xes-xenthermochronology of the rayner metamorphic complex, enderby land (east antarctica,...

ISSN 0869�5911, Petrology, 2014, Vol. 22, No. 5, pp. 438–449. © Pleiades Publishing, Ltd., 2014.Original Russian Text © D.P. Krylov, Yu.A. Shukolyukov, 2014, published in Petrologiya, 2014, Vol. 22, No. 5, pp. 469–481.

438

† INTRODUCTION

The Rayner Complex of the Enderby Land com�prises Precambrian sedimentary and igneous rocksmetamorphosed under amphibolite and granulitefacies conditions. Similar to many other Precambrianmetamorphic terranes, ambiguous age estimates werereported for the main stages of the geologic develop�ment of the Rayner Complex. The most controversialissue is the evolution of this complex within the inter�val 1.0–0.5 Ga. In particular, there are no reliable ageestimates for amphibolite and granulite metamor�phism, which is regionally manifested in the RaynerComplex. Nonetheless, it is generally accepted thatthe maximum metamorphic temperatures corre�sponded to the Grenville stage of the development ofthe complex (Black et al., 1987). Pan�African age esti�mates reported for the metamorphic rocks of theRayner Complex were previously interpreted as aresult of the influence of magmatic activity (formationof pegmatite fields), which caused local alterations inthe country rocks under amphibolite facies conditions

† Deceased.

(Grew, 1978, 1981). Subsequently, the pan�Africanage estimates were alternatively interpreted as reflect�ing the regional retrograde metamorphism of theRayner Complex under greenschist facies conditions(400–500°С) (Black et al., 1987).

This paper addresses the problem of the age ofendogenous processes manifested in the Rayner Com�plex and discusses the results of the Xes�Xen thermo�chronological investigation of zircon from the meta�morphic, ultrametamorphic, and igneous rocks of theMolodezhnaya Station area. The main advantage ofthe Xes�Xen thermochronology method, which wasdeveloped by Shukolyukov et al. (1974a, 1974b, 1975,1976, 1977), is the possibility to estimate the age of“primary” processes even in the case of partial naturalloss of xenon. In addition, the wide spectrum of xenonisotopes provides an opportunity to compare age esti�mates calculated from different isotope ratios and con�trol atmospheric contamination. Since the Xes�Xenmethod is based on xenon isotopes from the spontane�ous fission of uranium, the minerals used for Xes�Xengeochronometry are also suitable for U–Pb dating,which provides an opportunity to compare directly the

Xes–Xen Thermochronology of the Rayner Metamorphic Complex, Enderby Land (East Antarctica, Molodezhnaya Station Area)

D. P. Krylov and Yu. A. Shukolyukov†

Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, nab. Makarova 2, St. Petersburg, 199034 Russia

e�mail: [email protected] January 17, 2014; in final form March 27, 2014

Abstract—The Xes–Xen dating of zircons from rocks of the Rayner Complex of the Enderby Land at the Molo�dezhnaya Station area (coast of the Alasheyev Bight) yielded age estimates of 550 ± 50 and 1040 ± 30 Ma. Themetamorphic rocks of the Rayner Complex record two main events: first, the crystallization of the magmaticprotoliths of charnockitic and enderbitic gneisses and, second, superimposed structural and metamorphicalterations under conditions transitional from the amphibolite to granulite facies (metamorphism manifestedregionally in the rocks of the Rayner Complex). The most reliable Xes–Xen age estimates for magmatic zir�cons from the charnockitic and enderbitic gneisses correspond to the Grenville stage of the development ofthe Rayner Complex (~1.0 Ga). The Xe isotopic systematics of metamorphic zircons reflect a pan�Africanstage in the evolution of the Rayner Complex (600–550 Ma). Pan�African events are reflected in the U–Xeisotope system in two cases: if metamorphic zircons crystallized at the same time (which probably resulted inthe formation of a plateau in the Xes–Xen age spectrum) and if the initial isotopic systems were disturbed(which resulted in a decrease in apparent age toward low�temperature gas fractions). It is important that sec�ondary alterations and a decrease in apparent ages to 600–550 Ma affected only those components(i.e., caused xenon release only from those traps) that were unstable under the maximum metamorphic tem�peratures and yielded Tcl values lower than 750°C (conditions transitional from the amphibolite to granulitefacies). At a higher xenon retention, “primary” isotopic systems are preserved. Consequently, the age ofmetamorphism transitional between the amphibolite and granulite facies can be estimated at 600–550 Ma onthe basis of Xes–Xen dating. In general, the results of our study indicate that the age of regional metamor�phism of the Rayner complex at the Molodezhnaya area is approximately 600–550 Ma rather than ~1.0 Ga,as was previously supposed.

DOI: 10.1134/S0869591114050051

PETROLOGY Vol. 22 No. 5 2014

Xes–Xen THERMOCHRONOLOGY OF THE RAYNER METAMORPHIC COMPLEX 439

results obtained by these methods and, thus, estimatetheir accuracy.

CHARACTERISTICS OF SAMPLES FOR Xes–Xen THERMOCHRONOLOGY

The Molodezhnaya Station area (Alasheyev Bightcoast) is located in the western Enderby Land andcomprises the Thala Hills, Widdows Promontory,Konovalov–Gorodkov Mountains, and adjacentislands (Fig. 1). The geologic structure of this region ismade up of compositionally diverse metamorphicrocks, which are usually assigned to the Rayner meta�morphic complex. They are represented (Grew, 1978,1981; Black et al., 1987) by variably migmatizedpyroxene, hornblende, garnet–biotite, and garnet–pyroxene gneisses, as well as charnockitic, enderbitic,and granodioritic gneisses, and their metamorphicgrade correspond to the granulite and amphibolitefacies.

The sites of sampling for Xes�Xen thermochrono�logical investigations are shown in Fig. 1.

Migmatite leucosome (western coast of Freeth Bay,sample 43/32). Zircon was observed in a migmatiteleucosome of granitic composition mainly betweenorthoclase grains and occasionally in biotite aggre�gates. Smaller zircon grains were also found as inclu�sions in biotite. The zircon occurs usually as large(0.15–0.25 mm), well�faceted, short prismatic (aspectratio of 1.5), and slightly turbid grains of a honey yel�low color. The preservation of faces, absence of dis�tinct cores, and association with typical magmaticorthoclase grains support the formation of this zircongeneration during the crystallization of anatectic meltparental for the leucosome. The morphometricparameters (Pupin, 1985) of most zircon grains corre�spond to four types, S19, S20, S24, and S25, and thefirst of them is most common. The morphometricmean point is shifted toward the S25 field. The evolu�tionary trend of zircon is directed from more alkaline

Widdows

Freeth Bay

Konovalov Mt

Mount Vechernyaya

McMahon Islands

Enderby Land

Antarctica

Fluted Rock

A l a s h e y e v B i g h t

T h a l a H i l l s

11/32

43/32

N

37/32

1

2

1

2

3

4

E

05km

46°00′

67°40′

67°50′45°30′ E

N70°

60°50°

Promontory

Fig. 1. Geological sketch map of the region of the Alasheyev Bight, Enderby Land. (1) and (2) Rayner metamorphic complex:(1) enderbitic, charnockitic, and granodioritic gneisses; (2) migmatized pyroxene, hornblende, garnet–biotite, and garnet–pyroxene gneisses; (3) sampling site for Xes�Xen geochronological investigation and its number; and (4) Molodezhnaya Station.The inset shows the location of the Enderby Land: (1) Rayner metamorphic complex and (2) Napier metamorphic complex.

440


KRYLOV, SHUKOLYUKOV

crystallization conditions at high temperatures of~850°С toward lower alkalinity and a temperature of~800°С.

Biotite–amphibole plagiogneiss (Widdows Prom�ontory, sample 37/32). Zircon grains occur mainly inaggregates of mafic minerals (biotite and amphibole).The zircon is clear and colorless or slightly pinkish.Morphometric analysis is hampered by the poor pres�ervation of faces. Elongated zircon grains are usuallyoriented parallel to the crystallization schistosity. Thismay indicate that its formation (transformation)occurred under metamorphic conditions, which issupported by low U contents of approximately 200–300 ppm.

Enderbitic and charnockitic gneisses (McMahonIslands). Zircons from enderbitic gneiss (sample 11v/32)and garnet�bearing charnockitic gneiss (sample 11g/32),which originated from magmatic protoliths, areassigned to two morphometric types.

I. Long prismatic grains of zircon habits, transpar�ent or slightly turbid, colorless or yellowish, with asmall amount of dustlike inclusions. Noteworthy isthat the longer axes of the crystals are aligned parallelto crystallization schistosity.

II. Subhedral and sometimes reniform grains withcorroded faces, lilac–yellow and turbid. They wereoccasionally observed within orthopyroxene grains.These two zircon varieties are most likely of metamor�phic and magmatic origin, respectively.

BASICS OF THE Xes�Xen METHOD

The idea to use xenon in geochronology (Khlopinet al., 1947) is based on the spontaneous fissionof 238U:

(1)

where t is the sample age; Xes and 238U are the atomicconcentrations of xenon from spontaneous fission and238U, respectively; λ

α is the decay constant of 238U,

1.55125 × 10–10 yr–1 (Steiger and Jager, 1977); λs is theconstant of 238U spontaneous fission; ys is the frac�tional yield of Xes per one spontaneous fission; and136Xe λsys for 136Xe is 5.677 × 10–18 yr–1.

Equation (1) provides the lower constraint for agein the case of xenon loss. The U�Xe method requiresprecise Xe and U measurements in a single sample.

The content of uranium is not determined in theХеs�Xen method with stepwise heating (Shukolyukovet al., 1974, 1976, 1977; Teitsma et al., 1975; Teitsmaand Clarke, 1978), and the sample is irradiated bythermal neutrons, which induce 235U fission but do notaffect 238U. The xenon component produced duringirradiation (Xen) is isotopically different from Xes, and

tU–Xe1

λα

��Xes

U238��

λα

λsys

�� 1+⎝ ⎠⎛ ⎞ ,ln=

the Xes/Xen ratio can be determined from the massbalance equation:

(2)

where the n, s, and m subscripts denote neutron�induced, spontaneous, and measured xenon isotoperatios, respectively. The firs two ratios are tabulated byLedere and Shirley (1978) and Shukolyukov et al.(1994). The Хеs/Xen ratio can be calculated by threeindependent methods; for instance, for i = 134, 132,and 131 at fixed j = 136. More accurate results can beobtained for i = 134 and 131, because the difference inisotopic ratios between the s� and n�components of132Xe is very small. Hence, instead of the direct mea�surement of 238U content, which is necessary for agedetermination using Eq. (1), it can be calculated fromthe relation

(3)

where Φ is the neutron fluence (integrated neutronflux); yn is the xenon yield at the neutron�induced 235Ufission; σ is the thermal neutron cross�section for theinduced fission of 235U; R is the modern 235U/238Uratio, which is a constant of 1/137.88 (Shields, 1960).

If a standard sample (monitor) of known age is irra�diated in the same neutron flux, then

(4)

where the “sam” and “mon” subscripts stand for sam�ple and monitor, respectively. Hence, the Хеs�Xen ageis a function of a single variable, the Хеs/Xen ratio.

Using the notation

(5)

Eq. (4) can be more conveniently rewritten as

(6)

The value of F = (Хеs/Xen)sam can be determined foreach particular gas increment during stepwise heating.

The Хеs�Xen method with stepwise heating is simi�lar to the widely used method of 39Ar�40Ar geochronol�ogy. Neutron activation and stepwise heating areemployed in both cases, which enables obtaining rea�sonable age estimates at a partial loss of daughter iso�topes. However, the Хеs�Xen method has certainadvantages over 39Ar�40Ar, because (1) xenon is char�acterized by better preservation owing to larger atomicradius and mass, (2) Хеs�Xen ages can be calculated bythree independent ways, and (3) the content of excessradiogenic xenon in uranium�bearing minerals is verylow, because the abundance of xenon in the Earth’s

Xes

Xen

�� Xei

/ Xej( )n Xe

i/ Xe

j( )m–

Xei

/ Xej( )m Xe

i/ Xe

j( )s–��,=

Xen U235 Φσ235yn U

238RΦσ235yn,= =

tXes–Xen

= 1λα

��Xes

Xen

��⎝ ⎠⎛ ⎞

sam

Xen

Xes

��⎝ ⎠⎛ ⎞

mon

tmonλα

( )exp 1–( ) 1+⎝ ⎠⎛ ⎞ ,ln

J tmonλα

( )exp 1–( )/ Xes/Xen( )mon,=

t 1λα

�� FJ 1+( )ln .=



crust is several orders of magnitude lower that the con�centrations measured.

ANALYTICAL PROCEDURE

Accessory zircon was extracted from 15–20�kgsamples using heavy liquids and magnetic separation.Zircons of a single grain�size fraction, 100–150 μm,in which the absence of phase inclusions was con�trolled by X�ray diffraction were selected for Хеs�Xengeochronological investigations. Zircons werewrapped in Al foil and loaded together with a monitorinto a quartz ampoule for subsequent irradiation. Zir�cons from the rapakivi granite of the Berdiaush massifand Afshar Mount granite (eastern Afghanistan) wereused as monitors. The zircon of the Berdiaush massifhave a concordant U–Pb age of 1.37 ± 0.02 Ga(Krasnobaev et al., 1984) and an identical U–Xe ageof 1.4 Ga. The U–Pb age of zircon from the AfsharMount granite is 2060 ± 30 Ma (Bibikova et al.,1990).

Samples were irradiated by thermal neutrons (fluencevaried between sessions from 3 × 1016 to 6 × 1016 n/cm2)in the research reactor of the Moscow Institute ofPhysical Engineering. The maximum temperature ofsamples reached during irradiation did not exceed300°С. In order to check the uniformity of the neutronflux, several monitor samples were placed regularlyalong the length of the ampoule. Neutron flux gradi�ents exceeding the experimental errors were neverdetected. Therefore, mean irradiation factor, J, wascalculated for each session.

Xenon was extracted in a resistance�heated Ta fur�nace. Temperature was controlled within ±15°С usinga W–Re thermocouple, which was placed directlynear the sample container. In addition, temperaturewas determined using an optical pyrometer. The dura�tion of each heating step varied between the experi�mental series and was commonly 30–45 min. Thetemperature increment between heating steps was usu�ally 100 ± 10°С. The maximum experimental temper�ature depended on the cessation of xenon release dur�ing zircon decomposition and was usually 1800–1900°С. In order to eliminate the memory effect, thevessel was annealed between measurements at a tem�perature approximately 100°С higher than the maxi�mum temperature of the previous experiment. Theextracted gas was purified using metallic Zr–Ti getterscooled from 700 to 200°С and activated charcoal trapsheld at liquid nitrogen temperature. Xenon isotoperatios were measured using a modernized MI�1201mass spectrometer operated under static vacuum con�ditions. Analytical blanks were experimentally deter�mined using Al foil without a zircon sample and thesame procedure as for a sample heated up to 1800°С.The gas yield in the blank experiment was no higherthan 5 × 10–14 cm3 STP 136Xe, which is 1–2 orders ofmagnitude lower than the amount of gas released dur�ing any heating step. The measurement of each gas

increment was performing by scanning the magneticfield over the mass range 136–124 (or 136–129). Theduration of peak measurement was ~10 s. An addi�tional peak adjustment provided a sensitivity ofapproximately 10–14 cm3 STP 136Xe. The results ofmeasurements were extrapolated to the moment of gasintroduction into the chamber. The xenon isotoperatios were corrected for mass discrimination, whichwas no higher than 0.3% per mass unit. The mass dis�crimination was in turn determined using an airpipette, which was measured before and after eachsample and sometimes also between increments. Cor�rection for atmospheric contamination was based on130Xe, because this isotope is absent in the s� andn�components. The radiogenic xenon component wascalculated from the mass balance condition:

(7)

where i denotes 134Xe, 132Xe, or 131Xe; “0” denotes130Хе; “6” denotes 136Xe; and the “rad”, “atm”, and“m” subscripts refer to the radiogenic and atmo�spheric components and measured isotope ratio,respectively. The fraction of the atmospheric compo�nent did not exceed 1% of the radiogenic component,except for the low�temperature gas fractions, whichmake up no more than 3% of all xenon released. Theuncertainty of Хеs/Xen determination was estimatedby differentiating Eqs. (7) and (6):

(8a)

and

(8b)

where σ is the standard deviation, F = 136Xes/136Xen,

and the other symbols were defined above. For i = 134,taking into account the standard isotope ratios ofxenon (Lederer and Shirley, 1978) and that the (i/6)radvalue is close to one, Eqs. (8a) and (8b) can be recast as

(9a)

and

(9b)

i/6( )radi/6( )m 0/6( )atm i/6( )atm 0/6( )m–

0/6( )atm 0/6( )m–��,=

σrad2 i/6( )rad i/6( )atm–

0/6( )atm 0/6( )m–��⎝ ⎠

⎛ ⎞ σ0/62=

+0/6( )atm

0/6( )atm 0/6( )m–��⎝ ⎠

⎛ ⎞2

σi/62

,

σFi/6( )s i/6( )n–

i/6( )rad i/6( )s–��σrad,=

σrad2 4/6( )rad 1.176–

0.458 0/6( )m–��⎝ ⎠

⎛ ⎞2

σ0/62≈

+ 0.4580.458 0/6( )m–��⎝ ⎠

⎛ ⎞ 2

σ4/62

,

σF 13.6σrad.≈

442


KRYLOV, SHUKOLYUKOV

The errors of age estimates were calculated using theequation of Dalrymple et al. (1981):

(10)

RESULTS OF Xes–Xen THERMOCHRONOLOGY

The results of Xes�Xenn thermochronology for zir�con from the rocks of the Rayner polymetamorphiccomplex are shown in Table 1 and Fig. 2.

Migmatite leucosome (western coast of FreethBay). The age spectra of zircon from the migmatiteleucosome (sample 43/32) obtained from the134Хе/136Хе and 131Хе/136Хе ratios show distinct pla�teaus for xenon released between 1400 and 1750°С,which accounts for more than 70% of total xenon(Table 1, Fig. 2a). The 134Хе/136Хе results for the high�est temperature increment do not differ significantlyfrom those for any gas fraction of the plateau. Theweighted mean plateau ages are 569 ± 57 Ma(134Хе/136Хе) and 667 ± 18 Ma (131Хе/136Хе). Xenonreleased at a lower temperature of 1300°С yields some�what lower (compared with the plateau) apparent agescalculated from the 134Хе/136Хе ratio (<500 Ma). Theages obtained from 131Хе/136Хе are in general higherthan those from 134Хе/136Хе, and the statistical error ofage estimate is lower in the case of the 131Хе/136Хеratio. The ages based on the total xenon released fromthe zircon (559 ± 55 Ma) are almost identical to theplateau age, which implies that only a negligible xenonfraction could be lost after the process considered.This provides an opportunity to estimate indepen�dently the U–Xe age of zircon on the basis of U con�tent obtained during U–Pb dating (1156 ppm) and thecontent of the 136Xe component of spontaneous fis�sion, which was determined from the Xe isotope ratios.The obtained ages of 590 Ma (from Хеs/Xen calculatedon the basis of 134Хе/136Хе) and 600 Ma (from Хеs/Xencalculated on the basis of 131Хе/136Хе) can be consid�ered as the best estimates, because they are consistentwith each other and correspond to values obtained bythe Xes�Xen method on the basis of the independentisotope ratios 131Хе/136Хе and 134Хе/136Хе.

Biotite–amphibole plagiogneiss (Widdows Promon�tory). The age spectrum of zircon from the biotite–amphibole plagiogneiss (sample 37/32, Fig. 2b) doesnot differ significantly from the spectrum of the previ�ous sample. Gas increments from 1500 to 1700°Сconstitute more than 60% of total released xenon andyield distinct age plateaus calculated on the basis ofboth 134Хе/136Хе (526 ± 71 Ma) and 131Хе/136Хе(575 ± 33 Ma). The 134Хе/136Хе age is almost identicalto that obtained for this zircon by the U�Pb method,and an age estimate of 575 ± 33 Ma for it is consistentwith its U–Xe age. It is conceivable that a decrease inapparent age toward gas fractions extracted at the low�est temperatures relative to the plateau age of this sam�

σt2 J2σF

2 F2σJ2+

λα

21 FJ+( )

��.=

ple estimated from the 131Хе/136Хе ratio indicates aconsiderable xenon loss during superimposed pro�cesses. This is also suggested by a younger age obtainedfor bulk zircon (488 ± 69 and 482 ± 26 Ma for134Хе/136Хе and 131Хе/136Хе TF, respectively; Table 1)compared with the plateau age.

Enderbite (McMahon Islands). Xenon geochronologi�cal data for zircon from the enderbite (sample 11v/32) aresignificantly different from those for the samplesdescribed above (Table 1, Figs. 2c, 2d). Except for thelowest temperature gas fractions, a classic staircase agespectrum was observed in this sample, which could berelated to the diffusive xenon loss during superim�posed processes. However, the results obtained fromthe 131Хе/136Хе ratio for increments between 1600 and1880°С (60% of total xenon) do not differ significantly,which allowed us to calculate plateau ages for type I(1026 ± 27 Ma) and II (1023 ± 49 Ma) zircons fromsample 11v/32.

Charnockitic gneiss (McMahon Islands). Thehigh�temperature (>1400°С) xenon fractions from zir�con of the charnockitic gneiss (Table 1, sample 11g/32;Fig. 2e) also show staircase patterns. However, similarto the above sample (11v/32), the differences of theapparent ages of sequential xenon increments are sta�tistically insignificant, and plateau ages can be esti�mated at 1034 ± 73 and 1060 ± 34 Ma from134Хе/136Хе and 131Хе/136Хе, respectively. The rela�tively low�temperature xenon fractions (up to 1400°C)form a plateau with ages of 573 ± 70 and 574 ± 29 Maestimated from the 134Хе/136Хе and 131Хе/136Хе ratios,respectively.

DISCUSSION

The geological interpretation of the results ofXes�Хеn zircon dating requires a comparison of thestability of xenon in zircon and the conditions ofendogenous processes which accompanied its forma�tion and (or) reworking. In some cases, the similarityof Xes�Хеn plateau ages to the total fusion (TF) ages,as well as U–Pb isotopic ages indicates that xenon waswell preserved in zircon even under high�temperaturemetamorphic conditions. However, given the consid�erable scatter of kinetic parameters for xenon loss(Krylov and Shukolyukov, 1994) the spectra of gasrelease should be scrutinized in each particular sam�ple. To quantify xenon stability, the closure tempera�tures (Tcl) of the xenon isotope system were estimatedfor the zircons on the basis of the diffusion model ofDodson (1973). They were calculated from the diffu�sion activation energy Eact, preexponent D0, effectivediffusion length a, grain�shape parameter A, and cool�ing rate of a rock. The diffusion model provides anadequate description of xenon loss from zircon underboth experimental (Krylov and Shukolyukov, 1994)and natural conditions (Krylov et al., 1993).



0 0.2 0.4 0.6 0.8 1.0

0.5

1.0

F

Т

Tp(1/6) = 671 ± 18 Ma

Tp(4/6) = 559 ± 55 Ma

Sample43/32

0 0.2 0.4 0.6 0.8 1.0

0.5

1.0

F

Т

Tp(1/6) = 575 ± 32 Ma

Tp(4/6) = 541 ± 72 Ma

Sample37/32

(a) (b)

0 0.2 0.4 0.6 0.8 1.0

1.2

1.6

F

Т

Tp(1/6) = 1026 ± 27 Ma

Sample11v/32(I)0.4

0.8

(c)

00.2

0.2 0.4 0.6 0.8 1.0

0.5

1.0

F

Т

Tp(1/6) = 1060 ± 27 Ma

Tp(4/6) = 1023 ± 49 Ma

Sample11v/32(II)

(d)

0 0.2 0.4 0.6 0.8 1.0

0.5

1.0

F

Т

Tp(1/6) = 1060 ± 34

Tp(4/6) = 573 ± 70

Sample11g/32

Tp(1/6) = 574 ± 29Ma

Tp(4/6) = 1034 ± 73Ma

Ma

(e)

Fig. 2. Xes�Xen age spectra of zircon. Tp is the plateau age. The Хеs/Xen ratios were calculated from 131Xe/136Xe (1/6, hatchingsloping to right), 134Xe/136Xe (4/6, hatching sloping to left). The height of rectangles corresponds to the standard error (2σ) ofthe age of the increment. T is the apparent age. F is the cumulative fraction of released xenon relative to the bulk xenon contentof zircon.

444


KRYLOV, SHUKOLYUKOV

Table 1. Xenon isotope ratios and Xes–Xen ages of zircons: stepwise heating results

T, °C

Xe isotope ratios136Xe = 1000

Xes–Xen age, Ma

134Xe 131Xe 134Xe 131Xe

Migmatite leucosome (western coast of Freeth Bay, sample 43/32)

J = 0.4105, 119.5 mg, 1.9 × 10–9 cm3 g–1 136Xe STP (5.097 × 1010 at/g); U = 1030 ppm

900 (–260 ± 299) (–255 ± 62)

1100 (–304 ± 258) (–123 ± 22)

1200 (–104 ± 204) (225 ± 33)

1300 154 ± 95 221 ± 14 395 ± 237 560 ± 37

1400 208 ± 68 270 ± 19 529 ± 166 677 ± 48

1520 234 ± 41 251 ± 19 590 ± 99 632 ± 47

1650 241 ± 54 266 ± 12 608 ± 132 668 ± 31

1750 212 ± 41 270 ± 12 537 ± 100 677 ± 31

1800 234 ± 122 344 ± 52 590 ± 296 852 ± 123

1880 (609 ± 150) (725 ± 122) (1439 ± 318) (1679 ± 251)

TF 232 258 586 ± 55 650 ± 16

P1(1400–1800°C) 221 559 ± 55

P2(1400–1750°C) 266 671 ± 18

Biotite–amphibole plagiogneiss (Widdows Promontory, sample 37/32)

J = 0.4105, 82 mg, 3.87 × 10–10 cm3 g–1 136Xe STP (1.040 × 1010 at/g); U = 230 ppm

1100 (451 ± 639)

1200 (–12 ± 394) (–571 ± 8)

1300 (–31 ± 408) (–307 ± 13)

1400 (–116 ± 150) (52 ± 21)

1500 212 ± 68 251 ± 26 537 ± 166 632 ± 65

1600 204 ± 41 207 ± 21 519 ± 100 526 ± 53

1700 229 ± 54 232 ± 22 578 ± 132 587 ± 55

1800 43 ± 150 605 ± 56 113 ± 389 1429 ± 122

1880 –140 ± 204 62 ± 35 161 ± 91

TF 192 189 488 ± 69 482 ± 26

P(1500–1700°C) 207 227 541 ± 72 575 ± 32

Charnockitic gneiss (McMahon Islands, sample 11g/32)

J = 0.4105, 39 m, 2.77 × 10–9 cm3 g–1 136Xe STP (7.448 × 1010 at/g); U = 1350 ppm

1100 252 ± 122 268 ± 54 635 ± 294 672 ± 128

1200 201 ± 68 220 ± 29 511 ± 167 558 ± 71

1300 226 ± 41 241 ± 21 572 ± 100 607 ± 53

1400 237 ± 54 216 ± 16 599 ± 132 548 ± 42

1500 272 ± 54 412 ± 39 681 ± 130 1007 ± 90

1600 378 ± 68 395 ± 22 930 ± 157 969 ± 53

1700 406 ± 53 494 ± 25 994 ± 125 1190 ± 60

1800 456 ± 70 400 ± 1110 1106 ± 123 982 ± 77

1880 477 ± 109 482 ± 49 1152 ± 242 1163 ± 110

P1(1000–1400°C) 226 227 573 ± 70 574 ± 29

P2(1500–1800°C) 424 435 1034 ± 73 1060 ± 34



Table 1. (Contd.)

T, °C

Xe isotope ratios136Xe = 1000

Xes–Xen age, Ma

134Xe 131Xe 134Xe 131Xe

Enderbitic gneiss (McMahon Islands, sample 11v/32, type I zircon)


1100 (256 ± 231) (1070 ± 186)

1200 346 ± 109 344 ± 48 857 ± 253 852 ± 113

1300 477 ± 54 515 ± 39 1152 ± 123 1236 ± 88

1400 223 ± 21 330 ± 13 567 ± 52 815 ± 34

1500 300 ± 55 368 ± 18 750 ± 105 907 ± 46

1600 329 ± 27 402 ± 19 817 ± 66 985 ± 48

1700 342 ± 51 414 ± 14 847 ± 96 1013 ± 39

1800 415 ± 55 469 ± 31 977 ± 132 1135 ± 73

P1(1500–1700°C) 324 806 ± 47

P2(1600–1800°C) 420 1026 ± 27

Enderbitic gneiss (McMahon Islands, sample 11v/32, type II zircon)


1100 223 ± 129 202 ± 72 564 ± 314 513 ± 177

1200 179 ± 80 212 ± 45 457 ± 198 539 ± 110

1300 218 ± 34 241 ± 30 552 ± 83 607 ± 73

1400 230 ± 27 287 ± 14 581 ± 67 719 ± 36

1500 296 ± 41 448 ± 23 738 ± 97 1089 ± 57

1600 417 ± 27 447 ± 23 1018 ± 66 1086 ± 56

1700 430 ± 41 418 ± 17 1047 ± 94 1021 ± 43

1800 436 ± 68 434 ± 34 1060 ± 154 1057 ± 80

1880 355 ± 95 460 ± 53 878 ± 221 1115 ± 121

P1(1000–1400°C) 222 229 562 ± 49 578 ± 58

P2(1600–1800°C) 419 435 1023 ± 49 1060 ± 27

Monitors (bulk analyses of zircon)

B1 1104 ± 4 400 ± 5

B2 1103 ± 4 357 ± 7 1400

B3 1108 ± 6 439 ± 7

Reference values (Lederer and Shirley, 1978)

Xeatm 1176 2394

Xes 828 88

Xen 1238 456

The Xes/Xen values are corrected for mass discrimination by the triple isotope dilution method (using a mixed spike of 124Xe, 126Xe, and128Xe) and for atmospheric contamination (Eq. (7)). All errors reported are 1σ. STP denotes standard T and P conditions (0°C and100 kPa). J is the neutron fluence (Eq. (3)). The results in parentheses are insignificant because of strong atmospheric contamination (ini�tial steps of heating). P1 and P2 are plateau ages (numbers in parentheses show the temperature range of the plateau); TF denotes agescorresponding to bulk Xe released (Total Fusion). B1, B2, and B3 are replicate measurements of zircon from granites of the BerdyaushMassif.

446


KRYLOV, SHUKOLYUKOV

Based on the cumulative fractions of releasedxenon (F in Fig. 2), we calculated differential xenonlosses (rate of xenon loss, i.e., the interpolated tem�perature derivative of F, ∂F/∂T, Krylov et al., 1993).Table 2 gives the characteristic experimental maximaof differential xenon losses. Xenon release from all zir�con samples analyzed is characterized by polymodaldifferential losses, typically with two peaks. Morecomplex patterns of xenon release were observed inzircon from the enderbitic gneiss (sample 11v/32 (II))with a minor low�temperature (1200°С) tail. Thehighest rates of xenon release were observed in all thezircon samples between 1330 and 1550°С and at1650–1790°С. The existence of several maxima of dif�ferential losses in a single zircon sample can be relatedto the zoning of grains and the existence of phases withdifferent crystal chemical parameters or differentpositions of xenon in each phase. In any case, it can beconcluded that xenon from spontaneous fission occursin energetically nonequivalent sites in the crystalstructure of the zircon. Each peak of gas release can be

described by the two Arrhenius parameters (activationenergy and preexponential factor, Table 2), whichquantify the state of xenon atoms during their migra�tion through the mineral lattice. Based on the resultsof stepwise heating experiments, we calculated theclosure temperatures of the Xes–Хеn system in the zir�con studied separately for each peak of gas release atdifferent cooling rates (Table 2).

Under natural conditions, secondary processes (inthe present context, processes occurring after thebeginning of zircon formation) may have differenteffects on the Xes–Хеn isotope system depending onthe energy state of xenon. Part of xenon occurring inhighly mobile states (lower temperature peaks on thecurves of differential xenon release) can be expelledfrom the crystal, whereas xenon corresponding to thehigh�temperature peaks of gas release can remain sta�ble. Such differential xenon mobility in a single samplemay affect the age spectrum (plateau disturbance).

Table 2. Kinetic parameters of xenon loss at stepwise heating and estimation of the closure temperature of the xenon iso�topic system during metamorphism

Sample no. Tmax, °C Ea, kcal ln(D0/a2)

Tcl, °C

0.1(1) 1 10

43/32 1550 113 23.6 630 670 700

1700 272 57.3 1140 1170 1210

37/32 1450 210 49.8 910 940 980

1670 316 70.2 1180 1210 1240

11v/32(I) 1400 113 22.5 620 660 700

1800 177 31.6 >900 >900 >900

11v/32(II) 1200 90 19.1 500 530 560

1390 113 22.6 640 680 720

1540 134 25.6 760 800 840

1790 177 31.7 980 1020 1070

11g/32 1340 105 20.3 580 620 660

1650 165 37.7 700 740 780

(1) Cooling rate, °/Myr. Arrhenius parameters were calculated for each release peak separately after the deconvolution of the integratedcurve composed of overlapping peaks (Krylov et al., 1993). In further calculations, the fractions of gas release, f, were normalized tothe amount of gas related to a particular peak. Thus, any pair of Arrhenius parameters characterizes similar energy states of atoms ofmigrating gas. Diffusion parameters were calculated taking into account the disturbance of the concentration profile in grains(Fechtig and Kalbitzer, 1996) owing to xenon release during the previous heating step.



The main problem of the geological interpretationof the obtained experimental results is the substantia�tion of the plausibility of their extrapolation to naturalsystems. For instance, the effective diffusion length(parameter а in the Arrhenius equation) cannot bedetermined from the results of stepwise heating. Onthe other hand, part of xenon can be lost in naturalprocesses from traps with relatively low activationenergies, which are not observed under experimentalconditions because of the short heating time. Theasymmetry of gas release peaks could be related to var�ious factors, which was demonstrated, for example, forargon (Baadsgaard et al., 1961). However, since step�wise heating experiments are performed with the samesamples that are used for age estimation, the errors ofsome parameters compensate one another during cal�culations. For instance, the effective diffusion length (а)is combined with D (as D/a2) during the calculation ofboth kinetic parameters from stepwise heating dataand closure temperatures. Therefore, the two parame�ters need not be determined separately. The pressuredependence of Тсl is usually ignored (McDougall andHarrison, 1988). The calculation of Тсl is aimed atdetermining some boundary constraints for theparameters of xenon redistribution rather than obtain�ing accurate values for these parameters.

Migmatite leucosome (western coast of Freeth Bay,sample 43/32). The results of morphometric analysisand calculation of Zr solubility (Watson and Harrison,1983) show that zircon was formed in the migmatiteleucosome during the crystallization of its parentalgranite melt at temperatures of 750–850°С. The dif�ferential xenon losses show two peaks (Table 2). Xenonis mostly released under experimental conditions at atemperature of ~1790°С. The obtained plateau age(based on gas increments corresponding to the highertemperature peak) is 667 ± 18 Ma (Table 1). The Tclvalue calculated for this peak (Table 2) is higher thanthe possible temperatures of crystallization and meta�morphism; hence, this estimate can be interpreted asthe zircon crystallization age (correspondingly, as theage of leucosome formation). Part of xenon was prob�ably released from sites with lower potential energies(maximum of differential losses at ~1300°С), whichcorrespond to Tcl of 720–750°С; the xenon isotopesystem could be unstable above these temperatures dur�ing superimposed metamorphic processes. This gasfraction yields a Хеs�Хеn age of 560 ± 37 Ma (Table 1).This age estimate coincides with the U–Pb age of thesame zircon (535 Ma; Krylov, 1994). Thus, the U–Xe–Pb isotope system of zircon from the migmatiteleucosome preserves evidence on evolution within670–535 Ma.

Biotite–amphibole plagiogneiss (Widdows Promon�tory, sample 37/32). Spontaneous fission Xe is distrib�uted probably over two energy states in zircon from thebiotite–amphibole plagiogneiss, although the Tcl values

calculated for both of them (>900°С) are significantlyhigher than the possible metamorphic temperatures ofthe Rayner Complex. It is unlikely that the zircon maycontain an inherited component, because the Zr con�tent of the host rock (62 ppm) is significantly lower thanthe Zr content of zircon�saturated melt at temperatureshigher than 750°С (200–300 ppm). Therefore, theobtained plateau age (575 ± 32 Ma) is the crystalliza�tion age of metamorphic zircon.

Enderbitic gneiss (McMahon Islands, sample 11v/32).Xenon released from this zircon occurred probably inseveral types of traps, which is indicated by the com�plex patterns of differential losses. Based on closuretemperatures and possible natural losses, high�tem�perature fractions with Tcl higher than the maximumtemperatures of the Rayner Complex (Tmax) and rela�tively low�temperature fractions corresponding to Tclequal or lower than Tmax can be distinguished. Thehigh�temperature gas fractions define a plateau age of1030 ± 30 Ma. The Zr content of the rock (259 ppm)is higher than the level of melt saturation in zircon(87–156 ppm); hence, this zircon is most likely mag�matic in origin, which was noted above. The minimumage (approximately 600 Ma) suggests possible zirconloss from relatively low�energy sites.

Charnockitic gneiss (McMahon Islands, sample11g/32). Xenon is probably released from two traps ofdifferent energy. It is characteristic that the higher�temperature xenon fractions are released from trapscorresponding to Tcl comparable with the conditionsof amphibolite�facies metamorphism recorded in therock (740–780°C). Therefore, superimposed pro�cesses had affected the high�temperature xenon iso�tope system only to a limited extent, which resulted incomplex staircase�like patterns of xenon release.However, several sequential fractions in the range1500–1800°C form a plateau with 131Xe/136Xe and134Xe/136Xe ages of 1060 ± 34 and 1034 ± 73 Ma,respectively. The xenon fractions that are unstableunder amphibolite�facies conditions (Tcl of 580–660°C, Table 2) yield a well�defined plateau age of574 ± 29 Ma (Table 1), corresponding to the age of asuperimposed metamorphism.

CONCLUSIONS

The obtained results show that the metamorphicrocks of the Rayner Complex at the MolodezhnayaStation area provide a record of two main events: first,the crystallization of the magmatic protoliths of char�nockitic and enderbitic gneisses and, second, super�imposed structural and metamorphic transformationsunder conditions transitional between the amphiboliteand granulite facies (maximum metamorphic condi�tions manifested regionally in the rocks of the RaynerComplex). The most reliable Xes�Xen ages obtainedfor magmatic zircon from the charnockitic and ender�

448


KRYLOV, SHUKOLYUKOV

bitic gneisses correspond to the Grenville stage of thedevelopment of the Rayner Complex (at approxi�mately 1.0 Ga). The xenon isotopic systematics ofmetamorphic zircon indicate a pan�African stage ofthe Rayner Complex formation (600–550 Ma).

Pan�African processes are imprinted in the U–Xeisotope system in two cases: if metamorphic zirconcrystallized at the same time (in such a case, a plateauin the Xes�Xen age spectrum can be formed) and if theinitial isotopic systems were disturbed (the apparentage decreases then toward low�temperature gas frac�tions). It is important that the secondary processesthat caused the decrease of apparent age to 600–550 Ma affected only those components (i.e., resultedin xenon removal only from those traps) that wereunstable under the maximum metamorphic tempera�tures and for which the calculated Tcl is lower than750°С (conditions transitional between the amphibo�lite and granulite facies). In the case of higher xenonstability, primary isotope systems are preserved.

The “older” xenon isotopic system (~1.0 Ga) cor�responds to high�temperature increments (calculatedclosure temperature in excess of 750°С). It is possiblethat processes of the same age are also recorded inoxygen isotope systematics (high temperatures of18O/16O fractionation at elevated δ18O values in fluid;Krylov and Mineev, 1994; Krylov et al., 1998). Thepan�African stage corresponds to the xenon releasepeaks with closure temperatures of up to 700–750°Сand caused homogenization of the oxygen isotopecompositions of minerals over distances of no less thana few meters in most rocks of the complex (Krylov andMineev, 1994; Krylov et al., 1998) and intense alter�ation of the rocks. Consequently, the age of metamor�phism transitional between the amphibolite and gran�ulite facies can be estimated on the basis of Xes�Хеn

dating as 600–550 Ma. Thus, the results of our studyindicate that the regional metamorphism of theRayner Complex at the Molodezhnaya area has an ageof 600–550 Ma rather than ~1.0 Ga, as was supposedpreviously.

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Translated by A. Girnis

xes-xenthermochronology of the rayner metamorphic complex, enderby land (east antarctica,...

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