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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. 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