th/u zonation in zircon derived from evaporation analysis: a model and its implications

Ž .Chemical Geology 158 1999 325–333

ThrU zonation in zircon derived from evaporation analysis: amodel and its implications

U.S. Klotzli )¨Laboratory for Geochronology, Institute of Geology, UniÕersity of Vienna, Geozentrum, Althanstrasse 14, A-1090 Vienna, Austria

Received 8 December 1997; accepted 5 March 1999

Abstract

Zircon evaporation analyses often exhibit the existence of spatial extraneous variations in 208Pbr206Pb compared to207Pbr206Pb as revealed by increasing evaporation temperature and duration of analysis. As 208Pb solely stems from thedecay of 232Th, variations are interpreted to directly reflect changing ThrU ratios within different zircon domains which areprobed at different evaporation temperatures and evaporation duration. Such strongly varying ThrU is interpreted asreflecting the magmatic growth zonation in the zircon crystal. Constant ThrU over several evaporation steps is interpretedas being characteristic for homogeneous zircon domains which probably originate from zircon recrystallisation or growthduring a metamorphic event. Stepwise evaporation of zircon crystals and the monitoring of ThrU ratios thus provide somemeans to decipher the internal structure of the analysed zircon and therefore allows some conclusions about magmatic vs.recrystallised or newly grown metamorphic origin to be drawn. q 1999 Elsevier Science B.V. All rights reserved.

Keywords: Geochronology; Zircon evaporation analysis; Zonation; Crystal chemistry

1. Introduction

Single zircon evaporation analysis has proven tobe a very useful tool to establish the geochronologyof polymetamorphic rock series, especially of acidicto intermediate orthogneisses. Nevertheless, interpre-tation of whether the determined 207Pbr206 Pb agesactually do represent magmatic or probably meta-morphic ages is not always straightforward.

Zircon evaporation analyses often exhibit spatialextraneous variations in 208Pbr206 Pb compared to207Pbr206 Pb. The variations are twofold.

) Tel.: q43-1-31336-1960; fax: q43-1-31336-782; e-mail:[email protected]

Ž .1 Strong variations of the absolute amounts of208 Pb when compared to the absolute amounts of207Pb and 206 Pb are observed leading to ‘jumps’ in208 Pbr206 Pb at stable 207Pbr206 Pb.

Ž . 2082 The ion beam stability of Pb is often re-markably lower than the ion beam stability of 207Pband 206 Pb, reflecting extraneous small-scale varia-tions of 208 Pb and leading to comparably large ana-lytical errors.

As 208 Pb solely stems from the decay of 232 Th,the variations of 208 Pbr206 Pb are thus thought toreflect changing ThrU ratios within different zircondomains. Zircon evaporation and ion-microprobe datasuggest a wide range of ThrU ratios in zircon from0.01 up to 5 within a single crystal andror sampleŽBlack et al., 1986; Kober et al., 1989; Ansdell and

0009-2541r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0009-2541 99 00049-2

( )U.S. KlotzlirChemical Geology 158 1999 325–333¨326

Kyser, 1991; Silver, 1992; Kroner et al., 1994;¨Klotzli, 1997; Klotzli and Parrish, 1996; Klotzli-¨ ¨ ¨

.Chowanetz et al., 1997 .This contribution discusses occurrences and im-

plications of the variations of ThrU in zircon usingarguments from mass spectrometry, natural and theo-retical zircon zonation, zircon evaporation analysis,and examples.

For detailed zircon evaporation TIMS proceduresŽ .the reader is referred to Kober 1987 , Roddick

Ž . Ž .1994 and Klotzli 1997 .¨

2. Arguments from mass spectrometry

In order to be able to propose a chemical androrcrystallographic origin of the observed 208Pb varia-tions all possible isotope fractionation effects stem-ming from mass spectrometry have first to be dis-proved.

2.1. Mass fractionation

Assuming a common Rayleigh-fractionation pro-cess during thermal ionisation, theoretical fractiona-tion factors for the Pb masses are:

'a s 208r206 s1.004843 and a208,206 207,206

's 207r206 s1.002424 and a207,206

This leads to a fractionation behaviour for the Pbisotopes which is nearly linear proportional to rela-tive mass differences, at least within the bounds of

Žthe achievable analytical reproducibility 2s sm.0.1%; Klotzli, 1997 .¨

Thus, the fractionation effect from mass spectro-metric analysis should be around twice as large for208 206 Ž . 207 206 Ž .Pbr Pb Dms2 as for Pbr Pb Dms1 .Empirical fractionation factors found for SEM-ICand Faraday cup ion detection systems are in goodagreement with these theoretical considerationsŽ .Klotzli, 1997 .¨

Time dependent fractionation is in the order of0.001%ramu and hour for the equal-lead standardSRM NBS 982 irrespective of the absolute amountof lead, silicagel, and H PO loaded on the filament.3 4

No dependence on ionisation temperature is ob-served.

The conclusion, therefore, is that no commonmass fractionation effect can be made responsible forthe observed extraneous variation of 208 Pb.

2.2. Counting statistics

The absolute errors on the individual mass scansare calculated as the square root of the measured

Ž .intensity in counts per seconds . For many naturalzircons, especially for Palaeozoic or younger ones,the ion beam intensity is higher for 208Pb than for207Pb. The resulting relative counting error for 208 Pbis thus smaller than the one for 207Pb. This is in clearcontrast to the empirically observed scatter. Addi-tionally, the ratio of the relative errors of 208 Pbr206 Pband 207Pbr206 Pb is not dependent on the absoluteion beam intensity. It only depends on 208 Pbr207Pb.Even for the few cases where the counting statisticsfor 208 Pbr206 Pb are worse than for 207Pbr206 Pb thedegree of variation of the respective errors cannot befully explained.

2.3. Reaction kinetics

A possible source of isotopic fractionation due tomass dependent reaction kinetics could be the inter-action of the different lead isotopes with the silicaand possibly other elements evaporated from the

Žzircon prior to isotopic analysis i.e., Hf, Lu, U, Zr,.REE, Y; Kober, 1987; Roddick, 1994; Klotzli, 1997 .¨

But conventional lead analysis using the silicalgel–phosphorous acid technique does not show sucheffects, neither do experiments with evaporationanalysis of SRM NBS 982.

2.4. Isobaric oÕerlaps

Isobaric overlaps of 96Zr 16O, 176 Hf16O , and2 2207Pb1H can theoretically be responsible for the ex-cess scatter of 208Pb. The mass resolution of theMAT 262 mass spectrometer used in the presentstudy is not sufficient to resolve these possible iso-bars. During analysis masses 196, 202, 203, 205, and209 are monitored to recognise the isobars.

In TIMS stable ion beams of Zr O are not attain-2

able below 16008C, therefore the most critical isobarof 96Zr 16O should not interfere during Pb isotopic2

analysis. At least there would be a dependence be-

( )U.S. KlotzlirChemical Geology 158 1999 325–333¨ 327

tween the amount of mass 208 and raising evapora-tion temperature, a fact never observed so far. Even

Ž90 16 .the most abundant Zr-oxide ion Zr O , mass2

196, is not detected.176 16 ŽThe same is true for Hf O and other REE2

.oxides which requires even higher ionisation tem-Ž .peratures )20008C, Klotzli, 1997 . Problems with¨

isobars of BaPO , Tl and Hg can arise when2Ž .Clerici-solution TlHCO –Tl C H O or Hg bear-2 2 3 2 4Ž .ing heavy liquids Thoulet-, Rohrbach-solution were

used for heavy mineral separation. PbH is sometimesŽ .present at very low temperatures -8508C but dis-

Ž .appears above 9008C Klotzli, 1997 .¨Further evidence for the insignificance of isobaric

overlaps can be drawn from the excellent agreementbetween conventional UrPb, ion microprobe, and

Žsingle zircon evaporation TIMS age dating Kober etal., 1989; Kroner et al., 1994; Muller et al., 1995;¨ ¨

.Klotzli, 1997; Klotzli and Parrish, 1996 .¨ ¨

3. Arguments from zircon structure and chem-istry

By excluding any analytical artefacts being re-sponsible for the observed large and small-scalefluctuations in 208 Pb, variations in the internal zirconstructure and crystal chemistry must lead to theobserved scatter. Arguments for a mineralogicalrcrystal-chemical provenance come from the verycommonly observed chemical zonation patterns ofzircon.

3.1. Chemical zircon zonation

Zircons normally reveal two distinct chemicalzonation patterns.

Ž .1 Large scale, first order, andror small-scalesecond order oscillatory zoning parallel the major

Ž� 4 � 4. Ž� 4prism faces 100 , 110 and pyramidal faces 101 ,� 4.211 , respectively, either ordered, disordered or acombination thereof. Often the formation of a dis-tinct sector zoning can be seen. These zonations areusually interpreted as representing magmatic growthzonations characterised by changing relative propor-

Žtions of minor and trace elements i.e., Hf, U, Th, Y,. ŽREE, Pb, P in the crystal lattice Vavra, 1990, 1993;

.Benisek and Finger, 1993 . The small-scale oscilla-tory zoning, called second order zonation here, can

best be understood in terms of self-organisationalŽcrystallisation Ortoleva et al., 1987; Mattinson et

.al., 1996Ž .2 Nebulitic, patchy zircon domains, usually in-

terpreted as representing some sort of partial recrys-tallisation andror redistribution of elements, mostprobably during metamorphic overprinting at moder-

Žate to high temperatures Vavra, 1990, 1993; Pid-.geon, 1992; Benisek and Finger, 1993 .

Examples of magmatic oscillatory zircon zoningwith large differences in ThrU are given by Hinton

Ž . Ž .and Upton 1991 , Benisek and Finger 1993 , andŽ .Kroner et al. 1994 .¨

3.2. Causes and degrees of ThrU Õariations

The causes and the degrees of the ThrU varia-tions are highly hypothetical. Probable sources ofThrU variations are the contemporaneous crystalli-sation of zircon and Th andror U-rich mineralsŽ .monazite, thorite, etc. leading to local geochemicaldisequilibrium and associated replenishment effectsin the parental magma bodies, different diffusionrates of Th and U in melt and crystal, changes in theassociated K -values depending on degree of super-D

saturation, crystallisation kinetics controlled by meltvolume and solid–solid vs. melt–solid reactions, the

� 4growth blocking effect of U on 110 prism faces,and other crystallographic and crystal-chemical fea-

Žtures Vavra, 1990; Benisek and Finger, 1993; Mat-.tinson et al., 1996 .

ŽElectron microprobe studies Wayne et al., 1992;.Benisek and Finger, 1993; Hanchar and Miller, 1993

have revealed a distinct positive correlation of Uwith other trace elements, such as P and REE. Thevalence state of REE-ions is predominantly 3q . Soa coupled substitution of Zr 4qqSi4q with REE3qqP5q, in combination with the substitution of Zr 4q

4q Ž 4q .with U and Th ? , was often proposed as beingthe major mechanism for trace element incorporation

Ž .into the zircon lattice Romans et al., 1975 . ButŽ .Hinton and Upton 1991 in their ion-microprobe

study find no correlation of P with the REEs andthus conclude that incorporation of REE must not becoupled with P5q or other anions. Benisek and

Ž .Finger 1993 come to the same conclusion. Mattin-Ž .son et al. 1996 suggest that, under oxidising condi-

tions, the substitution with U6q could also providethe charge balance necessary for the incorporation of


REE3q. U6q has an ionic radius of 86 nm foreight-fold co-ordination, practically identical to theZr 4q ionic radius of 84 nm. The similar size and thestronger bonding, due to higher charge for U6q,make this a very favourable coupled substitution inthe zircon lattice. This would probably result in a farhigher partition coefficient for U6q than for U4q

Ž .100 nm ionic radius under the same conditions,additionally enhancing any processes leading to Uzonation. The overall K for U is a combination ofD

the respective U6q- and U4q-K values. This parti-D

tion coefficient itself is then strongly dependent onthe oxygen fugacity of the melt, whereas the distribu-tion of Th, which is only present in the 4q state,should not be changed by varying f . Such a mecha-O

nism would therefore strongly decouple U from Th.At least the oscillatory, second order zonation can beunderstood in terms of such a self-organisationalcrystallisation behaviour resulting in the juxtaposi-tion of high-ThrU and low-ThrU zircon zones dur-

Ž .ing crystal growth Mattinson et al., 1996 .Magmatic zircon composition also could be con-

trolled to some extent by whole rock chemistryŽ .Heaman et al., 1990 , although the reported valuesare not very conclusive.

Often the evaporation analysis of a single-agezircon domain results in an overall increase in208 206 ŽPbr Pb from the outer low evaporation tem-

. Ž .perature to the inner high evaporation temperaturepart of the domain. This most probably reflects theoverall geochemical evolution of the melt leading toa relative enrichment of U over Th during zirconcrystallisation. The reason for the relative U enrich-ment in the melt is the higher incompatibility of U

Ž .due to its larger ion size Silver and Deutsch, 1963 .The influence of temperature and pressure are notknown. The influence on zircon chemistry of thecontemporaneous crystallisation of other Th- and

ŽU-bearing mineral phases i.e., monazite, xenotime,.thorite, etc. is of potential importance but has not

been investigated yet.Empirically, ThrU ratios of metamorphically

grown or recrystallised zircon are often lower thanŽmagmatic ThrU ratios Kroner et al., 1994; Klotzli,¨ ¨

.1995; Klotzli-Chowanetz et al., 1997 , possibly due¨to contemporaneous crystallisation of monazite. Atleast for granulite facies zircons this somehow con-tradicts the commonly observed U-depletion of the

lower crustal rocks. Thus, it seems possible thatcrystallographic and crystal-chemical features aremore important for fixation of the ThrU ratio than isthe geochemical environment.

Ž .Hinton and Upton 1991 report Th-distributioncoefficients between zircon and melt of 3 to 15 timesthe REE-distribution coefficients. For U this factor isonly 1 to 5. Thus, geochemical variations due tomagmatic fractionation and local disequilibrium ef-fects are more profoundly reflected in the Th contentof zircon than in the U content and ThrU variationsof the parental magma are magnified in the zircon.

ŽIon probe data from Enderby Land Black et al.,.1986 , on the other hand, suggest that there exists no

direct correlation between Th and U concentrationsand ThrU ratios. ThrU variations in co-magmaticzircons often are as large as internal variations within

Žindividual zircon crystals Kober et al., 1989; Ans-.dell and Kyser, 1991 .

3.3. Inclusions

A source for excess 208 Pb are Th-rich inclusionsliberating their Pb during evaporation in an unsys-tematic mode. The complex mixing of Pb fromzircon and inclusions during evaporation could thentheoretically lead to the observed additional scatter

Ž .in ThrU. For instance, Hinton and Upton 1991positively demonstrated the substantial influence on

Ž .zircon crystal chemistry i.e., on ThrU of inclu-sions. High-Th candidates and therefore a possible

208 Ž .source for excess Pb are thorite ThSiO , thorian-4ŽŽ . . ŽŽ . .ite Th, U O , monazite Ce, La, Y, Th, U PO ,2 4

ŽŽ . . ŽŽxenotime Y, REE, Th, U PO , and coffinite U,4. .Th SiO . All these minerals are commonly found as4

inclusions in zircon, but are easily recognised bydiffering colour, biriferenge, grade of metamictisa-tion during the procedures of zircon classificationand evaluation. Evaporation analysis of such inclu-sion bearing zircons can therefore be completely

Ž .avoided Klotzli, 1997 . U-rich inclusions could pre-¨sumably be identified by the presence of additionalscatter in 207Pbr206 Pb too.

4. Arguments from zircon evaporation

It has been demonstrated, that variations of sev-eral percent of ThrU in zircon in fact do exist.


Probably such variations are quite common. Thequestion arises to what extent these variations can bemade visible by zircon evaporation TIMS.

Theoretically, evaporation, i.e., mobilisation ofSiO , U, Th, Pb, REE, and Hf, of zircons normally2

proceeds shell-like from the rim to the core of thecrystal. So, with time, the evaporation process mo-bilises elements from more internal parts of the

Žcrystal for a detailed description see Kober, 1987;Roddick, 1994; Roddick and Chapman, 1991; Ans-

.dell and Kyser, 1993; Klotzli, 1997 . Temporal vari-¨ations in 208Pbr206 Pb should therefore corresponddirectly to spatial changes in the UrTh-ratios of thezircon lattice. Thus, a complete evaporation analysisresults in a spatial depth-profile of the apparent207Pbr206 Pb age and the UrTh ratio. One of the bigadvantages of zircon evaporation TIMS is the farbetter spatial resolution achieved during routine anal-yses than is possible with LA-ICP-MS or ion-micro-probe techniques and the establishment of continu-ous age profiles.

4.1. First order-, second-order zonations, and theo-retical models

Zircon evaporation spectra very often show spe-cific features, which greatly help to interpret theobtained age data. To clarify the influence of zirconstructure and zonation on evaporation spectra, sometheoretical end-member cases are presented.

The indicated features are discussed below.

Fig. 1. Schematic representation of the six possible zircon typesand the resulting theoretical 207Pbr206 Pb and 208 Pbr206 Pb spec-tra of stepwise zircon evaporation. Different zircon types arecharacterised by the presence or absence of core andror rim,either with or without first and second order zonation with respectto ThrU and chemical composition, respectively. For simplicityonly first order zonations are shown. Darker shading indicates

Ž . Ž .higher ThrU. A Homogeneous zircon without core. B ZonedŽ .zircon without core. C Zircon with homogeneous rounded core

Ž .and homogeneous overgrowth. D Zircon with homogeneousŽ .rounded core and zoned overgrowth. E Zircon with zoned

Ž .rounded core and homogeneous overgrowth. F Zircon withzoned rounded core and zoned overgrowth. For better visualisa-tion zoned zircon cores are rotated a bit, although this must not benaturally so. T –T denote five evaporation steps with increasing1 5

evaporation temperature.


Ž .1 Strong variations of the absolute amounts of208 Pb, leading to ‘jumps’ in 208 Pbr206 Pb at stable207Pbr206 Pb.

Ž . 2082 The ion beam stability of Pb is often re-markably lower than the ion beam stability of 207Pband 206 Pb, reflecting extraneous small-scale varia-tions of 208 Pb and leading to comparably large ana-lytical errors. Often both features are combined.

Using a very simplified approach, zircons can berepresented as a two phase system built up by a coreand a surrounding rim. Both parts can be present ornot, and both parts can be homogeneous or zonedwith respect to chemical composition. On this basissix possible zircon types can be distinguished. Fig. 1gives a schematic representation of these six differ-ent zircon types and the resulting theoretical207Pbr206 Pb and 208 Pbr206 Pb spectra of stepwisezircon evaporation. In these models only first orderzonations are shown.

5. Examples

Three zircon evaporation spectra are presented todemonstrate the applicability of zircon evaporationTIMS in recognising igneous zircon growth. Fig. 2A

shows the analysis from a granite gneiss from Sha-dat, Sibai metamorphic core complex, Eastern Desert,

Ž .Egypt Bregar et al., 1999 . This zircon exhibitstypical first order magmatic growth zonation, an

Ž .exemplary reversed deposit Klotzli, 1999 and some¨extraneous, second order zonation. The age of 646"

17 Ma is thus interpreted as a possible igneousformation age of the gneiss protolith. Fig. 2B pre-sents an evaporation spectrum of a gneiss from theAustroalpine basement of Southern Tyrol, ItalyŽ .Klotzli, 1995 . The first four evaporation steps de-¨fine an age of 453"12 Ma with no recognisablefirst or second order zonation. Thus, the age isinterpreted as representing metamorphic zircongrowth or recrystallisation. The core age found in thelast two evaporation steps shows some first orderzonation. The age thus probably represents a mini-mum age for igneous zircon growth in the Protero-zoic. Fig. 2C gives a spectrum from a granodioritezircon from the Variscan South Bohemian pluton,

Ž .Austria Klotzli and Parrish, 1996 . This zircon shows¨a magmatic core and a magmatic overgrowth, firstorder magmatic growth zonation, practically no sec-ond order zonation and a small reversed deposit.Both ages are interpreted as magmatic formationages.

Fig. 2. Three single zircon evaporation spectra as examples for the typical 208 Pbr206 Pb and 207Pbr206 Pb distributions obtained with stepwise evaporation analysis. Data points are weighted block means of 10 mass scans. Indicated errors are propagated 2 standard errors of thecorresponding block mean. Error on the reported evaporation temperature is assumed to be "108C. Errors on 207Pbr206 Pb ages are

Ž .propagated 2 standard errors taken from the mass scans. A Evaporation spectrum of a zircon exhibiting magmatic growth zonation withstrongly varying ThrU at constant 207Pbr206 Pb. No core is present. The example comes from a granite gneiss from Shadat, Sibai

Ž . 208 206metamorphic core complex, Eastern Desert, Egypt AB281rC; Bregar et al., 1999 . Additionally to the variation of Pbr Pb, typicallyreflecting a first order magmatic growth zonation, some extraneous scatter of 208 Pbr206 Pb is found, reflecting a second order, fine

Ž .oscillatory magmatic zonation. A very prominent reversed deposit Klotzli, 1999 at temperature step 14508C pictures the transition between¨the Th-poor outer and Th-rich inner parts of the crystal. As no change in 207Pbr206 Pb takes place, the reversed deposit is not detectable withthis ratio. Whether the evaporation step directly corresponds to a transition zone between Th-poor and Th-rich parts of the crystal or just

Ž .reflects some mixing effects during evaporation is not clear. B Evaporation spectrum of a zircon showing a core and a metamorphicŽ .overgrowth. The example comes from a white-mica bearing orthogneiss from the Gsieser valley, Southern Tyrol 70r93rD; Klotzli, 1995 .¨

During the first four evaporation steps comparatively low 208 Pbr206 Pb increases steadily at constant 207Pbr206 Pb. No extraneous scatter isobserved. The spectrum reflects homogeneous zircon domains with practically no zonation. Thus, the age of 453"12 Ma is interpreted as

Ž .dating a metamorphic event leading either to zircon growth or to partial zircon homogenisation. Zircon typology subtype S7 favours the208 206 Ž .first possibility. The evaporation step at 14738C shows the typical Pbr Pb spectrum of a reversed deposit Klotzli, 1999 . The last two¨

Ž .high temperature evaporation steps show an inherited core with respect to the metamorphic overgrowth with a probable magmaticŽ .zonation. C Evaporation spectrum interpreted as reflecting a magmatic core and magmatic overgrowth. The example comes from the

Ž .Rastenberg granodiorite, South Bohemian pluton, Austria Klotzli and Parrish, 1996 . Apart from comparably larger analytical errors on¨208 Pbr206 Pb indicating second order zonation, no extreme first order variance of 208 Pb is observed. Labels bracketing 208 Pbr206 Pb aremaximum and minimum ThrU ratios today and at the respective 207Pbr206 Pb age. A not very prominent reversed deposit is present at theend of the evaporation step at 15158C, possibly demonstrating the existence of a small mixing zone between the already largely evaporatedovergrowth and the core.


6. Conclusions

In zircon evaporation TIMS, constant 207Pbr206 Pbratios over a certain time span or several evaporationsteps are normally interpreted as representing intactUrPb systematics with no unsupported U or PbŽ .Kober, 1987; Klotzli, 1997 . If this holds true and¨by excluding any analytical artefacts, variations ofthe 208 Pb abundance, resulting in extraneous firstorder and second order variations of 208Pbr206 Pb,must directly reflect primary spatial variations inUrTh within the zircon crystal.

Evaporation analyses of a large number of mag-matic zircons from granitoids of the Bohemian Mas-sif have revealed that extraneous scatter in 208 Pbr206

Pb at constant 207Pbr206 Pb is almost always present.Using theoretical models and additional data fromion-microprobe, LA-ICP-MS, and EMP investiga-tions, the observed variations in 208 Pbr206 Pb areinterpreted as representing the magmatic growthzonation and directly support the model of shell-like

Želement mobilisation during evaporation Klotzli,¨. 208 2061997 . Overall trends of Pbr Pb are interpreted

as possibly representing the batch magma evolutionduring zircon growth andror co-crystallisation ofTh-rich minerals. ‘Jumps’ of 208Pbr206 Pb are inter-preted as representing the first order ThrU varia-tions of the magmatic oscillatory zoning or of indi-vidual sector zones. Extraneous second order vari-ance of 208 Pbr206 Pb is interpreted as representingdirectly the small-scale magmatic oscillatory zona-tion pictured by varying ThrU due to self-organisa-tional crystallisation.

� 4The growth blocking effect of U on 110 leading� 4to more U-rich 100 growth zones cannot be quanti-

� 4fied at the moment. The question whether 110dominated zircons are more variable in respect to

� 4ThrU than 100 dominated zircons remains to beanswered.

Constant 208Pbr206 Pb at constant 207Pbr206 Pb isthen interpreted as representing homogeneous do-mains, with respect to UrTh, thus representing re-crystallised zircon domains where complete Pb ho-mogenisation, and probably also U and Th ho-mogenisation, have occurred.

Using this line of argumentation, evaporation agesfrom zircons of unknown provenance exhibiting largevariations in 208Pbr206 Pb at constant 207Pbr206 Pb

can be interpreted as representing magmatic ages.Vice versa, evaporation ages from zircons showingno or only very little variations in 208Pbr206 Pb cantentatively be interpreted as representing metamor-phic recrystallisation ages. The method even allows,in certain cases, to distinguish between inheritedzircon cores of magmatic or recrystallisedrnewlygrown metamorphic origin.

208 Pbr206 Pb variations in the range of 0.1% canbe made visible routinely. This reflects ThrU varia-tions in the range of 0.1%. Thus, zircon evaporationTIMS is a very sensitive tool for monitoring even thesmallest variations in ThrU.

Ž .Mattinson et al. 1996 concluded that internalinverse discordance results entirely from internal re-distribution of Pb. In contrast, external inverse dis-cordance results from the removal of U, and possiblyTh, from radiation-damaged, trace-element rich zir-con domains. The unanswered question is whether ornot especially internal inverse discordance can berecognised by means of zircon evaporation analysis.A problem with the approach of Mattinson et al.Ž .1996 is that the authors provide good evidence forthe spatial variation in U concentrations in zircons.Evaporation analysis, however, suggests that there isadditional scatter in Th-concentration and not neces-sarily in U-concentration. This suggests that ThrUvariations in zircon probably are dominated by exter-

Žnal effects e.g., melt composition, co-crystallisation.of other minerals rather than by internal effects

stemming from self-organisational crystal growth.This somewhat contradictory situation has yet to beresolved. Future investigations will have to focusdirectly on the relationship of Th4q, U4q, and U6q

in the crystal lattice.In conclusion, complete stepwise zircon evapora-

tion analysis allows to recognise magmatic zonationsvs. metamorphic recrystallisation and thus gives riseto ameliorated age data interpretation.

Acknowledgements

The author would like to thank the followingpersons for spending tedious hours at the mass spec-trometer, for discussion, criticism, advice, a steadyhand during zircon mounting, and critical reviews ofan earlier version of this paper: F. Bernhard, W.


Frank, V. Hock, G. Hoinkes, M. Jelenc, E. Klotzli-¨ ¨Chowanetz, J. Mattinson, S. Meli, B. Muller, S.¨Scharbert, R. Vocke, and two anonymous reviewers.Financial support from the Austrian Science Founda-tion and the Austrian National Bank is also acknowl-

[ ]edged. PD

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