Zircon solubility in aqueous fluids at high temperatures and pressures (original) (raw)
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Zircon solubility in alkaline aqueous fluids at upper crustal conditions
Field evidence and thermodynamic data at ambient conditions suggest that complexing of Zr with hydroxyl ions at high pH enhances the solubility of zircon. We tested this hypothesis by measuring the solubilities of the assemblages zircon (ZrSiO4) + baddeleyite (Z + B) and zircon + quartz (Z + Q) in neutral to alkaline fluids at 0.2 GPa and 450–750 °C. In neutral to alkaline fluids (0, 0.1, 1m NaOH) zircon dissolved incongruently in quartz-undersaturated fluids to form baddeleyite. At 450 °C various Na–Zr–silicates precipitated from fluids saturated and undersaturated in quartz. We observed no significant dependence of solubility on temperature. The measured solubilities of zircon in pure H 2 O at 600 and 750 °C are only slightly higher than the procedural blank of 3.3 Â 10 À7 m Zr. However, the measured solubility of zircon increases with increasing aqueous silica, suggesting that zirconium complexes with silica in the fluid. At 600 °C linear regression of experimental results yielded: ln (Zr) = 0.25 + 2.9*ln(Si) where terms in parentheses represent molal concentrations. Zircon solubility also seems to increase with increasing hydroxyl concentration in the fluid. Enhanced solubility in silica-rich, alkaline fluids may cause dissolution–reprecipitation of zircon and resetting of its isotopic clocks, suggesting that some zircon U–Pb ages may correspond to fluid events.
The Canadian Mineralogist, 2006
The results of ~4000 LA-ICP-MS analyses in 152 thin sections from common crustal rocks reveal that several rock-forming minerals contain tens to a few thousand ppm of Zr. The highest concentrations of Zr are found in xenotime, followed by titanite, ilmenite, rutile, allanite, amphibole, clinopyroxene, garnet, magnetite and, less commonly, plagioclase, K-feldspar and orthopyroxene. Olivine, cordierite, biotite, muscovite, apatite, epidote and monazite have low levels of Zr (<5 ppm, generally <1 ppm). The minerals with the highest K D Hf /K D Zr are titanite (2.5), orthopyroxene (2.0), amphibole and clinopyroxene (1.8), and epidote and rutile (1.6-1.7). Ilmenite, magnetite, the feldspars and apatite have K D Hf /K D Zr ≈ 1, and values less than one were found in xenotime and zircon (0.8), garnet (0.7), and allanite (0.6). The most important implications of these results follow. First, the growth of a Zr-bearing phase during partial melting does not influence the Zr concentration of the melt, but increases the fraction of zircon that can be dissolved at a given temperature. This accelerates the disappearance of zircon from the protolith or, in melts already segregated, the dissolution of inherited zircon. The effect will be more marked in metaluminous magmas precipitating amphibole and titanite than in any other type of magma. Second, the presence of Zr-bearing phases has little influence of the zircon-saturation "geothermometer". It may cause somewhat higher (20-30°C) results in metaluminous rocks, but has no effect on peraluminous rocks. Third, the uptake of Zr by major minerals in crystallizing magmas may reduce both the concentration of Zr in the melt available to form zircon and the temperature at which zircon begins to precipitate. Mineral-melt reactions involving Zr-bearing phases may lead to zircon grains with complicated patterns of zoning and texturally discordant zones, apparently diachronous. Fourth, higher-than-chondrite Zr/Hf fractionates may arise from titanite, amphibole or clinopyroxene fractionation, but this requires very little or no crystallization of zircon. Significantly lower-than-chondrite Zr/Hf magmas only result from zircon fractionation. Lastly, two new examples of mineral reactions that involve the formation of a mass-balancing accessory phase, useful for high-resolution geochronology, are described: the formation of xenotime as a product of the breakdown of garnet in amphibolite-grade metapelites, and the subsolidus growth of a new rim on zircon included in Zr-bearing feldspars.
Lithos
Mineral inclusions, e.g., apatite, titanite, monazite, K-feldspar, are common in magmatic zircons. Although many studies mention that light rare earth element (LREE) contents of zircons could be compromised by an analytical artefact of the accidental sampling of mineral inclusions, how and to what degree these inclusions influence analysis of zircon composition is still not well constrained. Here we report U-Pb ages and trace element abundances for zircon crystals, where apatite and K-feldspar inclusions are observed, from diorite porphyry in the Weibao deposit, East Kunlun Mountains, Northern Tibetan Plateau. Although zircon morphological and chronological evidence consistently advocates a magmatic origin without undergoing significant hydrothermal alteration, 7 of 15 analytical spots show LREE-enriched patterns and low Ce/Ce* ratios which are comparable to those for published "hydrothermal" zircon. Quantitative modelling in this study manifests that these LREE-enriched patterns and low Ce/Ce* ratios can be achieved with only 0.1 to 2 vol% contamination from sub-micrometer apatite inclusions, which in practice are hard to monitor under the LA-ICP-MS (normally with large pit diameter and depth) and conventional microscopes. Titanite, monazite, xenotime, and allanite have similar roles to apatite, and LREE contents of zircon can be significantly elevated with only 0.05 vol% contamination from these inclusions. We therefore suggest that the widely used geochemical discrimination criteria for magmatic and hydrothermal zircon, e.g., (Sm/La) N vs. La and Ce/Ce* vs. (Sm/La) N diagrams and the degree of Ce anomalies, are ambiguous since they are extremely susceptible to contamination by mineral inclusions, and that within single samples only Ce 4+ / Ce 3+ values calculated from zircons of low LREE values probably represent the oxidation state of magmas.
Journal of Petrology, 2016
Improved geochronological methods and in situ isotopic (O, Hf) and trace element studies of zircon require a new physical model that explains its behaviour during crustal melting. We present results of numerical modeling of zircon dissolution in melts of variable composition, water content, temperature, and thermal history. The model is implemented in spherical coordinates with two moving boundaries (for the crystal and the surrounding melt cell outer edge) using simplified mineral phase relationships, and accounting for melt proportion histories as a function of melting and crystallization of major minerals. We explore in detail the dissolution of variably sized zircons and zircon growth inside rock cells of different size, held at different temperatures and undersaturations, and provide an equation for zircon survivability. Similar modeling is performed for other accessory minerals: apatite and monazite. We observe the critical role of rock cell size surrounding zircons in their survivability. Diffusive fill away from a dissolving 100 lm zircon into a large >3 mm cell takes 10 2-10 4 years at 750-950 C, but zircon cores may survive infinitely in smaller than 1 mm cells. Heating followed by cooling for a similar amount of time leads to dissolution followed by nucleation and growth, but new zircon growth remains smaller than the original within the cell. The final zircon size is also investigated as a function of microzircons crystallizing on a front of major minerals, leading to shrinking cell sizes and bulldozing of Zr onto the growing zircon surface. We explore in detail the survivability and regrowth of zircon inside and outside dikes and sills of different sizes and temperatures, and in different rock compositions, on timescales of their conductive cooling and heating, respectively. For zircon-rich rocks, only the largest >200 m igneous bodies are capable of complete dissolution-reprecipitation of typically sized zircons at significant distances from the intrusion. Smaller intrusions result in partial dissolution and rim overgrowth. Zircons captured near the contact of conductively cooling sills undergo more overgrowth than dissolution. In contrast, heat wave propagation from the sill will completely dissolve and reprecipitate zircons in Zr-poorer rocks many diameters of the sill away and often 10 3-10 4 years after the sill intrusion. A single thermal spike and melting episode is capable of generating the observed complexity of isotopically diverse and geochronologically zoned zircons. A MATLAB program is presented for users to apply in their specific situations.
Field observations and solubility experiments show evidence for the efficient mobilization of nominally insoluble HFSE (i.e., Ti, Zr, Nb and Hf) by high pressure fluids, probably via complexation with polymerized alkali-silica dissolved species and halogens (F and Cl). Here we investigate the complexation of Zr in subduction-related fluids (aqueous fluids and hydrous haplogranite melts) up to 800°C and 2.4 GPa using X-ray absorption spectroscopy (XANES and EXAFS) in a hydrothermal diamond anvil cell and provide evidence for the formation of Zr-O-Si/Na polymeric species in alkali-(alumino)silicate fluids at high pressure. Zr 4+ speciation in dilute fluids (2.5 wt% HCl) is dominated by 8-fold-coordinated [Zr(H 2 O) 8 ] 4+ hydrated complexes at room conditions and no evidence for extensive Zr-Cl complexation in the fluid was found up to 420°C, as confirmed by ab initio XANES calculations of various ZrO 8Àx Cl x clusters. The addition of Na and Si dissolved species (from 35 to 60 wt% dissolved Na 2 Si 2 O 5 , NS2) into the fluid favors the formation of alkali-zirconosilicate clusters Zr-O-Si/Na similar to those found in vlasovite (Na 2 ZrSi 4 O 11 ), with Zr 4+ in octahedral coordination with oxygen (Zr-O distance = 2.09 ± 0.04 Å ) and $6 Si (Na) second neighbors (Zr-Si/Na distance = 3.66 ± 0.06 Å ). This coordination environment also dominates Zr speciation in F-free and F-bearing NS2 and haplogranite glasses and high pressure hydrous haplogranite melts (15.5-33 wt% dissolved H 2 O) in the investigated pressure-temperature range. The XAS analyses, assisted by ab initio XANES calculations, are not conclusive concerning the extent of Zr-F complexation in hydrous granitic melts. Alkali-zirconosilicate Zr-O-Si/Na clusters such as those identified in this study may explain the enhanced solubility of zircon ZrSiO 4 (and other HFSE-bearing minerals) in alkali-aluminosilicate-bearing aqueous fluids produced by dehydration and melting of the slab and provide a favorable mechanism for the mobilization of HFSE in subduction zones. Fluid-rock interactions and/or P/T variations as fluids migrate through the mantle wedge could affect the stability of these complexes, triggering the precipitation of HFSE-bearing accessory phases that are eventually recycled into the mantle, contributing to the dispersion of HFSE. These processes provide a possible explanation for the characteristic HFSE depletion recorded in arc magmas.
Chemical Geology, 2010
Zircons recovered from oceanic gabbro exposed on Atlantis Bank, Southwest Indian Ridge, typically display oscillatory and sector zoning consistent with igneous crystallization from mafic magmas. In one rock (of twenty investigated), weak-oscillatory-zonation patterns are overprinted by secondary textural features characterized by mottled, convoluted and wavy internal zonation patterns that are frequently associated with secondary micron-to submicron-scale micro-porosity. These zircons are hosted in a felsic vein that intruded an oxide gabbro, both of which are cross-cut by monomineralic amphibole-and quartz-rich veinlets. Zircons with weakoscillatory-zonation patterns record a weighted-average 206 Pb/ 238 U age of 12.76 ± 0.20 Ma (mswd = 1.5), and have high trace element concentrations [e.g., ΣREEs (∼0.4-2.2 wt.%), Y (∼0.6-2.8 wt.%), P (∼0.4-0.9 wt.%)], and Th/U (0.1-0.5). These zircons are anomalously old (≥1 Myr) relative to the magnetic age for this portion of oceanic crust (11.75 Ma). In contrast, zircons with non-igneous, secondary textures have a younger weightedaverage 206 Pb/ 238 U age of 12.00± 0.16 Ma (mswd = 1.7), and have lower trace element concentrations [e.g., ΣREEs (∼0.2-0.8 wt.%), Y (∼0.3-1.0 wt.%), P (∼0.1-0.3 wt.%)], and slightly lower Th/U (0.1-0.3). The weightedaverage age of these zircons is similar to the magnetic anomaly age, and other 206 Pb/ 238 U ages of nearby rocks. We do not observe a correlation between crystallographic misorientation, internal texture, or trace element chemistry. We suggest that the decrease in trace element concentrations associated with the development of non-igneous alteration textures is attributed to the purging of non-essential structural constituent cations from the zircon crystal lattice at amphibolite-facies conditions. The mechanism of alteration/re-equilibration was likely an interface-coupled dissolution-reprecipitation processes that affected pre-existing, anomalously old zircons during shallow-level magmatic construction of Atlantis Bank at ∼12.0 Ma.
Progress in Earth and Planetary Science, 2015
Throughout the Earth's history, mass transport involved fluids. In order to address the circumstances under which Zr 4+ may have been transported in this manner, its solubility behavior in aqueous fluid with and without NaOH and SiO 2 in equilibrium with crystalline ZrO 2 was determined from 550 to 950°C and 60 to 1200 MPa. The measurements were carried out in situ while the samples were at the temperatures and pressures of interest. In ZrO 2-H 2 O and ZrO 2-SiO 2-H 2 O fluids, the Zr 4+ concentration ranges from ≤10 to~70 ppm with increasing temperature and pressure. Addition of SiO 2 to the ZrO 2-H 2 O system does not affect these values appreciably. In these two environments, Zr 4+ forms simple oxide complexes in the H 2 O fluid with ΔH~40 kJ/mol for the solution equilibrium, ZrO 2 (solid) = ZrO 2 (fluid). The Zr 4+ concentration in aqueous fluid increases about an order of magnitude upon addition of 1 M NaOH, which reflects the formation of zirconate complexes. The principal solution mechanism is ZrO 2 + 4NaOH = Na 4 ZrO 4 + 2H 2 O with ΔH~200 kJ/mol. Addition of both SiO 2 and NaOH to ZrO 2-H 2 O enhances the Zr 4+ by an additional factor of about 5 with the formation of partially protonated alkali zircon silicate complexes in the fluid. The principal solution mechanism is 2ZrO 2 + 2NaOH + 2SiO 2 = Na 2 Zr 2 Si 2 O 9 + H 2 O with ΔH~40 kJ/mol. These results, in combination with other published experimental data, imply that fluid released during high-temperature/high-pressure dehydration of hydrous mineral assemblages in the Earth's interior under some circumstances may carry significant concentrations of Zr and probably other high field strength elements (HFSEs). This suggestion is consistent with the occurrence of Zr-rich veins in high-grade metamorphic eclogite and granulite terranes. Moreover, aqueous fluids transported from dehydrating oceanic crust into overlying mantle source rocks of partial melting also may carry high-abundance HFSE of fluids released from dehydrating slabs and transported to the source rock of partial melting in the overlying mantle wedge. These processes may have been operational in the Earth's history within which subduction resembling that observed today was operational.
Geology, 2009
Zircon from a microgranular enclave in the ca. 315 Ma postcollisional Karkonosze pluton (Western Sudetes, northeastern Bohemian Massif) is characterized by unusual morphologies and reequilibration textures. Blocky, clustered, and skeletal Th-U-rich zircon grains are internally corroded along discrete boundary zones, and subsequently replaced by porous microgranular aggregates of zircon and various other minerals, including thorite. The boundary zones have textures and compositions characteristic of diffusion-controlled chemical reaction fronts, including enrichment in Ca, Ba, and light rare earth elements, whereas microgranular domains are typical of zircon replacement and regrowth by coupled dissolution and precipitation. Initial zircon crystallization occurred with the mingling of mafi c magma into a cooler granitic melt, whereas zircon modifi cation is attributed to the reaction of late magmatic fl uids from the host granite with the enclave. Precise dating of reequilibrated zircon as 304 ± 2 Ma indicates that fl uid activity, which is also responsible for scheelite mineralization, postdates the emplacement of the main part of the pluton by several millions of years.
Geochimica et Cosmochimica Acta, 1991
Single grains of zircon can contain zones indicating several generations of crystal growth, each of which should reflect the chemical and physical conditions occurring at the time of its formation. Trace element analyses have been made of large zircon crystals from rocks of alkaline affinities by ion microprobe. The chondrite-normalised rare earth element (REE) concentrations increase rapidly from La to Lu, as would be expected from the decrease in ionic radius and consequent easier substitution into the Zr site within the zircon lattice. Lanthanum, praseodymium, and neodymium are considerably lower than values observed in bulk analyses of zircon. The partition coefficients for the light rare earth elements ( LREEs), between zircon and melt or whole rock, must therefore be significantly lower than those calculated using bulk analyses. Cerium is enriched relative to neighbouring REEs due to the presence of Ce4+. Estimates of partition coefficients of Ce3+ and Ce4+ between zircon and melt demonstrate that although the Ce anomalies are large the Ce4+/Ce3+ ratio is very small (less than 3 X 10e3). The size of the Ce anomaly is variable and should be capable of providing information on oxygen fugacity changes. 11 La Pl SmGdDy ErYb Ce Nd Eu Tb Ho Tm Lu Decreasing Ionic Radius ----> FIG. 12. Average calculated "melt" REE concentrations, normalised to chondrites, against ionic radius for Elie Ness ENLM zircon.