Zircon is of fundamental importance in the investigation of deeply

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1 Zircon Behaviour in Deeply Subducted Rocks Daniela Rubatto and Jörg Hermann * Photomicrograph of a zircon crystal containing inclusions of diamond, Kokchetav Massif, Kazakhstan Zircon is of fundamental importance in the investigation of deeply subducted crustal rocks in which it is a trace constituent. Tiny mineral inclusions within zircon may be the only indicators that rocks were subducted to a depth of up to 150 km. Because zircon is resistant to physical and chemical changes, it preserves stages of the subduction and exhumation history within submillimetre-size grains. Advanced in situ techniques allow us to date zircon domains and to determine their trace element composition. We can thus acquire a detailed knowledge of the temperature pressure time paths that these extraordinary rocks have experienced. Zircon studies provide evidence that subduction and exhumation act at plate tectonic speeds of 1 3 cm/year. THE ROLE OF ZIRCON IN HIGH-PRESSURE ROCKS KEYWORDS: zircon, high-pressure metamorphism, trace elements, eclogite, U Pb geochronology In the last twenty years, the discovery of coesite and diamond in metamorphosed continental crust has completely changed the paradigm concerning the depth in the mantle to which the least dense parts of the lithosphere can be subducted. The formation of coesite a mineral with the same composition as quartz but with denser structure requires pressures of 2.5 GPa, corresponding to a depth of at least 90 km. Microdiamonds in gneisses provide evidence for even deeper subduction of crustal rocks, to depths of up to ~150 km. These extreme conditions (i.e. great depth) result in ultrahigh-pressure (UHP) metamorphism. Exposures of high-pressure (HP) and UHP rocks provide a unique natural laboratory to study an important aspect of plate tectonics: subduction and exhumation of crustal rocks. While the presence of coesite and diamond unequivocally demonstrates that crustal rocks have been subducted to great depth, the processes acting at depth were, until recently, essentially unknown. How can minerals formed at such great depth be preserved? How fast can crustal rocks be buried and exhumed? Zircon, a common accessory mineral in HP rocks, has been fundamental in constraining processes acting at such extreme metamorphic conditions because of three extraordinary characteristics it possesses: (1) zircon contains measurable amounts of the radioactive element uranium and hence can be used as a chronometer for metamorphic processes; (2) zircon often preserves different growth zones within a single grain, and thus may document different stages of the subduction exhumation cycle; and (3) zircon protects mineral inclusions formed at high pressures from * Research School of Earth Sciences, The Australian National University, Canberra 0200, Australia daniela.rubatto@anu.edu.au retrogression during exhumation. In this contribution we summarise processes and conditions that can lead to formation of zircon during deep subduction and highlight the role of zircon as a mineral container, chemical tracer and time capsule. We show that the wealth of information contained in zircon can only be exploited if the conditions of zircon formation can be linked to the metamorphic evolution of its host rock. WHAT HAPPENS TO ZIRCON DURING SUBDUCTION? Metamorphic zircon that forms during the subduction and exhumation of the crust is texturally distinctive. Two main features are observed: partial or complete replacement of a zircon crystal by a zircon of different composition (also called recrystallization; see Geisler at al this issue) and new growth of zircon, often forming on relict (inherited) grains (FIG. 1). The processes involved in the formation of HP zircon are still poorly understood. Dissolution precipitation, i.e. dissolution of existing crystals, or parts of them, in a fluid or melt and the coupled reprecipitation of zircon with distinct texture and composition, appears to play a key role in both replacement and modification of zircon. Zirconium liberated from the breakdown of other phases (e.g. garnet, magmatic pyroxene, volcanic glass) may contribute to new zircon growth. As a general rule, replacement is common under subsolidus conditions (when no melt is present), whereas new growth is very common when melt is present. FIGURE 1 illustrates increasing degrees of structural modification of zircon in response to HP metamorphism. Zircon in equilibrated eclogite-facies rocks may be unaffected by metamorphism and represents the only magmatic relict in the mineral assemblage. The preservation of older zircon grains (inheritance) is the rule, particularly in HP rocks that experienced relatively low temperatures (<650 C). Metamorphic zircon first forms along fractures, probably in the presence of fluids (FIG. 1A). Commonly, inherited magmatic crystals have irregular domains where the original zoning is replaced by chaotic, patchy zircon (FIGS. 1B, E). The altered zircon is often porous and rich in micro-inclusions, and shows signs of corrosion (FIG. 1C). As a result, such altered zircon may be isotopically disturbed, yielding ages that are geologically meaningless (Rubatto and Hermann 2003; Tomaschek et al. 2003; Spandler et al. 2004). An insight into zircon recrystallization is provided by FIGURE 1D, in which a magmatic zircon has been replaced by an aggregate E LEMENTS, VOL. 3, PP

2 A B C D E F G H I FIGURE 1 Internal structure of zircon crystals from subducted rocks. A, B and E I are cathodoluminescence images (see Glossary), C is a secondary electron image and D is a backscattered electron image. (A) Zircon with preserved magmatic shape and zoning, transected by fractures filled with metamorphic zircon (Zermatt, Switzerland; Rubatto et al. 1998). (B) Zircon in eclogite with metamorphic alteration replacing the original magmatic zoning (Monviso, Italy; Rubatto and Hermann 2003). (C) Morphology of a zircon from an eclogite-facies rock showing a surface cut by corrosion channels (Syros, Greece; Tomaschek et al. 2003). (D) Zircon recrystallized as small crystals intergrown with HP allanite and omphacite (Lanzo, Italy). (E) Zircon from eclogite with a preserved magmatic core exhibiting low-grade alteration, subsequently overgrown by a HP metamorphic rim (New Caledonia; Spandler et al. 2004). (F) Zircon in eclogitic metasediment with a HP metamorphic rim on a detrital core (Aosta Valley, Italy; Rubatto et al. 1999). The ovals indicate the location of the SHRIMP analyses, which are small enough to resolve rim from core. (G) Hydrothermal zircon with rutile inclusions from a HP vein (Monviso, Italy; Rubatto and Hermann 2003). (H and I) Metamorphic zircon crystals from UHP rocks (Kokchetav Massif, Kazakhstan; Hermann et al. 2001). The crystal in H contains inclusions of coesite in the core and quartz in the rim. The crystal in I has two distinct metamorphic growth zones, which formed at different pressures and temperatures. of small zircon crystals, intergrown with HP minerals. Discrete zircon rims or domains are common features of zircon in subducted rocks. These rims form on inherited magmatic (FIG. 1E) or detrital cores (FIG. 1F) and often provide reliable ages for the metamorphism (see below). Occasionally, completely new zircon grains are found in HP metamorphic veins (e.g. Liati and Gebauer 1999; Rubatto et al. 1999; Rubatto and Hermann 2003). This growth requires dissolution of Zr from other sources (most likely magmatic zircon in the country rock) and very high fluid/rock ratios. These hydrothermal zircon crystals lack inheritance, are euhedral and polygonally zoned, and may contain inclusions of HP minerals (FIG. 1G). In subducted rocks that have reached temperatures high enough to cause partial melting (T > 650 C), inherited zircon can be completely dissolved or lost, and new metamorphic zircon, which tends to be euhedral and exhibits regular zoning, may be precipitated (FIGS. 1H, I; Hermann et al. 2001; Katayama et al. 2001). These features resemble those of melt-related zircon formed at high temperatures but lower pressures in granulites and migmatites (see Harley et al this issue). Metamorphic zircon found in HP rocks has usually been interpreted as forming at the pressure peak, i.e. the maximum subduction depth, which in most cases also corresponds to the temperature peak. However, zircon in HP rocks can in fact form over a wide range of conditions, from prograde subduction through to post-peak exhumation (FIG. 2). This explains why zircon often preserves multiple growth zones formed at different stages of HP metamorphism (FIGS. 1E, H, I). In rare cases, metamorphic zircon can form in low-pressure (<1.0 GPa) veins that are produced on the prograde burial path during subduction and accompanying dehydration. This occurs before the subducting crust reaches the depth at which eclogite-facies mineral assemblages are formed (Liati and Gebauer 1999; Rubatto et al. 1999; Spandler et al. 2004). There is some evidence that, once the rock is at great depth, metamorphic zircon forms even before the pressure peak. The formation of prograde HP zircon can explain the range of ages (~ Ma) documented in HP zircon of the Dabie-Sulu orogen of eastern China (e.g. Wan et al. 2005; Wu et al. 2006). Importantly, zircon formation has also been documented during postpeak exhumation of subducted crust, i.e. at pressures less E LEMENTS 32

3 than the maximum attained. This is particularly common in rocks that maintained relatively high temperatures during exhumation, such as those of the Kokchetav Massif in Kazakhstan or the Dabie-Sulu belt in China. In these cases, the retrograde formation of zircon is generally attributed to recrystallization in the presence of melts produced as a result of decompression-melting reactions. FIGURE 2 illustrates the fact that the conditions at which zircon forms in subducted rocks vary over a wide P T range ( GPa and C), suggesting that zircon does not form by a single reaction. In these rocks, zircon forms by processes that can act over a variety of P T conditions, whenever fluids or external factors (deformation, reaction kinetics, time) permit. Despite recent advances, our understanding of the influence of such factors on zircon formation is still quite limited. ZIRCON AS A MINERAL CONTAINER Zircon is also important in the study of HP metamorphism because of its extraordinary capacity to preserve mineral inclusions. This is due to its physical and chemical stability and its reluctance to re-equilibrate or dissolve at lower pressure conditions. In deeply subducted rocks that subsequently underwent intense lower-pressure recrystallization, zircon may contain the best or only evidence of the HP stage. For example, in the crustal rocks of the Kokchetav Massif, phases such as diamond and coesite have been almost exclusively found as small inclusions in zircon (FIG. 1H; Parkinson and Katayama 1999; Hermann et al. 2001; Katayama et al. 2001; Ogasawara 2005). Similarly, in the UHP rocks of the Dabie-Sulu orogen in China, coesite is extensively preserved as inclusions in metamorphic zircon (Wan et al. 2005), even in gneisses that have been completely retrogressed under lower-pressure conditions and preserve no other UHP relicts (Ye et al. 2000). Because of the remarkable confining strength of zircon, the coesite inclusions it contains can remain under high internal pressures even when the rocks are exposed at Earth s surface. For example, Parkinson and Katayama (1999) demonstrated using laser Raman spectroscopy that some coesite inclusions in zircon retain pressures up to 2 GPa. Similarly, zircon formed during low-temperature hydrothermal alteration is able to protect low-grade minerals from later HP metamorphism, providing a window into pre-subduction processes (Spandler et al. 2004). Identification of pressure-indicator minerals as inclusions in zircon has become increasingly important, not only for the diagnosis of UHP metamorphism, but particularly for the interpretation of zircon ages in the context of the metamorphic P T path. Mineral inclusions provide an excellent link between zircon formation and P T conditions, and thus between age and metamorphism. The use of mineral inclusions as a tool to link the age of zircon to P T conditions in subducted rocks has grown considerably from the pioneering work of Gebauer et al. (1997). Identifying pressure-indicator minerals such as coesite, zircon, rutile, omphacite, HP garnet and Ti-rich phengite in zircon (FIGS. 1D, G H) requires detailed microscopy, spot chemical analysis and in some cases Raman microspectroscopy. These techniques are widely available, low-cost and nondestructive, and assist greatly in relating ages to metamorphic evolution (e.g. Hermann et al. 2001; Katayama et al. 2001; Rubatto and Hermann 2003; Gilotti et al. 2004; Spandler et al. 2004; Wan et al. 2005; Zhang et al. 2005), particularly where multiple zircon growth stages with different inclusion generations are preserved (FIG. 1H). FIGURE 2 Schematic representation of zircon preservation and formation in subducted rocks. Zircon in subducted rocks records a number of different processes and forms over a wide range of P T conditions. Symbols represent different zircon types. The various P T paths refer to specific localities: pink Kokchetav Massif, Kazakhstan (Hermann et al. 2001; Katayama et al. 2001); brown Rhodope, Greece (Liati and Gebauer 1999); red Dabieshan, China (e.g. Wu et al. 2006); light blue Western Alps, Italy (e.g. Rubatto et al. 1998; Rubatto et al. 1999; Rubatto and Hermann 2003); dark blue New Caledonia (Spandler et al. 2004); green Syros, Greece (Tomaschek et al. 2003). ZIRCON AS A CHEMICAL TRACER In the last decade there has been an ever-increasing capability of microbeam techniques to measure in situ the chemical and isotopic compositions of small volumes of materials. As a consequence, the analysis of small grains of accessory minerals such as zircon has become increasingly important to geochemical studies. Zircon contains significant amounts of a number of key elements [such as Hf, Y, the heavy rare earth elements (HREE) and of course Zr] that are present in trace quantities in rocks but are important geochemical tracers. The composition of zircon is relevant to the study of subducted rocks for two main reasons: (1) the dissolution and crystallization of zircon strongly influence the transport and release of these trace elements at depth in the subduction factory, and (2) the trace element composition of zircon may assist in age interpretation. Despite comprising only 0.01% by volume of subducted oceanic crust, zircon, where present, carries most of the Zr (>95%) and Hf (~90%) and a significant amount of the U (~25%) contained in the bulk rock (Rubatto and Hermann 2003). Dissolution of accessory zircon in liberated fluids and melts will therefore play a fundamental role in the transfer of Zr and Hf from the subducted slab to the overlying mantle. E LEMENTS 33

4 Rubatto (2002) first documented that zircon formed at HP has a peculiar trace element pattern, with no enrichment in HREE and no Eu anomaly, two features common in magmatic zircon (FIG. 3). The negative Eu anomaly in the REE pattern of magmatic zircon can be explained by the presence of coexisting feldspars, which have a strong positive Eu anomaly, whereas HREE enrichment is related to the preferred substitution of the smaller REEs on the Zr site. Rocks equilibrated under HP conditions are characterised by the presence of abundant garnet (a mineral rich in HREE) and omphacite, and by the absence of feldspars. Zircon formed in equilibrium with these eclogitic minerals will thus be depleted in HREE relative to the middle REE (giving a flat chondrite-normalised HREE pattern with Lu/Gd < 3) and enriched in Eu (producing no negative Eu anomaly and with Eu/Eu expected > 0.75; FIG. 3). The peculiar trace element composition of HP zircon distinguishes it not only from most magmatic zircon but also from metamorphic zircon crystallized under lower-pressure conditions, where feldspar is present. The chemical signature of HP zircon is remarkably reproducible and has been observed in a wide range of rock types that have experienced HP metamorphism. This diagnostic tool is expected to be very robust because diffusion of REE in zircon is slow enough to be insignificant under most geological conditions experienced by crustal rocks. FIGURE 3 Rare earth element (REE) composition of zircon normalized to chondrite abundances. Thick lines represent HP zircon from different rocks and conditions. When compared to magmatic zircon (gabbro from Monviso) and low-pressure metamorphic zircon (amphibolite facies, Kokchetav Massif), HP zircon has relatively low HREE contents, shows flat HREE patterns and lacks a negative Eu anomaly. The value for La in magmatic and Sesia Zone metasediment was not measured, but inferred from detection limits. Sources: light blue HP metasediment, Sesia Zone (Rubatto 2002); pink UHP gneiss, Kokchetav Massif (Hermann et al. 2001); dark blue HP vein, Monviso (Rubatto and Hermann 2003); red kyanite eclogite, Greenland (Gilotti et al. 2004); brown retrogressed eclogite, Rhodope (Liati 2005). LREE: light REE, MREE: middle REE; HREE: heavy REE. ZIRCON AS A TIME CAPSULE Zircon has long been one of the most commonly used geochronometers for crustal rocks, but its application to HP rocks has become prominent only in the last 15 years. This is because zircon in HP rocks generally consists of multiple domains formed at different stages of metamorphism (FIG. 1), such that dating of entire crystals will often result in mixed ages. This problem was surmounted with the development of the sensitive high resolution ion microprobe (SHRIMP), the first instrument capable of dating mineral domains as small as 20 µm. Since it was first applied to the study of HP rocks (Gebauer 1996; Gebauer et al. 1997), there has been an explosion of ion microprobe U Pb analysis to date zircon growth zones in subduction-related rocks. In dating HP rocks that have experienced temperatures above 650 C, the use of zircon is particularly important because the U Pb system is one of the few chronometers that is resistant to such high temperatures. Dating of UHP rocks is particularly difficult because a large part of the decompression history occurs at nearly constant temperature (FIG. 2), making it essential that formation and not cooling ages are determined to constrain the time span of exhumation. U Pb ages date the formation of a particular zircon domain and not its cooling below the closure temperature of diffusion of the daughter product of the radioactive decay. This temperature resistance, together with the chemical robustness, allows preservation of multiple zircon ages, from inherited to prograde, peak and retrograde conditions. Because zircon in HP rocks can preserve inheritance and can form at different stages of the subduction exhumation cycle (FIG. 2), the age of any zircon domain cannot simply be assumed to date the pressure or temperature peak of the rock, but must be linked to P T conditions. In order to link ages to metamorphic conditions, a number of methods can be used. Initially and most commonly, the shape and internal zoning of zircon have been used to distinguish between inherited and metamorphic zircon. A Th/U ratio < 0.1 has long been recognised as a common feature of metamorphic zircon, in contrast to Th/U ratios of in magmatic zircon. The most direct way of determining the conditions of zircon formation is by documenting metamorphic mineral inclusions contained in distinct zircon domains. These mineral inclusions (for example coesite, diamond and garnet) are trapped during the growth of a zircon and permit the zircon domain to be related to the stable paragenesis in the host rock (e.g. Hermann et al. 2001; Katayama et al. 2001). On the other hand, textural relationships between zircon and other HP minerals have often proven to be inconclusive because of common inheritance a zircon included in a HP mineral might easily be the relict of a magmatic or detrital crystal. The trace element signature of HP zircon discussed above is effective in distinguishing it from inherited or low-pressure zircon. Additionally, a recently developed geothermometer based on the Ti content in zircon in equilibrium with rutile (TiO 2 ) is an important tool in relating zircon growth to temperature (Watson et al. 2006). As is often the case in science, there is no magic wand for interpreting the age obtained from zircon. The most successful zircon dating studies of deeply subducted rocks are those that combine a number of methods to interpret the age and that consider the complexity of the rock system. ZIRCON SPEEDOMETRY Zircon is often the only mineral that retains age information from different stages in the subduction exhumation cycle, and thus zircon has been used to determine the speed at which rock units are subducted and exhumed. The diamond-bearing gneisses of the Kokchetav Massif are an excellent example where multiple zircon growth stages have been documented (FIG. 2). Surprisingly, the age obtained from zircon domains containing diamonds is indistinguishable within error from the age of zircon domains formed at lower pressure, indicating that exhumation from depths of >140 km to 30 km took place in less E LEMENTS 34

5 then 6 Ma. This implies very fast exhumation rates of more than 1.8 cm/year (Hermann et al. 2001). Such high exhumation rates have also been found in the Eocene Oligocene UHP Dora Maira unit in the Alps ( cm/year; Gebauer et al. 1997; Rubatto and Hermann 2001). The discovery of very young zircon of 4 Ma in (U)HP rocks from Papua New Guinea (Baldwin et al. 2004) provides unequivocal proof of fast exhumation rates for deeply subducted crust. These rates are comparable to rates of seafloor spreading and plate convergence, indicating that exhumation can be as fast as subduction. Such fast exhumation rates exceed the known rates of erosion, providing evidence that exhumation of deeply subducted crust must be related to tectonic processes within the slab. What remains unresolved is the time that HP rocks spend at depth. This problem is due to the relative scarcity of prograde geochronological information from zircon or any other geochronometer. The initial data available indicate that the duration of an entire subduction exhumation cycle varies significantly and may last from a few million years (Liati and Gebauer 1999) to tens of millions of years (Wu et al. 2006). ACKNOWLEDGMENTS We thank guest editors Simon Harley and Nigel Kelly for the invitation to contribute to this issue. The work presented is the result of collaborations over the years with R. Compagnoni (Torino, Italy), D. Gebauer (Zürich, Switzerland), A. Korsakov (Novosibirsk, Russia), and C. Spandler and O. Müntener (Bern, Switzerland). Internal reviews by M. Norman and R. Rapp, and journal reviews by I. Parsons, J. Mattinson and D. Root helped to improve the manuscript. This work was supported by the Australian Research Council. REFERENCES Baldwin SL, Monteleone BD, Webb LE, Fitzgerald PG, Grove M, Hill EJ (2004) Pliocene eclogite exhumation at plate tectonic rates in eastern Papua New Guinea. 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