287

ISSN 1028-334X, Doklady Earth Sciences, 2009, Vol. 425, No. 2, pp. 287290. Pleiades Publishing, Ltd., 2009.Original Russian Text Yu.A. Shukolyukov, M.M. Fugzan, I.P. Paderin, S.A. Sergeev, D.P. Krylov, 2009, published in Doklady Akademii Nauk, 2009, Vol. 424, No. 6, pp. 814817.

Modern isotopic geology requires new approachesto geothermochronological problems for decipheringtemperature evolution of magmatic and sedimentaryrocks and related geological processes. One of thefuture research avenues in this field is the simultaneousapplication of genetically interrelated isotopic geochro-nological systems of noble gases: not only UThHe[1], but also

UXe

s

, UXe

n

, UKr

s

,

and

UKr

n

[2, 3] incombination with the UThPb isotopic system (Xe andKr are the products of spontaneous and neutron-induced fission [4]). This will maximally increase thegeochronological reliability of the obtained ages andgive opportunities unique for geothermogeochronologyowing to a common mechanism of migration of differ-ent radiogenic isotopes having different kinetic param-eters of migration. In order to make this approach appli-cable to practice, it is necessary to carry out extensivestudies of migration characteristics of He, Xe, and Krand the stability of the Xe/U, He/U, and Kr/U isotopicsystems in mineral geochronometers, primarily, zir-cons.

In this work, we experimentally determined themigration parameters of radiogenic xenon and ener-getic characteristics of the UXe isotopic system in thenonmetamict zircons with perfect crystalline structure;demonstrated for the first time the unique stability ofthe uraniumxenon isotopic system in the nonmetamictzircon samples; studied and compared the state of

genetically linked isotopic systems: UXe and UPb(using local methods of ion microprobe).

The study objects

were thoroughly selected zirconswith perfect structure, without signs of metamictiza-tion: a large fragment of a gem-quality zircon crystalfrom Sri Lanka (Ceylon Island); a zircon from Nigeriathat had a nonmetamict undisturbed structure of grains;two samples from plagiogranites (enderbites), Mac-Maxon Island, Antarctica: sample 11v32-b representedby opaque lilacyellow grains with fused crystal faces;and sample 11g-1/32 from plagiogranite (enderbite)from MacMaxon Island, Antarctica, consisting ofalmost transparent long-prismatic crystals with veryrare inclusions.

Experimental technique.

The principles of the UXe(

Xe

s

Xe

n

) method of isotopic geochronology, whichwas first proposed by one of the authors of this paper,and the fundamentals of the experimental methodologywere described in much detail in our earlier publication[3]. In order to calculate the age

, we determinedexperimentally the ratio of the

Xe

n

concentration pro-duced by spontaneous fission of

238

U (

s

)

to that pro-duced by neutron-induced fission of

235

U (Xe

n

)

in thestudied sample and the mineral monitor of a known ageirradiated by heat neutrons (

t

mon

) (zircon from rapakivisampled at the Berdyaush Massif, Southern Urals, asthe monitor mineral):

In order to determine the (

Xe

s

Xe

n

) age and param-eters of xenon migration (activation energy and fre-quency factor), we applied stepwise annealing withinthe temperature range from 300 to

1900

. The hetero-geneity of the zircon grains was examined and photo-graphed in transmitted and reflected light at 2550

magnification. The cathodoluminescence of the crys-tals was examined to get insight into their inner struc-ture and the type of their zoning. These studies were

tXesXen

tXesXen1-----

XesXen( )sampleXesXen( )mon

------------------------------------- etmon 1( ) .ln=

Unusually High Thermal Stability of the UraniumXenon Isotopic System in Nonmetamict Zircons

Yu. A. Shukolyukov

a

, M. M. Fugzan

b

, I. P. Paderin

c

, S. A. Sergeev

c

, and D. P. Krylov

Presented by Academician V.I. Kovalenko March 3, 2008

Received March 11, 2008

DOI:

10.1134/S1028334X09020251

a

Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, nab. Makarova 2, St. Petersburg, 199034 Russia; e-mail: xekrarne@JS10093.spb.edu

b

Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina 19, Moscow, 119991 Russia; e-mail: fugzan@geokhi.ru

c

Karpinskii All-Russia Research Institute of Geology (VSEGEI), Srednii pr. 74, St. Petersburg, 199026 Russia; e-mail: Sergey_Sergeev@vsegei.ru

GEOCHEMISTRY

288

DOKLADY EARTH SCIENCES

Vol. 425

No. 2

2009

SHUKOLYUKOV

et al.

carried out with the use of a CamScan 2500 MX scan-ning electron microscope. The UPb dating of zirconswas made on a SHRIMP II SIMS.

Results.

It was established that the studied zirconshave an extremely homogeneous structure. Detailedimages in transmitted and reflected light showed theabsence of visible inclusions or structural defects. Itscathodoluminescence images demonstrate only thin,weakly contrasting zoning of the magmatic type. Thedistribution of the U and Th concentrations is veryhomogeneous, with variations of U and Th concentra-tions at various sites in the grain within 23% and 5%,respectively, variations of Th concentration within 5%,and a

238

U/

232

Th

ratio, of 2.8%. The migration parame-ters of xenon in the crystalline structure, the stability ofthe uraniumxenon isotopic system, and the isotopicuraniumxenon age by the

Xe

s

Xe

n

method were deter-mined for each zircon sample. For the gem-qualitytransparent zircon from Sri Lanka, its UXe age wascompared with the SIMS SHRIMP-II UPb age. Basedon data of step annealing (concentration of xenon andits isotopic composition), we constructed curves for theescape kinetics of radiogenic Xe (Fig. 1). The integralXe escape curve is a superposition of symmetricalcurves (peaks of the independent escape rates from var-ious crystal-chemical sites in real zircon crystals). The

zircons from Ceylon define three maxima of radiogenicXe escape rates at temperature ranges of 12501450

C,16301700, and 18001850

, and the zircons fromAntarctica display four maxima of Xe escape rates (at12201250, 14001430, 16001620, and

18001830

)(Fig. 1). The presence of distinct energy states of radio-genic gas in the ideal nonmetamict structure withextremely homogeneous chemical composition isunexpected and even paradoxical. This indicates its,possibly, nanoscale heterogeneity. This possibility ismentioned in the literature [5]. Such a specificity of zir-con implies that all calculations of migration parametersof noble gases, for instance, radiogenic

4

He, according tothe classic diffusion model are inaccurate [1], because itrequires an equal energy state and homogeneous distri-bution of gas atoms in the mineral structure.

Therefore, this work is based on the concept of theexponential law of the migration kinetics of radiogenicxenon atoms, when the rate of migration is proportionalto the xenon concentration in zircon:

The

integral

form of the dependence of the current

Xe

t

concentration on the initial

Xe

0

has the following view:

where

k

=

k

0

is the frequency factor,

E

is activationenergy of migration,

R

is the gas constant, and

T

isabsolute temperature. These equations are transformedinto an equation for a straight line in the

.

The parameters of these lines can be used to calcu-late the values of the activation energy

E

and frequencyfactor

k

0

. The activation energy of Xe escape and thevalue of the frequency factor in the studied zircons varystrongly, primarily for distinct energy states in the samesample. For instance, the activation energy of theescape of

Xe

s

atoms from the low- and high-tempera-ture states of the zircon from Nigeria differs by a factorof more than 6 (18 and 116 kcal/mol, respectively).Even greater differences occur between the values ofthe frequency factor in the Xe escape equation for theseenergy states:

k

0

= 2.80

10

1

and

1.6

10

12

s

1

.

There are strong differences in the activation energyof the same energy state in the crystal lattice of differentzircon samples, for example, high-temperature ones: inparticular, those in zircons from Antarctica and Nigeriaare 78 and 116 kcal/mol, respectively, while the gem-quality zircon from Sri Lanka is characterized by thehighest activation energy

E

= 152 kcal/mol at

1790

(29% of total radiogenic Xe).

d Xe[ ]dt--------------- k Xet[ ].=

Xet[ ] Xe0[ ]e kt ,=

e

ERT-------

Xe0[ ]Xet[ ]

-------------lnln 1T---

1

1000 1200 1400 1600 1800 2000

T

, C

0

2

3

4

5

6

Zircon from Antarctica

2

0

4

6

8

Zircon from Sri Lanka

[

136

Xe

n

], 10

11

cm

3

/g

Fig. 1.

Escape curves of Xe produced by neutron-inducedfission of

235

U from nonmetamict zircons with perfectstructure.

DOKLADY EARTH SCIENCES

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No. 2 2009

UNUSUALLY HIGH THERMAL STABILITY OF THE URANIUMXENON ISOTOPIC SYSTEM 289

Knowledge of the migration parameters of xenonmade it possible to estimate the relative thermal stabil-ity of the uraniumxenon isotopic system in the studiedzircon samples. Because of this, the concept of the con-ventional closure temperature of the mineral system Tcwas used as a criterion of the stability of the UXe iso-topic systems. This temperature is understood as thetemperature at which 98% radiogenic Xes is retained inthe mineral for a specified time (for example, a timeequal to the age of the mineral). It was found that muchXe produced by the spontaneous fission of U (Xes) inour samples is generally tightly held by the crystalstructures. For example, the conventional closure tem-perature of the zircon from Nigeria is 860 and pro-

vides >98% retention of radiogenic Xes at a high-tem-perature energy state for 1 Ga, i.e., the whole lifetimeof the sample (Fig. 2).

Amazing stability is displayed by the UXe isotopicsystem of zircon from Sri Lanka. The crystal structureof this sample is such that its heating to 1200 during650 Ma ensured the preservation of 98% Xes at a high-temperature state (28% radiogenic Xe). At a tempera-ture of 1100, this zircon could retain all of its radio-genic Xe for a much longer time. At 1300, the zirconcould hold close to 70% of its Xe for 600 Ma.

Thus, the obtained data demonstrate that zirconswith perfect structures provide the preservation of all(or much of) their radiogenic Xe in the structure, inspite of the possibility of the partial loss of spontaneousfission Xe from the mineral during its geological life-time. This feature of Xe migration kinetics in the zirconcrystal structure confirms the principle of dating of

0.1

11001000 1200 1300 1400 1500T, C

0

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0Zircon from Sri Lanka

Tc

0.1

700 800 900 1000 1100T, C

0

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0Preservation Xes

Tc

Fig. 2. Conventional closure temperatures (Tc) correspond-ing to retention of no less than 98% radiogenic Xe in twozircon samples (860 and 1200C).

Zircon from Nigeria

100

0.10 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

200

300

400

500

600

700

Zircon from Sri Lanka (Ceylon Island)

Cumulative fraction of released Xen

XeXe621 20 Ma

616 6 MaUPb

200

0.10 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

300

500

600

1000

1100

1200

Zircon from Antarctica,sample 11v32-b

900

800

700

400

100

Age, Ma

Fig. 3. Age XesXen spectra of two zircon samples: the pla-teau corresponds to the true mineral age, which was confirmedfor zircon from Sri Lanka by UPb SHRIMP-II dating.

290

DOKLADY EARTH SCIENCES Vol. 425 No. 2 2009

SHUKOLYUKOV et al.

minerals by the XesXen method with the use of agespectra. One of the distinctive features of these spectrais young ages at relatively low annealing temperaturesbecause of the partial loss of radiogenic Xe in thecourse of its geological history. Another characteristicfeature is the occurrence of an age plateau at high tem-peratures, which corresponds to the crystallization timeof the mineral. All the studied samples, for instance, thezircons from Antarctica and Sri Lanka, showed thistype of age spectra (Fig. 3).

The study of the Sri Lanka zircon (Ceylon Island) ona SHRIMP-II secondary-ion mass spectrometer con-firmed XesXen plateau ages: the average UPb age is616 6 Ma while the age obtained by the Xes-Xenmethod is 621 22 Ma.

ACKNOWLEDGMENTSThis study was supported by the Russian Founda-

tion for Basic Research, project no. 07-05-00634-a.

REFERENCES

1. Low-Temperature Thermochronology: Techniques, Inter-pretations, and Applications, Ed. by J. J. Rosso, Rev. Min-eral. Geochem. 58 (2005).

2. Yu. A. Shukolyukov, S. A. Sergeev, M. M. Fugzan, andI. P. Paderin, Proceedings of 18th Vinogradov Sympo-sium on Isotope Geochemistry, Moscow, Russia, 2007(GEOKHI RAN, Moscow, 2007), pp. 292294.

3. D. P. Krylov and Yu. A. Shukolyukov, Petrology 11, 94(2003) [Petrologiya 11, 102 (2003)].

4. Yu. A. Shukolyukov, Fission Products of Heavy Ele-ments at the Earth (Energoizdat, Moscow, 1982) [inRussian].

5. S. Utsunomiya, J. W. Valley, A. J. Cavosie, et al., Radi-ation Damage and Alteration of Zircon from a 3.3 GaPorphyritic Granite from the Jack Hills, Western Austra-lia, Chem. Geol. 236, 92111 (2007).

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