The nature of zircon inheritance in two granite plutons

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The nature of zircon inheritance in two granite plutons

B. A. Paterson, W. E. Stephens, G. Rogers, I. S. Williams, R. W. Hinton and D. A. Herd

Transactions of the Royal Society of Edinburgh: Earth Sciences / Volume 83 / Issue 12 / January 1992, pp 459 471DOI: 10.1017/S0263593300008130, Published online: 03 November 2011

Link to this article: http://journals.cambridge.org/abstract_S0263593300008130

How to cite this article:B. A. Paterson, W. E. Stephens, G. Rogers, I. S. Williams, R. W. Hinton and D. A. Herd (1992). The nature of zircon inheritance in two granite plutons. Transactions of the Royal Society of Edinburgh: Earth Sciences, 83, pp 459471 doi:10.1017/S0263593300008130

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Transactions of the Royal Society of Edinburgh: Earth Sciences, 83, 459-471, 1992

The nature of zircon inheritance in two granite plutons

B. A. Paterson, W. E. Stephens, G. Rogers, I. S. Williams, R. W. Hintonand D. A. Herd

ABSTRACT: Using zircons taken from two granite plutons, Strontian (Caledonian, northwes-tern Scotland) and Kameruka (Bega Batholith, southeastern Australia), this study presentsobservations that have a bearing on refractory zircons as provenance indicators. Two broadtextural types of refractory zircon were identified: (1) those which show simple two-stage growthhistories; and (2) those which have apparently undergone repeated periods of growth,resorption, mechanical abrasion, fracturing and fracture-healing. SHRIMP U-Pb ages obtainedfrom the Kameruka zircons indicate that the cores are the textural manifestation of inheritance.The shapes of refractory cores are not unambiguously indicative of their ultimate origin, sincethey may also be modified by processes that occur before and after incorporation into themagma. The cores within the two populations show a great diversity in types and styles ofzoning, and in composition, implying that they have not chemically equilibrated internally, orexternally with their host melt.

KEY WORDS: Bega Batholith, Caledonian, core, electron microprobe, growth histories, Hf,ion microprobe, mineral zoning, Nd isotopes, REE, rim, SHRIMP, Th, U.

The occurrence of old, so-called inherited radiogenic Pb(Pb*) within zircons from otherwise younger granites is acommonly reported phenomenon (Grauert & Arnold 1968;Pasteels 1970; Gulson & Krogh 1973; Pidgeon & Aftalion1978; and many others). This indicates: (1) that it is possiblefor zircons to remain undissolved within crustal melts(Watson & Harrison 1983); and (2) that at the temperaturesattained by crustal melts (700-900°C) the diffusion of Pb inzircon is very slow.

Despite the extensive reporting within the geologicalliterature of occurrences of inherited Pb* within zircon, andthe acknowledged isotopic complexity of some zirconpopulations (Harrison et al. 1987; Williams et al. 1988; Chen& Williams 1990), surprisingly little is known about thetextural characteristics of refractory zircon. Moreover, ifrefractory zircons also remain closed with respect to theexchange of other elements, then they could be a bountifulsource of age, isotopic and chemical information, charac-teristic of their previous environment(s) of growth. Incombination with experimental data on intra-zircon elementdiffusion rates, they could also yield information on thetime-temperature history of crustal melting episodes(Watson & Harrison 1984).

This paper presents textural, compositional and isotopicobservations on zircons from two granite plutons that areknown to contain a component of inherited Pb*. Theprincipal aims are to demonstrate: (1) that granite zirconsoften have complex, protracted growth histories; (2) thatrefractory zircons in crustal melts apparently undergo littleexchange with respect to Hf, Y, the REE, Th and U; and(3) that an understanding of zircon growth histories isimportant to studies which use high spatial resolutionanalysis techniques.

1. Zircon inheritance in granites

Zircons that contain a component of inherited Pb* do notplot on the U-Pb concordia line and are said to be

discordant. Early workers (e.g. Grauert & Arnold 1968;Pidgeon & Aftalion 1978) observed that different zirconfractions from the same rock often defined a linear patternof discordance which had upper and lower intercepts withconcordia. The lower intercept was usually interpreted asrepresenting the crystallisation age of the granite, whereasthe upper intercept was interpreted as representing the ageof the inherited component. A necessary, although oftenunstated, assumption in the extrapolation of reversediscordia is that there is only one age of inherited zircon.However, there is mounting evidence (e.g. Harrison et al.1987; Williams et al. 1988; Foster et al. 1989; Chen &Williams 1990; Rogers & Dunning 1991) that suggests thatthis assumption is wrong in many cases, and that multipleage (or mixed) inheritance (Miller et al. 1988) in granites iscommon. Mixed inheritance in granites has only recentlybeen unambiguously identified, largely through the in situanalysis of U-Pb isotopes using SHRIMP (Compston et al.1984). The existence of mixed inheritance is not altogethersurprising for the following reasons:

(1) Melting of a heterogeneous crust containing rocks of anumber of different ages would result in a granite withmixed zircon inheritance. Williams et al. (1988) and Chenand Williams (1990) interpreted the mixed inheritance theyidentified within the I-type granites of the Bega Batholith(southeastern Australia) as resulting from the melting ofigneous rocks with a variety of ages plus a small componentof sediment in the source.

(2) Many individual granite source rocks (igneous,sedimentary and metamorphic) may themselves containzircon populations with multiple ages. Sedimentary proven-ance studies using single-grain dating of zircons have shownthat detrital zircons commonly have disparate ages andtherefore disparate original sources (Scharer & Allegre1982; Krogh 1986; Rogers et al. 1990). Harrison et al. (1987)explained the presence of mixed inheritance in twoperaluminous granites from New Hampshire as being causedby the original magmas being derived from sedimentarysource rocks.

460 B. A. PATERSON ET AL.

Williams and Claesson (1987) showed that detrital zirconpopulations with multiple ages are highly resistant tometamorphism, even to granulite fades conditions; detritalzircons from paragneisses of the Seve Nappes (ScandinavianCaledonides) retained a variety of ages indicative of theiroriginal sources. Other studies of high-grade gneisses withboth sedimentary and igneous protoliths have shown thatrepeated high-grade metamorphism can create zirconpopulations with multiple ages (Black et al. 1986; Kinny etal. 1988).

In summary, therefore, we would expect granitescommonly to contain mixed zircon inheritance. A corollaryof this is that zircon inheritance should be complex in termsof its textural, compositional and isotopic characteristics.

2. Case studies and methods

2.1. SamplesThe zircon populations from two samples were studied: onefrom the Strontian pluton (Caledonian, northwesternScotland), the other from the Kameruka pluton (LachlanFold Belt, southeastern Australia).

Strontian is a composite pluton consisting of two mainintrusions (Sabine 1963), namely the outer Loch SunartGranodiorite and the inner, cross-cutting Glen SandaGranodiorite (Paterson et al. 1992). Pidgeon and Aftalion(1978) and Halliday et al. (1979) reported that the LochSunart Granodiorite did not contain inherited zircon.However, using the same sample as that analysed byHalliday et al. (1979), and by carefully abrading selectedeuhedral prisms and prism tips, Rogers and Dunning (1991)demonstrated that the Loch Sunart Granodiorite doescontain zircon inheritance. A concordant U-Pb zircon ageof 425 ± 3 Ma (2a) was obtained by Rogers and Dunning(1991) for this intrusion. The later Glen Sanda Granodioriteis known to contain substantial zircon inheritance (Hallidayet al. 1979) and zircons from this intrusion (sampleBPSRG1) have been used for this study.

Kameruka is the larger of two plutons which comprise theKameruka Supersuite of the Siluro-Devonian Bega

Batholith. Like the majority of the plutons of the BegaBatholith, Kameruka is an I-type granite in the sense ofChappell and White (1974). Its petrography and chemicalfeatures have been described by Chappell et al. (1991). Theage of the Kameruka sample (AB40), based on an averageof 26 SHRIMP U-Pb analyses obtained from melt-precipitated zircon rims, is 421 ± 3 Ma (2a) (Williams thisvolume).

2.2. MethodsThe internal zoning textures of zircons were observed usingbackscattered electron (BSE) imaging on a JEOLJCXA-733 electron microprobe at St Andrews University;the images obtained primarily reflect the compositionalcontribution to electron backscattering and are termedatomic number (Z) contrast images (ZCI) (see Paterson etal. (1989) and Paterson & Stephens (1992) for details of themethod used). Studies were carried out on separated,hand-picked grains mounted in epoxy.

Quantitative analyses of zircons were obtained using awavelength dispersive electron microprobe (JEOL JCXA-733 at St Andrews University) and an ion microprobe(Cameca ims-4f at Edinburgh University). A combinedapproach using both techniques was employed in order tomaximise the spatial and compositional information. Theelectron microprobe, while having good spatial resolution(2-3 jum), generally has poor detection limits (for the REE,Th and U the detection limits are in the order of200-400 ppm). The ion microprobe has considerably betterdetection limits (generally < 1 ppm) but poorer spatialresolution (—25/xm).

Electron microprobe analyses were obtained using afocused beam of 20 kV and 50 nA. Interferences in the REEL-series X-ray spectra were avoided by the use of La- linesfor some of the REE elements and L)3 lines for others(Exley 1980). Peaks were considered significant if thecount-rate of the net peak was more than three times thestandard deviation of the error of that net peak. Peak andbackground positions are given in Table 1. Count times of100 s were used for both peak and background positions forall elements on standards and unknowns.

Table 1 Analysis conditions used in the electron microprobe analysis of zircon

Element

MgAlSiPCaMnFeYZrLaCePrNdSmDyErYbHfThU

Analysingcrystal

TAPTAPTAPPETPETLiFLiFTAPPETLiFLiFLiFLiFLiFLiFLiFLiFTAPPETPET

X-rayline

K«,K«,Ko-,K « ,Ko-,Ka ,Kff,Lar,La-,La,La ,L/3,L/S,L/3,L/3,La-,La-,Ma-,Ma-,Ma-,

Peakposition

(nA)

9-890008-339347-125426-157003-358392-101821-936046-448806-070502-665702-561502-258802-166901-998061-710621-784251-671897-539004-138103-91000

Background(nA)

9-613947-788006-527726-126303-295652-072041-864106-355906-007202-637002-532762-237102-153971-868541-698681-698681-635607-401104-075923-84870

positions(nA)

10-165608-585407-401106-219253-421242-153542-015006-540396-132802-694042-590502-280422181602-016871-831411-831411-698687-676974-200494-00248

Standard

PericlaseCorundumZirconApatiteWollastoniteMn metalFe metalREE-3ZirconREE-3REE-3REE-3REE-2REE-2REE-4REE-4REE-2ZirconThO2

UO2

(wt%)

0-0040-002

—0-0140-00500090-0080-018

—0-033002600390-0370-036003900210-021

0-0380035

Standard errors in % attwo concentrations

(0-1 wt%)

——

6-2—7-3

29——

27

—

—3321

2021

(0-5 wt%)

5-1—

—9-8

—

—

5-66 0

Notes: REE-1, 2, 3 and 4 are REE-doped glass standards of Drake and Weill (1972). Minimum detectable limits (CMDL) were calculated frompeak and background counts obtained on the relevant standard using the method of Scott and Love (1983). Values of analytical precisioncalculated from the standard errors obtained from both standards and unknowns.

ZIRCON INHERITANCE 461

Ion microprobe analyses were obtained using a 14.5 kVprimary O beam, focused to spot of approximately 25 /jmin diameter; for complete details of the ion microprobeanalysis procedure, see Hinton and Upton (1991).

U-Pb isotopic analyses were performed on the SHRIMPusing a 10 kV primary O beam focused to a spot of 25 jumin diameter; details of the analysis procedure have beengiven by Compston et al. (1984), Williams and Claesson(1987) and Chen and Williams (1990).

3. Textural observations

All the zircon grains described here contain core structureswhich have an epitaxial relationship with a euhedral rim.The terms "core" and "rim" are used to denote parts of acrystal which are distinct in terms of their composition ortexture, and which have a generally concentric relationship.This definition allows for the occurrence of several core/rimstructures within a single crystal. The nature of the interfacebetween core and rim or between two cores can bedescribed, firstly, by its shape, and secondly, by whether thechange in composition across the interface is continuous(gradual) or discontinuous (sharp). The ability to judgewhether a compositional boundary is continuous ordiscontinuous is determined by the resolution of the imagingtechnique. In the case of BSE imaging, this is determined bythe electron beam operating conditions, the width of theelectron beam, the mean atomic number (Z) of thespecimen and detector characteristics. The resolution of theZ-contrast images shown here is estimated to be better than1 /im.

3.1. Examples of core structuresThe examples described have been specifically chosen toillustrate the range of zoning textures seen in the two zirconpopulations. It is our experience that the zircon zoningtextures from the Strontian and Kameruka plutons are by nomeans unusual.

3.1.1. Example 1 (Fig. l(a)). This zircon contains manyof the basic features by which core structures are defined. Itcontains a relatively small, rounded core within an otherwiseeuhedral crystal. The core/rim interface is convex outwardsand is defined by a distinct change in grey-level at therounded core/rim interface. The truncation of linearcompositional discontinuities within the core (they would beplanar in three dimensions) also defines the core/riminterface. The arrange nent of face-parallel zoning withinthe rim immediately adjacent to the core/rim interface isclearly controlled by the external shape of the core.

3.1.2. Example 2 (Fig. l(b)). The core/rim interface,which is defined principally by changes in grey-level, has avery irregular shape with both convex and concavesegments. As with example 1, the face-parallel zoning withinthe rim is controlled by the shape of the core. The zoningwithin the core is complex; there are both planar andnon-planar compositional discontinuities, and there arenumerous small apatite inclusions.

3.1.3. Example 3 (Fig. l(c)). The core in this example isdifferent from those of examples 1 and 2. The zoning withinthe centre of the grain is truncated by a non-planarboundary towards one end of the crystal; note that part ofthis boundary is concave outwards. The position of the othercore boundaries is less obvious; they are euhedral and donot truncate the zoning within the core. The rim material ofthis grain is concentrated at the terminations of the crystal.

3.1.4. Example 4 (Fig. l(d)). This example has two

approximately concentric cores. The interfaces between theinner and outer cores and the outer core and the rim aregenerally rounded and defined principally on changes ingrey-level. Note that the outer core/rim interface truncatesboth the inner core/outer core interface and the zoningwithin the inner core. The two interfaces within this grainare compositionally discontinuous, which is particularlynoticeable at the interface between the inner and outercores. The fact that the rim to this grain is not entirelyeuhedral is probably due to fracturing during mineralseparation.

3.1.5. Example 5 (Fig. l(e)). The core/rim boundary isdefined by the truncation of the marked face-parallel zoningwithin the core. In addition to this, the zoning within thecore is cross-cut by several vein-like areas with generallyhigher Z (lighter grey). However, these areas are truncatedby the zoning within the rim of the crystal. The margins tothe vein-like areas are compositionally continuous, and theboundaries are not as sharp as other compositionalboundaries within this grain; compare the boundaries of thevein-like areas with the boundaries within the face-parallelzoning of the crystal rim.

3.1.6. Example 6 (Fig. l(f)). The zoning within thiseuhedral zircon is complex, and in places the texturalrelationships are ambiguous and thus difficult to describeobjectively. The zoning consists of a number of approxi-mately concentric core structures separated by non-planarboundaries. Some of these non-planar compositionaldiscontinuities truncate planar zoning structures, whereasothers separate areas of distinct grey-level. Although someof the cracks seen in zircon grains are artefacts of samplepreparation, most of those in this example are integralfeatures of the grain. The pattern of cracks appears to becontrolled by the zoning; many are seen to terminate atnon-planar boundaries. There are only a few cracks in theoutermost rim and in the innermost core.

3.2. Zoning within zircon coresHere we are concerned with the zoning inside the outermostcore/rim boundary of otherwise euhedral zircons. The coreswithin both zircon populations that were studied display awide variety of types and styles of zoning. Figures l(a),l(b), l(e), l(f), 2 and 3 show 11 zircons from the GlenSanda Granodiorite (Strontian) sample, each with adistinctive core structure. Some cores are comparativelydevoid of any internal structure (Fig. 3(a)), whereas otherscontain strong face-parallel zoning markedly different inspacing and style from the zoning within the rim (Fig. 2(b)).

3.3. SummaryAlthough zircon cores display a wide variety of types andstyles of zoning, for the purposes of the interpretation ofindividual growth histories, two broad textural types can beidentified: (1) those which have simple two-stage growthhistories; and (2) those which have multi-stage growthhistories.

4. Compositional observations

The great diversity in types and styles of zoning displayed bythe zircon cores from the Glen Sanda Granodiorite indicatesdiversity of composition, both within individual cores andbetween cores. However, since BSE grey-levels are notunique to a particular composition (many differentcompositions may give rise to the same BSE intensity), it isnecessary to use microanalysis techniques to document the


Figure 1 Z-constrast images of zircons from the Strontian and Kameruka plutons. See text for description: (a) example 1,Strontian, scale 100 ^m; (b) example 2, Strontian, scale 100 fim; (c) example 3, Kameruka, scale 100 fim. Numbered pits areSHRIMP analysis areas; (d) example 4, Kameruka, scale lOfim. Numbered pits are SHRIMP analysis areas; (e) example 5,Strontian, scale 10 /un; (f) example 6, Strontian, scale 100 ^m.


Figure 1 (Continued)

within-grain compositional variations indicated by theZ-contrast images.

Many of the apparent variations in the trace elementchemistry of zircon that have been documented can beexplained by the presence of mineral inclusions, the effectsof alteration and mineral zoning. For example, Hinton andUpton (1991) recently demonstrated that the relatively flatchondrite-normalised LREE patterns often observed in bulkanalyses of zircons (e.g. Nagasawa 1970; Mahood &Hildreth 1983; Heaman et al. 1990) are due to the presenceof inclusions, and do not accurately reflect the partitioningof REE into the zircon lattice. The use of in situmicroanalysis techniques means that it is possible to avoidthese problems. With the exception of positive Ce anomalies(due to minor Ce4+) and negative Eu anomalies (due tominor Eu2+), zircons are commonly HREE-enriched withpatterns that are uniformly steep from Lu down to La(Hinton & Upton 1991). The REE patterns obtained fromrefractory zircons may give information about parental meltchemistries (Hinton & Upton 1991) and, in the case of Ceand Eu anomalies, oxygen fugacities.

Owing to the relatively large diameter (—25 fim) primarybeam of an ion microprobe, the analyses presentedrepresent averages of relatively large volumes. However,using electron microprobe analysis, which has a spatialresolution of 2-3 ^m, and step-scans on the Cameca ionmicroprobe (~2 (im), it can be shown that Y, the REE, Uand Th concentrations in zircons tend to behave coherently:their spatial distribution is responsible for the fine-scalezoning seen on Z-contrast images (Paterson 1990; Hinton &Upton 1991). Furthermore, Hinton and Upton (1991) alsoshowed that the chondrite-normalised REE patterns of

adjacent zones within a single crystal are similar. This meansthat while absolute element abundances within zircons mayshow variations due to the presence of fine-scale zoning,element ratios and REE patterns may potentially containgeologically useful information.

4.1. Electron microprobe analysesFour cores (Fig. 2) from the Glen Sanda Granodiorite wereanalysed by electron microprobe, the purpose being toillustrate the diversity of compositional variation typicallypresent within zircon cores. The analysis numbers on thephotographs are cross-referenced with Table 2. The analysisareas, two or three per core, were chosen to reflect both thestyle of zoning within the core and the observed range of Z.

4.1.1. Example 7 (Fig. 2(a)). The core of this examplegenerally has a much lower Z than the thin euhedral rim.Apart from Si and Zr, only Hf was detectable in significantquantities; other trace elements such as Y, the REE, Th andU were either not detected or are present in quantities thatare very close to the minimum detectable limit. The analysesindicate that this core has a relatively restricted range ofHfO2 concentration from 1-73-1-93 wt%.

4.1.2. Example 8 (Fig. 2(b)). The core in this zircon alsoshows a restricted range of HfO2 concentration, signifi-cantly lower than those of example 6. Analysis 5, which wasobtained from an area within the core with the highest Z,also contains significant quantities of Y and the HREE (Erand Yb). The high Z area (analysis 6) that truncates thezoning within the core of this grain has much higher HfO2

(2-15 wt%) and UO2 concentrations (0-16 wt%) than theinner core, although no Y or HREE were detected.

4.1.3. Example 9 (Fig. 2(c)). Texturally, this grain


Figure 2 Z-contrast images of zircons from the Strontian pluton. Numbered spots are electron microprobeareas (Table 2), lettered spots are ion microprobe analysis areas (Table 3): (a) example 7, scale 100 fim; (b)example 8, scale 100 ,um; (c) example 9, scale 10 ^m; (d) example 10, scale 10 ^m.

ZIRCON INHERITANCE

Table 2 Electron microprobe analyses of zircon cores from Strontian. Only element detected in significant quantities are shown

465

SiO2

ZrO2

HfO2

Y2O3

UO2

TOTAL

1

32-1065-18

1-83n.d.004

9915

2

32-496609

1-930040-04

100-59

3

32-4365-21

1-73n.d.n.d.

99-37

4

31-656603

114019005

9913

5

31-8865-75

1-120-880-05

100-04

6

32-0066-35

2-15n.d.016

100-71

7

32-486519

1-540 1 3n.d.

99-39

8

32-506519

1-920 0 3018

99-86

9

32-1165-97

1-91n.d.n.d.

99-99

10

32-1665-86

1-43n.d.n.d.

99-45

11

31-8064-58

1-200-510-43

99-24

Notes: All concentrations in wt%. Analysis numbers cross-referenced with Figure 2. n.d. not detected.Analysis 4 includes: 0-07 wt% Yb2O3.Analysis 5 includes: 0-12 wt% Er2O3; 0-19 wt% Yb2O3; 0-05 wt% ThO2.Analysis 6 includes: 005 wt% Yb2O3.Analysis 7 includes: 005 wt% Yb2O3.Analysis 8 includes: 0-04 wt% Yb2O3.Analysis 11 includes: 0-04wt% A12O3; 0 0 8 FeO; 0-07 CaO; 0-05 Ce2O3; 0-07 Er2O3; 0-12wt% Yb2O3; 0-29wt% ThO2.

appears to consist of two approximately concentric cores.The area with low Z (analysis 7) contains 1-54 wt% HfO2

and 1-3 wt% Y2O3 and a small amount of Yb2O3 close to theminimum detectable limit. This contrasts with the outer highZ area (analysis 8) which has 1-92 wt% HfO2, and significantquantities of UO2 (018 wt%).

4.1.4. Example 10 (Fig. 2(d)). The core of this zircon istexturally complex. The pattern of cracks is closely relatedto the pattern of Z variations. Many of the cracks appear tooriginate in three distinct areas of the grain, and at least oneof these areas (bottom right) contains distinct high Z phases,including thorite and uraninite. Only Si, Zr and Hf werepresent in detectable quantities in analyses 9 and 10, which,however, gave significantly different concentrations of HfO2.Analysis 11 detected the presence of relatively largequantities of Y2O3, Yb2O3, Er2O3, ThO2 and UO2.Unusually, analysis 11 also detected the presence ofsignificant concentrations of elements such as Al, Fe, Caand Ce. The HfO2 concentration of analysis 11 (1-20 wt%) issignificantly lower than the concentrations of analyses 9 and10.

4.2. Ion microprobe analysesFour cores were also analysed by ion microprobe (Figs 2(a),3, Table 3), the main purpose being to obtain completeREE patterns for four distinctly different zircon cores (Fig.4). The areas analysed were selected without obviousfine-scale structures in view of the limited spatial resolutionof the primary ion beam. With the exception of the Eu andCe anomalies and variations in absolute REE abundances,the cores have very similar chondrite-normalised REEpatterns. In addition to variations in the REE, the fourcores also show marked differences in the absoluteconcentrations of Hf, Th, and U.

5. Isotopic observations

5.1. U-Pb ages obtained by SHRIMPUsing SHRIMP, the different growth stages of the zirconsfrom the Kameruka Granodiorite were analysed for theirU-Pb isotopic composition (see Figs l(c), l(d), 5 and Table4). These analyses were obtained as part of a much largerstudy of zircons from the granites of the Bega Batholith(Williams et al. 1988; Chen & Williams 1990). The results inTable 4 do not represent the complete data set available forKameruka; only the analyses that are cross-referenced withthe photographs in this paper are given.

U-Pb ages obtained from zircon rims (Figs l(c)-analysis2; 5(a)-analysis 7) give values which are interpreted asrepresenting the emplacement age of Kameruka (421 ±

3 Ma). A variety of ages was obtained from the cores,ranging from 509 Ma to 1263 Ma. The zircon in Figure l(d)contains two approximately concentric cores; the inner coregave an age of 929 Ma (analysis 4), whereas the outer coregave an age of 626 Ma (analysis 3). These are thought to bereal; the 626 Ma age does not represent loss of Pb* nor amixed analysis between the 929 Ma core and the 421 Mamelt-precipitated overgrowth.

5.2. Sm-Nd isotopic composition of granite zirconsAs part of a study of the influence that accessory mineralshave on the behaviour of REE and Sm-Nd isotopes inmagmatic processes (Paterson et al. 1992), REE-bearingphases from the Glen Sanda Granodiorite (Strontian) wereanalysed for their Sm-Nd isotopic composition byconventional techniques. The eNd425 values obtained fromapatite, —4-0 and —4-9, and monazite, —5-2, are inequilibrium with each other and are similar to their hostrock, —4-4. However, the eNd425 values of —7-8 and —7-9 forzircon are not in equilibrium with the apatite, monazite orwhole-rock values, a difference beyond that expected fromanalytical uncertainty. The zircon contains only a smallproportion of the whole-rock Nd (<0-3%), and thus itcannot cause a measurable shift in the whole-rock Ndisotopic composition.

6. Interpretation

The subhedral and anhedral zircon core structures fromStrontian and Kameruka are interpreted as representingrefractory material that was incorporated into the magmaeither during partial melting, or between that time and finalcrystallisation, and were subsequently the sites of new,melt-precipitated zircon growth. Although this has been theassumption of many U-Pb zircon studies, it has only beenthrough the selection of grains on the basis of morphology(e.g. Scharer & Allegre 1983) and analysis by SHRIMP thatthe basic truth of this assumption has been proved. Thereare several lines of evidence which support theinterpretation.

(1) The subhedral and anhedral core structures in theKameruka zircons all give pre-emplacement U-Pb ageswhen analysed by SHRIMP, whereas the rims give agesconsistent with emplacement. However, it should be notedthat one of the Kameruka zircons with inheritance, example3 (Fig. l(c)), had a euhedral core with a euhedralovergrowth, indicating that zircon inheritance need notalways occur as anhedral cores (see also Pidgeon & Aftalion1978). Although there are no SHRIMP U-Pb ages from theGlen Sanda Granodiorite zircons, this part of the Strontianpluton is known from a previous U-Pb study (Halliday et al.1979) to contain a substantial component of inherited Pb*.


(2) Internal zoning structures are commonly truncated bycore/rim boundaries, indicating that the core has, at sometime, undergone a reduction in size, due to dissolution,mechanical abrasion or fracturing. It is important to notethat it is theoretically possible for non-planar boundaries todevelop under conditions of crystal growth rather thandissolution, and that the existence of non-planar crystalinterfaces should never be considered prima facie evidencefor crystal dissolution (Donaldson 1985a) and/or mechanicalabrasion; other evidence is required.

(3) The fact that the arrangement of the face-parallelzoning within the zircon rims often reflects the shape of the

Table 3 Ion microprobe analyses of zircons from Strontian.Concentrations in ppm unless stated. Analysis letters cross-referenced with Figures 2(a) and 3

Figure 3 Z-contrast images of zircons from the Strontian pluton.Lettered spots are ion microprobe analysis areas (Table 3): (a) scale10 ^m; (b) scale 10,um; (c) scale 10 ^m.

YLaCePrNdSmEuGdTbDyHoErTmYbLuThU

Hf

A

518013

21-80 1 30-811590-529-363-36

46-817-89020-4

17946-3

109398

1-37%

B

1020014

30-30-231-503030-73

17-26-78

9035-1

17438-0

3257773

138

1-20%

C

10200-37

42-60-534-724-991-85

26-58-24

10036-5

17538-8

33479

146226

1-10%

D

15800-35

40-40-334145-850-97

30-310-89

15158

29570

594115103385

1-42%


core, implies that the core/rim boundary represents aprevious exterior surface on which the rim material wasgrown. The core/rim boundaries are in effect small-scaleunconformities.

(4) There is usually a marked contrast between the coreand rim in terms of the type and style of zoning present.This would be expected if the core and rim grew in differentenvironments.

(5) The textures obviously give no direct indication of theactual time-gap between different growth stages. However,many of the cracks within cores terminate at the core/rimboundary (see Fig. l(e) and (f)), indicating that the crackingoccurred prior to growth of the rim. Such cracking is causedby the differential expansion of the zircon structure due toslowly accumulated ar-particle damage.

For the zircons shown here, only the high spatialresolution U-Pb analyses obtained using SHRIMP can beconsidered to be proof that the cores represent material thatwas refractory during magmatism. The other lines ofevidence do not have a unique interpretation, but arecharacteristics that might be expected of an inherited zirconpopulation. It should be stressed that conventional U-Pbisotopic analyses of zircons selected on the basis ofmorphology also indicate that zircon cores commonlyrepresent the textural manifestation of inherited zircon. Thepresence of visible cores, however, is not proof of thepresence of inheritance.

7. Discussion

7.1. Zircon dissolution in silicate melts: theoreticalconsiderationsThe initial shape of an older zircon incorporated into a meltwill depend on the crystallisation history of the parent rockfrom which it was derived. In general zircons fromsub-alkaline rocks develop crystal faces (see Pupin 1980),whereas those from alkaline rocks are generally moreirregular. The shapes of zircons found in sedimentary rocksare determined by their original source and the effects ofsedimentary transport (Robson 1987).

10000

o 1000

1

o

75

o

|

IO

100

10

J I I I I I I I ILa Pr

Ce NdSm Gd Dy Er Yb

Eu Tb Ho Tm Lu

Decreasing Atomic Radius •

Figure 4 Chondrite-normalised REE patterns obtained from fourzircon cores (analyses A, B, C and D-Figs 2(a), 3 and Table 3).

When zircons are incorporated into a melt, dissolutionwill take place only if the melt is undersaturated withrespect to Zr (Watson & Harrison 1983). The rate of crystaldissolution in a melt may be controlled either by the rate ofinterface reaction or by the rate at which elements aretransported, either by diffusion or bulk flow, away from thedissolving interface and replaced by components in the bulkmelt. If element transport in the melt is more rapid thaninterface reaction, then interface reaction will be therate-controlling step, and vice versa. Where transport is therate-controlling step, dissolution will be more rapid atcrystal corners and edges, due to better bulk melt access,and rounded morphologies will result (see Donaldson(1985a) for discussion about dissolution morphologies).Where interface reaction is the rate-controlling step and theinterface kinetics are anisotropic in different crystallographicdirections, then, theoretically, faceted morphologies coulddevelop during dissolution (Donaldson 1985a), althoughthere are few documented cases involving dissolution ofsilicate minerals in silicate melts. In the case of zircondissolution, other, non-crystallographic, anisotropies (e.g.compositional zoning, cracks, fission tracks and metamictareas) may be important where interface reaction is therate-controlling step (Harrison & Watson 1983), andcomplex dissolution shapes could result.

Harrison and Watson (1983) concluded from theirdissolution experiments, that in the absence of bulk flow,zircon dissolution rates are controlled by Zr diffusion ratherthan interface kinetics. If this is commonly the case, thencrystal anisotropies would be unimportant and roundedmorphologies favoured. A number of studies (Harrison &Watson 1983; Donaldson 1985b; Zhang et al. 1989), haveobserved that, in general, interface reaction is unimportantin determining the dissolution rates of silicate minerals insilicate melts. Interface reaction may only become importanteither during bulk magma flow—in which a crystal is incontinual contact with the bulk melt—or where the melt isonly slightly undersaturated with respect to the componentsof the dissolving crystal (Donaldson 1985a; Zhang et al.1989).

7.2. Zircon dissolution in silicate melts: texturalobservationsThe majority of the zircons in the Strontian and Kamerukasamples have cores with smooth, rounded outlines whichtruncate zoning within the cores. Such core/rim boundariesmust have been produced by dissolution and/or mechanicalabrasion. If dissolution were responsible for the truncationof core zoning structures and the rounded outlines, thentransport must have been the rate-determining step in thedissolution reaction. If mechanical abrasion were respon-sible for the rounded shapes, then the zircons did notdissolve when incorporated into the melt. Extensivedissolution of a zircon which had been rounded duringsedimentary transport would modify its shape, with theresultant shape being dependent on dissolution kinetics.

A small number of zircon cores from the Strontian andKameruka plutons do not have smooth, rounded outlines.The cores with highly irregular outlines could havedeveloped during a regime of dissolution in which interfacereaction was the rate-determining step, controlled byheterogeneities such as zoning and state of metamictisation.If this is the case, then it would indicate dissolution underconditions where interface reaction is the rate-controllingstep, possibly due to low degrees of Zr undersaturation,after the bulk of the zircons had partially dissolved. An


Figure 5 Z-contrast images of zircons from the Kameruka pluton. Numbered pits are SHRIMP analysis areas, see Table 4: (a)scale 100 ^m; (b) scale 100 firn; (c) scale 10 /jm; (d) scale 10,um.


Table 4 SHRIMP U-Pb ages from the Kam-eruka grains. Analyses cross-referenced withFigures l(c), (d) and 6. Ages calculated from themeasured 206Pb/238U ratios. Analyses 2 and 7,from zircon rims, give ages which are interpretedas the emplacement age of Kameruka (421 ±3 Ma). Errors are 2a

Analysis number

12

3

4

5

6

7

8

9

10

Age (Ma)

509 ±18emplacement626 ± 22

929 ± 32

1263 ± 42

1212 ±40

emplacement

633 ± 22

592 ± 20

590 ± 22

alternative possibility is that irregular shapes are a primarypre-incorporation feature and no dissolution occurredbecause the melt was already Zr-saturated.

7.3. Textural, compositional and isotopicdisequilibriumThe zircon cores from both the Strontian and Kamerukaplutons appear to retain the primary zoning structures andchemistries developed in their original growthenvironment(s). The preservation of compositional hetero-geneities implies that there was no significant internaldiffusion of the elements responsible for the zoning. Thestrong similarities in the chondrite-normalised REE patternsobtained from four texturally diverse cores implies that theyoriginally grew from melts with similar REE patterns,although the variety of zoning styles suggests that theenvironments of growth were different in other respects.The only marked differences in the REE patterns of thecores are in the magnitudes of the Eu and Ce anomalies.These probably reflect variations in the prevailing redoxconditions in the environments in which the inherited corescrystallised.

Complete chemical equilibration of the refractory zirconcomponent with the bulk magma would have resulted inhomogeneous cores, the composition of which would havebeen determined by prevailing zircon/melt partitioncoefficients. The marked difference in the compositions ofindividual cores in the Strontian sample also argues thatextensive chemical equilibration did not occur.

The Sm-Nd isotopic disequilibrium between zircon andcoexisting apatite and monazite from the Glen SandaGranodiorite (Strontian) is thought to be due to thepresence of an inherited Nd isotopic component in thezircon, rather than being due to a post-crystallisationisotopic disturbance (Paterson et al. 1992). The disequi-librium is consistent with the preservation of zoningstructures within the cores, and the presence of an inheritedU-Pb zircon component in the Glen Sanda Granodiorite(Halliday et al. 1979). If the Nd isotopic disequilibrium inthe zircons is due to an inherited Nd isotopic component, itwould imply that zircon did not completely equilibrate andthat diffusion of Sm and Nd (and presumably the otherREE) at temperatures in excess of 700°C is very slow(Paterson et al. 1992).

There is evidence that at high metamorphic grades,zircons remain closed to exchange of Lu-Hf isotopes

(Pettingill & Patchett 1981; Stille & Steiger 1991; Kinny etal. 1991). The work of Kinny et al. (1991), which usedSHRIMP, demonstrated that the core and rim of individualzircons can have different Hf isotopic compositions.Although no similar studies have been carried out on zirconswith known U-Pb inheritance, it seems likely that refractoryzircons also inherit their original Hf isotopic composition.

8. Conclusions

(1) In terms of growth histories, two broad textural types ofzircon can be identified in the Strontian and Kamerukaplutons: (a) those that contain a single core and a euhedralovergrowth (i.e. grains which have simple two-stage growthhistories); and (b) those which have undergone repeatedperiods of resorption/mechanical abrasion and growth (i.e.grains which have multi-stage growth histories). Individualcores can be defined by their shape, which can range fromrounded to highly irregular, and by the zoning on either sideof the core/rim interface.

(2) U-Pb isotopic analyses by SHRIMP demonstrate thatin general anhedral zircon cores from the Kamerukaintrusion represent the textural manifestation of inheritance,although one of the inherited grains contained a euhedralcore structure. The fact that the SHRIMP U-Pb dataobtained from the zircon cores have a range of ages, alsoshows that the Kameruka zircons have mixed inheritance.

(3) On textural and compositional criteria, it is arguedthat the U-Pb inheritance that is known to exist in the GlenSanda Granodiorite (Strontian pluton), is present in theform of subhedral and anhedral core structures. Thecompositional and textural diversity of individual zirconcores and the presence of grains with multi-stage growthhistories is consistent with the zircon population havingmixed inheritance. Although this interpretation can only beproved by selective U-Pb analysis, it is worth noting thatRogers and Dunning (1991) interpreted the pattern ofinheritance in the outer Loch Sunart Granodiorite of theStrontian pluton as being due to mixed inheritance.

(4) The shapes of refractory cores are not unambiguouslyindicative of their ultimate origin, since shape is also afunction of dissolution kinetics. For example, the roundedshapes of cores could result from partial dissolution wheretransport in the melt is the rate-determining step, or theycould be due to mechanical abrasion during sedimentarytransport sometime before their incorporation in the melt.

(5) The preservation of zoning within refractory zirconcores, the compositional diversity that is indicated by thevariety of zoning styles, and the quantitative electron andion microprobe analyses, imply that the cores have neitherchemically equilibrated internally, nor externally with theirhost melt. If the refractory cores had completelyequilibrated, they would be unzoned and compositionallyidentical.

(6) The Sm-Nd isotopic disequilibrium between zirconand coexisting apatite and monazite from Strontian isinterpreted as being due an inherited Nd isotopiccomponent in the zircon, rather than post-crystallisationisotopic disturbance (Paterson et al. 1992). The results implythat zircons remain closed systems with respect to Sm, Ndand REE diffusion at the elevated temperatures of crustalmelts.

(7) The knowledge derived from studies of refractoryzircon populations which combine textural, compositional,geochronological and isotopic information, should prove apowerful way of determining granite provenance. However,for this ideal to be a realistic objective there needs to be a


better understanding of element partitioning and diffusivityin zircon at high temperatures. Part of this understandingwill come from laboratory experiments, and part fromisotopic and chemical studies of zircons from well-characterised magmatic systems. In parallel to this, thereneed to be technical advances in the ability to measurefine-scale isotopic and chemical variations within singlezircons.

9. Acknowledgements

Part of this work was carried out whilst BP held a NERCstudentship (GT4/86/GS/93) at the University of StAndrews. NERC also supported this work through thegrants to GR (GR3/7222), WES (GR3/4633) and theEdinburgh University ion microprobe laboratory. SURRC isalso supported by the Scottish Universities. Two construc-tive reviews greatly improved the final manuscript.

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BRUCE A. PATERSON, Isotope Geology Unit, Scottish Universities Research and Reactor Centre, EastKilbride, Glasgow G75 OQU, Scotland and Department of Geography and Geology, Division of Geology,University of St Andrews, St Andrews, Fife KY16 9ST, Scotland; present address: Isotope Geology Unit,SURRC.W. EDRYD STEPHENS, Department of Geography and Geology, Division of Geology, University of StAndrews, St Andrews, Fife KY16 9ST, Scotland.GRAEME ROGERS, Isotope Geology Unit, Scottish Universities Research and Reactor Centre, EastKilbride, Glasgow G75 OQU, Scotland.IAN S. WILLIAMS, Research School of Earth Sciences, Australian National University, GPO Box 4,Canberra ACT 2601, Australia.RICHARD W. HINTON, Department of Geology and Geophysics, University of Edinburgh, EdinburghEH9 3JW, Scotland.DONALD A. HERD, Department of Geography and Geology, Division of Geology, University of StAndrews, St Andrews, Fife KY16 9ST, Scotland.

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The nature of zircon inheritance in two granite plutons