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The entire population of magmatic oscillatory zoned zircons in a migmatitic granite from the Tjarnesjo intrusion, southwest Sweden, reveal fine-scale brittle fracturing. The oscillatory zoned fragments are rotated but not dispersed. Fractures between individual fragments are sealed by newly formed CL-bright zircon. Hydraulic fracturing is the most probable mechanism. The internal structure of fractured zircons and the LREE-enriched, low Th character of CL-bright zircon both suggest that cracks between oscillatory zoned zircon fragments were rapidly sealed after fracturing by CL-bright zircon, precipitated from hydrothermal fluids. Zircon fracturing and crack-sealing has been dated by SIMS ion-probe and U-Th-Pb isotopes to 920 [+ or -] 5] Ma (lower intercept age, 2[sigma], MSWD = 1.09) with a limit for the youngest possible age of 960 [+ or -] 16 Ma ([sup.207]Pb/[sup.206]Pb, 2[sigma], MSWD = 0.23) dated by sector-zoned rims forming overgrowths on the fractured cores. Keywords: Zircon, hydrothermal, cathodoluminescence, U-Pb, Sveconorwegian, SIMS, EBSD, fracturing, hydraulic

Generally, it has been assumed that signal intensity variations in back-scattered electron (BSE) images of minerals are mainly controlled by chemical heterogeneity. This is especially true for images of single crystals, where effects of different crystal orientations with respect to the incident beam on the observed BSE are excluded. In contrast, we show that local variations of the structural state within single-crystals (i.e., degree of lattice order or lattice imperfectness) may also have dramatic effects on the back-scattering of electrons. As an example, we present BSE images of single-crystals of natural zircon, ZrSi[O.sub.4], whose intensity patterns are predominantly controlled by structural heterogeneity, whereas effects of chemical variations are mostly negligible. In the case of natural zircon, structural heterogeneity affecting the BSE patterns is predominantly due to heterogeneous accumulation of radiation damage. We attempt to explain our observations with lowered penetration and channeling and, thus, enhanced back-scattering of electrons in more radiation-damaged internal zones and microareas. Back-scattered electron contrast of natural zircon is, therefore, considered as a special case of electron channeling contrast. This phenomenon seems to have been generally underappreciated in the discussion of BSE images of radiation-damaged minerals thus far. Keywords: Electron microscopy, BSE imaging, Raman spectroscopy, zircon

The release of Zr, U, and Pb from nearly metamict zircon and its recrystallized analog and of Zr from fully crystalline and slightly radiation-damaged zircon in [H.sub.]O + HCl [+ or -] Si[O.sub.2] fluids was investigated in situ at temperatures between 200 and 500[degrees]C using a hydrothermal diamond-anvil cell and time-resolved synchrotron-radiation XRF analyses. Dissolution of nearly metamict zircon proceeded much faster than that of zircon with little or no radiation damage and resulted in a 1.5 to 2 log units higher Zr molality in 6 to 7 m HCl fluids. Extensive recrystallization of the almost fully amorphous material started at 260 to 300[degrees]C in [H.sub.2]O + HCl, and at about 360[degrees]C if quartz was added, and was coupled with a decrease of the Zr concentration in the fluid by more than an order of magnitude. Recrystallization in 7 m HCl had little effect on the aqueous U and Pb concentrations, whereas addition of quartz caused a more sluggish decrease of the Zr concentration in the fluid upon recrystallization and lowered the release of U. The data presented here support the interpretation that enhanced Zr mobility in low-grade metamorphic rocks may be related to dissolution of metamict zircon by aqueous fluids and illustrate the significance of the silica activity on the kinetics of dissolution and recrystallization during zircon-fluid interaction. Keywords: High-temperature studies, zircon-aqueous fluid interactions, hydrothermal diamond-anvil cell, in-situ SR-XRF analysis, dissolution kinetics, order-disorder, metamictization, recrystallization, fluid phase, [H.sub.2]O + HCl [+ or -] Si[O.sub.2], recrystallization kinetics

The free energy of formation of zircon (ZrSi[O.sub.4]) from its oxides was determined between 1100 and 1300 K by an electrochemical method, in which values of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] defined by the two assemblages [Fe.sub.2]Si[O.sub.4]-Fe-Si[O.sub.2] (fayalite-iron-quartz) and [Fe.sub.2] Si[O.sub.4]-Fe-Zr[O.sub.4]-ZrSi[O.sub.4] were each measured using oxygen concentration cells with calcia-stabilized zirconia solid electrolytes. The difference in [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] between these two assemblages corresponds to the reaction Zr[O.sub.2] + Si[O.sub.2](qz) = ZrSi[O.sub.4]. The results, when analyzed using calorimetric data for the entropies and high-temperature heat capacities of ZrSi[O.sub.4], Zr[O.sub.2], and Si[O.sub.2](quartz), yields [[DELTA].sub.f,ox][H.sup.0.sub.298K] = -24.0 [+ or -] 0.2 kJ/mol for ZrSi[O.sub.4], in good agreement with the calorimetric value of Ellison and Navrotsky (1992). ZrSi[O.sub.4] is predicted to decompose to Zr[O.sub.2] plus Si[O.sub.2] (cristobalite) at 1938 K, assuming a temperature of 1430 K for the martensitic phase transition between the tetragonal and monoclinic forms of Zr[O.sub.2] (baddeleyite), with an enthalpy of transition of 8.67 kJ/mol. The same experimental approach was used also to determine the free energy of formation of hafnon (HfSi[O.sub.4]). The entropy of hafnon ([S.sup.0.sub.298K] = 93.6 J/mol x K) is similar to that for zircon, but the enthalpy of formation is slightly more exothermic ([[DELTA].sub.f.ox]/[H.sup.0.sub.298K] = -25.0 [+ or -] 0.2 kJ/mol). The cells with either ZrSi[O.sub.4] + Zr[O.sub.2] or HfSi[O.sub.4] + Hf[O.sub.2] produce an anomalous excursion in EMF when the temperature of the [alpha]-[gamma] transition in Fe metal at 1184 K is traversed; this excursion takes >12 hours to decay back to the equilibrium value. This behavior is presumably related to strain caused by the volume change of the [alpha]-[gamma] transition. The redetermination of the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] of the [Fe.sub.2] Si[O.sub.4]-Fe-Si[O.sub.2](qz) equilibrium (the quartz-fayalite-iron or QFI oxygen buffer) carried out in the course of this study gave results in reasonable agreement with previous work, but with a different slope vs. temperature, implying a slightly higher value of [S.sup.0.sub.298K] for [Fe.sub.2]Si[O.sub.4] than the currently accepted calorimetric datum (i.e., 153.5 vs. 151.0 [+ or -] 0.2 J/K x mol). Keywords: Zircon, hafnon, free energy of formation, fayalite, thermodynamic data

For more than 100 years mineralogists, physicists, chemists, geologists, and biologists have discussed the formation of periodic Liesegang patterns observed in natural and experimental systems. Spectacular examples of minerals showing complex periodic patterns are agate, malachite, and sphalerite. Here we report the first observation of Liesegang-like patterns in hydrothermally treated metamict (i.e., amorphous) zircon. The structures observed show curved bands, radial sets of pocket-like wave fronts or irregular curved patterns in both cathodoluminescence and backscattered electron images. They are composed of alternating zones of crystallographically well-aligned, polycrystalline zircon along with remnant amorphous pockets and a phase assemblage of randomly oriented zircon crystallites, monoclinic Zr[O.sub.2], and amorphous Si[O.sub.2], as revealed by transmission electron microscopy. Analyses by secondary ion mass spectrometry and electron microprobe reveal that the latter zones are characterized by higher hydrogen concentrations and higher Zr-Si ratios. Both zones are also distinguishable by a distinctly different crystallite size. We propose a possible pattern-forming mechanism that is based on a feedback of hydrogen diffusion, zircon nucleation, and the displacement of hydrogen atoms from growing crystallites.

We theoretically investigated the local structure of the aperiodic states of zircon (ZrSi[O.sub.4]) using ab initio quantum mechanical calculations. The low and high density liquid and solid glassy phases were obtained by constant volume Car-Parrinello molecular dynamics simulations, using the molar volume of metamict and crystalline zircon, respectively. As in naturally metamict zircons, the polymerization of Si units, the segregation of Zr atoms, and an overall decrease of the Zr coordination were observed. However, the local ordering of our theoretical glasses differs from that of the natural amorphous samples. In the theoretical glasses, the Zr-O distances in the first coordination polyhedra are significantly more distributed and five- and six-coordinated Si species were observed. The relaxation of the glass structure on a time scale exceeding the possibility of molecular modeling is a possible explanation for these discrepancies. The dielectric and (29)Si NMR responses of the glassy phases were computed, providing new constraints for the analysis of experimental data recorded from metamict zircons. The calculated NMR spectra are in good agreement with the experimental NMR spectra in the four-coordinated Si region. Our results show that the usual regular systematic decrease of the (29)Si chemical shift as a function of the polymerization of Si units ([Q.sup.n] species) cannot be used to interpret the (29)Si NMR spectrum of amorphous zircon. In particular, whereas the empirical scale would indicate an average polymerization of [Q.sup.3], the average polymerization of [Q.sup.1.5] observed in the low density zircon glass can account for the experimental data. In a non-uniform model of the structure of metamict zircon, this lower average polymerization is consistent with a clustering of cations limited to few coordination polyhedra.

Periodic density functional theory (DFT) was used to calculate the structure of zircon (ZrSi[O.sub.4]) and reidite (scheelite polymorph of ZrSi[O.sub.4]) at pressures from 0-41 GPa. Subsequently, the [29.sup] Si nuclear magnetic resonance (NMR) chemical shifts at each pressure were determined using the GIPAW method. The results show that in both zircon and reidite the chemical shifts become more negative with increasing pressure. The chemical shift of reidite is predicted by calculation to be -91.8 ppm and measured as -91.1 ppm. The results are used to interpret the previously observed systematic decrease in [sup.29] Si NMR chemical shifts (increasingly negative) in the crystalline fraction of radiation-damaged zircon with increasing [alpha]-dose. The original hypothesis that strain induced by the compression of the crystal by internal regions of damaged, amorphized material at high pressure was the origin of the shifts was disproved. An alternative source of internal pressure in the radiation-damaged crystals linked to '' stuffing' of the lattice with interstitial O or He atoms is proposed.

We present the results of a series of hydrothermal experiments on grains from two partially metamict zircon samples from Sri Lanka in the temperature range 350 to 650 [degrees]C and with different solutions (2 M Al[Cl.sub.3], 2 M Ca[Cl.sub.2], pure [H.sub.2]0, and a multi-cation solution). Under these conditions, sharply bounded reaction fronts penetrated into the zircon grains and developed complex lobate and rim structures that resemble structures found in natural zircon systems. The reaction zones are characterized by a marked increase in the cathodoluminescence intensity, a decrease of the back-scattered electron emission, and an increased degree of structural order, as revealed by micro-Raman and infrared spectroscopy. Sensitive high-resolution ion microprobe and electron microprobe measurements revealed that the altered areas gained solvent cations (e.g., [Ca.sup.2+], [Ba.sup.2+], [Mg.sup.2+], [Al.sup.3+]) from the solution and lost variable amounts of Zr, Si, Hr, the REE, U, Th as well as radiogenic Pb. A comparison between 'dry' and 'hydrothermal' annealing trends shows that the kinetics of structural recovery, including recrystallization of the amorphous phase in metamict zircon, is strongly enhanced under hydrothermal conditions. This finding suggests that water 'catalyzes' structural recovery processes in metamict zircon. We found that the structure of the reacted areas does not resemble that of crystalline zircon, i.e., is still characterized by a temperature-dependent degree of disorder, which would not be expected if the reaction is controlled by a coupled dissolution and re-precipitation process. Instead, the alteration process can be described best by a diffusion-reaction-recrystallization model. In this model, it is postulated that the diffusion of water into the metamict structure is the driving force for moving recrystallization fronts. We found that the rate and the extent of solid-state recrystallization of the amorphous phase is an important factor in determining the mobility of trace elements. This interpretation is indicated by the observation that trace elements, including U and Th, were preferentially lost during the reaction with a fluid at low temperatures, where recrystallization of the amorphous material was slow or not activated at all. The observed chemical alteration patterns are believed to reflect a competition between the kinetics of long-range diffusion and ion exchange and the kinetics of the short-range diffusion necessary lot the recrystallization process.

Reidite is a high-pressure polymorph of zircon with the scheelite structure. It has been found in an upper Eocene impact ejecta layer in marine sediments on the upper continental slope off New Jersey and on Barbados. Reidite occurs (epitaxially oriented) in shock-metamorphosed zircons. It is associated with impact glass (tektites), shocked quartz and feldspar with multiple sets of planar deformation features, coesite, and trace amounts of stishovite. This phase was first produced in high-pressure laboratory experiments in 1969 and has also been produced in shock recovery experiments. Reidite is brittle with an irregular fracture, a hardness of 7.5, a calculated density of 5.2 g/[cm.sup.3], a white streak, adamantine luster, and it does not fluoresce. In index oil in transmitted light, shocked zircon grains consisting almost entirely of reidite are transparent. Pleochroism was not observed. Reidite appears to have parallel extinction and is length slow. The maximum birefringence is roughly 0.015. Reidite appears to be uniaxial positive. It is tetragonal, space group I[4.sub.1]/a, a = 4.738 (1) [Angstrom], c = 10.506 (2) [Angstrom], V = 235.84(2) [[Angstrom].sup.3]. Previous shock-loading experiments on zircons indicate that the transition to reidite starts at about 30 GPa and is completed around 53 GPa. Reidite should be a useful indicator of peak pressure in shock metamorphosed rocks. Reidite is named after Alan Reid who first produced this phase in the laboratory.

We have determined major (Si, Zr, Hf), minor (Al, Y, Fe, P), and trace element (Ca, Sc, Ti, Ba, REE, Th, U) concentrations and Raman spectra of a zoned, 200 [[micro]meter] zircon grain in lunar sample 14161,7069, a quartz monzodiorite breccia collected at the Apollo 14 site. Analyses were obtained on a thin section in situ with an ion microprobe, an electron microprobe, and a laser Raman microprobe. The zircon grain is optically zoned in birefringence, a reflection of variable (incomplete) metamictization resulting from zonation in U and Th concentrations. Variations in the concentrations of U and Th correlate strongly with those of other high-field-strength trace elements and with changes in Raman spectral parameters. Concentrations of U and Th range from 21 to 55 ppm and 6 to 31 ppm, respectively, and correlate with lower Raman peak intensities, wider Raman peaks, and shifted Si-O peak positions. Concentrations of heavy rare earth elements range over a factor of three to four and correlate with intensities of fluorescence peaks. Correlated variations in trace element concentrations reflect the original magmatic differentiation of the parental melt [approximately]4 b.y. ago. Degradation of the zircon structure, as reflected by the observed Raman spectral parameters, has occurred in this sample over a range of [Alpha]-decay event dose from [approximately]5.2 x [10.sup.14] to 1.4 x [10.sup.15] decay events per milligram of zircon, as calculated from the U and Th concentrations. This dose is well below the [approximately][10.sup.16] events per milligram cumulative dose that causes complete metamictization and indicates that laser Raman microprobe spectroscopy is an analytical technique that is very sensitive to the radiation-induced damage in zircon.

The oscillatory zoning patterns (OZP) in crystals with respect to self-affine fractals are described by a quantitative method yielded by spectroscopic studies. The fractal dimension, a quantitative calculation of the nonrandom behavior of the pattern, characterizes the OZP, and are the quantitative characteristics along with compositional variations. These quantitative characteristics are reproduced by the oscillatory crystallization. The fractal dimensions of the OZP crystals are similar to the spectral patterns of the crystals.

The morphological and compositional changes undergone by zircon resulting from heating to 1650 degrees celsius under vacuum during Pb-isotope analysis were studied. Results revealed that at low temperatures, zircon reacted more near margins, cracks and inclusions, while its crystalline internal portions reacted more at high temperatures. During the reaction, zircon was broken down into a porous zirconium oxide and silica, Pb and other trace elements were released. As temperature rose, an inward migration from the sample's margin by the reaction front was observed. Zircon's age was found to be best reflected by 207Pb/206Pb ratios obtained at the highest temperatures.

Zircon crystals synthesized in a Li-Mo oxide melt and doped with trivalent lanthanides and Y (REE), both with and without P, were examined by single-crystal X-ray diffraction (XRD). REE are incorporated into the Zr site in the zircon structure, and some Zr appears to be displaced to the Si site. Crystals doped with middle REE (MREE, Sm to Dy) and Y, plus P follow the xenotime substitution ([REE.sup.3+] + [P.sup.5+] = [Zr.sup.4+] + [Si.sup.4+]) rather closely, whereas crystals doped with heavy REE (HREE, Er to Lu) deviate from the xenotime substitution, having REE:P atomic ratios significantly greater than one. Xenotime substitution requires that [P.sup.5+] replace [Si.sup.4+], but this substitution becomes limited by strain at the Si site in HREE-doped crystals. As Si sites become saturated with [P.sup.5+], additional charge balance in synthetic zircon crystals may be provided by [Mo.sup.6+] and [Li.sup.+] from the flux entering interstitial sites, accounting for an additional 0.3 to 0.6 at% HREE beyond that balanced by [P.sup.5+] ions. Heavy REE are more compatible in the zircon structure than are LREE and MREE, and HREE substitution is ultimately limited by the inability of the zircon structure to further accommodate charge-compensating elements. Thus the limit on REE concentrations in zircon is not a simple function of [REE.sup.3+] ionic radii but depends in a complex way on structural strain at Zr and Si sites, which act together to limit REE and P incorporation. The mechanisms that limit the coupled xenotime substitution change from LREE to HREE. This change means that REE fractionation in zircon may vary according to the availability of charge-compensating elements. REE partition coefficients between zircon and melt must also depend in part on the availability of charge-compensating elements and their compatibility in the zircon structure.

Zircon crystals were grown from a Li-Mo flux (7.5 mo1% [Li.sub.2][MoO.sub.4]; 86.5 mo1% [MoO.sub.3]) to which equal molar proportions [ZrO.sub.2] and [Li.sub.2][SiO.sub.3] were added (3 mo1% each). The crystals were initially grown undoped, but later Dy was added to the flux without any other charge-compensating species. With Dy and P added, in equal molar proportions, the zircon crystals incorporated 1.37 mo1% (6.99 wt%) Dy and 1.36 mo1% (1.33 wt%) P, a factor of 5.3 increase in Dy over the crystals grown without P intentionally added to the flux. The other P+REE-doped zircon crystals displayed an approximately 1000-fold increase in REE and P from La through Lu, a result of decreasing ionic radii. The incorporation of ps+ allowed, in a general sense, the xenotime-type coupled substitution of ([REE.sup.3+], [Y.sup.3+]) + [P.sup.5+] = [Zr.sup.4+] + [Si.sup.4+]. From La to Nd, however, P exceeds REE, from Sm to Gd, the REE are generally equal to P, and from Tb to Lu the REE exceed P. The Y- and P-doped zircon behaved more like middle-REE (MREE)-doped zircon than heavy-REE (HREE)-doped zircon crystals in their ability to incorporate Y (and P) and to maintain charge neutrality. To investigate the incorporation of Dy with no P added to the flux, the P to LREE excess, and the HREE to P excess in the doped zircon, secondary ion mass spectrometry (SIMS) analyses were completed on selected crystals. In the Dydoped crystals, the SIMS analyses revealed minor amounts of P, Li, and Mo in the crystals. These elements contributed to charge balance required by the excess Dy. In REE- and P-doped zircon, the SIMS analyses detected Li and Mo, and the Li and Mo may also provide charge balance for excess REE in the HREE+P-doped crystals.

Diffuse X-ray scattering from single crystals of metamict zircon reveals residual crystallinity even at high fluences (up to 7.2 x [10.sup.18] [Alpha]-decay events/g). The experimental evidence does not suggest that radiation-induced amorphization is a 'phase transition.' The observations are in good agreement with a nonconvergent, heterogeneous model of amorphization in which damage production is a random process of cascade formation and overlap at increasing fluence. Instead of an amorphization transition, the existence of a percolation transition is postulated. At the level of radiation damage near the percolation point, the heterogeneous strain broadening of X-ray diffraction profiles is reduced whereas the particle-size broadening increases. Simultaneously, the macroscopic swelling of the zircon becomes larger than the maximum expansion of the unit-cell parameters. A suitable empirical parameter that characterizes this transition is the flux, [D.sub.s], at which the macroscopic expansion is identical to the maximum expansion of the crystallographic unit cell. In zircon, [D.sub.s] = 3.5[multiplied by][10.sup.18] [Alpha]-decay events/g.