Geochemistry of magmatic and hydrothermal zircon from the highly evolved Baerzhe alkaline granite: implications for Zr–REE–Nb mineralization

The Baerzhe alkaline granite pluton hosts one of the largest rare metal (Zr, rare earth elements, and Nb) deposits in Asia. It contains a geological resource of about 100 Mt at 1.84 % ZrO₂, 0.30 % Ce₂O₃, and 0.26 % Nb₂O₅. Zirconium, rare earth elements (REE), and Nb are primarily hosted by zircon, yttroceberysite, fergusonite, ferrocolumbite, and pyrochlore. Three types of zircon can be identified in the deposit: magmatic, metamict, and hydrothermal. Primary magmatic zircon grains occur in the barren hypersolvus granite and are commonly prismatic, with oscillatory zones and abundant melt and mineral inclusions. The occurrence of aegirine and fluorite in the recrystallized melt inclusions hosted in the magmatic zircon indicates that the parental magma of the Baerzhe pluton is alkali- and F-rich. Metamict zircon grains occur in the mineralized subsolvus granite and are commonly prismatic and murky with cracks, pores, and mineral inclusions. They commonly show dissolution textures, indicating a magmatic origin with later metamictization due to deuteric hydrothermal alteration. Hydrothermal zircon grains occur in mineralized subsolvus granite and are dipyramidal with quartz inclusions, with murky CL images. They have 608 to 2,502 ppm light REE and 787 to 2,521 ppm Nb, much higher than magmatic zircon. The texture and composition of the three types of zircon indicate that they experienced remobilization and recrystallization during the transition from a magmatic to a hydrothermal system. Large amounts of Zr, REE, and Nb were enriched and precipitated during the transitional period to form the giant low-grade Baerzhe Zr–REE–Nb deposit.     

Metamict transformations in some niobates and zircons according to X-ray absorption spectra

The niobium and zirconium L _III-absorption spectra in some niobates and zircons were obtained with a vacuum focusing crystal spectrometer. The effective charges of Nb and Zr in these minerals were derived from the X-ray absorption spectra. The fine structure of the absorption spectra and effective charges Nb and Zr in metamict, partly-metamict minerals and crystalline analogues made it possible to draw a conclusion as to the nature of the first coordination sphere of Nb and Zr during metamict decay and subsequent recrystallization under annealing of these minerals.     

Alpha-decay damage in minerals of the pyrochlore group

X-ray diffraction analysis and transmission electron microscopy have been used to study the effects of alpha-decay damage in pyrochlore group minerals, characterized by the general formula A _1?m B ₂O₆(O,OH,F)_1?n ·pH₂O. As defined by the XRD intensity ratio I/I ₀ , both the saturation dose (for which I/I ₀ =0.1?0.0) and the dose which signifies the initial loss of crystallinity (for which I/I ₀ =1.0?0.8) increase as a function of geologic age. The increase is attributed to annealing of isolated alpha-recoil tracks back to the original crystalline structure. The tracks have calculated mean lives, τ_a, on the order of 10⁸ years. In contrast, minerals which remain crystalline (e.g., uraninite, UO₂) despite doses of up to 10¹⁸ alpha-events/mg have mean alpha-recoil track lives ≈10⁴ years (Eyal and Fleischer 1985). After correcting the calculated dose for annealing of alpha-recoil damage, I/I ₀ is observed to decrease exponentially to zero over the dose range 0.02–1.0 × 10¹⁶ alpha-events/mg. The relationship between I/I ₀ and “corrected” dose was used to calculate an average alpha-recoil track diameter of 4.6 nm, in which < 2600 atoms are displaced. XRD line broadening due to strain dominates the first half of the crystalline-to-metamict transition, reaching a maximum of 0.003, then decreasing to < 0.001. Line broadening due to decreasing crystallite size dominates the latter half of the transition. Estimated crystallite dimensions decrease from 450 nm to < 15 nm prior to reaching the fully metamict state. With increasing dose HRTEM images of microlites from the Harding pegmatite sequentially exhibit: 1) mottled diffraction contrast, 2) isolated 1–5 nm aperiodic areas, 3) coexisting aperiodic and crystalline areas, 4) relict “islands” of crystalline material in an aperiodic matrix, and 5) complete loss of lattice fringe periodicity. With no consideration given to alpha-recoil track fading, the transition covers a dose range of 0.04–1.7 × 10¹⁷ alpha-events/mg. Using a value of τ_a=10⁸ years, this dose range is corrected down to 0.02–1.2 × 10¹⁶ alpha-events/mg. The metamict state is characterized by a range of M-M and M-O distances which give rise to bands of diffuse scattering centered at 0.30 nm and 0.18 nm, respectively, in x-ray and electron diffraction patterns. Random image contrast shown by HRTEM is consistent with a random network type structure, an interpretation supported by EXAFS/XANES studies (Greegor et al. 1985a, b, 1987). The structure of metamict pyrochlore consists of an aperiodic framework of corner-sharing B-O polyhedra. Compared to the crystalline precursor, the metamict state displays a reduced M-O coordination number and mean bond length, increased distortion of the B-site, and a slight increase in the average M-M distance.     

The structure of metamict zircon: A temperature-dependent EXAFS study

The thermal annealing (300–1700 K) of two metamict zircons (Ampagabe, Madagascar and Näegy, Japan) has been studied using X-Ray Diffraction (XRD) and Extended X-ray Absorption Fine Structure spectroscopy (EXAFS) at Zr K-edge. Two stages of thermal annealing within the aperiodic zircon are evidenced between 293 and 1700 K. The first stage (up to 600° C) shows a decrease of the a ₀-cell parameter from 6.674 (at 300° C) to 6.610 (at 600° C)± 0.005 Å. In that temperature range, the average local environment around Zr (presence of ^VIIZr and d(Zr-Zr) 3.3–3.6 Å) shows a weak, but significant increase of the Zr-Zr correlations located at 3.3–3.4 Å, undetectable by XRD. At temperatures up to 700° C (stage 2), the XRD-Bragg component arising from crystalline zircon increases in magnitude, whereas, Zr-K EXAFS analysis indicates a progressive ^VIIZr^VIIIZr transition, associated with a recovery of the crystalline zircon medium-range environment. For both techniques, the zircon structure is fully recovered at annealing temperatures up to 900° C.Electrostatic modelings suggest that the ^VIIIZr^VIIZr transition observed in zircon with increasing alpha-decay damage creates significantly overbonded oxygen atoms around Zr. With increasing temperature, those oxygen atoms are better bonded to ^VIIZr, due to the thermal expansion of the Zr-O bond. The congruent recovery of the zircon structure should therefore be favoured with increasing temperature. On the other hand, the metamict network can be also partially reorganized around 400–500° C, with the creation of Zr-rich domains, as measured by EXAFS. However, the growth of these domains after 3 hours annealing affects only minor portions of the aperiodic network. This model is corroborated by a similar thermal behaviour observed for a synthetic sol-gel of ZrO₂ · SiO₂ composition.     

Formation Process of Zircon Associated with REE‐Fluorocarbonate and Niobium Minerals in the Nechalacho REE Deposit,Thor Lake,Canada

The two drill holes, which penetrated sub‐horizontal rare earth element (REE) ore units at the Nechalacho REE in the Proterozoic Thor Lake syenite, Canada, were studied in order to clarify the enrichment mechanism of the high‐field‐strength elements (HFSE: Zr, Nb and REE). The REE ore units occur in the albitized and potassic altered miaskitic syenite. Zircon is the most common REE mineral in the REE ore units, and is divided into five types as follows: Type‐1 zircon occurs as discrete grains in phlogopite, and has a chemical character similar to igneous zircon. Type‐2 zircon consists of a porous HREE‐rich core and LREE–Nb–F‐rich rim. Enrichment of F in the rim of type‐2 zircon suggests that F was related to the enrichment of HFSE. The core of type‐2 zircon is regarded to be magmatic and the rim to be hydrothermal in origin. Type‐3 zircon is characterized by euhedral to anhedral crystals, which occur in a complex intergrowth with REE fluorocarbonates. Type‐3 zircon has high REE, Nb and F contents. Type‐4 zircon consists of porous‐core and ‐rim, but their chemical compositions are similar to each other. This zircon is a subhedral crystal rimmed by fergusonite. Type‐5 zircon is characterized by smaller, porous and subhedral to anhedral crystals. The interstices between small zircon grains are filled by fergusonite. Type‐4 and type‐5 zircon grains have low REE, Nb and F contents. Type‐1 zircon is only included in one unit, which is less hydrothermally altered and mineralized. Type‐2 and type‐3 zircon grains mainly occur in the shallow units, while those of type‐4 and type‐5 are found in the deep units. The deep units have high HFSE contents and strongly altered mineral textures (type‐4 and type‐5) compared to the shallow units. Occurrences of these five types of zircon are different according to the depth and degree of the hydrothermal alteration by solutions rich in F and CO₃, which permit a model for the evolution of the zircon crystallization in the Nechalacho REE deposit as follows: (i) type‐1 (discrete magmatic zircon) is formed in miaskitic syenite. (ii) LREE–Nb–F‐rich hydrothermal zircon formed around HREE‐rich magmatic zircon (type‐2). (iii) type‐3 zircon crystallized through the F and CO₃‐rich hydrothermal alteration of type‐2 zircon which formed the complex intergrowth with REE fluorocarbonates; (iv) the CO₃‐rich hydrothermal fluid corroded type‐3, forming REE–Nb‐poor zircon (type‐4). Niobium and REE were no longer stable in the zircon structure and crystallized as fergusonite around the REE–Nb‐leached zircon (type‐4); (v) type‐5 zircon is formed by the more CO₃‐rich hydrothermal alteration of type‐4 zircon, suggested by the fact that type‐4 and type‐5 zircon grains are often included in ankerite. Type‐3 to type‐5 zircon grains at the Nechalacho REE deposit were continuously formed by leaching and/or dissolution of type‐2 zircon in the presence of F‐ and/or CO₃‐rich hydrothermal fluid. These mineral associations indicate that three representative hydrothermal stages were present and related to HFSE enrichment in the Nechalacho REE deposit: (i) F‐rich hydrothermal stage caused the crystallization of REE–Nb‐rich zircon (type‐2 rim and type‐3), with abundant formation of phlogopite and fluorite; (ii) F‐ and CO₃‐rich hydrothermal stage led to the replacement of a part of REE–Nb–F‐rich zircon by REE fluorocarbonate; and (iii) CO₃‐rich hydrothermal stage resulted in crystallization of the REE–Nb–F‐poor zircon and fergusonite, with ankerite. REE and Nb in hydrothermal fluid at the Nechalacho REE deposit were finally concentrated into fergusonite by way of REE–Nb–F‐rich zircon in the hydrothermally altered units.     

陕西丹凤富铷伟晶岩中褐钇铌矿矿物学及地球化学特征

东秦岭的商南—丹凤地区分布着众多含稀有金属矿化的花岗伟晶岩,在该地区的富铷花岗伟晶岩中首次鉴定出褐钇铌矿,其主要产出于富石榴子石的微斜长石花岗伟晶岩中,多与石榴子石、锆石、磷钇矿、铌钽铁矿等副矿物伴生,为花岗质岩浆晚期结晶产物.电子探针分析表明,褐钇铌矿除了含有Y、Nb、REE、Ta、Ti等主要元素,还含有较高含量的放...     

Zircon reaction and stability of the U-Pb isotope system during interaction with carbonate fluid: experimental hydrothermal study

 The interpretation of metamorphically induced U-Pb isotopic discordance requires a thorough understanding of zircon-fluid interactions. With this aim we have studied the behaviour of metamict and crystalline zircon phases and their U-Pb systems by cathodoluminescence after treatment by 2M Na₂CO₃ solution at T = 200–800 °C and P = 1–5 kbar for 3–14 days, X-ray diffraction, microprobe and isotope dilution analysis. The data indicate that zircon transformation under hydrothermal conditions depends on the experimental conditions and the degree of structural damage. Reconstitution of defective and impurity-enriched zones of metamict zircon (homogenization of impure element concentrations and increase of crystallinity) was observed at 400 °C and P = 1 kbar. Considerable lead and uranium loss occurred under these conditions. As a result of zircon dissolution, newly formed baddeleyite accommodating U from 2M Na₂CO₃ solution and Zr-Na-silicate were recognized. This process intensified with increasing pressure. Study of crystalline zircon indicates that migration of U and Pb took place only during dissolution of zircon at T above 650 °C. In the presence of carbonate-ions essential U and Pb amounts are lost from metamict zircon at a lower P-T than is typical for greenschist facies metamorphism. Received: 4 October 1997 / Accepted: 6 December 1999     

Pressure-induced structural evolution of pyrite-type SiO<Subscript>2</Subscript>

The equation of state and pressure-induced structural evolution of pyrite-type SiO₂ have been investigated based on synchrotron X-ray diffraction measurements in a diamond-anvil cell. The Rietveld refinement revealed that the oxygen coordinate x of pyrite-type SiO₂ increases with increasing pressure. The SiO₆ coordination polyhedra of pyrite-type SiO₂ is less compressible than the unit cell, and the increase in x induces a rotation of the SiO₆ coordination polyhedra to fill the blank space in the unit cell. Thus, the volume reduction in pyrite-type SiO₂ is achieved mainly by the rotation of the SiO₆ polyhedra, rather than by the compression of the SiO₆ polyhedra. In addition, the increase in x with increasing pressure enhances a distortion of the coordination polyhedra of pyrite-type SiO₂, implying that pyrite-type SiO₂ is not likely to transform into a fluorite-type structure at higher pressures.     

The crystal structure of ilinskite, NaCu5O2(SeO3)2Cl3, and review of mixed-ligand CuOmCln coordination geometries in minerals and inorganic compounds

The crystal structure of ilinskite, NaCu₅O₂(SeO₃)₂Cl₃, a rare copper selenite chloride from volcanic fumaroles of the Great fissure Tolbachik eruption (Kamchatka peninsula, Russia), has been solved by direct methods and refined to R ₁?=?0.044 on the basis of 2720 unique observed reflections. The mineral is orthorhombic, Pnma, a?=?17.769(7), b?=?6.448(3), c?=?10.522(4) Å, V?=?1205.6(8) ų, Z?=?4. The The CuO_mCl_n coordination polyhedra share edges to form tetramers that have 'additional' O1 and O2 atoms as centers. The O1Cu₄ and O2Cu₄ tetrahedra share common Cu atoms to form [O₂Cu₅]⁶⁺ sheets. The SeO₃ groups and Cl atoms are adjacent to the [O₂Cu₅]⁶⁺ sheets to form complex layers parallel to (100). The Na⁺ cations are located in between the layers. A review of mixed-ligand CuO_mCl_n coordination polyhedra in minerals and inorganic compounds is given. There are in total 26 stereochemically different mixed-ligand Cu-O-Cl coordinations.     

Interaction of metamict zircon with fluids of various composition

Interaction of metamict zircon with solutions of various composition (H₂O, 1 m NaCl + 0.5 m HCl, 1 m NaCl, 1 m NaHCO₃, 0.1 m Na₂CO₃, 1 m Na₂CO₃, 2 m Na₂CO₃, and 1 m NaOH) was examined at temperatures of 25–800°C, pressures of 1 and 5 kbar, and exposure times of 3, 7, and 14 days. The choice of fluid compositions was determined by the fact that aqueous, chloride, and carbonate solutions are the principal components of postmagmatic, metamorphic, and metasomatic fluids that can affect zircon in nature. Occurring in contact with these solutions, metamict zircon congruently and incongruently dissolves competing with the sorption of elements, crystallization of newly formed phases, and the restoration of the crystal structure. The intensities of these processes are controlled by the temperature, pressure, exposure time, and the pH and composition of the solution.     

一种新产状的含铈褐钇铌矿

关于褐钇铌矿族的研究,国内外积累了不少资料。其主要产在黑云母花岗岩、花岗伟晶岩、微斜长石岩、交代变质岩、蚀变花岗岩、白岗岩及花岗岩的残坡积和冲积砂中。1979年我们在进行内蒙白云鄂博铌、稀土、铁矿床物质成分的研究时,在白云石型铌、稀土矿石中发现含铈褐钇铌矿,与该区原已发现的褐铈铌矿等矿物组成了褐钇铌矿-褐铈铌矿系列。     

Neoproterozoic Rare-Metal Subalkali Leucogranite from the Northern Yenisei Ridge

Petrochemical study and U–Pb SIMS (SHRIMP–II) zircon analyses of subalkaline leucogranite of the Khariusikha Massif have been carried out. They have revealed for the first time a rare-metal mineralization. The elevated concentrations of rare elements (wt %) are Nb (0.5–0.7), Ta (0.12–0.16), REEs (0.08–0.24), Y (0.06–0, 1), Zr (2.3–2.6), Hf (0.1–0.12), U (0.05–0.1), and Th (0.08–0.1) and are confined to albitized granites. The main mineral phases concentrating the rare elements, U and Th, are tantalo–niobates: fergusonite, euxenite, U–pyrochlore, tantalite, as well as thorite, monazite, zircon, and sphene. These minerals associate with cassiterite, sulfides, and gold. The simultaneity of the intraplate granitoid magmatism (753 ± 4 Ma) and bimodal rhyolite–basalt volcanism (753 ± 6 Ma) in the neighboring rift structure has been demonstrated. Presumably, the Neoproterozoic rifting and intraplate magmatism relate to the plume activity that caused the supercontinent Rodinia to break up.     

我国稀土铌钽矿物学研究回顾与展望

作为高新技术原料用的稀土铌钽是国家发展的支柱之一, 稀土铌钽矿物学研究在与国家的技术进步同步前进。 通过研究,已经获得了我国产出的全部百余种稀土铌钽矿物的化学组成、物理性质、稀土配 分、结晶参数、共生组合和产状成因的全面系统的鉴定描述成果;发现了多种稀土铌钽新物,并提出许多新规律和新理论,建立了易解石和褐钇铌矿两个新的矿物族;确定了稀土铌钽铁锰钨的复杂氧化物的晶体结构关系;确立变生矿物学为矿物学研究的一个特殊分支,探讨了稀土次生富集的离子型稀土矿成矿机理。     

Residence of REE, Y, Th and U in Granites and Crustal Protoliths; Implications for the Chemistry of Crustal Melts

A systematic study with laser ablation—ICP-MS, scanningelectron microscopy and electron microprobe revealed that 70–95wt% of REE (except Eu), Y, Th and U in granite rocks and crustalprotoliths reside within REEYThU-rich accessories whose nature,composition and associations change with the rock aluminosity.The accessory assemblage of peraluminous granites, migmatitesand high-grade rocks is composed of monazite, xenotime (in low-Cavarieties), apatite, zircon, Thorthosilicate, uraninite andbetafite-pyrochlore. Metaluminous granites have allanite, sphene,apatite, zircon, monazite and Thorthosilicaie. Peralkaline graniteshave aeschinite, fergusonite, samarskite, bastnaesite, fluocerite,allanite, sphene, zircon, monazite, xenotime and Th-orthosilicate.Granulite-grade garnets are enriched in Nd and Sm by no lessthan one order of magnitude with respect to amphibolite-gradegarnets. Granulitegrade feldspars are also enriched in LREEwith respect to amphibolite-grade feldspars. Accessories causenon-Henrian behaviour of REE, Y, Th and U during melt—solidpartitioning. Because elevated fractions of monazite, xenotimeand zircon in common migmatites are included within major minerals,their behaviour during anatexis is controlled by that of theirhost. Settling curves calculated for a convecting magma showthat accessories are too small to settle appreciably, beingseparated from the melt as inclusions within larger minerals.Biotite has the greatest tendency to include accessories, therebyindirectly controlling the geochemistry of REE, Y, Th and U.We conclude that REE, Y, Th and U are unsuitable for petrogeneticalmodelling of granitoids through equilibrium-based trace-elementfractionation equations. KEY WORDS: accessory minerals; geochemical modelling; granitoids; REE, Y, Th, U     

Vibrational spectra of olivine composition glasses: The Mg-Mn join

The vibrational frequencies of a series of splatquenched, olivine glasses spanning the compositional range from Mg₂SiO₄ to Mn₂SiO₄ have been determined using both infrared and Raman spectroscopies. The spectra of all glasses show evidence of tetrahedral coordination of silicon (possibly with some slight distortions), and largely octahedral coordination of magnesium. Spectra of Mn-rich glasses indicate that there is some manganese in 4 or 5-fold coordination. The frequencies observed for the fundamental vibrations of the silica tetrahedra are similar to those previously observed for SiO₄ groups in both crystalline and glassy orthosilicates. Additionally, there is evidence for a small amount of silicate polymerization in all glasses characterized: vibrations attributable to Si₂O₇ groups are visible in both infrared and Raman spectra.     

The crystal structure of bøgvadite (Na2SrBa2Al4F20)

The crystal structure of bøgvadite, Na₂SrBa₂Al₄F₂₀, has been solved and refined to a R₁ factor of 4.4 % from single-crystal data (MoKα X-ray diffraction, CCD area detector) on a sample from the cryolite deposit at Ivittuut, SW Greenland. Bøgvadite is monoclinic, P2₁/n space group, with unit cell parameters a?=?7.134(1), b?=?19.996(3) and c?=?5.3440(8) Å, β?=?90.02(1)^o. A close proximity of the crystal structure to an orthorhombic symmetry and the presence of the two twin components in a nearly 1:1 ratio suggest that the investigated bøgvadite crystal has originally formed as a high-temperature orthorhombic polymorph which on cooling transformed to the stable low temperature monoclinic structure. The bøgvadite crystal structure has groupings of cation-fluoride coordination polyhedra similar to those found in the crystal structures of the genetically closely associated minerals jarlite and jørgensenite. However, its structure type is different from the latter two. The fluoridoaluminate framework of bøgvadite consists of infinite zig-zag chains of cis-connected AlF₆ coordination octahedra. The ¹ _∞[AlF₅] chains are interconnected by infinite chains of Na-F coordination polyhedra which extend in the same direction. Na is coordinated by nine F atoms if its full surrounding is taken in consideration, but makes significant chemical bonds only to closest five. The chains of AlF₆ and NaF₉ coordination polyhedra form double layers. In the centre of layers, relatively large voids in the form of pentagonal antiprisms are occupied by Sr atoms which make chemical bonds with the closest six F atoms. Between the SrF₁₀ coordinations in the centre of layers run empty channels. The double layers are interconnected by Ba atoms which are coordinated by eight F atoms and fill the spaces between the layers. Bøgvadite belongs to the group of fluoridoaluminates with infinite chains of cis-connected AlF₆ coordination octahedra, alike those found in the crystal structures of Ba-fluoridoaluminates.     

Trace-element composition of hydrothermal zircon and the alteration of Hadean zircon from the Jack Hills,Australia

Hydrothermal zircon can be used to date fluid-infiltration events and water/rock interaction. At the Boggy Plain zoned pluton (BPZP), eastern Australia, hydrothermal zircon occurs with hydrothermal scheelite, molybdenite, thorite and rutile in incipiently altered aplite and monzogranite. The hydrothermal zircon is texturally distinct from magmatic zircon in the same rocks, occurring as murky-brown translucent 20–50 μm-thick mantles on magmatic cores and less commonly as individual crystals. The hydrothermal mantles are internally textureless in back-scatter electron and cathodoluminescence images whereas magmatic zircon is oscillatory zoned. The age of the hydrothermal zircon is indistinguishable from magmatic zircon, indicating precipitation from a fluid evolved from the magma during the final stages of crystallization. Despite indistinguishable U-Pb isotopic compositions, the trace-element compositions of the hydrothermal and magmatic zircon are distinct. Hydrothermal zircon is enriched in all measured trace-elements relative to magmatic zircon in the same rock, including V, Ti, Nb, Hf, Sc, Mn, U, Y, Th and the rare-earth elements (REE). Chondrite-normalized REE abundances form two distinct pattern groupings: type-1 (magmatic) patterns increase steeply from La to Lu and have Ce and Eu anomalies—these are patterns typical for unaltered magmatic zircon in continental crust rock types; type-2 (hydrothermal) patterns generally have higher abundances of the REE, flatter light-REE patterns [(Sm/La)_N = 1.5–4.4 vs. 22–110 for magmatic zircon] and smaller Ce anomalies (Ce/Ce* = 1.8–3.5 vs. 32–49 for magmatic zircon). Type-2 patterns have also been described for hydrothermally-altered zircon from the Gabel Hamradom granite, Egypt, and a granitic dyke from the Acasta Gneiss Complex, Canada.Hadean (∼4.5–4.0 Ga) zircon from the Jack Hills, Western Australia, have variable normalized REE patterns. In particular, the oldest piece of Earth—zircon crystal W74/2-36 (dated at 4.4 Ga)—contains both type-1 and type-2 patterns on a 50 μm scale, a phenomenon not yet reported for unaltered magmatic zircon. In the context of documented magmatic and hydrothermal zircon compositions from constrained samples from the BPZP and the literature, the type-2 patterns in crystal W74/2-36 and other Jack Hills Hadean (JHH) zircon are interpreted as hydrothermally-altered magmatic compositions. An alteration scenario, constrained by isotope and trace-element data, as well as α-decay event calculations, involving fluid/zircon cation and oxygen isotope exchange within partially metamict zones and minor dissolution/reprecipitation, may have occurred episodically for some JHH zircon and at ∼4.27 Ga for zircon W74/2-36. Type-2 compositions in JHH zircon are interpreted to represent localized exchange with a light-REE-bearing, high δ¹⁸O (∼6–10‰ or higher) fluid. Thus, a complex explanation involving “permanent” liquid water oceans, large-scale water/rock interaction and plate tectonics in the very early Archean is not necessary as the zircon textures and compositions are simply explained by exchange between partially metamict zircon and a low volume ephemeral fluid.     

Single-zircon evaporation combined with Pb+ emitter bedding for 207Pb/206Pb-age investigations using thermal ion mass spectrometry,and implications to zirconology

The internal precision of Pb isotope analyses using single-zircon evaporation in a double-filament solid source mass spectrometer (Kober 1986) can be improved combining the evaporation of Pb directly from the single grain with a suitable Pb⁺ emitter-bedding technique. This is most easily done by step-wise evaporating the investigated grain at temperatures of 1700–1800 K generating on the ‘cold’ ionization filament a deposit of radiogenic Pb together with further elements and compounds derived directly from the crystal. The heating of the deposit on the ionization filament to 1400–1500 K results in long-lived and stable Pb⁺ ion beams. The ‘activating reagents’ in the deposit are HfO₂ and SiO₂. Their release from the zircon grain together with the radiogenic Pb, which presumably is sited in the crystalline zircon domains as Pb⁴⁺, is probably due to disintegration reactions of trace-element silicates hosted in the grain. In the bedding deposited on the ionization filament thermally stable Pb/Hf/SiO₂ compounds are formed (PbHfSiO₅(?)). They retain the Pb isotopes on the (Re) filament up to 1400 K–1500 K and are highly efficient Pb⁺ ion emitters similar to the ‘Si-gel’-method (Cameron et al. 1969). The combined evaporation/emitter-bedding technique has been applied to natural zircons of different genesis and to isotope standards. Routinely, a Pb⁺ ion yield of 2*10^?4-1*10^?3 and a relative standard deviation of the ²⁰⁷Pb/²⁰⁶Pb ratio in the order of 1% have been obtained for sub-ng- to ng-amounts of Pb from standards and samples. The method rapidly can yield Pb isotope information on the ‘concordant’ zircon phases with a standard deviation of ±15–20 Ma of the derived ages also in the case of Paleozoic zircon populations.     

Molecular orbital (MSXα) calculations of s-electron densities of tetrahedrally coordinated ferric iron: Comparison with experimental isomer shifts

The MSXα method has been used to calculate the s-electron densities at the nucleus for tetrahedrally coordinated ferric iron, (FeO₄)^5?, comparing the observed increase in isomer shift values with increasing Fe-O separation. The results give an isomer shift calibration constant of ?0.3 (a.u. mm×s^?1) assuming a constant ratio for the iron and oxygen sphere radii for the different polyhedra sizes. It is suggested that increasing bonding distances in tetrahedral coordination polyhedra are the dominant factors determining the value of the isomer shifts in Fe-Mg-silicates.