Zircon Lattice Research Articles

Multi-mode Li diffusion in natural zircons: Evidence for diffusion in the presence of step-function concentration boundaries

Micron- to submicron-scale observations of Li distribution and Li isotope composition profiles can be used to infer the mechanisms of Li diffusion in natural zircon. Extreme fractionation (20–30‰) within each single crystal studied here confirms that Li diffusion commonly occurs in zircon. Sharp Li concentration gradients frequently seen in zircons suggest that the effective diffusivity of Li is significantly slower than experimentally determined (Cherniak and Watson, 2010; Trail et al., 2016), otherwise the crystallization/metamorphic heating of these zircons would have to be unrealistically fast (years to tens of years). Charge coupling with REE and Y has been suggested as a mechanism that may considerably reduce Li diffusivity in zircon (Ushikubo et al., 2008; Bouvier et al., 2012). We show that Li diffused in the direction of decreasing Li/Y ratio and increasing Li concentration (uphill diffusion) in one of the zircons, demonstrating charge coupling with REE and Y. Quantitative modeling reveals that Li may diffuse in at least two modes in natural zircons: one being slow and possibly coupled with REE+Y, and the other one being fast and not coupled with REE+Y. The partitioning of Li between these two modes during its diffusion may depend on the pre-diffusion substitution mechanism of REE and Y in the zircon lattice. Based on our results, sharp Li concentration gradients are not indicative of limited diffusion, and can be preserved at temperatures >700 °C on geologic timescales. Finally, large δ7Li variations observed in the Hadean Jack Hills zircons may record kinetic fractionation, rather than a record of ancient intense weathering in the granite source materials.

Effect of pressure on the Raman-active modes of zircon (ZrSiO4): a first-principles study

Density-functional theory (DFT) was employed in a first-principles study of the effects of pressure on the Raman-active modes of zircon (ZrSiO4), using both the generalized gradient and local density approximations (GGA and LDA, respectively). Beginning with the equilibrium structure at zero pressure, we conducted a calibration of the effect of pressure in a manner procedurally similar to an experimental calibration. For pressures between 0 and 7 GPa, we find excellent qualitative agreement of frequency–pressure slopes $$\partial \omega /\partial P$$ calculated from GGA DFT with results of previous experimental studies. In addition, we were able to rationalize the ω vs. P behavior based on details of the vibrational modes and their atomic displacements. Most of the $$\partial \omega /\partial P$$ slopes are positive as expected, but the symmetry of the zircon lattice also results in two negative slopes for modes that involve slight shearing and rigid rotation of SiO4 tetrahedra. Overall, LDA yields absolute values of the frequencies of the Raman-active modes in good agreement with experimental values, while GGA reproduces the shift in frequency with pressure especially well.

Phase and microstructural evolutions of the CeO2–ZrO2–SiO2 system synthesized by the sol–gel process

In this paper, a sol–gel method was employed to synthesis the zircon and Ce-doped zircon in the (CeO2–)ZrO2–SiO2 system. The evolutions of their phase and microstructure with the changing composition and the sintering temperature were studied. It was found that the formation rates of zircon increase initially and then decrease with the increase of sintering temperature, and reach their maximum at 1500–1550°C, while those of Ce-doped zircon present a decrease trend from 1400°C to 1600°C. The results of XRD patterns and Raman spectrum suggest that the loadings of cerium in the zircon lattices decrease with the increase of sintering temperature from 1400°C to 1600°C. SEM images show that very compact structure of (CeO2–)ZrO2–SiO2 can be achieved by a sol–gel process, and both the increase of sintering temperature and the adding of cerium are beneficial to the growth of crystalline grains.

Finite lattice distortion patterns in plastically deformed zircon grains

Abstract. This study examines finite deformation patterns of zircon grains from high-temperature natural shear zones. Various zircon-bearing rocks were collected in the Western Tauern Window, eastern Alps, where they were deformed under amphibolite facies conditions, and in the Ivrea–Verbano Zone (IVZ), southern Alps, where deformation is related with granulite-facies metamorphism. Among the sampled rocks are granitic orthogneisses, metalamprophyres and paragneisses, all of which are strongly deformed. The investigated zircon grains ranging from 10 to 50 μm were studied in situ using a combination of scanning electron microscope (SEM) techniques, backscattered electron (BSE) imaging, forward-scattered electron (FSE) imaging, cathodoluminescence (CL) imaging, and crystallographic orientation mapping by electron backscatter diffraction (EBSD), as well as micro-Raman spectroscopy. Energy-dispersive X-ray spectrometry (EDS) was applied to host phases. Microstructural analysis of crystal-plastically deformed zircon grains was based on high-resolution EBSD maps. Three general types of finite lattice distortion patterns were detected: type (I) is defined by gradual bending of the zircon lattice with orientation changes of about 0.6–1.8° per micrometer without subgrain boundary formation. Cumulative grain-internal orientation variations range from 7 to 25° within single grains. Type (II) represents local gradual bending of the crystal lattice accompanied by the formation of subgrain boundaries that have concentric semicircular shapes in 2-D sections. Cumulative grain-internal orientation variations range from 15 to 40° within single grains. Type (III) is characterized by formation of subgrains separated by a well-defined subgrain boundary network, where subgrain boundaries show a characteristic angular closed contour. The cumulative orientation variation within a single grain ranges from 3 to 10°. Types (I) and (II) predominate in granulite facies rocks, whereas type (III) is restricted to the amphibolite facies rocks. The difference in distortion patterns is controlled by strain rate and by ratio between dislocation formation and dislocation motion rates, conditioned by the amount of differential stress. Investigated microstructures demonstrate that misorientation axes are usually parallel to the < 001 > and < 100 > crystallographic directions; dominant slips are < 010 > {001}, < 010 > {100} and < 001 > {010}, whereas in some grains cross-slip takes place. This study demonstrates that activation of energetically preferable slip systems is facilitated if zircon grain is decoupled from the host matrix and/or hosted by a soft phase.

Lattice distortion in a zircon population and its effects on trace element mobility and U–Th–Pb isotope systematics: examples from the Lewisian Gneiss Complex, northwest Scotland

Zircon is a key mineral in geochemical and geochronological studies in a range of geological settings as it is mechanically and chemically robust. However, distortion of its crystal lattice can facilitate enhanced diffusion of key elements such as U and Pb. Electron backscatter diffraction (EBSD) analysis of ninety-nine zircons from the Lewisian Gneiss Complex (LGC) of northwest Scotland has revealed five zircons with lattice distortion. The distortion can take the form of gradual bending of the lattice or division of the crystal into subgrains. Zircon lattices are distorted because of either post-crystallisation plastic distortion or growth defects. Three of the five distorted zircons, along with many of the undistorted zircons in the population, were analysed by ion microprobe to measure U and Pb isotopes, Ti and REEs. Comparison of Th/U ratio, 207Pb/206Pb age, REE profile and Ti concentration between zircons with and without lattice distortion suggests that the distortion is variably affecting the concentration of these trace elements and isotopes within single crystals, within samples and between localities. REE patterns vary heterogeneously, sometimes relatively depleted in heavy REEs or lacking a Eu anomaly. Ti-in-zircon thermometry records temperatures that were either low (~700 °C) or high (>900 °C) relative to undistorted zircons. One distorted zircon records apparent 207Pb/206Pb isotopic ages (−3.0 to +0.3 % discordance) in the range of ~2,420–2,450 Ma but this does not correlate with any previously dated tectonothermal event in the LGC. Two other distorted zircons give discordant ages of 2,331 ± 22 and 2,266 ± 40 Ma, defining a discordia lower intercept within error of a late amphibolite-facies tectonothermal event. This illustrates that Pb may be mobilised in distorted zircons at lower metamorphic grade than in undistorted zircons. These differences in trace element abundances and isotope systematics in distorted zircons relative to undistorted zircons are generally interpreted to have been facilitated by subgrain walls. Trace elements and isotopes would have moved from undistorted lattice into these subgrain walls as their chemical potential is modified due to the presence of the dislocations which make up the subgrain wall. Subgrain walls provided pathways for chemical exchange between crystal and surroundings. Only five per cent of zircons in this population have lattice distortion suggesting it will not have a major impact on zircon geochronology studies, particularly as three of the five distorted zircons are from strongly deformed rocks not normally sampled in such studies. However, this does suggest there may be a case for EBSD analysis of zircons prior to geochemical analysis when zircons from highly deformed rocks are to be investigated.

Ce and Eu anomalies in zircon as proxies for the oxidation state of magmas

Chondrite normalized rare earth element (REE) patterns of zircons generally have enriched Ce values relative to La and Pr, and depleted Eu values relative to Sm and Gd. High Ce contents in zircon may imply oxidizing conditions (Ce4+ is more compatible than Ce3+), whereas depleted Eu contents may imply reducing conditions (Eu2+ does not substitute into the zircon lattice). We report 41 experiments in which temperature, melt composition, and oxygen fugacity (fO2) were varied in order to explore the details of Ce and Eu incorporation into zircon. Crystals were synthesized in hydrous silicate melts at 10kbar and 800–1300°C. Synthetic rock mixes were doped with La+Ce+Pr (±P) or Sm+Eu+Gd and buffered at oxygen fugacities ranging from ∼IW (iron–wüstite) to >MH (magnetite–hematite); the run products were analyzed by electron microprobe to obtain crystal/melt partition coefficients. Cerium anomalies increase with higher oxygen fugacities and lower crystallization temperatures. In agreement with other experimental studies, peralkaline melts yield the largest zircon grains but show only modest Ce anomalies even at fO2s>MH. The same reason that zircons grown in peralkaline melts are easy to synthesize in the laboratory (these melts are capable of dissolving wt.% levels of Zr before zircon saturation due to high alkali content) makes the melt structure/composition atypical and not representative of most natural magmas. With this in mind, we synthesized zircons in a granitic melt with more modest alkali contents that require geologically plausible Zr contents for saturation. We obtained the following empirical relationship: lnCeCe∗D=(0.1156±0.0050)×ln(fO2)+13,860±708T(K)-6.125±0.484where (Ce/Ce∗)D is the Ce anomaly in zircon calculated from partition coefficients, and T is the zircon crystallization temperature in K. Europium anomalies from the same melt composition are more negative at lower oxygen fugacities, but with no resolvable temperature dependence, and can be described by the following empirical relationship: EuEu∗D=11+10-0.14±0.01×ΔNNO+0.47±0.04where (Eu/Eu∗)D is the Eu partitioning anomaly and ΔNNO is the difference in log units from the NNO buffer. If both Eu and Ce anomalies in zircons can be used as proxies for the oxidation state of Ce and Eu in the host melts, then it is clear that Eu2+ and Ce4+ can coexist in most zircon-saturated magmas. This implies that depletion of Eu melt contents by feldspar crystallization fractionation prior to (or during) zircon crystallization is not required to produce Eu anomalies. Thus, zircon Eu anomalies are a function of the oxygen fugacity and the Eu anomaly of the melt. Cerium anomalies of natural melts are not predicted to be as common because no major rock-forming phase depletes or enriches magmas in Ce compared to neighboring elements La and Pr. Thus, (Ce/Ce∗)D may be most readily applied to constrain the oxidation state of natural melts.

Synthesis of Self-glowing Crystals of Zircon and Zirconia Doped with Plutonium-238 and Terbium

ABSTRACTDurable crystalline actinide host phases of ceramic waste forms are considered as advanced materials which are prospective for safe use of Pu and minor actinides before their final disposal. Development of self-glowing actinide-doped materials with matrices that are chemically inert and resistant to radiation damage may significantly change the approaches to actinide immobilization. Single crystals of zircon doped with different amount of Tb and 238Pu were synthesized by the flux method. Different non-radioactive crystals of Tb-doped zircon were studied first by cathodoluminescence method in order to identify the optimal content of Tb3+ that provides the highest luminescence emission. Then self-glowing crystals of zircon were grown with the optimal Tb content and small admixture of 238Pu (less than 0.1 wt. %). It was proposed that the valence state of Tb incorporated into zircon crystals can be (3+) and (4+), but only trivalent Tb is responsible for intensive luminescence. It is demonstrated that a small addition of Zr-phosphate to the flux supports Tb incorporation into zircon lattice and stabilizes preferably Tb3+. At the same time the addition of Zr-phosphate caused the crystallization of zirconia as a minor phase. Zircon crystals with very intensive self-glowing were successfully synthesized. The 238Pu content was 0.02 wt.% and the Tb concentration varied between 0.2 and 0.3 wt.%. Zirconia crystals obtained from the same experiment are characterized by weak self-glowing, although the Tb content was only 0.02 wt.%, while the content of 238Pu was comparable to that of zircon, i.e. 0.03 wt. %.

U and Th Contents and Th/U Ratios of Zircon in Felsic and Mafic Magmatic Rocks: Improved Zircon‐Melt Distribution Coefficients

Abstract:High‐precision data on U and Th contents and Th/U ratios of zircon obtained using secondary ion mass spectrometry analysis have been collected from the literature. Zircon in the granitic rocks has median values of 350 ppm U, 140 ppm Th, and Th/U=0.52; the recommended zircon‐melt partition coefficients are 81 for DU and 8.2 for DTh. In zircon from mafic and intermediate rocks, the median values are 270 ppm U, 170 ppm Th, and Th/U=0.81, and the recommended zirconmelt partition coefficients are 169 for DU and 59 for DTh The U and Th contents and Th/U ratios of magmatic zircon are low when zircon crystallizes in equilibrium with the melt. Increasing magma temperature should promote higher Th contents relative to U contents, resulting in higher Th/U ratios for zircon in mafic to intermediate rocks than in granitic rocks. However, when zircon crystallizes in disequilibrium with the melt, U and Th are more easily able to enter the zircon lattice, and their contents and Th/U ratios depend mainly on the degree of disequilibrium. The behavior of U and Th in magmatic zircon can be used as a geochemical indicator to determine the origins and crystallization environments of magmatic zircon.

Geothermochronology based on noble gases: II. Stability of the (U-Th)/He isotope system in zircon

The kinetics of He migration from zircon of variable degree of metamictization was investigated. The migration parameters of He were experimentally determined, the influence of radiation damage and the degree of metamictization on the stability of the (U-Th)/He isotope system was evaluated, the mechanisms of noble gas escape from zircon were investigated, new data on the kinetics of He migration were obtained and compared with previous results for the kinetics of Xe migration from zircon of the same geologic objects. It was shown that He occurs in two energy positions in the zircon lattice: the main position (more than 80% He) with an activation energy of ∼39 kcal/mol and k 0 = 1011 yr−1 and the second position with an activation energy for migration of 5–10 kcal/mol and k 0 ∼ 106 yr−1. It was concluded that He migration from the main energy position is better described by a single-jump mechanism. The migration of He from the second energy position is consistent with the diffusion mechanism. It was shown that deviations from the linear dependence in the lnln(He0/Het)-1/T coordinates are probably related to the destruction of volume defects containing He atoms at high temperatures (more than 1000°C on the experimental time scale) resulting in a change from the single-jump to diffusion mechanism and the presence of atoms migrating via the diffusion mechanism. It was shown that the peak width in the spectrum of radiogenic He release and the appearance of a second peak also depend on the fraction of atoms migrating in accordance with the diffusion mechanism. It was found that the low activation energy for He release from the second energy position indicates the existence of continuous He loss from the zircon lattice.

Comment on “Rheic Ocean ophiolitic remnants in southern Iberia questioned by SHRIMP U‐Pb zircon ages on the Beja‐Acebuches amphibolites” by A. Azor et al.

Comment on “Rheic Ocean ophiolitic remnants in southern Iberia questioned by SHRIMP U‐Pb zircon ages on the Beja‐Acebuches amphibolites” by A. Azor et al.

Influence of gadolinium concentration on the EMR spectrum of Gd3+ in zircon

Abstract Electron magnetic resonance (EMR) spectra of gadolinium-doped zircon (ZrSiO 4 ) powders have been studied at room temperature for gadolinium concentrations between 0.20 and 1.0 mol%. The results suggest that Gd 3+ ions occupy substitutional sites in the zircon lattice, that the electron magnetic resonance linewidth increases with increasing gadolinium concentration and that the range of the exchange interaction between Gd 3+ ions is about 1.17 nm, larger than that of the same ion in other host lattices, such as ceria (CeO 2 ), strontium oxide (SrO) and calcium oxide (CaO). The fact that the electron magnetic resonance linewidth of the Gd 3+ ion in polycrystalline zircon increases, regularly and predictably, with Gd concentration, shows that the Gd 3+ ion can be used as a probe to study, rapidly and non-destructively, the crystallinity and degradation of ZrSiO 4 .

He incorporation and diffusion pathways in pure and defective zircon ZrSiO 4: A density functional theory study

In this paper, we report a detailed study of helium (He) incorporation and diffusion pathways in the perfect and defective zircon lattice. Ab initio methods based on Density Functional Theory (DFT) were used to calculate the structural features, the solution energies in interstitial sites in the perfect zircon and in vacancy sites of the defective lattice and He diffusion pathways in these systems. We show that the mode of He incorporation in the perfect zircon is influenced mainly by the topological features of the lattice, promoting site preference of He towards accommodation in the interstitial sites present in the middle of c cylinder channels. The presence of defect species in the form of lattice vacancies and interstitials has a significant effect on He solubility and diffusivity in the lattice, where the ability of lattice vacancies to act as physical traps or repulsive sites depends greatly on the electronic nature of the vacancy, which can enhance solubility of He in the lattice and affect the kinetics of He mobility in zircon mineral.

On the valency state of radiogenic lead in zircon and its consequences

In zircon U–Pb systematics, extreme robustness up to the temperatures of granulite facies and anatexis contrasts with apparently easy loss of radiogenic Pb at low temperatures, often without any metamorphic event being in evidence. Here we propose that this paradoxical behaviour can be understood with the hypothesis that radiogenic Pb in zircon is tetravalent. We review data and arguments in favour of this hypothesis. Diffusion profiles calculated for Pb 2+ in a 25 μm radius zircon xenocryst in a melt at 1000 °C, combined with the incompatibility of Pb 2+, or for a zircon core inside a younger zircon rim at this temperature, show age effects that should have been observed in SIMS dating. Further, in zircon evaporation as well as in leaching experiments, common Pb is generally released preferentially to radiogenic Pb. After removal of less radiogenic Pb, the evaporation record generally shows pure radiogenic Pb during the final evaporation steps. The distribution of residual Pb in a leached titanite, revealed by PIXE, is similar to that of Ti. Lastly, XANES spectra of a 1 Ga old titanite (predominantly radiogenic Pb) and an Alpine one (predominantly common Pb) are significantly different, although the former does not resemble that of PbO 2. The arguments why radiogenic Pb should be tetravalent are based on analogies with studies relating to the tetravalent state of 234Th and the hexavalent state of 234U, which show that α-recoil in silicates generates a strongly oxidizing environment at the site where the recoiling nucleus comes to rest. Further, a zircon grain, being small, should remain highly oxidizing in its interior by the constant loss of β-particles, maintaining the 4+ state of radiogenic Pb. From its effective ion radius, similar to that of Zr 4+, and its charge, Pb 4+ has to be compatible in the zircon lattice. Also, by analogy with U 4+, Th 4+ and Hf 4+, its diffusivity should be several orders of magnitude lower than that measured for Pb 2+ in zircon. These factors can account for the extreme retentivity of U–Pb clocks even at high temperatures. On the other hand, radiogenic Pb situated in α-recoil damaged sites could be leached out by any electrolyte solution that reduces it to the divalent state, making it both incompatible and soluble. Thus, discordia can be generated in weathering. The curious observation that discordant Archaean zircon suites generally define trends to lower intercepts at up to 800 Ma instead of zero, appears to reflect a c. 14 to 28% better average accessibility to leaching of radiogenic 207Pb compared to 206Pb, in accord with the somewhat higher recoil energy of the last (branched) α-decays in the 235U chain.

Pb+ irradiation of synthetic zircon (ZrSiO4): Infrared spectroscopic investigation

The structural variations of synthetic zircon (ZrSiO 4 ) single crystals irradiated at room temperature by 280 keV Pb + ions (with fluences up to 1 × 10 15 ions/cm 2 ) were investigated using infrared (IR) spectroscopy. Like metamict zircon whose crystal structure is damaged and amorphized by naturally occurring α-decay events, the Pb + -irradiated zircon crystals show a dramatic decrease in reflectivity. However, no significant decrease in wavenumbers of the stretching vibrations of SiO 4 tetrahedra in zircon was detected. The Pb + -implanted zircon exhibits new IR bands, indicating irradiation-induced new vibrations or domains, clusters or phases in addition to SiO 2 and ZrO 2 . IR features consistent with those of Pb silicates (with a divalent state, i.e., Pb 2+ ) are also found in the irradiated sample. This finding implies that some of the radiogenic Pb in natural zircon might not actually reside in the zircon lattice or in ZrSiO 4 phases, but form new local domains or clusters. Infrared bands of OH-stretching vibrations were also detected in the irradiated synthetic zircon, which was originally free from OH features prior to the irradiation. These results indicate that H can easily diffuse into the irradiated layer or into irradiated-induced phases to form OH or and hydrous species after the irradiated material is damaged. The type and content of hydrous species vary with irradiation fluences.

Zircon formation versus zircon alteration — New insights from combined U–Pb and Lu–Hf in-situ LA-ICP-MS analyses, and consequences for the interpretation of Archean zircon from the Central Zone of the Limpopo Belt

In this study we demonstrate that the combination of U–Pb and Lu–Hf isotope data obtained by laser ablation-inductive coupled plasma-mass spectrometry provides a powerful tool for distinguishing zircon domains grown during several metamorphic or magmatic events from altered domains affected by complete single or multiple Pb-loss. This discrimination is possible because the U–Pb and Lu–Hf isotope systems are decoupled during zircon alteration. Initial 176Hf/ 177Hf once “incorporated” into the zircon lattice remains nearly unaffected during later alteration processes which cause Pb-loss. Zircon overgrowths always have higher initial 176Hf/ 177Hf than previously grown domains, due to the incorporation of additional radiogenic 176Hf formed by 176Lu decay in the rock's matrix between successive zircon growth events. Incorporation of such 176Hf into zircon domains affected by post-growth alteration is negligible. By applying the U–Pb and Lu–Hf systems, in combination with cathodoluminescence imaging, we show that zircon in the Sand River orthogneiss of the Limpopo Belt grew during magma crystallization at 3.28 Ga and anatectic events at 2.64 Ga and 2.02 Ga. Our data demonstrate that abundant zircon domains underwent multiple Pb-loss, and that many of the 2.02 Ga domains result from complete Pb-loss from zircon overgrowth domains already formed at 2.64 Ga. In contrast, all domains of complex zoned zircon grains from the Zanzibar granodiorite gneiss were formed during a single magmatic event at 2.61 Ga, as indicated by their uniform Hf isotope composition, but most subsequently suffered multiple Pb-loss.

Pb and rare earth element diffusion in xenotime

Diffusion of Pb and the rare earth elements Sm, Dy and Yb have been characterized in synthetic xenotime under dry conditions. The synthetic xenotime was grown via a Na 2CO 3–MoO 3 flux method. The sources of diffusant for the rare earth diffusion experiments were REE phosphate powders, with experiments run using sources containing a single REE. For Pb, the source consisted a mixture of YPO 4 and PbTiO 3. Experiments were performed by placing source and xenotime in Pt capsules, and annealing capsules in 1 atm furnaces for times ranging from 30 min to several weeks, at temperatures from 1000 to 1500 °C. The REE and Pb distributions in the xenotime were profiled by Rutherford Backscattering Spectrometry (RBS). The following Arrhenius relations are obtained for diffusion in xenotime, normal to (101): D Sm = 1.5 × 10 − 4 exp ⁡ ( − 441 ± 12 kJmol − 1 / R ⁢ T ) m 2 s − 1 . D Dy = 9.0 × 10 − 8 exp ⁡ ( − 349 ± 16 kJmol − 1 / R ⁢ T ) m 2 s − 1 . D Yb = 3.9 × 10 − 7 exp ⁡ ( − 362 ± 13 kJmol − 1 / R ⁢ T ) m 2 s − 1 . Diffusivities among the REE do not differ greatly in xenotime over the investigated temperature range, in contrast to findings for the REE in zircon [Cherniak, D.J., Hanchar, J.M., Watson, E.B., 1997. Rare earth diffusion in zircon. Chem. Geol. 134, 289–301.], where the LREE diffuse more slowly, and with higher activation energies for diffusion, than the heavier rare earths. In zircon, these differences among diffusion of the rare earths are attributed to the relatively large size of the REE with respect to Zr, for which they likely substitute in the zircon lattice. With the systematic increase in ionic radius from the heavy to lighter REE, this size mismatch becomes more pronounced and diffusivities of the LREE are as consequence slower. Although xenotime is isostructural with zircon, the REE are more closely matched in size to Y, so in xenotime this effect appears much smaller and the REE diffuse at similar rates. In addition, the process of diffusion in xenotime likely involves simple REE + 3 → Y + 3 exchange, without charge compensation as needed for REE + 3 → Zr + 4 exchange in zircon. This latter factor may also contribute to the large activation energies for diffusion of the REE in zircon (i.e., 691–841 kJ mol − 1 , [Cherniak, D.J., Hanchar, J.M., Watson, E.B., 1997. Rare earth diffusion in zircon. Chem. Geol. 134, 289–301.]), in comparison with those for xenotime. For Pb, the following Arrhenius relation is obtained (also normal to (101)): D Pb = 3.0 × 10 − 9 exp ⁡ ( − 382 ± 64 kJmol − 1 / R ⁢ T ) m 2 s 1 . These measurements suggest that Pb diffusion in xenotime is quite slow, even slower than Pb diffusion in monazite and zircon, and considerably slower than diffusion of the REE in xenotime. Xenotime may therefore be even more retentive of Pb isotope signatures than either monazite or zircon in cases where Pb isotopes are altered solely by volume diffusion. However, because the activation energy for Pb diffusion in xenotime is lower than those for monazite and zircon, Pb diffusion may be somewhat faster at many temperatures of geologic interest in xenotime than in monazite or zircon.

Contrasting response of monazite and zircon to a high- T thermal overprint

Contact metamorphism in the aureole of the 1322 Ma Makhavinekh Lake Pluton, northern Labrador, affected monazite and zircon in the adjacent 1850 Ma metapelitic gneisses. Transformation of regional garnet and sillimanite to lower-pressure symplectitic intergrowths of cordierite, orthopyroxene, and spinel was accompanied by resorption of inherited monazite inclusions in garnet coupled with the appearance of coronitic high-Y monazite rims. In situ ion-microprobe dating is used to show that high-Y rims formed during contact metamorphism. Liberation of Y and HREE from garnet also gave rise to new xenotime growth. The coronitic nature of monazite overgrowths reflects the diffusion-controlled nature of net-transfer reactions whereas its higher Y composition reflects equilibration with xenotime at peak T (> 800 °C) conditions in the inner aureole. Very thin overgrowths on inherited zircon were also encountered, but only where zircon is surrounded by the symplectitic assemblage, reflecting liberation of Zr from garnet. Although these overgrowths are too thin to date using conventional ion-microprobe techniques, well-developed triple junctions between zircon and orthopyroxene suggests that they grew in textural equilibrium with the contact metamorphic assemblage. In contrast to monazite, inherited zircon remained intact during contact metamorphism, exhibiting no change in morphology (other than the growth of thin rims) or internal zoning throughout the aureole. However, inherited sector-zoned zircons of anatectic origin display evidence for intracrystalline Pb redistribution in the inner aureole. In these samples, ion-microprobe analyses encountered heterogeneous Pb signals and a dispersion of 207Pb / 206Pb dates away from the well constrained 1850 Ma age of regional metamorphism. Whereas analyses from the outer aureole faithfully record the age of regional metamorphism, those from the inner aureole are normally and reversely discordant and distributed along a line collinear with a 1850 to 1322 Ma discordia. This disturbance is correlated with proximity to the pluton implying that Pb was mobile in the zircon lattice during contact metamorphism. Most grains are characterized by apparent Pb loss from low-U domains and apparent Pb gain in higher-U domains. These data are interpreted to reflect recovery of strained crystalline domains leading to expulsion of Pb* that was able to efficiently diffuse into higher-U domains that were partly amorphous prior to rapid reheating in the inner aureole.

Oxygen isotopic composition and U-Pb discordance in zircon

We have investigated U-Pb discordance and oxygen isotopic composition of zircon using high–spatial resolution δ 18O measurement by ion microprobe. δ 18O in both concordant and discordant zircon grains provides an indication of the relationship between fluid interaction and discordance. Our results suggest that three characteristics of zircon are interrelated: (1) U-Pb systematics and concomitant age discordance, (2) δ 18O and the water-rock interactions implied therein, and (3) zircon texture, as revealed by cathodoluminescence and BSE imaging. A key observation is that U-Pb–disturbed zircons are often also variably depleted in 18O, but the relationship between discordance and δ 18O is not systematic. δ 18O values of discordant zircons are generally lighter but irregular in their distribution. Textural differences between zircon grains can be correlated with both U-Pb discordance and δ 18O. Discordant grains exhibit either a recrystallized, fractured, or strongly zoned CL texture, and are characteristic of 18O depletion. We interpret this to be a result of metamictization, leading to destruction of the zircon lattice and an increased susceptibility to lead loss. Conversely, grains that are concordant have less-expressed zoning and a smoother CL texture and are enriched in 18O. From this it is apparent that various stages of water-rock interaction, as evidenced by systematic variations in δ 18O, leave their imprint on both the texture and U-Pb systematics of zircon.

Exsolution of thortveitite, yttrialite, and xenotime during low-temperature recrystallization of zircon from New Caledonia, and their significance for trace element incorporation in zircon

Recrystallized zircon grains in a phengite-epidote-chlorite schist from north-eastern New Caledonia contain as inclusions a mineral assemblage consisting of celadonite, kaolinite, quartz, Fe-oxy-hydroxide, smectite, chlorite, xenotime-(Y), thortveitite, yttrialite, and allanite-(Ce). This assemblage formed during low-temperature (<100 °C) seafloor alteration of a plagioclase-rich mafic rock and represents the first documented evidence of this alteration event in the high-P belt of northeastern New Caledonia. The survival of this low-temperature mineral paragenesis in zircon of a rock that has undergone subsequent eclogite-facies metamorphism testifies to the strength and value of zircon as a container for mineral inclusions. Thortveitite (Sc 2 Si 2 O 7 ) and yttrialite (Y 2 Si 2 O 7 ) inclusions in the altered zircon cores represent a new occurrence for these minerals and suggest that they may be more common than is currently recognized. The altered zircon cores generally have lower trace-element contents than the pristine igneous zircon cores and we suggest that thortveitite, yttrialite, and xenotime-(Y) formed as a result of trace-element expulsion from zircon during low-temperature recrystallization. Trace-element concentrations in the zircon cores indicate that trivalent cations (REE, Y, Sc) in zircon cannot be charge-balanced by xenotime substitution alone. Pentavalent cation concentrations (Nb, As) are also insufficient for charge balance. The presence of thortveitite (Sc 2 Si 2 O 7 ) and yttrialite (Y 2 Si 2 O 7 ) suggests that trivalent cations in zircon might be charge-balanced either by monovalent anions substituting for oxygen or by small monovalent cations such as H occurring interstitially in the zircon lattice.

Unusual Hf contents in metamorphic zircon from coesite‐bearing eclogites of the Dabie Mountains, east‐central China: implications for the dating of ultrahigh‐pressure metamorphism

AbstractMetamorphic zircon from coesite‐bearing eclogites in the Dabie Mountains encloses high‐P phases, and may have formed at the peak of ultrahigh‐pressure (UHP) metamorphism. Morphologically, the metamorphic zircon typically occurs as small, multi‐faceted, near‐spherical grains with homogeneous internal structure and weak backscattered electron (BSE) luminescence. Geochemically, it is characterized by extremely high and relatively constant contents of hafnium (Hf) and very low contents of Y, U and Th, reflecting the contraction of the zircon lattice under the UHP conditions. High contents of Hf may be characteristic of zircon formed during UHP metamorphism, which has important consequences for interpretation of geochronological results. We propose that the metamorphic zircon extremely enriched in Hf may be used to date the peak of UHP metamorphism that produced the coesite‐bearing eclogites in the Dabie Mountains, and potentially in other UHP terranes.