American Mineralogist, Volume 78, pages36-41, 1993
Textural and chemical changesundergoneby zircon during the
Pb-evaporationtechnique
KnvrNM. ANsonr,r*.T. Kunrrs Kvsrn
Department of Geological Sciences,University of Saskatchewan,Saskatoon,SaskatchewanS7N 0W0, Canada
Ansrucr
This study provides details of the textural and compositional changesin zircon as a
result of heating from approximately 1400 to 1650 'C under vacuum in a mass spectrometer during analysis by Pb-evaporation from a single zircon grain. Electron microprobe
analysesofpartially analyzedzircon grains show that the zircon reactspreferentiallyaround
the margins, along cracks, and around inclusions at low temperatures,whereasthe more
crystalline internal portions of the zircon react at higher temperatures.The reaction involves the breakdown of zircon to form a porous zirconium oxide with concomitant release
of silica and other trace elements, including Pb, which are collected on a filament and
subsequentlyionized to measure the isotopic composition of the Pb. The reaction front
migrates inwards from the margin of the zircon sample with increasingtemperature,such
that Pb from the most retentive sites in the crystal is releasedat the highesttemperatures.
207Pbl2o6Pb
These domains are the least likely to have undergone Pb loss, and thus the
ratios obtained at theseremperaruresare consideredto representbest the ageofthe zircon.
The Hf isotopic composition of zircon cannot be obtained using the evaporation technique
becauseof uncorrectablemass interferenceswith '76Hf. The use of this technique to deage of other U-bearing minerals such as titanite, baddeleyite,and
termine the 20?Pb/206Pb
apatite is complicated by different breakdown reactions within these minerals, especially
the lack of silica, which is needed for successfulanalysis of the isotopic composition of
rhe Pb.
ber, 1986, 1987; Kroner and Todt, 1988; Ansdell and
l99l; Ansdellet al., 1991).
Zircon is the most commonly used mineral for U-Pb Kyser,20?Pb/2o6Pb
ageprofiles obtained during step heatThe
geochronologybecauseit has a high U/Pb ratio, is gen(e.g.,Kober, 1986;Kober et al., 1989)are analogous
ing
erally resistant to isotopic resetting, and occurs in a vaageprofiles obtained during 40Ar/3eAtstep heating,
riety of rock types. The single-zircon Pb-evaporation to the
which
are consideredto representa profile ofthe distritechnique(Kober, I 986, 1987)is now beingusedfor rouof radiogenic o0Arwithin a given sample (Turner
bution
tine analysis of single zircon grains in a number of laboRecently, the interpretation that Ar-Ar proratories worldwide. The technique consistsof loading an et al., 1966).
files are related to volume diffirsion of Ar during step
untreated singlezircon grain in an outgassedRe filament.
questionedby Gaber et al. (1988) and
The sample is then heated in steps from about 1400 to heatinghas been
(1991). These workers show that during step
Lee
et
al.
about 1650'C. The Pb driven offfrom the zircon is desuch as biotite
posited on a secondfilament and subsequentlyanalyzed heating in a vacuum, hydrous minerals
phase changesat various temundergo
amphibole
and
207Pbl206Pb
for its isotopic composition. However, only
peraturesthat are concurrent with extensive Ar release.
agesare obtained becausethe zircon grains are not spiked.
The internal character of hydrous phasesand how they
2o7Pb/206Pb
Conventional U-Pb geochronologistsconsider
are affectedby heating to temperaturesofabout 1200 "C
agesobtained by Pb-evaporation to representminimum
vacuum are thus important in correctly interpreting
crystallization agesfor zircon becausethere is no inde- in
Ar-Ar
age profiles. The objective of this study is to docpendent evidence that zircon agesare concordant (e.g.,
the morphological and compositional changesthat
ument
Heaman and Parrish, 1991). However, a variety of studas a result ofheating to temperaturesof
zircon
undergoes
ies comparing agesfrom the single-zirconPb-evaporation
'C under vacuum during Pb-isotope analysis.This
1650
technique with agesobtained using conventional and highprocessby which
precision U-Pb geochronologyindicate that the 2o1Pb/2o6Pbinformation is vital to determine the
provide constraints on
zircon
and
to
from
Pb
is
released
agesare usually identical to concordant U-Pb ages(Koagesobtained
the geologicalsignificanceofthe 207Pbl206Pb
* Present address:Geological Survey of Canada, 601 Booth
(seeAnsdell et al., l99l). Resultsof prelirninaryattempts
agesfrom baddeleyite,titanite,
to obtain both 207Pb/206Pb
Street,Ottawa, Ontario KIA 0E8, Canada.
INtnooucrroN
0003-004x/93/0 l 02-0036$02.00
36
ANSDELL AND KYSER: CHANGES IN ZIRCON DURING Pb EVAPORATION
37
Fig. 1. Secondaryelectron images of zircons. (A) Typical euhedral zircon from a suite of detrital zircon gxains analyzed by
Ansdell et al. (1992). (B) Typical appearanceof zircon after heating in mass spectrometerto 162O"C. Euhedral zircon from Missi
5 suite, which yielded a 2o1Pb/2o6Pb
age of I 865 + 25 Ma (Grain 2, Missi 5; Ansdell et al., 1992).(C) Close-up of surfaceshowing
porous texture. (D) Close-up of surfaceshowing channelsand annealedtexture with triple junctions.
and apatite and Hf isotopic compositions of singlezircon
grains are also reported.
Axlr,v:tlc.q,r, METHoDS
The analysis of single zircon grains by the Pb-evaporation technique at the University of Saskatchewanfollows the procedureof Kober (1987),with slight modifications as outlined by Ansdell and Kyser (1991) and
Ansdell et al. (1992). Analysis of Pb from the zircon is
usually continued until all the Pb has been removed, and
the2oTPb/2o6Pb
age for each heating step is calculatedusing the techniqueoutlined by Kroner and Todt (1988).
An age plateau is consideredto representthe age of the
zircon, similar to the step-heatingAr-Ar method. If a
well-formed plateauis not attained, then the highesttemperature step representsthe minimum age of the zircon.
A selectionof zircon samples,someof which still retained
some Pb, were removed from their filament and analyzed
using a Jeol JXA-8600 electron microprobe at the University of Saskatchewan.Backscatteredelectron images
and wavelength-dispersiveanalyseswere obtained from
C-coated samples, and secondary electron images were
obtained from Au-coated samples.
RBsur,rs
Inspection of zircon after its removal from the mass
spectrometerindicates that every zircon sample turned a
milky white color irrespective of original color and irrespective of the number of heating steps.The majority of
samplesretained their original morphology, but became
very friable after heating, primarily as a result of the
changesin the chemical composition of zircon during
heating. Secondaryelectron microscopy ofthe surfaceof
partially reacted zircon reveals that the surface is more
irregular and porous than unreactedzircon (Fig. 1A, lB).
Under high magnification, the surfaceis seento consist
of channelsinto the crystal,up to a few micrometerswide,
indicating a highly porous material (Fig. lC, lD). The
foam texture ofthe surfaceis suggestiveofannealing with
formation of triple junctions (Fig. I D). The high porosity
38
ANSDELL AND KYSER: CHANGES IN ZIRCON DURING Pb EVAPORATION
Fig. 2. Backscatteredelectron images of partially and completely reactedzircon samples.(A) Partially analyzedzircon that
was originally devoid of zoning prior to analysis(AS3, Table 1).
The reaction front progressesinward from the maryin. (B) Closeup of the reactedmargin of a zircon sample (FCl, Table 1) emphasizing the porous texture exhibited by the new zirconium
oxide phase. (C) Wavelength-dispersivemapping of Si and Zr
distribution emphasizesthe Si-poor character of the reacted
margin relative to the unreactedinternal portions of the zircon
(89-22,Table I of Ansdell and Kyser, l99l). Note that the zircon has also reactedpreferentially along cracks and around inclusions (D) Partially analyzed zircon containing typical magmatic growth zones (W506, Table 1). The zircon preferentially
reacts around the margin and along the more U-rich growth
zones.(E) Completely reactedOtto Stock, which yielded a'zo'Pbl
']o6Pbage of 268'1 + 12 Ma. An age of 2680 + I Ma has been
obtained by high-precision U-Pb geochronology(Corfu et al.,
l 989).
indicates that a volume decreaseoccurs during the alteratlon process.
Backscatteredelectron images and microprobe analysesshow the physicaland chemicaleffectsofthe heating
process on zircon. In partially analyzed zircon with no
compositional zoning or structural defects(e.g.,Fig. 2A),
the zircon has reacted preferentially from the margins
inwards, as shown by the porous texture of the margin
(Fig. 2B). Heating of the zircon converts the reacted portion to a porous zirconium oxide with an associated loss
of silica (Table l, Fig. 2C).
The effects of the breakdown reaction associated with
heating zircon are heterogeneous in more internally complex zircon. Figure 2C shows that the reaction process
39
ANSDELL AND KYSER: CHANGES IN ZIRCON DURING Pb EVAPORATION
Trele 1.
Electron microprobe analyses of representativeunreactedand reacted zircon, Pb isotopic compositions, and 'z07Pbl'?06Pb
technique
ages obtainedusing the single-zirconPb-evaporation
Sampleno.
Sample
Location
(wt%)
sio,
ZrO,
Hfo,
CaO
Tho,
U"ou
ToTAL
,07Pb/2o6Pbage
2o7Pbl26Pb
,03Pb/106Pb
,04Pb/r6Pb
U-Pb zircon U.l. age
U-Pb titanite U.l. age
Rb-.Sr whole rock age
AS3
Duluth anorthosite
Minnesota
Unreacted
30.81
66.04
1.62
Reacted
97.20
2.18
0.08
98.68
99.85
' 1 0 9 8+ 1 6
0 0 7 6 1 2+ 6 1
0.1473 + 27
<0.00009
1 0 9 9+ 1 -
FC1
Duluth anorthosite
Minnesota
Unreacted
32.80
66.49
1.05
0.05
0.05
100.50
Reacted
92.25
1.40
0.08
94.53
1 1 0 8+ 1 2
0.07648+ 48
0 . 1 6 7 2+ 8 5
<0.00002
1 0 9 9+ 1 .
w506
HoskinLakegranite
Saskatchewan
Wisconsin
Light gray
Dark gray
32.10
66.21
1.73
33.10
64 16
2.24
0.05
0.03
0.03
100.18
89-22
NeagleLakepluton
Reacted
0.22
99.89
1903+ 12
0 . 1 1 6 5+1 7 8
+ 50
0.1835
<0.00009
92.65
4.51
0.13
0.14
97.85
Unreacted
Reacted
32.30
65.58
1.37
94.41
2.76
99.26
97.17
1 9 3 1+ 1 5
0 . 1 1 1 9 +1 9 8
0.0538+ 15
<0.00007
1846 + 14l-61
1692+ 69..
Dashes indicate
Note; Standards used for electron microprobeanalyseswere quartz (Si), zircon (Zr, H0, diopside (Ca), monazite(Th), and U3O8(U).
that the element was not detectable. Operating conditions: acceleratingvoltage : 15 kV, probe current for wavelength-dispersiveanalysis and
backscatteredelectron imagery -10 nA, except for Hf (35 nA), probe current for secondaryelectron microscopy: 300 pA.
The Pb isotopic compositionsand the calculated2o7Pb/,o6Pb
age were taken from the highesttemperatureevaporationstep. All data are unpublished,
except for 89-22 (89-22 Grainl in Ansdell and Kyser, 1991). Ages are reported in Ma, and errors in Pb isotopic compositions.Ages are Quotedat 2o'
- J. Paces,personalcommunication,1991.
.* Petermanet al., 1985.
t Ashton et al.. 1992.
preferentially afects zircon adjacent to cracks and mineral inclusions, as well as the margins of the crystal. A
zircon grain with typical magmatic growth zoning is shown
in Figure 2D. Variations in composition are indicated by
differencesin level of gray, and there is preferential reaction ofthe growth zonesexhibiting the darker gray color. If step-heatinganalysisis continueduntil there is no
Pb left, the whole of the zircon exhibits a homogeneous
porous texture (Fig. 2E). Many of theseobservationshave
also been reported recently by Roddick and Chapman
099l).
The ionization filament is usually cleaned by heating
to high temperature (about 2200 "C) after each analysis.
The material depositedon the ionization filament is thus
burnt off, which prevents Pb from the next evaporation
step from mixing with Pb derived from the previous temperature step.Ionization filaments prior to thermal cleaning have a residue consistingof silica. No other elements
were detectable in the residue at the limit of the EDS
analysis.However, during the courseof some mass spectrometry analyses,mass scansindicate that Pb, REE, Hl
and a suite of other trace elementsare also deposited on
the ionization filament during each evaporation step.
Zircon is an important Hf reservoir in the continental
crust, and the Hf isotopic composition of zircon may thus
provide an important constraint on its source area (e.g.,
Patchettet al., l98l; Stevensonand Patchett,1990).The
possibility of determininglhe tl6Hf/177Hfratio of the same
zircon dated using the Pb-evaporation technique is obviously attractive. The most abundant Hf isotope, 180Hf,
was monitored during the course of Pb isotope analyses
ofzircon from Early Proterozoic granitoids from the Flin
Flon area, Canada(Ansdell and Kyser, l99l). At the filament temperaturesattained during a typical Pb-evaporation analysis, the '80Hf intensity was only marginally
above background levels on the secondaryelectron multiplier, but significant signalswere obtained by increasing
the evaporation and ionization filament temperaturesto
about 2000-2300 "C. However, electron microprobe
analysesof reactedzircon samples(Table l) indicate that
some Hf is retained within the zirconium oxide phase
that forms during the breakdown of zircon. In addition,
measurementof '?6Hfusing the evaporation technique is
intractable without prior element separation becauseof
mass interferencesfrom t76lu and 176Yb.
Ilrpr,rclrroNs FoR DATING zIRCoN UsING THE
TECHNIQUE
Pb-nv.lpourloN
Zircons are often internally heterogeneousand may
contain growth zones, resolption features, cores, overgrowths, metamict zones, alteration zones, and mineral
and fluid inclusions. Radiogenic Pb that accumulates
within the zircon as a result of decay of U may be held
in. or associatedwith. all of these internal features, although they have different capacitiesto retain Pb (Kober,
1986). For example, Pb is more easily removed from
metamict zonesthan crystalline domains in zircon. Conventional dissolution and analysisofzircon that contains
both domains result in discordant U-Pb ages.In most
conventional U-Pb laboratories, painstaking but fruitful
attempts are made to reduce the likelihood of analyzing
zircon containing domains that have undergonelater Pb
40
ANSDELL AND KYSER: CHANGES IN ZIRCON DURING Pb EVAPORATION
loss in order to obtain high-precision, concordant U-Pb
Overall, the observations made on partially analyzed
ages.The methods employed include abrasion (Krogh, zircon support the general hypothesis of Kober (1986)
1982a),magneticsusceptibility (Krogh, 1982b),and hand that Pb is derived from locations within the zircon at
picking. In contrast, the Pb-evaporation technique relies different temperatures,with the Pb from the most retenon temperature to separatePb from different domains tive sites being releasedonly at the highest temperature.
within the zircon. i.e.. Pb adsorbed onto the surface or These domains are the least likely to have undergonealratio
along cracks is driven off at low temperatures,Pb from teration and Pb loss. Accordingly, the 207Pbl206Pb
metamict domains at intermediate temperatures,and Pb derived from the higher temperaturestepsare considered
from crystalline portions of the zircon at high tempera- to be an accurateindicator ofthe age ofthe zircon.
The observations made during this study also shed light
tures(Kober, 1986,1987).
The observationsmade in this study indicate that the on the processby which Pb is releasedfrom zircon. In
breakdown of zircon preferentially takes place from the experiments performed at atmospheric pressure,Buttermargin inwards. Thus, any Pb adsorbedon the surfaceof man and Foster (1967) showed that zircon breaks down
'C. Kober
the zircon would be removed during the lowest temper- to form zirconium oxide and silica at 1676
(1987)
reaction
was unsuggestedthat this breakdown
ature step. This Pb is assumed to be analogousto that
removed during abrasion and acid leaching,prior to dis- likely to be the dominant source of silica becausethis
temperature is higher than the temperature of the evapsolution in conventional U-Pb geochronology.
In an internally homogeneouszircon (e.g., Fig. 2A), oration filament during the Pb-evaporation technique.
subsequenthigher temperature stepsresult in the inward However, it appears that under vacuum (10-8 bar), the
migration of the reaction front from the margins of the zircon structure breaks down to zirconium oxide, releaszircon, and each step should yield the same 207Pbl206Pbing silica and other trace elements, such as Pb, at temages.In zircon that is crackedand contains inclusions but peraturesas low as 1400'C. The zircon thus providesits
is otherwise relatively homogeneous(e.9., Fig. 2C), the own silica bed on the ionization filament, which results
age spectrum may be more complex. The reaction front in a stable Pb* ion beam. We suggestthat the viability of
migrates from the internal heterogeneities,as well as the the single-zirconPb-evaporation technique is dependent
margins of the zircon, and the last aliquot of Pb is derived upon the releaseof silica from the zircon and the forfrom the most crystalline portions of the zircon that are mation of the silicabed. Bogomolov(1991)assumedthat
also the farthest away from both the margin and any in- volume difrrsion was the main mechanism of lead mioC
ternal heterogeneities.In thesecases,the highesttemper- gration betweenapproximately I 200 and I 650 for var2olPb/
ious ground and unground singlezircon grains. However,
ature evaporation step yields the most accurate
206Pb
age for that zircon. Inclusion-rich zircon exhibiting the textural evidence reported here suggeststhat the remagmatic growth zoning (e.g., Fig. 2D) reacts preferen- leaseof Pb is likely related to the progession of the retially around the margins and inclusions, as well as along action front through the zircon.
specific growth zones. In zircon exhibiting magmatic
Appr,rc.c.rroN To orHER MTNERALS
g"rowthzoning in which specific growth zones have not
preferentially undergone Pb loss, 207Pb/'?06Pb
Attempts to use the Pb-evaporation technique to deages obtained from growth zonesthat react at lower temperature termine the age of single crystals of other U- and Pbdo not differ from those that react at higher temperature, bearing minerals were unsuccessful.Titanite from the
since the zircon grew over a relatively short time during Phantom Lake granite, Saskatchewan,Canada,which has
been dated at 1840 + 7 Ma using the Pb-evaporation
the crystallization of the magma.
The development of metamict zones in zircon com- technique on zircon (Ansdell and Kyser, 1991), was heatmonly includes the formation of microfractures that ed progressivelyfrom about 1200 to about 2000'C. Dewere obtainedat about I 250, I 300,
crosscut the more crystalline portions of the zircon and tectablebursts of 206Pb
radiate away from the metamict zones(Chakoumakoset and 1800 "C, but none produced a measurablePb signal
al., 1987).This phenomenonis likely relatedto the vol- after a l0 min evaporation period at thesetemperatures.
ume expansion of the metamict zones. The location of Apatite from the Annabel Lake pluton (1860 + 6 Ma;
these metamict or cracked crystalline zones within the Ansdell and Kyser, l99l) yielded a 206Pbsignal only
zircon controls the temperature at which Pb is derived slightly above backgroundup to about 1500'C and nothfrom them. Zones connectedto the surfaceby crackslose ing at higher temperatures.At thesetemperaturesthe apPb at relatively low temperatures,and the crystalline, most atite had melted. Baddeleyitefrom the Phalaborwacomcrack-freeinternal portions ofthe zircon lose Pb at higher plex, South Afica(2047 * I I or -8 Ma;Eriksson, 1984)
signal as the tempertemperatures,unless the metamict domains are located yielded a small but consistent'zo5Pb
toward the center ofthe zircon and are not connectedto ature of the evaporation filament was raised from 1200
the surface by cracks. Characteization of the internal to 2000 "C.
ages for
The lack of successin obtaining 2o7Pb/2o6Pb
structure and composition of a zircon grain are thus important in properly interpreting age spectraobtained us- U-bearing minerals other than zircon implies that the
silica releasedby zircon during breakdown may play an
ing the Pb-evaporation technique.
ANSDELL AND KYSER: CHANGES IN ZIRCON DURING Pb EVAPORATION
important role in the production of a stablePb* ion beam.
To test this theory, 12 pL of silica gel was dried on Re
filaments prior to loading apatite and baddeleyite. For
both minerals,the filamentswere heatedto 1500-1700
oC and allowed to evaporate for 20 min. Although
there
was no improvement in the overall Pb signal obtained
from the apatite, a significant ,06Pbsignal was attained
for a very short period from the baddeleyite.Higher temperatureevaporation stepsyielded 206Pb
intensitiesbarely
above background intensities. The addition of silica gel
to the filaments containing the minerals may improve the
beam stability, but the silica is removed completely during the flrst evaporation step.Therefore, the lack ofsilica
in baddeleyite and apatite indicates that successfulanalysis of theseminerals by the Pb-evaporation technique is
unlikely. Successfulanalysis of titanite requires detailed
documentation of the temperaturesat which breakdown
reactions occur, and successfulanalysiswill only be possible ifthere is coincidental releaseofboth Pb and silica.
AcxNowr,rocMENTS
Zircon sampleswere kindly provided by F. Corfu (Otto Stock),J. Paces
(FCl, AS3), and Z. Peterman (W506), and the baddeleyitesample was
provided by L. Hulbert. Technical assistancewas provided by A. Vuletich
(massspectrometry),B. Novakovski (microprobe plug preparation),and
R. George (electron microscopy). Financial assistancewas provided by
Natural Scienceand EngineeringResearchCouncil (NSERC) infrastructure and operating grants, and an NSERC-Industry (Cameco) research
grant to T.K.K. K.M.A. acknowledgesreceiptof a University of Saskatchewan graduatescholarship.D. O'Hanley is thanked for discussionsand a
review of the original manuscript. Reviewsby M.E. Bickford, H.E. Gaudette, and J. Longhi are appreciated
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MeNuscnryr
MaNuscnrpr
RECETvEDApp.t 27,1992
AccEprED Aucusr 28, 1992