 25 2.18 127 0.16 5000 26 1.65 158 0.70 55 27 1.73 199 0.17 2.5 28 1.37 5.5

10 1.31 0.38 29 0.73 1411 2.0 8.2 30 1.66 4112 3.95 0.10 31 1.60 8.413 0.19 2.2 32 0.92 1.514 1.77 0.3 33 <0.01 >72000015 1.79 0.77 34 2.65 1316 1.67 0.42 35 0.87 0.217 0.56 0.05 36 0.77 0.118 1.90 0.21 37 1.61 2.119 0.09 0.91 38 3.14 0.7

10


SHUKOLYUKOV et al.

Chudnoe

T, °C

Chudnoe 4

He,

arb

. un

its

Nesterovskoe Kitoyskiy uzel

400 600 800 1000 1200

Witwatersrand

400 600 800 1000 1200

T, °C

4H

e, a

rbit

rary

un

its

400 600 800 1000 1200

(a)Native gold

(b) Native copper

T, °C

Fig. 5. Kinetic curves of 4He thermal desorption from native metals. Release of radiogenic He from the crystal lattice of the sam�ples of (a) native gold and (b) native copper. Here and in further diagrams, the star symbol indicates melting temperature.



range. However, this interpretation is in conflict withexperimental data. First, similar maxima in migrationrate were observed during the thermal desorption ofHe from technical copper, aluminum, and steel free ofgas–liquid inclusions or another noble gas, radon,from aluminum (Fig. 8). Second, a close correlationwas observed between the experimentally determinedamounts of He released in the regions of the high�tem�perature and low�temperature peaks in the curves ofHe thermal desorption from native gold (Fig. 9). Thisindicates an obvious connection of the low�tempera�ture peaks to the mechanism of 4He migration in thecrystal lattices of native and technical metals. In ouropinion, the suggestion on the significant role of Hethermal desorption from gas–liquid inclusions innative gold (Pettke et al., 1997; Eugster et al., 1995) istherefore not plausible. This lends additional supportto the interpretation that the kinetics of 4He thermaldesorption from native metals is controlled by themechanism of its migration. In order to understandthe obtained data, a mechanism should be found forthe evaluation of the data on He thermal desorptionfrom the crystal lattices of native metals.

Model of Helium Migration in Solids

There are two main approaches for the descriptionof the migration of atoms of radiogenic noble gases,including He, in minerals. Since the pioneering publi�cations of Gerling (1939a, 1939b, 1961), most Russianresearchers (Shukolyukov, 1970; Ashkinadze, 1980;Shukolyukov et al., 2009; Yakubovich et al., 2010)have used the formalism of a monomolecular first�order chemical reaction. It is based on the assumptionthat the crystal structure of natural minerals (real crys�tals) is so defective in the region of the formation of aradiogenic isotope that a single jump of a migratingatom is sufficient for its transition into a mobile state;therefore, the behavior of a migrating radiogenic atomis formally identical to that of atoms in simple chemi�cal interactions. Indeed, a single event of α�decay of Uresulting in the sequential formation or 6–8 atoms ofradiogenic He is accompanied by the release and dis�persion of ~108 eV in the structure, as compared withbonding energies of 2–4 eV between atoms in a crystallattice (Holland, 1954). Thus, radioactive decay in thecrystal lattice of a mineral results in the release of tre�mendous amounts of energy and formation of adefect, which can serve as a repository for He atoms.

4 He,

arb

. un

its

T, °C

10

5

0 100 200 300

Tin

T, °C

10

5

0 200 400 600

Cadmium 4

He,

arb

. un

its

T, °C

80

0 500 1000 1500

Steel X18H10T

60

40

20

Technical metals:

T, °C

3.0

0 500 1000 1500

Ferrite steel

2.0

1.0

12Cr�ODS

Fig. 6. Kinetic curves of the release of He implanted into the crystal lattices of technical tin and cadmium (Klyavin et al., 2002),construction reactor steel X18H10T (Tolstolutskaya et al., 2004), and ferrite steel 12Cr�ODS (Kompaniets, 2009).

12


SHUKOLYUKOV et al.

Technical metals:Copper Steel OX16H15M3B Aluminum0.4

0.3

0.2

0200 600 800 1000 1200

4H

e, a

rb. u

nit

s

0.1

400

1.0

0.6

0.4

0 400 600 800 1000 1200

0.2

0.8

T, °C200

1.0

0.6

0.4

0 400 600 800 1000

0.2

0.8

200

Fig. 8. Kinetic curves of the release of artificially implanted He from technical metals in the high�temperature and low�temper�ature ranges: copper (Klyavin et al., 2008), steel OX16H15M3B (Zaluzhnyi and Suvorov, 2001), and aluminum (Brovko et al.,1979).

Tinserin Nesterovskoe Chudnoe

0.6

0.4

0.2

0

0.3

0.2

0.1

0.5

0.4

400 600 800 1000 1200 1400

4H

e, a

rb. u

nit

s

0.3

0.2

0

0.1

400 600 800 1000 1200 1400 400 600 800 1000 1200 14000

Native copper0.5

0.3

0.2

0400 600 800 1000 1200 1400

4H

e, a

rb. u

nit

s

0.1

0.4

Native gold

Native silver

T, °C

T, °C

1.0

0.6

0.4

0400 600 800 1000 1200

4H

e, a

rb. u

nit

s

0.2

0.8

T, °CFig. 7. Kinetic curves of He release from native gold, copper, and silver with local maxima at low and high temperatures.



The migration of an He atom from this accumulationcenter can be described in terms of the one�jumpmodel. This model of migration is based on theassumption that the energy necessary for the first jumpis much higher than the energy needed for subsequentdiffusion from the mineral grain through its damagedstructure. The mathematical apparatus of the theory ofa monomolecular first�order chemical reaction is usedin various isotope�geochemical models of the evolu�tion of the Earth’s mantle–crust–atmosphere systemaccounting also for other (not gaseous) elements (e.g.,Azbel’ and Tolstikhin, 1988).

Using this approach, the migration rate of radio�genic He atoms can be expressed as

(2)

(3)

where k0 is the frequency factor, E is the activationenergy for migration, R is the gas constant, Т is theabsolute temperature, and He is the helium content.

The differential form of the He migration law inminerals can be integrated to give

(4)

where Het is the He content at the moment of time t,and He0 is the initial He content.

In order to determine migration characteristics(activation energy Е and frequency factor k0), Eq. (4)can be recast as

(5)

(6)

Since this is the equation of a straight line in thelnln(He0/Het)–1/T coordinates, the parameters ofthis line can be used to calculate the activation energyfor migration, Е, and the frequency factor, k0.

One advantage of this presentation is that it sup�ports the existence of zones in real crystals with differ�ent degrees of lattice damage, the possibility of radio�genic atom migration along different directions in thestructure, the possibility of existence of various pri�mary defects and traps for such migrating atoms inminerals, the possibility of different forms of U occur�rence in mineral grains, and, correspondingly, the het�erogeneous distribution of radiogenic He in the crystallattice. In other words, the possibility of differentenergy states of radiogenic He atoms in real mineralstructures can be substantiated. Since the first studiesof the kinetics of radiogenic He and Ar release fromU�bearing minerals (Gerling, 1939a, 1939b, 1961), ithas been known that the rate of gas release at thermal

dHedt

�� kHe t( ),–=

k k0eE

RT��–

,=

Het He0e kt–,=

He0

Het

�� ek0e

ERT��–

t,=

He0

Het

��lnln k0 tln+ln ERT��.–=

treatment of a mineral shows several local maxima andminima. The kinetic curves of heating�induced gasrelease with several maxima of the migration rate ofXe, Kr, Ar, Ne, and He in U minerals and zircons havesubsequently been reported in a number of studies(Shukolyukov, 1970; Shukolyukov and Levsky, 1972;Shukolyukov and Ashkinadze, 1967; Krylov andShukolyukov, 1994; Reimold et al., 1995). The one�jump model explains more or less adequately such com�plex curves of radiogenic He migration in minerals.

Another advantage of the formalism of a monomo�lecular first�order chemical reaction (one�jumpmodel) is relatively simple mathematical proceduresfor the determination of the migration parameters, Еand k, on the basis of experimental data. A shortcom�ing is that the resolution of the method may be insuffi�cient for the reliable separation of migration processeswith different kinetic parameters.

An alternative approach is based on Fick’s diffu�sion laws, which describe variations in the concentra�tion of atoms of radiogenic gases in minerals at ther�mal events (Fechtig and Kalbitzer, 1966; Carslaw andJaeger, 1959). The difference between the two modelsis that the time needed for the detachment of an atomfrom the accumulation center is the controlling factorin the one�jump model, whereas the escape of an atomfrom the mineral lattice is defined in the classic diffu�sion model by the time of its walk in the grain. The lat�ter model is based on stringent conditions: it isassumed that diffusion occurs in spherical mineralgrains (or grains of other a priori defined shapes) ofidentical size with initially homogeneous distributionof both the radioactive (parent) isotope and radiogenicnoble gas. The concentration of remaining gas atomschanges with time in accordance with a complex func�tion depending on the diffusion coefficient D, temper�

0

–4

–8

–12

–16

–20 –18 –14 –10 –6 –2

Log

arit

hm

of 4 H

e co

nte

nt

at h

igh

�tem

pera

ture

pea

k

Logarithm of 4He content

at low�temperature peak

0–4–8–12–16

Fig. 9. Correlation between the experimentally determinedamounts of He in the region of high�temperature and low�temperature peaks of the thermal desorption curves of Hefrom native gold, which demonstrates a connectionbetween the low�temperature peaks with the mechanismof 4He migration in the crystal lattice of native and techni�cal metals.

14


SHUKOLYUKOV et al.

ature T, and grain size, for instance, the radius ofspherical grains (R) (Fechtig and Kalbitzer, 1966). Thefraction of lost gas can be calculated from an expres�sion corresponding to the accepted model of migra�tion in accordance with the laws of classic diffusion:

(7)

(8)

where D is the diffusion coefficient, t is the time, R isthe grain radius, and n is a natural number. If the tem�perature�dependent diffusion coefficient is known,diffusion parameters (activation energy for diffusionand frequency factor) can be calculated for certaineffective sizes of diffusion cells (which are often differ�ent from the real size of crystals) using the experimen�tal equation:

(9)

However, the main problem in the interpretation ofexperimental data on noble gas migration in the struc�ture of real minerals is that the assumptions of initiallyhomogeneous gas distribution and the identical energypositions of all gas atoms in the structure are idealizedand very far from the real migration conditions. Inparticular, it is known that the distribution of U and Th(parent isotopes of He) is very inhomogeneous in thecrystal structure of most zircons and other accessoryminerals (Shukolyukov, 1970; Tugarinov and Bibik�ova, 1980; etc.).

Owing to the different mechanisms of migration,the diffusion model depends on grain size, whereasthis parameter is unimportant for the one�jumpmodel. Most of recent isotope�geochemical studieshave relied on one of these models. However, the dif�fusion model is used much more frequently in geo�thermochronology (Rosso, 2005).

Svetukhin et al. (2005) proposed an approach com�bining the above two interpretations of noble gasmigration in solids: the real process of radiogenic Hemigration from the mineral lattice is most likely acombination of diffusion and one�jump processes.The solubility of He in the lattices of metals and someminerals is limited. The supersaturation of the metalsolid solution in He diminishes owing to He diffusiontoward various internal and surface defects, whichserve as He accumulation centers in the mineral lattice(Gol’tsev and Guseva, 1973; Neklyudov and Tolsto�lutskaya, 2003; Ghoniem et al., 1983). Thus, Hemigration from a real crystal can be considered as thedetachment of an atom from the accumulation center,He depository or defect (one�jump mechanism), and

F 1 6

π2�� 1

n2��e n

2Bt–

,

n 1=

∞

∑–=

B π2 D

R2��,=

D D0eE

RT��–

.=

its subsequent diffusion toward grain boundaries. Ingeneral, the rate of He loss can be described by the fol�lowing equation (Svetukhin et al., 2005):

(10)

where С(r, t) is the dependence of the concentration ofdissolved He atoms in the lattice on radius r and timet; D(T) is the diffusion coefficients of He, which isindependent of concentration, С; N is the concentra�tion of He atoms occurring in an He accumulationcenter; k is the kinetic coefficient; and r is the effectivegrain radius.

The temperature dependence of the kinetic coeffi�cient can be expressed as

(11)

where Еа is the activation energy for the escape of anHe atom from the defect, and k0 is proportional to theconcentration of accumulation centers and related tothe geometry of the center and the change of theentropy of the crystal at the removal of an He atomfrom the accumulation center.

According to Eq. (11), diffusion is the main mech�anism of He escape from the mineral lattice if k �D/r2, and the one�jump mechanism is rate�limiting forHe escape if k � D/r2. It is obvious that migration iscontrolled by diffusion in large monocrystals r ∞and D/r2 0), and the one�jump mechanism dom�inates at r 0. Numerical modeling by the exampleof He thermal desorption from irradiated nuclearreactor materials showed that the main controllingfactor of He release is the one�jump mechanism, andthe contribution of the diffusion mechanism can beneglected (Svetukhin et al., 2005). This is why we usedthe one�jump model for the determination of migra�tion characteristics.

Experimental Determination of Helium Migration Characteristics in Native Metals

The choice of the one�jump migration model fol�lowing the law of a first�order chemical reactionallowed us to elucidate the mechanism of the thermaldesorption of 4He in noble metals. The migrationparameters were determined from the dependence ofHe content after heating at a given temperature nor�malized to the initial He content on the temperature ofheating (Eq. (6)). The kinetics of He thermal desorp�tion was investigated separately in the low�tempera�ture region and during heating to the highest tempera�tures.

∂C r t,( )∂t

�� D T( )

r2�� ∂

∂r�� r2∂C r t,( )

∂r��⎝ ⎠

⎛ ⎞=

+ kN 0( ) kt–( ),exp

k T( ) k0Ea

RT��–⎝ ⎠

⎛ ⎞ ,exp=



In the low�temperature region (600–950°C), thekinetic curves of He thermal desorption from somesamples display more than one peak (two and eventhree in one case, Fig. 7). The migration parameterswere calculated separately for each peak supposingthat they are related to He release from different dis�crete energy positions of atoms in the noble metalstructure. In the case of poorly resolved peaks, theirshape was reconstructed assuming that they are sym�metrical relative to the extremum point. Calculationswere performed for typical samples with low�temper�ature peaks on the curves of 4He thermal desorption.According to the obtained data (Fig. 10, Table 4),three of four gold samples with sufficiently high Heamounts in the region of low�temperature peaksyielded rather high activation energies for 4He migra�tion in the low�temperature range. These parametersare sufficient for He retention in the structure at tem�peratures of 100–200°С for time periods from 3 billionyears (sample 6) to tens of million years (sample 3).

The high�temperature behavior of 4He in nativemetals is very different and unusual. There are twopeculiar features. First, the thermal desorption of 4Heoccurs differently in two temperature intervals: in theregion of Au premelting and melting and at tempera�tures above the melting point of this native metal. Thisis observed in almost all kinetic curves of thermal des�orption: the thermal treatment of samples results inthat part of 4He is desorbed at a certain rate below themelting point of Au, and part of 4He is desorbed at avery different rate above the melting point. In the caseof sample 6, there are two peaks in the high�tempera�ture range (Fig. 7). Correspondingly, an inflection isoften observed in the linear dependence oflnln(4He0/

4Het) versus 1/T (Fig. 11). A similar behav�ior is also characteristic of other native metals, forinstance, copper (Fig. 12). It can be seen from this dia�gram that the same features are observed during thethermal desorption of He from technical metals, forinstance, tin. Perhaps, this is characteristic of Hemigration from all metals.

Second, very high values of activation energy andfrequency factor were obtained for temperatures 100–200°С below the melting point of Au (Table 5). Themigration of Au is strongly suppressed below the melt�ing temperature. At higher temperatures, the activa�tion energy and frequency factor become significantlylower. In all cases, the inflection point is rather close tothe melting point of the metal. The temperature of theinflection point separates two different regions(Table 5). Helium migration is strongly suppressed inthe region of premelting and melting of native goldand copper (940–1083°С). The atoms of 4He very eas�ily escape from the crystal structures of ordinary min�erals, but are unusually strongly retained in the struc�ture of native metals, significantly stronger than evenAr atoms in minerals with the highest Ar retentivity.Some native gold and copper samples showed activa�tion energies as high as 180 and 170 kcal/mol. The val�ues of the pre�exponential frequency factor, k0, appear

Native gold1

0

–1

–2

–3

–40.0007 0.0008 0.0009 0.0010 0.0011 0.0012

1/T, K

lnln

(4 He 0

/4 He t

)

ChudnoeTinserin

Nesterovskoe

965°С 865°С 750°С

Fig. 10. Determination of kinetic parameters from thelow�temperature peaks of the kinetic curves of 4He ther�mal desorption from native gold: no. 3, Nesterovskoe lodedeposit (E = 41 kcal/mol and k0 = 1.3 × 106 s–1); no. 24, Tin�

serin lode deposit (E = 29 kcal/mol and k0 = 6.0 × 102 s–1);and no. 6, Chudnoe gold deposit (E = 61 kcal/mol andk0 = 6.6 × 107 s–1). T, °C is the temperature at the maxi�mum of the low�temperature peak.

Table 4. Kinetic parameters of 4He migration in the low�temperature range (600–900°C)

Sample no.(Table 1)

Low�temperatureextremum, °C

Fraction of 4He releasedin the temperature range

Activation energy, E, kcal/mol

Frequencyfactor, k0, s–1

2 600 ≈2 * *

7 649 ≈2 * *

5 760 ≈4 * *

8 720 ≈10 ≈24 8 × 102

24 865 23 29 6.0 × 102

3 750 11 41 1.3 × 106

6 965 16 61 6.6 × 107

* Migration parameters were not determined because of the insufficient amount of 4He in the low�temperature fractions.

16


SHUKOLYUKOV et al.

to be no less intriguing. The theoretically highest pos�sible value depends on the frequency of atomic vibra�tions and cannot exceed ≈1013 s–1 in the case of migra�tion of single atoms. During the migration of 4He innative gold and copper below their melting tempera�tures, the values of the pre�exponential frequency fac�tor, k0, are 5.1 × 1027 and 2.0 × 1025, respectively. Obvi�ously, this suggests that a specific mechanism of Hemigration different from jumps of individual atomsoperates in the temperature regions of pre�melting andthe beginning of melting of native metals. Most likely,this is related to the joint migration of many He atomsin clusters.

This specific mechanism of the thermal desorptionof radiogenic 4He from native metals is the same as inthe case of technical metals. The incorporation of Heatoms with their symmetric and strong electron shellsinto native metals results in considerable changes inthe distribution of valence charge density in their crys�tal lattices. This is why, similar to technical metals, Heis not dissolved in native metals but migrates towardgrain boundaries and dislocations and is released onthem as vesicles or He clusters. Such He clusters (ves�icles) have long been imaged in construction materialsand technical metals by various methods (Ruedl andSchiller, 1979; Jäger et al., 1983; Van Siclen et al.,1992; etc.). Perhaps, similar stable nanoclusters are

1100 1050 1000 950 T, °C

0

–1

–2

–3

–4

0.00072 0.00076 0.00080 0.00084

lnln

(4 He 0

/4 He t

)

E = 86k0 = 5.3 × 1011

Chudnoe

E = 27k0 = 2.4 × 101

1150 1100 T, °C

0

–1

–2

–3

–40.00070 0.00072 0.00074

E = 84k0 = 6.5 × 1012

Nesterovskoe

E = 181k0 = 5.1 × 1027

1100 1050 1000 T, °C

0

–1

–2

–3

–4

0.00072 0.00076 0.00080

lnln

(4 He 0

/4 He t

)

E = 110k0 = 6.4 × 1014

Chudnoe

E = 139k0 = 3.9 × 1019

1/T, K

1100 1050 1000 T, °C0

–1

–2

–3

–4

0.00068 0.00076 0.00084

E = 32k0 = 3.0 × 102

Nesterovskoe

E = 51k0 = 3.0 × 105

1/T, K

9501150

–5

Native gold

Fig. 11. Anomalous behavior of the lnln(4He0/4Het) versus 1/T (K) dependence in native gold: the appearance of an inflectionin the linear dependence, its separation into two straight segments corresponding to different He migration characteristics (acti�vation energy, E in kcal/mol, and frequency factor, k0 in s–1) above the melting temperature of gold. The samples are from theChudnoe (nos. 7 and 5) and Nestreovskoe (nos. 2 and 3) deposits.



formed in native metals, and their detection under amicroscope is an important task for future studies.

Previously, we pointed out (Figs. 5, 6) that ther�mally activated He migration in native metals isaccompanied by intense He release at high tempera�tures approaching or even higher than the meltingpoint of the metal, and a very similar phenomenon isobserved in technical metals. This phenomenon isreferred to as the burst�effect, explosion�like totaldestruction of microscopic He vesicles (clusters) withthe release of 4He (Barnes and Mazey, 1963).

When such temperatures are reached, the characterof 4He migration in native gold and copper changessharply. The values of the activation energy for migra�tion become normal, and, moreover, the pre�expo�nential frequency factor, k0, also decreases below thetheoretically permissible value.

The established ability of radiogenic 4He to bestrongly retained in the crystal structure of native met�als gives grounds to suggest that they can be dated bythe (U–Th)/He method. Of special interest are plati�num�group metals, the melting points of which (cor�respondingly, temperatures of the burst�like release ofradiogenic He) are from 1552°С(Pd)–1769°С(Pt) to3050°С(Os)–3453°С(Re).

Distribution of Helium and Uranium in Native Gold

In addition to information on the stability of radio�genic He in the crystal lattices of native metals, thecharacterization of U and, correspondingly, He distri�bution in these minerals is no less important. Thequestion is whether the distribution of U (and, corre�spondingly, radiogenic He) is sufficiently homoge�

Native copper1150 1100 1050 T, °C

0

–1

–2

–3

–40.00068 0.00072 0.00076

lnln

(4 He 0

/4 He t

)

E = 55k0 = 9.7 × 105

E = 176k0 = 2.1 × 1025

1/T, K

Technical tin240 230 220 T, °C

0

–1

–2

–30.0019 0.0020 0.0021

E = 14k0 = 5.0 × 1011

E = 52k0 = 2.0 × 1021

1/T, K0.0022

210

Fig. 12. Examples of the anomalous behavior of the lnln(4He0/4Het) versus 1/T (K) dependence during the thermal desorptionof He from native copper (our data) and technical tin (Klyavin et al., 2008).

Table 5. Kinetic parameters of 4He migration in the high�temperature range (900–1100°C)

Sample premelting and melting Temperature of transition between 4He migration

types, °C

Temperatures above the melting point

Sample no. (Table 1)


Frequency factor, k0, s–1


Frequencyfactor, k0, s–1

2 181 5.1 × 1027 1140 84 6.5 × 1012

7 86 5.3 × 1011 1048 27 2.4 × 101

5 139 3.9 × 1019 1064 110 6 × 1014

6 92 1.8 × 1011 1074 53 1.1 × 106

24 89 2.4 × 1013 1082 72 5 × 1010

3 51 3.0 × 105 1102 32 3 × 102

34 (copper) 176 2.0 × 1025 1085 55 9.8 × 105

18


SHUKOLYUKOV et al.

neous for the determination of their contents from ali�quots of a single sample.

The concentrations of He in the native gold sam�ples discussed here vary by three orders of magnitude,from 10–8 to 10–3 cm3/g (Table 3, Fig. 13a). Thesevariations are related to both the ages of deposits andvariations in the contents of parent U and Th isotopes.This is supported by the fact that, even within a singleoccurrence, He concentration varies from grain tograin by orders of magnitude (Fig. 13b). Considerableheterogeneity was observed even during the investiga�tion of He distribution within individual Au frag�ments, in which the He concentration may vary by afactor of 1.5–3.0. Perhaps, the main reason is theinhomogeneous distribution of U in the gold. Thisimplies that both 4He and U (Th) must be analyzed inthe same aliquot for the (U–Th)/He dating of nativegold. For this purpose, the second variant of a reactorfor sample fusion was designed and installed in themass spectrometric setup (Fig. 3).

CONCLUSIONS

The following conclusions can be drawn from theresults reported in this paper.

(1) It was experimentally established that thebehavior of 4He is very similar in native and technicalmetals. The atoms of the radiogenic inert gas heliumhave a symmetrical and strong electron shell and can�not gain electrons from other atoms or lose their ownelectrons; therefore, when such atoms appear in thecrystal lattice of native metal owing to radioactivedecay, they cannot occupy a stable position. They rap�idly migrate toward grain boundaries and dislocationsforming He vesicles (clusters). This is energeticallyfavorable compared with the dissolution of singleatoms.

(2) The microscopic He vesicles–clusters movewithin the crystal lattice of Au and other native metalsas a whole. The energy required for the movement ofsuch objects in the crystal structure of native metals isvery high. In order to extract He occurring as gas clus�ters in the crystal lattice of native metals, activationenergy as high as 100–180 kcal/mol is necessary inmany cases.

(3) It was shown that the character of the thermaldesorption of radiogenic 4He changes fundamentallywhen the melting temperature of the metal is reached.The destruction of the crystal structure of the metal isaccompanied by the disappearance of He clusters–vesicles. At very high temperatures, He migrates as

0.08

0.06

0.04

0.02

0

(a)

10–8 10–7 10–6 10–5 10–4 10–3

Arb

. un

its

0.08

0.04

0

(b)

10–7 10–6 10–5

Arb

. un

its

4He, cm3/g

Fig. 13. Variations in the content of radiogenic He in native gold from the deposits considered in this paper: (a) from differentdeposits and (b) in different samples from a single deposit.



single atoms relatively rapidly from the melt. The acti�vation energy and pre�exponential frequency factoracquire values typical of migration.

(4) Such properties of radiogenic He, i.e., stronglysuppressed thermal desorption up to melting temper�atures and He escape from native metals only aftertheir melting, provide unique opportunities for Heretention in the structure of gold and other native met�als at temperatures below their melting points. Thesecharacteristics make native metals unique amongminerals.

This work was supported by the Russian Founda�tion for Basic Research (project nos. 10�05�00321�aand 11�05�12046�ofi�m�2011) as well as by the the�matic plan of Research Works of the St. PetersburgState University.

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Carslaw, H.S. and Jaeger, J.C., Conduction of Heat in Solids,2nd Ed., Oxford: Clarendon, 1959.

Cherdantsev, Yu.P., Behavior of the Metal–Hydrogen Sys�tem at Radiation Impact, Doctoral (Physmath.) Disserta�tion, Tomsk: Tomsk. Tekhnol. Univ., 2005.

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