The hydrothermal stability of zircon: Preliminary experimental and isotopic studies (original) (raw)
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Contributions to Mineralogy and Petrology, 2001
We report results of hydrothermal experiments on four alluvial zircons from Sri Lanka, which cover a wide range of radiation damage, at 450°C and 1.3 kbar for 744 h with 2 M CaCl 2 solution as reactive¯uid. After the hydrothermal treatment, the most metamict samples show micrometer-thick reaction rims, which surround apparently unreacted zircon, as revealed by cathodoluminescence (CL) and Nomarski differential interference contrast (NDIC) images. These rims have sharp, curved, and transgressive boundaries with unreacted zircon and are, in some cases, spread out along cracks. The thickness of reaction rims increases with increasing cumulated a-dosage of the starting materials. The reaction rims are strongly enriched in Ca (up to 7000 ppm) and a water species and depleted in radiogenic Pb, Zr, and Si, as revealed by electron microprobe analyses. A signi®cant Th loss from the reaction rims was detected in the case of the most metamict sample, whereas U remained in the structure. FT-infrared spectrometry and X-ray diffraction measurements revealed that the bulk run products were recrystallized. Using micro-Raman spectrometry, we were able to demonstrate that dierential recrystallization took place. The reaction rims are strongly recrystallized, whereas the unreacted grain interiors underwent only minor recrystallization. Recrystallization of the rims is accompanied by an enhancement of the integral CL intensity. It is suggested that recrystallization in the reaction rims was catalyzed by water in®ltration and ion exchange and prevented significant congruent zircon dissolution under the given experimental conditions. Previous zircon studies have shown that (1) a transgressive morphology, (2) a reduced Th±U ratio, and (3) an enhanced CL emission are also characteristics of rims in zircons from high-grade metamorphic rocks. Based on these similarities between natural and experimentally produced rims, it is suggested that leaching-catalyzed recrystallization is an important alteration process in zircon under wet geological conditions and can account for many complex core-rim structures found in natural zircons. Furthermore, the strong enrichment of Ca in the reaction rims supports previous assumptions that high Ca concentrations in natural zircons are of secondary origin. It is suggested that lower U±Pb concordia intercept ages obtained from single-phase zircons with high Ca contents date a leaching event.
Chemical Geology, 2002
Understanding the stability of radiation-damaged zircon under low-temperature hydrothermal conditions is crucial to the application of zircon for U-Pb geochronology and as a host phase for the disposal of plutonium waste. We report the results of an investigation of the stability of partially metamict zircon by leaching experiments at 175 jC with a 2 M AlCl 3 and a 1 M HCl-CaCl 2 solution as hydrothermal fluids for 1340 h. Cathodoluminescence (CL) and backscattered electron (BSE) images show that the zircon grains have developed a reaction rim several micrometers thick or deeply penetrating reticulated alteration zones with sharp boundaries to unaltered metamict zircon. These zones have experienced severe loss of Si, U, Th, and Pb, and gain of Al or Ca, and a water species as revealed by electron microprobe, sensitive high-resolution ion microprobe (SHRIMP) analyses, and infrared spectroscopy. Micro-Raman and infrared measurements on the altered areas show that disordered crystalline remnants of the partially metamict zircon structure were partially recovered, whereas recrystallization of the embedding amorphous phase was not observed. No detectable structural or chemical changes were detected inside the unaltered areas. Intensive fracturing, which was most intense in the HCl-CaCl 2 experiment, occurred inside the altered areas due to the volume reduction associated with the recovery of the disordered crystalline material and probably with the leaching reactions. We explain the formation of deep penetrating alteration zones by a percolation-type diffusion model, which is based on the assumption that percolating interfaces or areas of low atomic density between crystalline and amorphous regions as well as between amorphous domains exist along which fast chemical transport is possible. The idea of the existence of fast diffusion interfaces is supported by the sharp chemical gradients at the margin to unreacted zircon. The model was used to estimate for the first time diffusion coefficients for U, Th, and Pb diffusion in amorphous zircon at 175 jC by assuming that volume diffusion inside the amorphous domains is the loss rate-limiting process. These estimates show that volume diffusion in metamict zircon can cause significant loss of U, Th, and especially loss of radiogenic Pb over geological time scales, even at temperatures as low as 175 jC. Our results show that recent Pb loss discordias can be generated (1) by predominate Pb loss from metamict zircon through volume diffusion at low temperatures where thermal healing of the structure is insignificant, and (2) by leaching of Pb (and U and Th) from metamict zircon through an external fluid.
On the breakdown of zircon upon “dry” thermal annealing
Mineralogy and Petrology, 2009
Zircon samples without and with secondary chemical alteration from diverse sources were subjected to heat treatment at 1400 °C for 96 hours. Resulting new phases and textures suggest that decomposition of zircon into component oxides occurred in all experiments to various degrees. The crucible material was found to have a strong influence on the extent of breakdown, especially in the case of altered starting materials. In this study the progressive stages of the breakdown of zircon grains are described. The factors that may govern the decomposition are discussed, including radiation damage, secondary alteration and external conditions (sample container, atmosphere). Based on the results, it is highly suggested to prefer platinum over alumina crucibles to avoid breakdown when dry annealing is done on zircon prior to U/Th-Pb isotopic analyses. * Calculated doses, given in 10 18 alpha events per gram. Values extracted from the references quoted. # Materials: Pt -platinum, Al 2 O 3 -sintered alumina.
American Mineralogist, 2010
The application of the Hf isotope composition of zircon as a geochemical tracer requires the preservation of Lu-Hf systematics within individual grains. We performed hydrothermal experiments on a self-irradiation-damaged zircon to test whether hydrothermal alteration would affect its Hf isotopic composition. Severely radiation-damaged zircon from Sri Lanka was reacted in either Teflon reactors or gold capsules at 200 °C (1080 h, autogeneous pressure), 400 °C (120 h, 1 kbar), and 600 °C (72 h, 1 kbar) in a 1 M HCl-0.2 M HF solution that was spiked with 300 ppm non-natural Hf (98.2% 180 Hf) and 970 ppm Yb. Laser ablation inductively coupled plasma mass spectrometry measurements of the Hf-and U-Pb isotope compositions of the altered domains revealed that the U-Pb system of such domains was severely disturbed, resulting in a discordia pointing toward the origin of the concordia diagram, but that the Hf isotope composition was unaffected. In addition, Yb enrichment was observed in the reacted zircon domains, predominantly near the zircon-solution interface. The Yb has apparently diffused into the altered domains. The new data are fully consistent with a diffusion-controlled aqueous alteration process occurring within radiation-damaged zircon.
Physical and chemical response of zircons to deformation
Contributions to Mineralogy and Petrology, 1988
An investigation of U--Pb isotopic systematics in zircons from mylonitized Henderson Gneiss revealed that selected zircon fractions from the mylonite zone suffered total loss of radiogenic Pb at 460 m.y. To further investigate the relationship between Pb loss, U gain, and grain size reduction associated with increasing strain in the shear zone, we have characterized the chemistry and morphology of zircons in the mylonitic rocks, using both electron microprobe analysis and scanning electron microscopy.
Contributions To Mineralogy and Petrology, 2010
A natural, altered zircon crystal from an alkaline pegmatite from the Zomba–Malosa Complex of the Chilwa Alkaline Province in Malawi has been studied by a wide range of analytical techniques to understand the alteration process. The investigated zircon shows two texturally and chemically different domains. Whereas the central parts of the grain (zircon I) appear homogeneous in backscattered electron images and are characterised by high concentrations of trace elements, particularly Th, U, and Y, the outer regions (zircon II) contain significantly less trace elements, numerous pores, and inclusions of thorite, ytttrialite, and fergusonite. Zircon II contains very low or undetectable concentrations of non-formula elements such as Ca, Al, and Fe, which are commonly observed in high concentrations in altered radiation-damaged zircon. U–Pb dating of both zircon domains by LA-ICPMS and SHRIMP yielded statistically indistinguishable U–Pb weighted average ages of 119.3 ± 2.1 (2σ) and 118 ± 1.2 (2σ) Ma, respectively, demonstrating that the zircon had not accumulated a significant amount of self-irradiation damage at the time of the alteration event. Electron microprobe dating of thorite inclusions in zircon II yielded a Th–U-total Pb model age of 122 ± 5 (2σ) Ma, supporting the age relationship between both zircon domains. The hydrothermal solution responsible for the alteration of the investigated zircon was alkaline and rich in CO32−, as suggested by the occurrence of REE carbonates and CO2-bearing fluid inclusions. The alteration of the crystalline, trace element-rich zircon is explained by an interface-coupled dissolution-reprecipitation mechanism. During such a process, the congruent dissolution of the trace element-rich parent zircon I was spatially and temporally coupled to the precipitation of the trace element-poor zircon II at an inward moving dissolution-precipitation front. The driving force for such a process was merely the difference between the solubility of the trace element-rich and -poor zircon in the hydrothermal solution. The replacement process and the occurrence of mineral inclusions and porosity in the product zircon II is explained by the thermodynamics of solid solution-aqueous solution systems.
Retention of uranium in complexly altered zircon: An example from Bancroft, Ontario
Chemical Geology, 2010
Mesoproterozoic (~1050 Ma; Stenian) zircon crystals from the Saranac Prospect, Bancroft, Ontario, contain up to~1 wt.% U and~0.15 wt.% Th and, correspondingly, they are for the most part extensively radiation-damaged (calculated total α-doses 2.3−35.3 × 10 18 /g). The crystals show textures of complex, intense chemical alteration that is attributed to multiple, low-T replacement events along fluid-controlled reaction fronts. Centers of crystals appear totally replaced; the primary zoning is virtually erased and the material has high porosity and numerous inclusions. Interior regions surrounding the central reworked areas still exhibit primary igneous-type zoning; in those regions the alteration emanates from fractures and then follows the more radiation-damaged growth zones. Altered areas are typically recognized by their high porosity, low BSE intensity, and deficient analytical totals. Those regions often have lost a significant fraction of their radiogenic Pb. They are in general somewhat depleted in Zr, Si, and U, and are notably enriched in Ca and Fe. Element maps reveal elevated concentrations of Al and Y within filled fractures. Our observations indicate that the fluiddriven ion exchange is mainly controlled by the accessibility of micro-areas with elevated levels of radiation damage to transporting fluids via "fast pathways". Most importantly, there is apparent Zr−Si−U equilibrium between initially existing and newly formed zircon. The retention of U after the chemical replacement (94 ± 14% relative to the original U content in the respective zones) does not significantly fall below the retention of two major cations Zr (95 ± 4%) and Si (95 ± 2%). In spite of the partially extreme hydrothermal alteration overprinting, the original U zoning in the crystals is well preserved. These observations suggest that preferential chemical leaching of U from zircon is clearly not a general feature of this mineral. This in turn seems to question the general validity of hydrothermal experiments to low-T, fluid-driven alteration of zircon in geological environments. The observed apparent immobility of U may affect the interpretation of U−Pb discordance in zircon, and the performance assessment of this mineral as potential waste form for actinides.
Oxygen isotopic composition and U-Pb discordance in zircon
Geochimica et Cosmochimica Acta, 2005
We have investigated U-Pb discordance and oxygen isotopic composition of zircon using highspatial resolution ␦ 18 O measurement by ion microprobe. ␦ 18 O 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) ␦ 18 O 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 18 O, but the relationship between discordance and ␦ 18 O is not systematic. ␦ 18 O 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 ␦ 18 O. Discordant grains exhibit either a recrystallized, fractured, or strongly zoned CL texture, and are characteristic of 18 O 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 18 O. From this it is apparent that various stages of water-rock interaction, as evidenced by systematic variations in ␦ 18 O, leave their imprint on both the texture and U-Pb systematics of zircon.
Studies Regarding Corrosion Mechanisms in Zirconium Alloys
Journal of ASTM International, 2011
Understanding the key corrosion mechanisms in a light water reactor primary water environment is critical to developing and exploiting improved zirconium alloy fuel cladding. In this paper, we report recent research highlights from a new collaborative research programme involving 3 U.K. universities and 5 partners from the nuclear industry. A major part of our strategy is to use the most advanced analytical tools to characterise the oxide and metal/oxide interface microstructure, residual stresses, as well as the transport properties of the oxide. These techniques include three-dimensional atom probe (3DAP), advanced transmission electron microscopy (TEM), synchrotron X-ray diffraction, Raman spectroscopy, and in situ electro-impedance spectroscopy. Synchrotron X-ray studies have enabled the characterisation of stresses, tetragonal phase fraction, and texture in the oxide as well as the stresses in the metal substrate. It was found that in the thick oxide (here, Optimized-ZIRLO, a trademark of the Westinghouse Electric Company, tested at 415 C in steam) a significant stress profile can be observed, which cannot be explained by metal substrate creep alone but that local delamination of the oxide layers due to crack formation must also play an important role. It was also found that the oxide stresses in the monoclinic and tetragonal phases grown on Zircaloy-4 (autoclave testing at 360 C) first relax during the pre-transition stage. Just before transition, the compressive stress in the monoclinic phase suddenly rises, which is interpreted as indirect evidence of significant tetragonal to monoclinic phase transformation taking place at this stage. TEM studies of pre-and post-transition oxides grown on ZIRLO, a trademark of the Westinghouse Electric Company, have used Fresnel contrast imaging to identify nano-sized pores along the columnar grain boundaries that form a network interconnected once the material goes through transition. The development of porosity during transition was further confirmed by in situ electrochemical impedance spectroscopy (EIS) studies. 3DAP analysis was used to identify a ZrO sub-oxide layer at the metal/oxide interface and to establish its three-dimensional morphology. It was possible to demonstrate that this sub-oxide structure develops with time and changes dramatically around transition. This observation was further confirmed by in situ EIS studies, which also suggest thinning of the sub-oxide/barrier layer around transition. Finally, 3DAP analysis was used to characterise segregation of alloying elements near the metal/oxide interface and to establish that the corroding metal near the interface (in this case ZIRLO) after 100 days at 360 C displays a substantially different chemistry and microstructure compared to the base alloy with Fe segregating to the Zr/ZrO interface.