Download Zircon Behaviour during Low-temperature

JOURNAL OF PETROLOGY
VOLUME 50
NUMBER 4
PAGES 571^589
2009
doi:10.1093/petrology/egp011
Zircon Behaviour during Low-temperature
Metamorphism
D. C. HAY AND T. J. DEMPSTER*
DEPARTMENT OF GEOGRAPHICAL AND EARTH SCIENCES, UNIVERSITY OF GLASGOW, GLASGOW G12 8QQ, UK
RECEIVED 2008; ACCEPTED 2009
ADVANCE ACCESS PUBLICATION MARCH 15, 2009
The discovery of zircons up to 44 Gyr old (Mojzsis et al.,
2001; Wilde et al., 2001; Utsunomiya et al., 2004; Nemchin
et al., 2006) is testament to the remarkable durability of
zircon. The ability of zircon to incorporate U and Th but
exclude Pb from the crystal lattice upon formation
(Watson et al., 1997), combined with sluggish diffusion
rates of trace elements within its structure (Cherniak &
Watson, 2001) are properties that have led to zircon being
the premier U^Pb geochronometer (Davis et al., 2003).
Moreover, the use of zircon as a geochronometer is linked
to it being highly refractory at the Earth’s surface (Fedo
et al., 2003). Multiple stages of growth can be revealed
using cathodoluminescence (CL) and back-scattered electron (BSE) imaging (e.g. Scharer et al., 1997; Corfu et al.,
2003; Tichomirowa et al., 2005; Harley et al., 2007). The
preservation of these features provides isotopic and geochemical insights into thermal histories and past environments (Harley et al., 2007). However, the robust nature of
zircon in low-temperature metamorphic environments has
recently been questioned (e.g. Dempster et al., 2004;
Rasmussen, 2005).
The stability of zircon at the Earth’s surface has led to
proposals of it being a potential storage material for plutonium (Ewing et al., 1995). A large body of work now exists
that has studied how radiation damage accumulates
within the crystal lattice (e.g. Farnan et al., 2003;
Trachenko et al., 2003) and its effect on the properties of
zircon (e.g. Geisler et al., 2007). Radiation-damaged (metamict) zircon is chemically and physically less stable than its
crystalline equivalent, causing enhanced susceptibility to
alteration by aqueous fluids (Pidgeon et al., 1966; Trocellier
& Delmas, 2001; Geisler et al., 2003a, 2007; Hoskin, 2005).
Hydrothermal alteration of metamict zircon results in the
structural recovery of amorphous areas and enrichment in
hydrous species and the non-formula elements Al, Mg, Ca,
Fe, Mn, Y and P (Speer, 1982; Geisler et al., 2003a, 2003b,
2007). The structural recovery of metamict zircon is
*Corresponding
author.
Telephone:
00441413305445.
Fax: 00441413304817. E-mail: [email protected]
ß The Author 2009. Published by Oxford University Press. All
rights reserved. For Permissions, please e-mail: journals.permissions@
oxfordjournals.org
Zircon in greenschist-facies metasedimentary rocks from the Scottish
Highlands displays a range of complex textures that reflect
low-temperature alteration of original detrital grains. In situ backscattered electron, cathodoluminescence, electron backscatter diffraction and chemical analyses show that altered zircon is porous,
weakly luminescent, enriched in non-formula elements such as Al
and Fe, and is associated with fractures within the host zircon. The
low-temperature zircon appears to be nano-crystalline and to replace
U-rich zircon via modification of whole grains or selective alteration
of parts of grains, and is linked to the development of zircon outgrowths. The altered zircon is also associated with epitaxial xenotime outgrowths and inclusions. Low-temperature zircon is
abundant in slates and other mica-rich samples and its formation is
linked to a dissolution^reprecipitation mechanism. Zircon within
quartz-rich host rocks typically shows evidence of deformation and
the resulting fractures enhance its dissolution, creating rounded
embayed morphologies. In contrast, zircon from phyllosilicate-rich
rocks contains more new low-temperature growth. Zircon alters
during both prograde and retrograde metamorphic events and its
development is controlled by both the progressive accumulation of
radiation damage in the host grain and the access of metamorphic
fluids to the metamict zircon.
KEY
WORDS:
zircon; metamorphism; dissolution; metamict;
greenschist facies
I N T RO D U C T I O N
JOURNAL OF PETROLOGY
VOLUME 50
thought to be the result of water diffusion promoting solidstate recrystallization (Geisler et al., 2003a), with the first
structural changes at c. 758C (Geisler et al., 2003c). Putnis
(2002) disputed a solid-state, diffusion-driven cationexchange mechanism and favoured simultaneous dissolution^reprecipitation that results in the development of a
porous product phase but ‘still preserves the external
shape of the parent’. Low-temperature alteration of metamict zircon results in a nano-crystalline structure of randomly oriented crystals (Schmidt et al., 2006; Utsunomiya
et al., 2007). Over geological time, metamict zircon may be
partly annealed at temperatures below 2008C (Nasdala
et al., 2001b) and possibly at Earth surface temperature
(Meldrum et al., 1998). Fission tracks in zircon are known
to anneal around 200^2508C (Garver, 2002; Yamada et al.,
2003; Rahn et al., 2004) and this may suggest that
-damage in zircon will be repaired at similarly low temperatures (Nasdala et al., 2001b).
The great majority of studies of zircon behaviour have
been conducted on mineral separates without regard to
their in situ petrological context. Investigations of zircon
within thin sections of high-grade metamorphic rocks provide evidence that it responds to metamorphic processes
(Vavra et al., 1996; Rubatto et al., 2001; Kelly & Harley,
2005; Harley et al., 2007; Dempster et al., 2008a) and studies
of hydrothermal settings suggest that zircon can be taken
into solution and precipitated (Fraser et al., 1997;
Tomaschek et al., 2003). Investigations of zircon in greenschist-facies (Dempster et al., 2004) and prehnite^
pumpellyite-facies (Rasmussen, 2005) metasedimentary
rocks revealed small metamorphic zircon outgrowths on
the margins of detrital zircon, which show that Zr can be
mobilized at low temperatures. This notion was also considered over 70 years ago when the discovery of outgrowths, with similar optical properties to the host
detrital zircon, was made in sedimentary rocks
(Butterfield, 1936). Such early studies of zircon outgrowths
failed to provide evidence for a source of Zr, but others
have since observed evidence of zircon dissolution in lowtemperature environments (e.g. Dempster et al., 2004;
Delattre et al., 2007). Despite these studies, relatively little
is known about the response of zircon to low-temperature
events in geological conditions. This study aims to document zircon behaviour during greenschist-facies
metamorphism.
GEOLOGIC A L S ET T I NG A N D
SAMPLI NG
Greenschist-facies metasedimentary rocks were sampled
from the Late Proterozoic Dalradian Supergroup in the
Scottish Highlands (Table 1). The rocks are from chlorite
and biotite zones and have experienced regional metamorphism during the Caledonian orogeny at c. 470 Ma,
NUMBER 4
APRIL 2009
although earlier low-grade metamorphic events are also
possible (Dempster et al., 2002). The Dalradian clastic
metasedimentary rocks were deposited from c. 750 Ma to
c. 590 Ma and contain detrital zircons with a range of
U^Pb ages (Cawood et al., 2003) indicating a provenance
from Archaean to Mid-Proterozoic sources. Several phases
of deformation have affected the rocks, although they are
dominated by a single penetrative fabric defined by phyllosilicates. After metamorphism the Dalradian rocks cooled
relatively rapidly at c. 450 Ma and most unroofing of the
western parts of the Scottish Highlands was probably complete by c. 400 Ma (Dempster, 1985), although small
amounts of Tertiary denudation were probably linked to
North Atlantic rifting (Persano et al., 2007).
Samples with the ‘KL’ prefix (Table 1) are Appin Group
quartz-rich metasedimentary rocks from the north shore of
Loch Leven (Grid Ref. NN 152 618). These typically contain c. 10% phyllosilicates (muscovite, biotite and chlorite)
within a weakly defined cleavage. Rare small (c. 05 mm)
garnets suggest these units experienced greenschist-facies
metamorphism at temperatures c. 4508C. Grain sizes are
typically 100^200 mm although micas of c. 500 mm are present in some samples. Heavy mineral layers dominated by
Fe-oxides are present in some rocks. Samples with the
prefix ‘Bal’ are from Ballachulish (Grid Ref. NN 085 583)
and are also part of the Appin Group. They are phyllosilicate-rich (450%), dark graphitic slates, with small lenses
of quartz aligned parallel to bedding, and phyllite with
millimetre-scale quartz-rich laminations. Both rock types
are dominated by fine-grained (c. 10 mm) muscovite, chlorite, quartz and plagioclase together with pyrite of variable
grain size (c. 01^5 mm) and rare biotite within the phyllite. The samples from Ballachulish are of lower grade
(c. 4008C) than the Loch Leven samples and their proximity to the c. 420 Ma Ballachulish and Glen Coe granites
suggests that they may have experienced mild reheating,
although this is not apparent from the mineral textures.
Samples with the prefix ‘E’ are finely laminated Easdale
slates from the Argyll Group (Grid Ref. NS 218 831) including a well-cleaved black graphitic slate and a carbonatecemented slate with a poorly defined cleavage. Both
contain muscovite, chlorite, quartz and plagioclase with
occasional pyrite (up to 3 mm). Samples with the prefix
‘Dh’ are from the Cove area on the Rosneath peninsula
(Grid Ref. NS 218 831) and are part of a sequence from
the Dunoon Phyllites in the Southern Highland Group.
These include three samples of fine-grained (c. 10 mm) graphitic slate together with a psammite. The slates have a
penetrative cleavage that is locally weakly crenulated and
are composed of muscovite and chlorite (together 450%),
quartz and plagioclase. The psammite has a weak cleavage
defined by muscovite and chlorite and contains coarse
polycrystalline quartz clasts (03^3 mm) in a matrix of
finer grained (30^50 mm) quartz and albite with patches
572
HAY & DEMPSTER
ZIRCON AND LOW-T METAMORPHISM
Table 1: Rock types and associated zircon abundance, size and textural character
Rock type
Grain size
and sample
(mm)
Zirc/cm2
Size (mm)
Light
Dark
% with zirc
% with xen
mean, (range)
BSE (%)
BSE (%)
outgrowths
outgrowths
Quartzite
KL1
50–100
40
20, (5–50)
90
10
0
0
KL2
100–200
15
40, (10–50)
91
9
0
0
KL3
100–300
11
40, (5–100)
81
19
0
0
KL4
30–300
71
30, (5–50)
85
15
1
0
KL5
100–300
17
40, (10–100)
64
36
25
0
KL5h
100–300
16
20, (5–120)
62
38
4
0
KL6
550–300
12
20, (10–30)
66
34
0
0
100–3000
88
30, (10–110)
55
45
18
5
5–100
225
20, (5–70)
82
18
3
40
E1
55–10
265
20, (10–40)
25
75
42
7
BAL1
55–10
152
10, (55–30)
42
58
58
63
BAL1a
55–10
134
5, (55–30)
43
57
48
61
BAL1c
55–10
150
5, (55–20)
0
100
17
42
BAL1d
Psammite
DH4
Phyllite
BAL3
Slate
55–10
105
10, (55–30)
37
63
53
47
DH1
55
171
5, (55–30)
44
56
67
0
DH2
55–10
263
10, (55–30)
63
37
73
3
DH3
55–10
250
10, (5–40)
53
47
33
6
of carbonate. Based on the muscovite- and chloritedominated mineral assemblage of the Dunoon Phyllites
and Easdale Slates, they have experienced low-grade
regional metamorphism that is unlikely to have exceeded
300^3508C. There is no evidence of late hydrothermal
veining in the samples.
M ET HODS
All zircon was characterized in polished thin sections of
the rocks using a FEI Quanta 200F field-emission environmental scanning electron microscope (SEM) operated at
20 kV with moderate beam currents. Brightness and contrast of each BSE image were set to maximize differences
within zircon grains and distinct groups based on BSE
intensity and other characteristics are identified.
Cathodoluminscence (CL) images of zircon were obtained
using a K.E. Developments Centaurus CL detector
attached to the SEM. The CL detector responds to wavelengths in the range 300^650 nm. The SEM has an
EDAX Pegasus 2000 energy-dispersive X-ray microanalyser (EDX) for qualitative element analysis. EDX scanned
chemical maps were obtained at 512 400 resolution.
To assess the relative proportions of zircon types, single
grains were characterized using the SEM, by point counting in transects perpendicular to layering at 50
magnification.
Electron probe microanalysis (EPMA) was conducted
on a Cameca SX-50 at University of Glasgow and a
Cameca SX-100 at University of Strathclyde. The former
was used to analyse the host zircon and outgrowths and
the latter for dark BSE zircon. The setup conditions for
both were 20 kV and 20 nA with a 30 s count time on
peaks. The detection limits based on background counting
statistics for EPMA are: Mg 100 ppm, Al 100 ppm, Si
150 ppm, Ca 70 ppm, Fe 300 ppm, Y 300 ppm, Hf
1000 ppm and Zr 500 ppm. Lack of suitable standards precluded the analysis of REE and actinides. Minor contamination by matrix minerals or micro-inclusions was
recognized by consistent and predictable deviation from
overall trends in a range of chemical plots, and such data
were excluded. Representative analyses are given in
Table 2 and the full set of 227 analyses are reported in the
electronic appendix (available for downloading at http://
petrology.oxfordjournals.org/)
Crystallographic information on zircon and xenotime
has been obtained by electron backscatter diffraction
573
JOURNAL OF PETROLOGY
VOLUME 50
NUMBER 4
APRIL 2009
Table 2: Representative chemical analyses of zircons
Sample
No.
wt % oxides
cations per formula unit (c.p.f.u.)
MgO
Al2O3
SiO2
CaO
FeO
Y2O3
ZrO2
HfO2
Total
Mg
Al
Si
Ca
Fe2þ
Y
Zr
Hf
Total
200
Light BSE
dh4zr14
2
000
000
3128
001
006
005
6566
138
9844
000
000
098
000
000
000
100
001
kl5zr16c
80
000
001
3134
001
015
020
6658
126
9955
000
000
097
000
000
000
101
001
200
bal1bz28c
35
0
000
3411
010
031
011
6318
147
9928
000
000
104
000
001
000
096
003
204
203
bal1bz47a
91
0
000
3360
002
020
004
6487
124
9997
000
000
102
000
001
000
098
002
Ezr5
176
0
000
3254
003
037
000
6597
102
9993
000
000
100
000
001
000
101
002
204
Ezr1-5a
181
0
002
3262
002
057
006
6413
158
9900
000
000
101
000
001
000
099
003
204
E1zr2
102
000
002
3125
002
028
009
6820
093
10079
000
000
096
000
001
000
102
001
200
dh4zr14
23
009
109
2524
106
076
245
5592
107
8768
001
005
091
004
002
005
098
001
206
dh4zr14
24
009
108
2531
098
077
244
5607
123
8797
000
005
091
004
002
005
098
001
206
dh4zr14
35
013
089
2624
112
072
165
5865
127
9067
001
004
091
004
002
003
099
001
205
dh4zr26
42
009
049
2634
074
050
300
5607
097
8820
000
002
094
003
001
006
097
001
205
dh4zr26
48
006
034
2871
057
055
135
6056
101
9315
000
001
096
002
002
002
099
001
203
dh4zr9
57
009
124
2407
107
066
419
5179
099
8410
000
005
090
004
002
008
095
001
207
dh4zr9
59
006
097
2333
108
052
404
5206
101
8307
000
004
089
004
002
008
097
001
206
dh4zr40
64
008
080
2404
106
056
341
5443
114
8552
000
003
089
004
002
007
099
001
205
kl5zr16c
85
002
033
2770
030
034
064
6137
122
9192
000
001
094
001
001
001
102
001
202
kl5zr16c
88
004
038
2742
040
049
082
6088
110
9153
000
002
094
001
001
001
101
001
203
kl5zr16c
89
003
072
2666
071
079
167
5801
103
8962
000
003
093
003
002
003
099
001
204
E1zr2
99
025
125
2493
142
081
318
5323
092
8599
001
005
091
006
002
006
095
001
208
E1zr2
100
026
120
2550
139
080
316
5378
094
8703
001
005
092
005
002
006
094
001
208
203
Dark BSE
Outgrowths
bal1bz28e
37
0
003
3341
016
030
041
6250
133
9814
000
000
103
001
001
001
096
002
bal1bz47d
94
0
006
3271
015
038
049
6234
145
9758
000
000
102
000
001
001
096
003
203
bal1bz47e
95
0
009
3246
009
032
034
6202
141
9673
000
000
102
000
001
001
097
003
203
204
bal1bz47g
97
0
012
3121
024
053
070
5948
156
9384
000
000
102
001
001
001
095
003
Ezr5b
177
0
000
3265
005
053
000
6533
167
10023
000
000
100
000
001
000
100
003
204
Ezr5c
178
0
003
3267
005
053
004
6487
170
9989
000
000
100
000
001
000
099
003
204
Ezr5d
179
0
018
3230
022
083
032
6329
146
9860
000
001
100
001
002
001
097
003
204
Porous
dh4zr17a
8
000
000
3252
001
029
003
6680
127
10092
000
000
099
000
001
000
099
001
200
dh4zr17a
9
000
000
3237
001
032
001
6634
127
10032
000
000
099
000
001
000
099
001
200
dh4zr17a
10
000
000
3259
000
032
000
6630
110
10032
000
000
100
000
001
000
099
001
201
(EBSD) on the SEM with samples tilted to 708. Sections
were given a final polish with colloidal silica for 10 min to
improve the quality of the diffraction pattern. Analysis in
low-vacuum mode, with no carbon coat, produces a high
detector signal pattern that allows for rapid scans and
results in high-quality patterns. However, these advantages
are offset by problems caused by charging. To minimize
this but still obtain a high-quality pattern, samples were
coated with a very thin (c. 10 nm) carbon coat and silver
was painted around the zircon grain prior to analysis in
high-vacuum mode. The effective resolution for EBSD analysis is c. 50 nm (Humphreys, 2001). Crystallographic orientations were assessed relative to a crystal reference frame
by plotting the inverse pole figures (IPF) of the kikuchi
patterns. Imperfections in the lattice can be monitored
using EBSD and can determine crystal quality through
the Image Quality (IQ) of the pattern. The latter is
defined as the average height of the detected peaks, which
is a direct function of the intensity of the pixel on the
Hough transform.
574
HAY & DEMPSTER
ZIRCON AND LOW-T METAMORPHISM
Fig. 1. Zircon types in greenschist-facies metasedimentary rocks. Images are BSE unless stated otherwise. (a) CL image of unmodified detrital
zircon exhibiting low-intensity core with a surrounding oscillatory-zoned domain (KL5). (b) Zircon with porous zones and radial fractures
(KL5). (c) Concentric zones of light BSE and dark BSE zircon with zircon outgrowths on margin. Dark BSE areas have abundant cavities
(E1). (d) Light BSE zircon surrounded by dark BSE zircon and zircon outgrowths. Dark BSE zircon contains bright xenotime inclusions and
pores (Bal1d). (e) Zircon of typical detrital shape but dominated by dark BSE zircon that has significant variation in BSE intensity and shows
weak zoning parallel to the grain edge. Domains of unmodified light BSE zircon have irregular margins. High BSE intensity inclusions are
xenotime (Dh4). (f) Central fractured zircon contains dark BSE and light BSE areas, with rounded zircon fragments. Unmodified light BSE
zircon in lower right of image is shown in (a) (KL5).
Z I RC O N A N D X E N O T I M E
C H A R AC T E R I S T I C S
A total of 585 zircon grains were characterized during this
study (Table 1). Zircon either has a random distribution or
is concentrated in heavy mineral bands. Zircon in slate
and phyllite ranges from 2 to 40 mm in diameter with an
average of 5^10 mm. Zircons in quartzite and psammite
are not as abundant but are larger, with an average diameter of c. 30 mm (Table 1). The larger zircons are subhedral
to well-rounded; occasional euhedral grains are found typically as inclusions in large (4100 mm) quartz grains.
Smaller zircons (530 mm) have a greater variety of
shapes, from well-rounded grains to those with angular
margins. No micro-zircon of the sort reported from
amphibolite-facies schists (Dempster et al., 2008a) has
been observed.
A variety of zircon types were observed (Fig. 1), with a
range of crystallinity, inclusion content and grain shapes.
BSE and CL images of zircon in both quartzite and slate
reveal a range of zoning types, including homogeneous
grains, those with simple concentric and oscillatory
zoning, and those with discrete cores and thick homogeneous rims.
Zircon that lacks inclusions and pores and has a characteristic high BSE signal intensity, regardless of minor variations caused by zoning, is referred to as ‘light BSE zircon’
(Fig. 1f). Grains composed entirely of light BSE zircon are
similar to the majority of detrital zircons obtained from
mineral separates from metasedimentary rock types
(e.g. Cawood et al., 2003). Many zircons contain areas
with a distinctive and complex lower BSE signal intensity
(Fig. 1c^e) and these are referred to as ‘dark BSE zircon’. A
further group of zircons containing high concentrations of
spherical pores (Fig. 1b) is also present. Irrespective of
zircon type, overall shapes suggest that the zircon grains
in the greenschist-facies metasedimentary rocks had a detrital origin.
Grains containing dark BSE zircon represent c. 62% of
the population by number, in slate and c. 23% in quartzite
(Fig. 2). Slate has a lower percentage of light BSE zircon
than quartzite, psammite and phyllite. Zircon outgrowths
(Fig. 1c), and more rarely overgrowths that completely
575
JOURNAL OF PETROLOGY
VOLUME 50
NUMBER 4
APRIL 2009
Fig. 2. Histogram showing proportions of different zircon types in slate and quartzite.
surround the host, are present on c. 50% of the zircon
grains in slate, but are much less common in quartzite
(Table 1). In rocks where dark BSE zircon is most abundant, zircon outgrowths are rare. However, when dark
BSE zircon is present in more modest amounts, zircon outgrowths are more abundant (Table 1). Xenotime outgrowths can be abundant, forming on 460% of the zircon
grains in some metapelites. In slate containing numerous
zircon outgrowths, xenotime is scarce or absent. Zircon in
metapelites rarely contains fractures that break up the
grains but c. 10% of the light BSE zircon in quartzite and
psammite is strongly fractured with cracks 410 mm wide.
Fractures are often quartz-filled and cross-cut the internal
zoning, and are generally associated with larger (430 mm)
zircon.
Light BSE zircon
Light BSE zircon generally has a composition typical of
unmodified zircon (e.g. Hoskin & Schaltegger, 2003), with
c. 1wt % Hf and trace amounts of Fe and Y. Light BSE
zircon is generally luminescent, commonly zoned (Fig. 1a)
and produces high IQ EBSD patterns. It typically lacks
major fractures but some grains contain radial and concentric fractures that are spatially linked to the BSE zoning
(Fig. 3a). These radiate from, terminate at, or parallel,
BSE zone boundaries and particularly around their tips
are marked by small elongate cavities (up to 10 mm long).
In such fractured zircon, the zones are also characterized
by variable levels of EBSD IQ (Fig. 3b) that correlate with
the BSE compositional zoning, with particularly low IQ
where cavities are observed.
Interpretation
Light BSE zircon typically shows high crystallinity and
little sign of chemical or structural modification. As such
it represents detrital grains or parts of grains that have
not experienced metamorphic changes. A few light BSE
zircons have zones with low EBSD IQ signatures that indicate poor crystallinity, and these are coupled to internal
fracturing that suggests volume changes within discrete
zones that are probably linked to radiation damage (Lee
& Tromp, 1995). The lack of any chemical modification suggests closed-system behaviour and a lack of fluid access that
may have inhibited recrystallization of the damaged
zircon.
Dark BSE zircon
Dark BSE zircon is often present in the cores of grains or
growth layers between zones of unmodified light BSE
zircon (Figs 1c and 4). In such instances, dark BSE zircon
is often enclosed within unmodified zircon and together
they have an overall shape typical of detrital zircon
(Fig. 4a). Zircon containing dark BSE domains occurs in
close proximity to entirely unmodified zircon with no difference in the adjacent minerals. Within grains, multiple
fractures generally radiate from the boundary between
dark and light BSE zircon to the grain edge (Fig. 4a).
Such fractures are typically perpendicular to the boundary
and may be filled with dark BSE zircon but are absent
from the dark BSE zircon zones (Figs 4a and 5b).
Irregular-shaped domains of unmodified zircon may
occur within or partly surrounded by dark BSE zircon
(Figs 1d and 4b, c). The boundary between light and dark
BSE zircon is sharply defined. It can parallel zoning within
576
HAY & DEMPSTER
ZIRCON AND LOW-T METAMORPHISM
Fig. 3. Zoned light BSE zircon. (a) BSE image of zircon in quartzite (KL5) with elongate cavities oriented parallel to zoning and radial and
concentric fractures. (b) EBSD IQ-inverse pole figure (IPF) map of (a). The low IQ dark band surrounding the middle of the grain has high
BSE intensity and represents a metamict zone where cavities have developed.
Fig. 4. BSE images of dark BSE zircon. (a) Zircon with outer light BSE radially fractured zone and dark BSE interior that is pore-rich, contains
xenotime inclusions and has patches of slightly higher BSE zircon. Rectangle shows the position of image (d) (Dh4). (b) Dark BSE zircon with
irregular-shaped pores and inclusions, and thin concentric laths of light BSE zircon that parallel the grain margins. Rectangle shows the position of the image (e) (Eas1). (c) Streaky dark BSE^light BSE zircon with high BSE intensity xenotime outgrowths forming on light BSE and
dark BSE zircon. Dark BSE zircon contains abundant pores (Bal1c). (d) High-magnification image of the dark BSE interior within (a), showing
variable BSE intensity and a frilly-edged higher BSE intensity area that is chemically unmodified. Both light BSE and dark BSE domains are
porous (Dh4). (e) Higher magnification image of (b) showing the delicate and irregular nature of the margins of dark BSE zircon interfingering
with the matrix. Microstructure contains minute inclusions and pores, and fractured light BSE zircon domains (Eas1).
577
JOURNAL OF PETROLOGY
VOLUME 50
NUMBER 4
APRIL 2009
Fig. 5. Chemical, CL and EBSD characterization of concentric light BSE and dark BSE zoned zircon from psammite (Dh4). (a) BSE image of
oscillatory-zoned light BSE^dark BSE zoned zircon with radial fractures; inset shows EBSD pattern from spot analysis of light (spot 1) and dark
BSE zircon (spot 2). (b) High-magnification BSE images of area defined in (a). High BSE intensity, small irregular xenotime grains within the
dark BSE zircon with porous zones in the light BSE domains. (c) CL image of (a). CL intensity is low in corresponding dark BSE zones. (d)
EDX element maps of (a) showing modified growth layers enriched in non-formula elements (Al, Ca) and U.
the unmodified light BSE zircon but irregular boundaries
with thin c. 1 mm laths of light BSE zircon fingering
between dark BSE zircon are common (Fig. 4). Dark BSE
zircon characteristically has a mottled appearance and
contains patches of slightly higher BSE intensity (Figs 1d,
e and 4b). Such patches form irregular frilly-edged
domains several microns across (Fig. 4d) but thin streaks
or nanometre-sized specks are also present (Fig. 4e).
Dark BSE zircon characteristically contains numerous
unevenly distributed subrounded pores (503 mm;
Rouquerol et al., 1994), larger more irregular isolated cavities and mineral inclusions (Fig. 4c). Mineral inclusions
are abundant and range from 03 mm to 12 mm long, and
occur as rounded, angular and highly irregular shapes.
High BSE intensity inclusions (typically505 mm) of xenotime are very common in dark BSE zircon (Fig. 1d). Other
inclusions where these are large enough to be identified
include quartz, mica and Fe-oxides.
The margins of dark BSE zircon have small (501^3 mm)
irregular jagged undulations where in contact with the
matrix (Figs 1d and 4e). U-shaped, angular and elongate
embayments typically up to 2 mm deep occur separated by
curved or irregular-shaped spines and 501 mm thin wisps
of dark BSE zircon that finger between matrix grains.
Despite the irregularities on the margins of dark BSE
zircon, most grains maintain an overall shape similar to
that of original detrital zircon and some preserve euhedral
edges.
Dark BSE zircon margins are typically more rounded in
quartzite (Fig. 6) with 505 mm deep nicks, deeper (05^
5 mm) U-shaped notches and larger embayments (2^5 mm
wide and deep) (Fig. 6b). Large smooth-sided channels
extend between some dark BSE zircon grains and form
networks through what were probably once single detrital
grains (Figs 1f and 6). Quartz and phyllosilicates typically
fill these channels. Apparently broken fragments of dark
578
HAY & DEMPSTER
ZIRCON AND LOW-T METAMORPHISM
Fig. 6. Embayed and veined zircon in quartzite (KL5). (a) Dark BSE zircon with large matrix-filled cavities. All margins are rounded and
small rounded fragments extend from the margin. (b) High-magnification image of area shown in (a). This shows the relationship between
rounded light BSE zircon domains and dark BSE zircon domains resulting in a mottled appearance. (c) Veined and fragmented dark BSE
zircon with irregular but slightly rounded edges and cavities. High BSE intensity inclusions are of thorite. (d) CL image of fractured dark
BSE zircon with bright CL ‘blotches’ with adjacent bright CL unmodified zircon (Fig. 1f).
BSE zircon are found between quartz grains and in cleavage domains of psammite and quartzite, and small grains
of dark BSE zircon may trail from larger (410 mm) light
BSE zircon cores (Fig. 1f). In these rocks, the contact
between light and dark BSE zircon typically does not
follow internal zoning of the grain. Some grains have
2 mm rounded domains of light BSE zircon within a
matrix of dark BSE zircon forming a mottled texture
(Fig. 6b). Dark BSE zircon in quartzite locally contains
thin cross-cutting veins (100^400 nm wide) filled with
darker BSE zircon (Fig. 6c), containing a series of small
interconnected pores and elongate holes (up to 2 mm long).
All dark BSE zircon is characteristically enriched in the
non-formula elements Y, Mg, Al, Ca and Fe (Table 2,
Fig. 5), although there is variation in their concentration,
both between grains and to a lesser extent within areas of
the same zircon. The enrichment is coupled to Zr and Si
depletion in dark BSE zircon. Y is the most enriched of
the non-formula elements and exhibits a strong correlation
with both Zr (Fig. 7) and Si depletion, following a 1:1 substitution trend. This trend is replicated by the other nonformula elements (Fig. 7). Dark BSE zircon is enriched in
U and Th relative to light BSE zircon within the same
grain (Fig. 5). However, numerous high BSE intensity
5500 nm grains are present that are probably xenotime
inclusions (Fig. 1d). The xenotime chemistry is masked by
zircon because the Y L and P K both occur close to the
Zr L peak, which also precludes P determination in the
zircon. This may result in an artificially high Zr/Si ratio.
Light BSE zircon has average analysis totals in EMPA
close to 100% but dark BSE zircon totals average 8975%
( 43). Other than the presence of small actinide
peaks, EDX analysis of dark BSE zircon fails to identify
X-ray peaks from elements not included in the EMPA.
579
JOURNAL OF PETROLOGY
VOLUME 50
NUMBER 4
APRIL 2009
Fig. 7. Plots showing the chemical composition (in cations per formula unit) of different zircon types. Black-filled symbols, unmodified light
BSE zircon; grey line crosses, porous zircon; grey-filled circles, dark BSE zircon; open crosses, zircon outgrowths. Plot in lower right shows a
positive correlation between average Hf (wt %) contents of zircon outgrowth and adjacent host grain.
580
HAY & DEMPSTER
ZIRCON AND LOW-T METAMORPHISM
Hf contents appear to be similar in both light BSE and
dark BSE zircon.
Typically, dark BSE zircon has a very low intensity in
CL and produces very weak EBSD patterns with low IQ
(Fig. 5). Slightly more intense CL parts of the dark BSE
zircon are correlated with a slight increase in EBSD IQ
and slightly brighter BSE areas have slightly higher IQ
EBSD patterns. Some dark BSE zircon in quartzite has a
typical very weak CL signal but with bright specks near
and around the grain edges (Fig. 6d), but this has no
correlation to either BSE or EBSD IQ. Imaging by SE
confirms this is not due to charging effects around the
grain.
Interpretation
Dark BSE zircon formation is in part controlled by the
composition of the detrital grain and hence spatially controlled by the original growth zoning. It appears to
develop within U-rich zones and is spatially linked to fractures that relate to internal volume changes. This strongly
suggests that dark BSE zircon replaces radiation-damaged
zircon. The low weight per cent totals and low mean
atomic number of dark BSE zircon could in part reflect
the presence of light elements (e.g. P, F, Cl) but are probably largely due to hydrous components as reported by other
studies of altered zircon (e.g. Nasdala et al., 2001a; Geisler
et al., 2003b; Utsunomiya et al., 2007). This, coupled to the
incorporation of non-formula elements in the dark BSE
zircon, the abundant pores and mineral inclusions and
the spatial link to fracture development, suggests that
fluids are needed to form dark BSE zircon. The consistent
weak EBSD IQ points to the dark BSE areas being nanocrystalline (see Capitani et al., 2000; Utsunomiya et al.,
2007). Consequently, dark BSE zircon is thought to replace
parts of original detrital grains that became metamict. The
textures of low-temperature, dark BSE zircon are very similar to those reported to have formed during replacement
of metamict zircon by dissolution^reprecipitation processes
(Geisler et al., 2007).
The quartzites experienced higher metamorphic temperatures (c. 4508C) than the metapelites, and the zircons
in the former have experienced more dissolution, producing embayed rounded margins. In these samples tectonic
deformation also appears to be effective in generating fluid
pathways and allowing both dissolution and dark BSE
zircon to form independently of fracturing linked to radiation damage. In contrast, the margins of dark BSE zircon
in lower temperature metapelites (c. 300^3508C) have
formed delicate intergrowths with the matrix and the
shape of the detrital grain is better preserved. The fragile
nature of these intergrowths suggests that the dark BSE
zircon formed at, or after, the time of the peak of
metamorphism.
Porous zircon
A small proportion of the zircon population (53%) has
some light BSE areas that appear porous and contain
small (up to 300 nm) rounded dark BSE spots (Fig. 1b).
These are often found in multiple zones within the same
grain (Fig. 8a). Spots have a relatively constant size and,
in contrast to pores within dark BSE zircon, are evenly distributed within the same growth layer, although their
abundance varies considerably. Grading in the spot size
occurs across some zones (Fig. 8a). Porous zircon is not
observed in contact with the matrix. Radial fractures are
found in light BSE zircon adjacent to porous zones
(Fig. 1b), perpendicular to the light BSE zircon^porous
layer boundary. Dark BSE zircon is often located in the
porous zone near the tip of the fractures (Fig. 8a) and
small zircon outgrowths may occur at the point where the
fracture meets the outer edge of the grain.
Porous zircon is slightly enriched in Fe at the expense of
Zr and Si. Such enrichment correlates with increasing size
and concentration of pores. Typically other non-formula
elements do not show significant enrichment in porous
zircon, except where pores are very abundant. The porous
zircon and light BSE zircon have similar weight per cent
totals. As the size and concentration of pores increases
EBSD IQ and CL intensity decreases (Fig. 8b and c). Not
all dark BSE zircon areas associated with the porous zones
show a low IQ EBSD pattern and may have a higher IQ
than adjacent porous areas.
Interpretation
Pores in zircon are confined to particular growth zones
and their formation must relate to the original zircon composition. Radial fractures in the adjacent light BSE zircon
are linked to the swelling of these zones. Limited amounts
of dark BSE zircon form where these fractures are in communication with the edge of the grain and hence fluids.
Although slightly enriched in Fe, porous zones show little
by way of chemical modification and the formation of
porous zircon may be associated with an absence of fluids
or at least restricted fluid access. Care must be taken in
attributing the reduced EBSD IQ pattern to a loss of crystallinity because the pitted surface is likely to degrade the
pattern. However, radial fractures spatially linked with
porous zircon and an association with dark BSE zircon
make it likely that low EBSD IQ patterns are in part a
result of radiation damage. This suggests that the formation of porous zircon is a response to metamictization possibly via a solid-state recrystallization mechanism.
Outgrowths
Zircon outgrowths (Fig. 9) on light BSE zircon are abundant in greenschist-facies metapelites. The outgrowths
truncate the zoning of the host grains and occur on
581
JOURNAL OF PETROLOGY
VOLUME 50
NUMBER 4
APRIL 2009
Fig. 8. Porous zircon from psammite (Dh4). (a) BSE image of large detrital zircon with porous zones. Concentration of pores varies between
zones, some showing a grading in pore size. Dark BSE zircon is present in porous zones where fractures communicate with the grain edge.
Zircon outgrowths on the edge of the grain are spatially linked to dark BSE zircon. (b) CL image of (a) showing low-intensity signal in
porous areas and very low luminescence levels in dark BSE zircon of porous zones. (c) IQ EBSD scanned map of lower part of image (a)
shows high IQ domain in unmodified light BSE zircon and lower IQ in porous areas. Areas where pores are heavily concentrated show lower
IQ than areas where pores are sparse. Step size was 01 mm.
broken surfaces of detrital zircon that has been fractured
after deposition. They typically protrude 2^3 mm into the
matrix but range from 51 mm to 5 mm, and have similar
morphology to dark BSE zircon where it is in contact
with the matrix. Outgrowths can be found as isolated crystals (Fig. 9a) or in clusters forming a crust that either completely surrounds (Fig. 9b) or partially encloses the host
(Fig. 9c). They have a wide range of habits including
rounded, sub-hexagonal blocks, jagged serrations, euhedral prisms, and wispy, bulbous and frilly shaped. A variety of these forms may occur on the same zircon. Euhedral
zircon outgrowths occur only adjacent to phyllosilicates.
Outgrowths are typically larger and more common where
adjacent to phyllosilicates rather than quartz (Fig. 9f).
Zircon outgrowths are absent when next to large quartz
grains, although in some instances outgrowths form adjacent to quartz in preference to other grains. They are not
oriented in the direction of the fabric in the rock and show
no preference for a particular crystal face or location on
the host. Outgrowths may coincide with the ends of prominent fractures in the host zircon and so are in contact with
internal zones of dark BSE zircon.
Outgrowths have a lower BSE intensity than their host
zircon, similar to dark BSE zircon (Fig. 9b) and have an
inclusion-rich microstructure and an intricate network of
pores. Although identification of inclusions is difficult
because of their size, outgrowths do contain quartz and
phyllosilicates. Outgrowths characteristically have a
mottled appearance in BSE images as a result of both
irregular-shaped small (02^1 mm) areas of brighter BSE
zircon similar to the host grain, and the inclusions. The
outer tips of zircon outgrowths typically appear slightly
brighter in BSE (Fig. 9b). Bright BSE zircon within the
outgrowth is most commonly found in conjunction with
xenotime outgrowths (Fig. 9d). Zircon outgrowths in xenotime-poor slates are typically more homogeneous than
those in rocks where xenotime is abundant. Outgrowths
are rare in quartzite and psammite (Fig. 2) and are smaller
(52 mm) than those in slates. Where outgrowths occur in
quartz-rich lithologies, the rocks are in a succession dominated by slates, which contain abundant zircon
outgrowths.
Thirty zircon grains with the largest outgrowths
were chosen for EPMA from samples Bal1, Bal1b and E1.
582
HAY & DEMPSTER
ZIRCON AND LOW-T METAMORPHISM
Fig. 9. Zircon outgrowths. BSE images unless stated otherwise. (a) Frilly (top of grain) and larger (bottom right margin) outgrowths on host
zircon with light BSE^dark BSE zoned interior. Fractures in light BSE zircon are filled by dark BSE zircon that connects with outgrowths
(Eas1). (b) Inclusion-rich, patchy BSE intensity zircon overgrowth on a light BSE zircon host (Bal1d). (c) Zircon with variable BSE intensity
zircon outgrowths containing (bright) xenotime (Bal1b). (d) Zircon with crack sealed by dark BSE zircon. Xenotime forms high BSE intensity
outgrowth on upper margin and zircon outgrowths form on remainder of margin. (e) IQ-Inverse pole figures (IPF) EBSD map of (d) showing
epitaxial xenotime outgrowths. Zircon outgrowths give no EBSD pattern. Step size was 01 mm. (f) Zircon outgrowth on host zircon adjacent to
matrix muscovite and lacking outgrowths on right-hand margin adjacent to quartz (Bal1a).
The Hf content of the outgrowth is approximately the
same as that of the host zircon (Fig. 7), but more variable
and higher than the dark BSE zircon. Zircon outgrowths
are enriched in non-formula elements but generally less so
than dark BSE zircon, and outgrowths display different
chemical trends to dark BSE zircon (Fig. 7).
Typically, zircon outgrowths are weakly luminescent
compared with the host zircon and produce very low IQ
EBSD patterns (Fig. 9e). Xenotime inclusions in outgrowths produce high IQ EBSD patterns.
Interpretation
Slates contain abundant dark BSE zircon and zircon outgrowths. This suggests that slate either contains inherently
more reactive detrital zircon or that the alteration and
growth of new zircon is enhanced by the presence of phyllosilicates (Dempster et al., 2008a, 2008b) and possibly controlled by local variation in fluid composition (Rasmussen,
2005).
Zircon outgrowths can be linked via a series of
metamict-generated fractures to internally modified dark
BSE zircon that displays evidence of dissolution (Fig. 1c).
These fractures are often filled with dark BSE zircon and
provide pathways for fluids to both form the dark BSE
zircon and transport Zr to the edge of the grain. The outgrowths have a similar poorly crystalline structure to the
dark BSE zircon. Many zircon outgrowths are found on
the margins of completely unmodified zircon (Fig. 9c),
which suggests that Zr is transported by fluids from
zircon that has experienced dissolution elsewhere. No link
between the characteristics of adjacent grains has been
identified in two dimensions. However, the similar Hf contents of the host zircons and the zircon outgrowths (Fig. 7)
may point to a local source. In comparison with dark BSE
zircon, the relative lack of non-formula elements in zircon
outgrowths may be due to these elements being more readily partitioned into adjacent matrix phases.
The delicate structure of outgrowths (Fig. 9b) would be
unable to survive sediment transport. Outgrowths contain
583
JOURNAL OF PETROLOGY
VOLUME 50
NUMBER 4
APRIL 2009
Fig. 10. BSE images of zircon with xenotime inclusions and outgrowths. Scale bars all represent 5 mm. (a) Dark BSE zircon with irregular
margins and abundant pores and bright BSE xenotime inclusions. Very bright central inclusion of coffinite (Bal1c). (b) Light BSE zircon with
fractures and outer dark BSE zircon rim and xenotime-filled fracture. Dark BSE zircon margins are irregular and contain inclusions and holes
(Bal3). (c) Zircon with xenotime infilling cavities in dark BSE interior. Xenotime also forms blocky outgrowth (Bal3). (d) Zircon with xenotime
outgrowth and small zircon outgrowths. Dark BSE zircon with numerous pores and xenotime inclusions has partly engulfed light BSE laths and
forms outgrowth-like features on the margins (Bal1a). (e) Zoned zircon with xenotime outgrowths that show three growth phases (Bal1b). Thin
spines of xenotime protrude along cleavage in adjacent mica (top right). (f) Detrital zircon with inclusion-rich zircon outgrowth and large
xenotime overgrowth (Bal1b). (g) High-contrast image of angular xenotime core with xenotime overgrowth (Bal1a).
inclusions of matrix minerals and project into, and finger
between, muscovite and chlorite, suggesting that growth
is contemporaneous with, or post-dates, the growth of
metamorphic minerals. This may indicate precipitation
from fluids that become supersaturated with Zr as temperatures fall during the retrograde part of the metamorphic cycle.
Xenotime
Xenotime is abundant in some slates and is commonly
associated with both dark BSE zircon and zircon outgrowths (Fig. 10). No xenotime has been observed as inclusions within light BSE or porous zircon. Inclusions of
xenotime in dark BSE zircon are typically 02^1 mm in
size (Fig. 10a), but are up to 8 mm long. They have irregular
shapes, often with small-scale (5100 nm) frilly edges and
are concentrated in thin bands, aligned parallel to zoning
in light BSE zircon. Larger (43 mm) xenotime is more
rounded and may fill cavities (Fig. 10c) and fractures
(Fig. 10b). Some dark BSE zircon contains smaller xenotime inclusions towards the rim of the grain (Fig. 10b) but
generally their size and distribution appears to be random.
Zircon with xenotime outgrowths is likely to contain
xenotime inclusions within dark BSE zircon areas
(Fig. 10d). In xenotime-poor rocks, the cavities and holes
in dark BSE zircon are typically empty (Figs 1c and 9a).
Rare inclusions of thorite (ThSiO4) and coffinite (USiO4)
occur within dark BSE zircon from slate (Fig. 10a), psammite and quartzite (Fig. 6c).
Typically, xenotime outgrowths (c. 5 mm) are larger than
zircon outgrowths (Fig. 10d and f). Xenotime outgrowths
may occur on a host zircon whereas neighbouring grains
lack outgrowths. In such instances, the zircon with the outgrowth is usually adjacent to phyllosilicates, whereas those
lacking outgrowths are surrounded by quartz. In psammite, xenotime outgrowths are not abundant, but are typically larger, and have a rounded morphology. In contrast,
metapelites contain, thin (5100 nm) spines of xenotime
that project between the cleavage planes of micas
(Fig. 10e). Fine-grained (52 mm) quartz is the most
common type of inclusion in the xenotime. Three distinct
domains can be identified within some xenotime outgrowths (Fig. 10e). An inclusion-rich inner domain is separated from a weakly zoned inclusion-poor domain by a thin
(502 mm) high-intensity BSE band. Fingers of the outer
part of the second domain may protrude out into the
584
HAY & DEMPSTER
ZIRCON AND LOW-T METAMORPHISM
matrix. On the fringes of some outgrowths, isolated euhedral micrometre-sized xenotime represents a third phase of
growth.
Xenotime outgrowths typically form on light BSE
zircon (Fig. 10d) but rarely occur on margins of dark BSE
zircon. In such circumstances the xenotime typically grows
on the most BSE intense part of the zircon (Fig. 4d).
Xenotime and zircon outgrowths are often contiguous
(Figs 9c and 10f). The contact between them varies from
straight to slightly irregular and curved to jagged, and is
usually sharply defined but occasionally gradational.
Rare, small (c. 10 mm), rounded to subhedral xenotime
cores of probable detrital origin may be encased by up to
5 mm thick xenotime overgrowths (Fig. 10g) in slate. No
zircon has been observed in outgrowths on xenotime host
grains.
Xenotime shows strong luminescence and high IQ
EBSD patterns, and inverse pole figure maps reveal that it
forms epitaxial outgrowths on the host zircon (Fig. 9e).
Interpretation
Xenotime crystallization is linked to the alteration of the
zircon. Xenotime occurs as a pore-filling phase and
within fractures, and hence forms after, or synchronous
with, the dark BSE zircon. In the case of the latter, the
nano-crystalline dark BSE zircon must be sufficiently permeable to allow xenotime to form in pores. Xenotime
growth preferentially occurs on a relatively crystalline
host zircon and in crystallographic continuity.
Phyllosilicates, and in particular muscovite, appear to promote xenotime growth and there is often a strong crystallographic control on the position of xenotime outgrowths
on the host zircon (Fig. 10e).
DISCUSSION
Alteration of metamict zircon
The survival of ancient zircon during high-grade metamorphism is testament to its durability. Therefore it is unlikely that fully crystalline zircon has become modified to
dark BSE zircon in these low-grade rocks. In its metamict
state, zircon is 2^3 times more susceptible to dissolution in
the presence of hydrothermal fluids than when fully crystalline (Ewing et al., 2003). The recoil of radioactive
nuclides during -particle decay generates local amorphous areas in the lattice (c. 3 nm in diameter; Ewing
et al., 2000). Metamict zircon is generated by the accumulation and overlap of these areas (Salje et al., 1999). This
damage accumulates in conditions below the annealing
temperature of zircon, c. 52508C (Garver et al., 2005;
Nasdala et al., 2005). There is some uncertainty as to the
annealing temperature that may in part be linked to the
amount of radiation damage in the lattice (Rahn et al.,
2004). Zircon with less radiation damage, whereby a vestige of the crystalline lattice remains intact, may anneal
at lower temperature. The amount of damage is both a
function of time and the concentration of U and Th. The
uneven distribution of actinides within zircon, and the possible different ages of zircon growth zones and of detrital
grains, can lead to range of crystalline states in any zircon
population. The result is that at any one time, some growth
zones or grains are particularly susceptible to dissolution or
alteration (completely metamict), others are slightly more
resistant to dissolution or alteration (partially metamict)
and others remain insoluble and inert (crystalline zircon).
The ability of zircon to react not only depends on temporal
and thermal constraints but is also controlled by fluid
access to the metamict zircon. Metamictization causes a
swelling of amorphous areas by up to 18% (Lee & Tromp,
1995; Ewing et al., 2003) and a series of fractures may be
generated in the surrounding crystalline zircon. These provide pathways for fluids to communicate with metamict
areas, allowing these areas to become modified into dark
BSE zircon. The mechanical strength of zircon is also
reduced in its metamict state (Chakoumakos et al., 1991).
Therefore metamict zircon grains are susceptible to breaking under tectonic stress and sediment transport. The
apparent retention of U by zones of dark BSE zircon
(Fig. 5d) may also allow these areas to become metamict
again, although in the examples studied this is unlikely
given the timescales involved (Salje et al., 1999). The combination of these factors has the potential to create uniquely
complex responses to metamorphic events by single zircon
grains.
The textural evidence suggests that dark BSE zircon
formed as a result of a dissolution^reprecipitation process
(Geisler et al., 2007). However, low IQ EBSD patterns
show that both dark BSE zircon and outgrowths
have poor crystallinity. This is compatible with a nanocrystalline structure identified in other studies of altered
zircon (e.g. Utsunomiya et al., 2007). Grain boundaries of
these small crystals are also likely to suppress luminescence. A nano-crystalline structure may provide a permeable medium that allows fluids to communicate with the
alteration front, and judging from the lack of dark BSE
zircon in equivalent rocks of higher metamorphic grade
(Dempster et al., 2008a) it also seems more prone to later
dissolution than the unmodified light BSE zircon.
Amorphous areas in radiation-damaged zircon are
likely to be out of equilibrium with the fluid and susceptible to dissolution (e.g. Ewing et al., 1982; Geisler et al.,
2003a; Tromans, 2006) provided the fluid phase is undersaturated with respect to Zr or Si. If enough metamict zircon
is dissolved to saturate the fluid in Zr, then the fluid may
become supersaturated and dark BSE zircon will precipitate; in effect, the metamict zircon recrystallizes. This
low-temperature dissolution and precipitation process
(Geisler et al., 2007) seems likely to be driven by inherent
differences in solubility of the forms of zircon involved
585
JOURNAL OF PETROLOGY
VOLUME 50
(Tromans, 2006); that is, metamict zircon is more soluble
than nano-crystalline, non-formula element rich, hydrous
zircon. The abundance of holes and inclusions in dark
BSE zircon suggests that significant volume changes occur
during alteration (Putnis, 2002; Tomaschek et al., 2003).
Where such changes take place within a previously fractured zircon, Zr can effectively be expelled along these
cracks and this process may allow the formation of some
outgrowths. Crystalline remnants of original zircon probably remain onto which dark BSE zircon can nucleate (Salje
et al., 1999). Without such nucleation sites, zircon may be
completely dissolved without reprecipitation. Rare grains
that have altered to dark BSE zircon apparently lack light
BSE remnants; however, imaging in thin section provides
only provides a two-dimensional view.
Behaviour of zircon during
greenschist-facies metamorphism
Populations of relatively strong zircon grains will dominate
clastic sedimentary rocks, because old metamict grains
will be prone to disintegration during transport. Finegrained sediments will in theory contain a high proportion
of small radiation-damaged zircon fragments that will be
more likely to react during metamorphism. Rounded
zircon dominates the original detrital zircon population
in the metasedimentary rocks examined in this study and
consequently light BSE zircon dominates the original
assemblage. A small proportion of zircon grains are euhedral and could represent input from proximal volcanic
sources or may have been protected as inclusions in detrital
silicate grains. Some grains of light BSE zircon remain
essentially unmodified since deposition and, together with
relict parts of unmodified zircon, such material would
dominate any mineral separate of zircon. However, the
other forms of zircon such as dark BSE zircon, outgrowths
and porous zircon have substantially modified texture and
composition and dominate most of the metamorphic rocks
sampled. The age, U content and structural condition of
zircon grains are crucial in determining their metamorphic response.
Any metamict detrital zircon exposed to fluid will probably alter to dark BSE zircon during prograde metamorphism. Although slates may contain a higher
proportion of such grains than quartz-rich metasedimentary rocks, typically prograde reactions in zircon will be
enhanced by a long time gap between deposition and
metamorphism. Metamict zircon may also be initially protected from fluids either encased within strong crystalline
zircon or as inclusions in other minerals. Such metamict
zircon will be exposed to the fluid if liberated from the
enclosing grains, either through metamorphic recrystallization of the host grain or via fractures induced by deformation or metamictization. Crushed or fragmented
zircons are virtually absent from slate and found only in
the quartzite and psammite. This may be due to the
NUMBER 4
APRIL 2009
abundant micas in the slate deforming more readily and
hence relieving the stress on the zircon. Smaller fracture
systems, such as the vein-like features, may become clogged
by dark BSE zircon, restricting fluid access and limiting
reaction progress. At metamorphic temperatures above
2508C, radiation damage may be repaired and further
alteration to dark BSE zircon may be limited to those
more highly damaged areas of zircon that are difficult to
anneal at relatively low temperatures [i.e. those where the
remnants of crystalline lattice are no longer interconnected
(Salje et al., 1999; Trachenko et al., 2003)]. During exhumation after peak metamorphism, radiation damage will
again accumulate in zircon below c. 2508C. Alteration
during cooling will affect those metamict grains that were
protected from alteration during the prograde history and
those that become metamict or partially metamict after
the peak of metamorphism. As a consequence of stresses
induced during cooling and unroofing (Haxby &
Turcotte, 1976; Evans, 1978; Zaraisky & Balashov, 1994) permeability will increase with a corresponding increase in
reaction of zircon.
Metamict zircon seems to show evidence of alteration in
a variety of lithologies, an indication that it is out of equilibrium with fluids in most low-temperature environments.
The metamict state of zircon may exert too strong an influence on the behaviour of zircon for any appreciable effect
of fluid composition (see Rasmussen, 2005) to be observed.
It is possible that fluid composition changes the level of Zr
saturation in the fluid rather than directly affecting the
zircon solubility. The release of fluids from dehydration
reactions and the changing mineralogy during metamorphism may ensure that fluids are in a constant flux
between states of undersaturation, saturation and supersaturation with respect to Zr. Consequently, the replacement
of metamict zircon by dark BSE zircon proceeds via a
series of dissolution and reprecipitation micro-steps.
CONC LUSIONS
Zircon has a uniquely varied response to low-temperature
greenschist-facies metamorphic events in metasedimentary
rocks. This is a result of the interaction between the characteristics of the detrital zircon population, the accumulation of radiation damage through time, fluid access,
temperature and rock type. The key conclusions from this
study are as follows.
586
(1) Low-temperature altered zircon forms during metamorphism by the replacement of metamict zircon
and is the dominant type of zircon in greenschistfacies metapelites. This dominance is probably a function of the original detrital zircon population.
(2) Fluid access to radiation-damaged zircon allows lowtemperature zircon to form, creating a porous, inclusion-rich, probably nano-crystalline and hydrous
HAY & DEMPSTER
(3)
(4)
(5)
(6)
(7)
ZIRCON AND LOW-T METAMORPHISM
zircon, enriched in non-formula elements. Alteration
appears to take place via a dissolution^reprecipitation
process.
Fracturing of zircon occurs via both tectonic deformation in quartz-rich lithologies and a self-induced fracture mechanism. This is critical in allowing fluids to
access the interior parts of zircon grains.
Porous zircon also appears to form from metamict
zircon and may represent recovery in the absence of
fluids.
Elevated temperatures during metamorphic events
appear to enhance dissolution of zircon and will
also be important in controlling the annealing of
radiation-damaged areas.
Xenotime is intimately associated with lowtemperature zircon in slates and occurs both as inclusions and as outgrowths on zircon.
Zr is mobilized from areas of low-temperature alteration and new zircon forms as outgrowths on the margins of unmodified zircon. Phyllosilicates enhance the
formation of both zircon and xenotime outgrowths.
The temporal controls on the accumulation of radiation
damage dictate that zircon will be potentially unresponsive at certain times but reactive at others. This behaviour,
coupled to the importance of fluid access to the alteration
of zircon, creates exceptionally varied and complex metamorphic textures. The complex nature of low-temperature
zircon has implications for reliable age-dating of whole
grains, and understanding the processes that produce
these features is essential for the meaningful interpretation
of U^Pb ages.
The zircon populations in the low-grade metamorphic
rocks are very different from those reported from higher
temperature conditions, despite the fact that such rocks
must have experienced an equivalent low-temperature history. Analysis of in situ zircon from lower amphibolitefacies Dalradian schists found that low-temperature
zircon forms were absent (Dempster et al., 2008a). The fate
of low-temperature zircon at higher metamorphic grades
remains uncertain, but dissolution appears to be more efficient at elevated temperatures. The nano-crystalline structure and high surface energy of these particles make them
susceptible to either dissolution or recrystallization.
Whether Zr released in such conditions is typically incorporated in other silicate phases such as garnet (Degeling
et al., 2001) or grows as new micro-zircon (Dempster et al.
2008a) or is lost from the rocks is unclear. However, zircon
alteration seems ubiquitous in low-grade metasedimentary
rocks where metamict zircon is present.
AC K N O W L E D G E M E N T S
John Gilleece, Martin Lee, Peter Chung and Robert
Macdonald are thanked for their technical assistance.
The study was supported by the University of Glasgow
and also benefited from the award of a Mineralogical
Society Postgraduate Bursary. Chris McFarlane, Craig
Grimes and Lutz Nasdala are thanked for their helpful
reviews, which substantially improved the manuscript.
S U P P L E M E N TA RY DATA
Supplementary data for this paper are available at Journal
of Petrology online.
R EF ER ENC ES
Butterfield, J. A. (1936). Outgrowths on zircon. Geological Magazine 73,
511^516.
Capitani, G. C., Leroux, H., Doukhan, J. C., Rios, S., Zhang, M. &
Salje, E. K. H. (2000). A TEM investigation of natural metamict
zircons: structure and recovery of amorphous domains. Physics and
Chemistry of Minerals 27, 545^556.
Cawood, P. A., Nemchin, A. A., Smith, M. & Loewy, S. (2003).
Source of the Dalradian Supergroup constrained by U^Pb dating
of detrital zircon and implications for the East Laurentian margin.
Journal of the Geological Society, London 160, 231^246.
Chakoumakos, B. C., Oliver, W. C., Lumpkin, G. R. & Ewing, R. C.
(1991). Hardness and elastic-modulus of zircon as a function of
heavy-particle irradiation dose. 1. In situ alpha-decay event
damage. Radiation Effects and Defects in Solids 118, 393^403.
Cherniak, D. J. & Watson, E. B. (2001). Pb diffusion in zircon. Chemical
Geology 172, 5^24.
Corfu, F., Hanchar, J. M., Hoskin, P. W. O. & Kinny, P. (2003). Atlas
of zircon textures. In: Hanchar, H. M. & Hoskin, P. W. O. (eds)
Zircon. Mineralogical Society of America, Reviews in Mineralogy and
Geochemistry 53, 469^500.
Davis, D. W., Williams, I. S. & Krogh, T. E. (2003). Historical development of zircon geochronology. In: Hanchar, H. M. & Hoskin, P.
W. O. (eds) Zircon. Mineralogical Society of America, Reviews in
Mineralogy and Geochemistry 53, 145^181.
Degeling, H., Eggins, S. & Ellis, D. J. (2001). Zr budgets for metamorphic reactions, and the formation of zircon from garnet breakdown. Mineralogical Magazine 65, 749^758.
Delattre, S., Utsunomiya, S., Ewing, R. C., Boeglin, J.-L., Braun, J.-J.,
Balan, E. & Calas, G. (2007). Dissolution of radiation-damaged
zircon in lateritic soils. American Mineralogist 92, 1978^1989.
Dempster, T. J. (1985). Uplift patterns and orogenic evolution of the
Scottish Dalradian. Journal of the Geological Society, London 142,
111^128.
Dempster, T. J., Hay, D. C. & Bluck, B. J. (2004). Zircon growth in
slate. Geology 32, 221^224.
Dempster, T. J., Rogers, G., Tanner, P. W. G., Bluck, B. J., Muir, R. J.,
Redwood, S. D., Ireland, T. R. & Paterson, B. A. (2002). Timing of
deposition, orogenesis and glaciation within the Dalradian rocks of
Scotland: constraints from U^Pb zircon ages. Journal of the
Geological Society, London 159, 83^94.
Dempster, T. J., Hay, D. C., Gordon, S. H. & Kelly, N. M. (2008a).
Micro-zircon: origin and evolution during metamorphism. Journal
of Metamorphic Geology 26, 499^507.
Dempster, T. J., Martin, J. C. & Shipton, Z. K. (2008b). Zircon dissolution in a ductile shear zone, Monte Rosa granite gneiss, northern
Italy. Mineralogical Magazine 74, 971^986.
Evans, A. G. (1978). Microfracture from thermal expansion anisotropy
I. Single pahse systems. Acta Metallurgica 26, 1845^1853.
587
JOURNAL OF PETROLOGY
VOLUME 50
Ewing, R. C., Haaker, R. F. & Lutze, W. (1982). Leachability of zircon
as a function of alpha dose. Scientific Basis for Radioactive Waste
Management 5, 387^397.
Ewing, R. C., Lutze, W. & Weber, W. J. (1995). Zircon: a host-phase for
the disposal of weapons plutonium. Journal of Materials Research 10,
243^246.
Ewing, R. C., Meldrum, A., Wang, L. M. & Wang, S. X. (2000).
Radiation-induced amorphization. In: Redfern, S. A. T. &
Carpenter, M. A. (eds) Mineralogical Society of America, Reviews in
Transformation Processes in Minerals 39, 319^361.
Ewing, R. C., Meldrum, A., Wang, L. M., Weber, W. J. & Corrales, L.
R. (2003). Radiation effects in zircon. In: Hanchar, H. M. &
Hoskin, P. W. O. (eds) Zircon. Mineralogical Society of America,
Reviews in Mineralogy and Geochemistry 53, 387^420.
Farnan, I., Balan, E., Pickard, C. J. & Mauri, F. (2003). The effect of
radiation damage on local structure in the crystalline fraction of
ZrSiO4: Investigating the Si-29 NMR response to pressure in
zircon and reidite. American Mineralogist 88, 1663^1667.
Fedo, C. M., Sircombe, K. N. & Rainbird, R. H. (2003). Detrital
zircon analysis of the sedimentary record. In: Hanchar, H. M. &
Hoskin, P. W. O. (eds) Zircon. Mineralogical Society of America,
Reviews in Mineralogy and Geochemistry 53, 277^303.
Fraser, G., Ellis, D. & Eggins, S. (1997). Zirconium abundance in granulite-facies minerals, with implications for zircon geochronology in
high-grade rocks. Geology 25, 607^610.
Garver, J. I. (2002). Discussion: ‘Metamictisation of natural zircon:
accumulation versus thermal annealing of radioactivity-induced
damage’. Contributions to Mineralogy and Petrology 143, 756^757.
Garver, J. I., Reiners, P. W., Walker, L. J., Ramage, J. M. & Perry, S.
E. (2005). Implications for timing of Andean uplift from thermal
resetting of radiation-damaged zircon in the Cordillera
Huayhuash, northern Peru. Journal of Geology 113, 117^138.
Geisler, T., Pidgeon, R. T., Kurtz, R., van Bronswijk, W. &
Schleicher, H. (2003a). Experimental hydrothermal alteration of
partially metamict zircon. American Mineralogist 88, 1496^1513.
Geisler, T., Rashwan, A. A., Rahn, M. K. W., Poller, U.,
Zwingmann, H., Pidgeon, R. T., Schleicher, H. & Tomaschek, F.
(2003b). Low-temperature hydrothermal alteration of natural
metamict zircons from the Eastern Desert, Egypt. Mineralogical
Magazine 67, 485^508.
Geisler, T., Zhang, M. & Salje, E. K. H. (2003c). Recrystallization of
almost fully amorphous zircon under hydrothermal conditions: An
infrared spectroscopic study. Journal of Nuclear Materials 320,
280^291.
Geisler, T., Schaltegger, U. & Tomaschek, F. (2007). Re-equilibration
of zircon in aqueous fluids and melts. Elements 3, 43^50.
Harley, S. L., Kelly, N. M. & Mo«ller, A. (2007). Zircon behaviour and
the thermal histories of mountain chains. Elements 3, 25^30.
Haxby, W. F. & Turcotte, D. L. (1976). Stresses induced by the addition
and removal of overburden and associated thermal effects. Geology
4, 181^184.
Hoskin, P. W. O. (2005). Trace-element composition of hydrothermal
zircon and the alteration of Hadean zircon from the Jack Hills,
Australia. Geochimica et Cosmochimica Acta 69, 637^648.
Hoskin, P. W. O. & Schaltegger, U. (2003). The composition of zircon
and igneous and metamorphic petrogenesis. In: Hanchar, H. M. &
Hoskin, P. W. O. (eds) Zircon. Mineralogical Society of America, Reviews
in Mineralogy and Geochemistry 53, 27^62.
Humphreys, F. J. (2001). ReviewçGrain and subgrain characterisation by electron backscatter diffraction. Journal of Materials Science
36, 3833^3854.
Kelly, N. M. & Harley, S. L. (2005). An integrated microtextural and
chemical approach to zircon geochronology: refining the Archaean
NUMBER 4
APRIL 2009
history of the Napier Complex, east Antarctica. Contributions to
Mineralogy and Petrology 149, 57^84.
Lee, J. K. W. & Tromp, J. (1995). Self-induced fracture generation in
zircon. Journal of Geophysical ResearchçSolid Earth 100, 17753^17770.
Meldrum, A., Boatner, L. A., Weber, W. J. & Ewing, R. C. (1998).
Radiation damage in zircon and monazite. Geochimica et
Cosmochimica Acta 62, 2509^2520.
Mojzsis, S. J., Harrison, T. M. & Pidgeon, R. T. (2001). Oxygenisotope evidence from ancient zircons for liquid water at the
Earth’s surface 4300 Ma ago. Nature 409, 178^181.
Nasdala, L., Beran, A., Libowitzky, E. & Wolf, D. (2001a). The incorporation of hydroxyl groups and molecular water in natural zircon
(ZrSiO4). AmericanJournal of Science 301, 831^857.
Nasdala, L., Wenzel, M., Vavra, G., Irmer, G., Wenzel, T. & Kober, B.
(2001b). Metamictisation of natural zircon: accumulation versus
thermal annealing of radioactivity-induced damage. Contributions
to Mineralogy and Petrology 141, 125^144.
Nasdala, L., Hanchar, J. M., Kronz, A. & Whitehouse, M. J. (2005).
Long-term stability of alpha particle damage in natural zircon.
Chemical Geology 220, 83^103.
Nemchin, A. A., Pidgeon, R. T. & Whitehouse, M. J. (2006). Reevaluation of the origin and evolution of 442 Ga zircons from the
Jack Hills metasedimentary rocks. Earth and Planetary Science Letters
244, 218^233.
Persano, C., Barfod, D. N., Stuart, F. M. & Bishop, P. (2007).
Constraints on early Cenozoic underplating-driven uplift and
denudation of western Scotland from low temperature thermochronology. Earth and Planetary Science Letters 263, 404^419.
Pidgeon, R. T., O’Neil, J. R. & Silver, L. T. (1966). Uranium and lead
isotopic stability in metamict zircon under experimental hydrothermal conditions. Science 154, 1538^1540.
Putnis, A. (2002). Mineral replacement reactions: from macroscopic
observations to microscopic mechanisms. Mineralogical Magazine
66, 689^708.
Rahn, M. K., Brandon, M. T., Batt, G. E. & Garver, J. I. (2004). A
zero-damage model for fission-track annealing in zircon. American
Mineralogist 89, 473^484.
Rasmussen, B. (2005). Zircon growth in very low grade metasedimentary rocks: evidence for zirconium mobility at similar to 2508C.
Contributions to Mineralogy and Petrology 150, 146^155.
Rouquerol, J., Avnir, D., Fairbridge, C. W., Everett, D. H., Haynes, J.
H., Pernicone, N., Ramsay, J. D. F., Sing, K. S. W. & Unger, K. K.
(1994). Recommendations for the characterization of porous solids.
Pure and Applied Chemistry 66, 1739^1758.
Rubatto, D., Williams, I. S. & Buick, I. S. (2001). Zircon and monazite
response to prograde metamorphism in the Reynolds Range, central Australia. Contributions to Mineralogy and Petrology 140, 458^468.
Salje, E. K. H., Chrosch, J. & Ewing, R. C. (1999). Is ‘metamictization’
of zircon a phase transition? American Mineralogist 84, 1107^1116.
Scharer, U., Corfu, F. & Demaiffe, D. (1997). U^Pb and Lu^Hf isotopes in baddeleyite and zircon megacrysts from the Mbuji-Mayi
kimberlite: constraints on the subcontinental mantle. Chemical
Geology 143, 1^16.
Schmidt, C., Rickers, K., Wirth, R., Nasdala, L. & Hanchar, J. M.
(2006). Low-temperature Zr mobility: An in situ synchrotron-radiation XRF study of the effect of radiation damage in zircon on the
element release in H2O þ HCl SiO2 fluids. American Mineralogist
91, 1211^1215.
Speer, J. A. (1982). Zircon. In: Ribbe, P. H. (ed.) Orthosilicates.
Mineralogical Society of America, Reviews in Mineralogy 5, 67^112.
Tichomirowa, M., Whitehouse, M. J. & Nasdala, L. (2005).
Resorption, growth, solid state recrystallisation, and annealing of
588
HAY & DEMPSTER
ZIRCON AND LOW-T METAMORPHISM
granulite facies zirconça case study from the Central Erzgebirge,
Bohemian Massif. Lithos 82, 25^50.
Tomaschek, F., Kennedy, A. K.,Villa, I. M., Lagos, M. & Ballhaus, C.
(2003). Zircons from Syros, Cyclades, GreeceçRecrystallization
and mobilization of zircon during high-pressure metamorphism.
Journal of Petrology 44, 1977^2002.
Trachenko, K., Dove, M. T. & Salje, E. K. H. (2003). Structural
changes in zircon under -decay irradiation. Physical Review B 65,
180102 (R).
Trocellier, P. & Delmas, R. (2001). Chemical durability of zircon.
Nuclear Instruments and Methods in Physics Research, Section B 181,
408^412.
Tromans, D. (2006). Solubility of crystalline and metamict
zircon: A thermodynamic analysis. Journal of Nuclear Materials 357,
221^233.
Utsunomiya, S., Palenik, C. S., Valley, J. W., Cavosie, A. J., Wilde, S.
A. & Ewing, R. C. (2004). Nanoscale occurrence of Pb in an
Archean zircon. Geochimica et Cosmochimica Acta 68, 4679^4686.
Utsunomiya, S., Valley, J. W., Cavosie, A. J., Wilde, S. A. & Ewing, R.
C. (2007). Radiation damage and alteration of zircon from a 33 Ga
porphyritic granite from the Jack Hills, Western Australia. Chemical
Geology 236, 92^111.
Vavra, G., Gebauer, D., Schmid, R. & Compston, W. (1996). Multiple
zircon growth and recrystallization during polyphase Late
Carboniferous to Triassic metamorphism in granulites of the Ivrea
Zone (Southern Alps): An ion microprobe (SHRIMP) study.
Contributions to Mineralogy and Petrology 122, 337^358.
Watson, E. B., Cherniak, D. J., Hanchar, J. M., Harrison, T. M. &
Wark, D. A. (1997). The incorporation of Pb into zircon. Chemical
Geology 141, 19^31.
Wilde, S. A., Valley, J. W., Peck, W. H. & Graham, C. M. (2001).
Evidence from detrital zircons for the existence of continental
crust and ocean on the Earth 44 Gyr ago. Nature 409, 175^177.
Yamada, K., Tagami, T. & Shimobayashi, N. (2003). Experimental
study on hydrothermal annealing of fission tracks in zircon.
Chemical Geology 201, 351^357.
Zaraisky, G. P. & Balashov, V. N. (1994). Thermal decompaction of
rocks. In: Shmulovich, K. I., Yardley, B. W. D. & Gonchar, G. G.
(eds) Fluids in the Crust: Equilibrium and Transport Properties. London:
Chapman & Hall, pp. 253^284.
589