Single Zircon Crystals Research Articles

The age of the basement of Arctic Alaska – Chukotka microplate on the example of the Velitkenay granite-migmatitic dome (NE Russia)

The basement of Arctic Alaska – Chukotka microcontinent (AACHM) is exposed in Velitkenay granite-metamorphic dome of Chukotka. U-Pb (SIMS) geochronological study of the orthogneisses of the basement, paragneisses after Paleozoic rocks and leucogranites of Velitkenay pluton in the central and southwestern parts of this structure is carried out. Neoproterozoic ages of basement orthogneisses (velitkenay complex) and Albian ages of granitoids of Velitkenay pluton are confirmed. The latter concides with the age of orthogneisses migmatization. The youngest zircon clasters from paragneisses within velitkenay complex have Cambrian and Silurian ages, single zircon crystals contain inherited 1.7 and 1.9 Ma cores. The сorrelation of Neoproterozoic orthogneisses of Velitkenay granite-metamorphic dome with similar complexes of the other AACHM regions indicates unity of its basement. It is offered to enter velitkenay metamorphic complex into the legend of Chukotka series of the third-generation geological map sheets.

Luminescence properties of zircon single crystals doped with europium and vanadium

Crystals of ZrSiO4; ZrSiO4: Eu3+; and ZrSiO4: (Eu3++V) have been grown by the flux method. Their structural and luminescent properties have been studied demonstrating heterogeneity in cathodoluminescence (CL) for all samples. Such a heterogeneity of CL in ZrSiO4; ZrSiO4: Eu3+ is related to irregular distribution of point defects associated presumably with oxygen vacancies. Uneven distribution of vacancies as assumed led to the formation of different sites for europium ion. Moreover, in different areas of the crystal matrix the number of sites and their types vary. Activation of zircon with europium and vanadium simultaneously caused an increase of europium incorporation in comparison with zircon doped with europium only. It was found that heterogeneity of CL properties of ZrSiO4: (Eu3++V) is related to the irregular distribution of vanadium in crystal matrix. Polarization studies of CL spectra suggest that there are at least four europium sites in ZrSiO4: (Eu3++V).

Single crystal zircon doped with europium and vanadium simultaneously in order to support intensive luminescence under UV-light

Crystals of zircon, ZrSiO4, doped with Eu and V simultaneously have been grown for the first time by the flux method. The use of both (Eu + V) has supported higher Eu3+ incorporation into zircon lattice and respective increase of luminescence intensity under UV-light (220 and 365 nm) in comparison with zircon doped with Eu3+ only. The irregular distribution of vanadium and appearance of blue color in some zones of crystal matrix have been observed and studied using optical and scanning electron microscopy, electron probe microanalysis, cathodoluminescence imaging and spectroscopy. Further research is proposed on study of valence states of vanadium in zircon lattice and possible improvement of synthesis conditions in order to avoid formation of blue color suppressing the luminescence.

Local Structure, Structural, Vibrational, and Optical Properties of Natural Zircon

Structural properties of Indian metamict single zircon crystals are studied by comparing their properties with those of synthetic zircon synthesized via solid‐state reaction method, which serves as the standard. X‐Ray diffraction (XRD) data reveal changes in lattice parameters and the presence of strain, metamictization, etc. It is found that the degree of crystallinity is ≈63.8% indicating the simultaneous presence of crystalline and disordered regions. These changes are likely due to the presence of several radioactive/nonradioactive elements. The presence of such elements is observed and the sample composition is analyzed through total reflection X‐Ray fluorescence. XRD crystallographic data are further utilized as input for analyzing X‐Ray absorption spectroscopy data recorded around Zr central absorbing atom. X‐Ray absorption near‐edge structure shows slight modification in the white line feature and some disorder compared to the synthetic one. Reduced coordination number observed from extended X‐Ray absorption fine structure suggests the presence of disorder in local structure. Micro‐Raman spectroscopy indicates that crystalline behavior varies across different locations, suggesting that the sample has undergone varying degrees of metamictization, confirming a nonuniform distribution of elements. The work documents and analyses structural evolution within isolated silica tetrahedron network in association of Zr4+ cation as a function of self‐irradiation and recovery over millions of years and correlating the same with optical properties.

Nanoscale imaging of Fe-rich inclusions in single-crystal zircon using X-ray ptycho-tomography

We apply X-ray ptycho-tomography to perform high-resolution, non-destructive, three-dimensional (3D) imaging of Fe-rich inclusions in paleomagnetically relevant materials (zircon single crystals from the Bishop Tuff ignimbrite). Correlative imaging using quantum diamond magnetic microscopy combined with X-ray fluorescence mapping was used to locate regions containing potential ferromagnetic remanence carriers. Ptycho-tomographic reconstructions with voxel sizes 85 nm and 21 nm were achievable across a field-of-view > 80 µm; voxel sizes as small as 5 nm were achievable over a limited field-of-view using local ptycho-tomography. Fe-rich inclusions 300 nm in size were clearly resolved. We estimate that particles as small as 100 nm—approaching single-domain threshold for magnetite—could be resolvable using this “dual-mode” methodology. Fe-rich inclusions (likely magnetite) are closely associated with apatite inclusions that have no visible connection to the exterior surface of the zircon (e.g., via intersecting cracks). There is no evidence of radiation damage, alteration, recrystallisation or deformation in the host zircon or apatite that could provide alternative pathways for Fe infiltration, indicating that magnetite and apatite grew separately as primary phases in the magma, that magnetite adhered to the surfaces of the apatite, and that the magnetite-coated apatite was then encapsulated as primary inclusions within the growing zircon. Rarer examples of Fe-rich inclusions entirely encapsulated by zircon are also observed. These observations support the presence of primary inclusions in relatively young and pristine zircon crystals. Combining magnetic and tomography results we deduce the presence of magnetic carriers that are in the optimal size range for carrying strong and stable paleomagnetic signals but that remain below the detection limits of even the highest-resolution X-ray tomography reconstructions. We recommend the use of focused ion beam nanotomography and/or correlative transmission electron microscopy to directly confirm the presence of primary magnetite in the sub 300 nm range as a necessary step in targeted paleomagnetic workflows.

Reconstructing the thermal history of the Morondava Basin, Madagascar, after Gondwanan breakup as resolved through detrital zircon (U–Th)/He and U–Pb geochronology

Formation of the Phanerozoic basins of Madagascar coincided with the initial stages of Permian break-up of Gondwana. The largest of these, the Morondava Basin, records the tectonic history of rifting, drifting, and uplift of the Malagasy terrane over the last 300 Myr. Herein, we have applied detrital zircon U–Pb geochronology and (U–Th)/He low-temperature thermochronology to resolve the thermal history of the basin as Madagascar separated from Africa and subsequently India. Coarse-grained siliciclastic samples were taken along two transects parallel to the Morondava River in the central basin, including from the Permo-Triassic failed rift basin and from post-Jurassic passive margin sequences. Detrital zircon U–Pb age distributions are defined by common Neoarchean and Neoproterozoic populations which suggests input from the basement terranes of the Malagasy Central Highlands (Antananarivo domain). Samples also contain Paleo-to Mesoarchean and Cambrian populations likely associated with metamorphic terranes amalgamated during the Pan-African orogeny. Single crystal zircon (U–Th)/He dates are 503 ± 17 Ma to 40 ± 1 Ma with effective uranium (eU) values ranging from 35 to 1760 ppm, which exhibit a strong negative date-eU relationship that indicates partial resetting of the He systematics in zircon throughout the basin. Inverse and forward thermal history models from the data reveal separate thermal histories for strata on either side of basin-dividing Illovo Fault. Under an elevated geothermal gradient, temperatures did not exceed 180 °C either during post-Gondwanan burial of Jurassic rift strata or during Late Cretaceous burial of Early Cretaceous passive margin strata. Intrusion of Turonian-age dikes and sills related to the Marion hot spot likely promoted subtle yet widespread heating. The time of latest cooling during the Paleogene recorded in the basin is poorly constrained by the detrital zircon thermochronology.

Thermoelastic properties of non-metamict zircon

Abstract The thermoelastic properties and thermal expansion of natural single-crystal zircon without detectable radiation damage were determined in the temperature range between 100 and 1650 K on five samples from Cambodia, Sri Lanka, and Tanzania. Up to about 1500 K, the results are in excellent agreement and fully reversible. The average adiabatic elastic stiffness coefficients at room temperature in GPa are c11 = 426.8(6), c12 = 68.1(4), c13 = 152.0(4), c33 = 490.8(9), c44 = 113.1(2), and c66 = 49.0(1). Polycrystalline averaging for the aggregate bulk and shear modulus (Voigt-Reuss-Hill average) yields K = 229.3(4) GPa and G = 109.3(1) GPa. From 293 to 1573 K, the elastic stiffnesses soften almost linearly with the temperature coefficients in MPa/K: dc11/dT = –45.36(7), dc12/dT = –2.03(7), dc13/dT = –9.59(7), dc33/dT = –42.85(12), dc44/dT = –9.60(3), and dc66/dT = –2.19(1). Below room temperature, the behavior of the cij is nonlinear, with decreasing absolute values of the temperature coefficients. Above about 1500 K, a temperature- and time-dependent irreversible elastic stiffening accompanied by an increasing ultrasound dissipation was observed in all samples. These anomalies are interpreted to be the consequence of increased dislocation densities.

Tracing magmatic genesis and evolution through single zircon crystals from successive supereruptions from the Socorro Caldera Complex, USA

Large volume rhyolitic ignimbrite volcanism is a significant contributor to the evolving crust. The introduction of high-silica material into the upper crust, differentiation within the middle crust, and partial melting in the lower crust contributes to geochemical and isotopic evolution of the crust. Developing accurate models for the genetic evolution of these events is dependent upon geochronology to determine rates of magmatic processes as model constraints. We present new zircon high-precision CA-ID-TIMS U-Pb geochronology and MC-ICPMS Hf isotope geochemistry for four ignimbrites from the nested caldera complex near Socorro, New Mexico (USA), within the Mogollon-Datil volcanic field. In agreement with past 40Ar-39Ar data, interpretations of new U-Pb data indicate eruptions from the Socorro caldera cluster were pulsed. These pulses were intermittently spaced, and a volcanic hiatus following the Hells Mesa Tuff at 33.442 ± 0.015 Ma was interrupted by four successive eruptions, beginning with the La Jencia Tuff at 29.158 ± 0.025 Ma and finishing with the South Canyon Tuff at 28.066 ± 0.021 Ma. Zircon age spectra became more protracted with each eruption, exhibiting age dispersions ranging from 0.347 Myr in the Hells Mesa Tuff to 4.502 Myr in the South Canyon Tuff. The increased dispersion is paralleled by an increase in the proportion of normally discordant grains, indicative of xenocryst incorporation. These protracted age spectra are not necessarily a function of thermal maturation in the middle to upper crust due to long-lived magma chambers. Rather, they are likely the result of increased melting of zircon-bearing lower crust due to deep thermal maturation from repeated juvenile magma injections based on the incorporation of zircon material at the melt source. In contrast, the Hf isotope record is volumetrically dominated by autocrystic zircon domains and becomes more radiogenic through time, recording juvenile replenishment of the lower crust during progressive melting. Together, these data record the protracted evolution of the lower crust sampled by ignimbrites, lend insight into that evolution, and emphasize the need for detailed interpretation of high-precision datasets to advance volcanic models.

Experimental Equilibrium and Fractional Crystallization of a H2O, CO2, Cl and S-Bearing Potassic Mafic Magma at 1.0 GPa, With Implications for the Origin of Porphyry Cu (Au, Mo)-Forming Potassic Magmas

ABSTRACT The origin of intermediate to felsic potassic magmas is debated, and not much is known about the volatile content of potassic magmas associated with porphyry Cu (Au, Mo) deposits. To better understand the liquid line of decent of mafic potassic magmas and the behavior of volatiles during magma differentiation, we performed 19 experiments at 1.0 GPa and 1150 °C to 850 °C using piston cylinder presses. We developed a new experimental technique that involves a capsule liner made of single-crystal zircon to prevent the loss of Fe and S in the starting material to the noble metal capsule. The starting material is a high-Mg, basaltic trachyandesite (52 wt% SiO2, 12 wt% MgO, 1.9 wt% Na2O and 5.3 wt% K2O) from the Sanjiang region in southwestern China, doped with geologically realistic amounts of volatiles (i.e. 4.0 wt% H2O, 0.34 wt% CO2, 0.27 wt% Cl and 0.25 wt% S). The addition of 0.25 wt% S in the form of anhydrite internally buffered the experiments at an oxygen fugacity of 2.0 ± 0.5 log units above the fayalite–magnetite–quartz buffer, which is similar to the redox state of the Sanjiang variously evolved potassic magmas. The experimentally produced silicate melts match well with the Sanjiang intermediate to felsic magmas in terms of major, minor and trace element compositions, and also with regard to the S and Cl contents. The sequence of crystallizing minerals (olivine + clinopyroxene –> biotite ± orthopyroxene –> apatite –> K-feldspar) also fits with the one observed in the Sanjiang mafic to intermediate magmas. These results suggest that the Sanjiang intermediate to felsic magmas, including the porphyry Cu (Au, Mo)-forming magmas, can form solely by differentiation of potassic mafic magmas without any involvement of old crustal material. During experimental differentiation at 1.0 GPa, the S content of the evolving silicate melt first increased until ~57 wt% melt SiO2, and then decreased in response to precipitation of sulfides, sulfate melt and/or anhydrite, whereas the H2O and Cl contents of the evolving silicate melt increased exponentially until saturation in a CO2-rich fluid was reached at 60 to 65 wt% melt SiO2 and ~ 8 wt% melt H2O. During further magma differentiation the H2O and Cl contents of the evolving silicate melt remained constant until ~70 wt% melt SiO2, after which point the Cl content of the silicate melt decreased due to increased partitioning of Cl into the fluid phase ± increased fluid/melt ratio. Based on these experimental results and petrographic and geochemical evidence from natural samples, the Sanjiang porphyry Cu (Au, Mo)-forming magmas (65–70 wt% SiO2) are interpreted to have formed through differentiation of primitive, mantle-derived, potassic magmas in the lower crust (≥1.0 GPa), and to have ascended ±directly from the lower crust to shallow crustal levels. They likely contained 8 to 13 wt% H2O, 0.37 to 0.90 wt% Cl and 0.07–0.29 wt% S. This case study on the magma evolution in the Sanjiang region may have implications for the origin and nature of intermediate to felsic potassic magmas in various tectonic settings.

Brillouin light scattering in niobium doped lead zirconate single crystal

Brillouin light scattering experiments were performed for lead zirconate single crystals doped with niobium. Special attention was paid to the elastic mode softening near phase transition temperatures. The results are compared with data obtained by Raman light scattering experiments. We observed that the interaction between acoustic and optic modes is responsible for symmetry breaking far above TC, leading to polar regions' appearance. No changes in the acoustic mode frequency and its damping are observed at TC, where ε(T) exhibits a maximum value. The absence of these changes and the central peak observed in Raman experiments suggest that the phase transition at TC is mainly of the order–disorder type. The origin of other phase transitions is discussed as well.

Pleistocene volcanism along the margins of the Canal de Ballenas transform fault, Gulf of California

Pleistocene subaerial volcanism along the margins of the Ballenas Channel, northern Gulf of California, is represented by two morphologically young dacite dome complexes exposed at the opposite edges of sheared continental crust: Isla Coronado in coastal Baja California, and the Lobera volcanic complex in west-central Isla Ángel de la Guarda. Single crystal zircon U-Pb crystallization ages of Coronado and Lobera volcanoes range between ca. 250 and 1000 ka, indicating maximum ages for the eruptions. Eruption ages are directly constrained by an 40Ar/39Ar whole-rock age of 692 ± 164 ka for one of the lava units in the Lobera volcanic complex. Trace elements in Pleistocene zircon indicate continental affinity, which supports radiogenic (Nd, Sr) isotopic data that were modeled using different mixing and AFC scenarios indicating a MORB-type primary magma with significant (~10–20 %) crustal assimilation involving tonalitic basement of the eastern Peninsular Ranges Batholith.

Rapid screening of Zr-containing particles from Chang’e-5 lunar soil samples for isotope geochronology: Technical roadmap for future study

New samples returned by China Chang’e-5 (CE-5) mission offer an opportunity for studying the lunar geologic longevity, space weathering, and regolith evolution. The age determination of the CE-5 samples was among the first scientific questions to be answered. However, the precious samples, most in the micrometer size range, challenge many traditional analyses on large single crystals of zircon developed for massive bulk samples. Here, we developed a non-destructive rapid screening of individual zirconium-containing particle for isotope geochronology based on a Micro X-ray fluorescence analysis (µXRF). The selected particles were verified via scanning electron microscopy (SEM), 3D X-ray microscopy (XRM), and focused ion beam scanning electron microscopy (FIB-SEM) techniques, which showed that zirconium-bearing minerals with several microns were precisely positioned and readily suitable for site-specific isotopic dating by second ion mass spectrometry (SIMS). Such protocol could be also applicable in non-destructively screening other types of particles for different scientific purposes. We therefore proposed a correlative workflow for comprehensively studying the CE-5 lunar samples from single particles on nanometer to atomic scales. Linking various microscopic and spectromicroscopic instruments together, this workflow consists of six steps: (1) single-particle selection with non-destructive µXRF technique, (2) 2D/3D morphological and structural characterization with a correlative submicron 3D XRM and nanoscale resolution FIB-SEM imaging methods, (3) SEM analysis of the surface morphology and chemistry of the selected particle, (4) a series of microscopic and microbeam analyses (e.g., SEM, electron probe microanalysis, and SIMS) on the cross-section of the selected particle to obtain structural, mineralogical, chemical, and isotopic features from the micron to nanometer scale, (5) advanced 2D/3D characterization and site-specific sample preparation of thin foil/tip specimens on a microregion of interest in the selected particle with FIB-SEM technique, and (6) comprehensive analyses on the FIB-milled specimens at nanometer to atomic scale with synchrotron-based scanning transmission X-ray microscopy, analytic transmission electron microscopy, and atom probe tomography. Following this technical roadmap, one can integrate multiple modalities into a uniform frame of multimodal and multiscale correlated datasets to acquire high-throughput information on the limited or precious terrestrial and extraterrestrial samples.

Zircon of Triassic Age in the Stuttgart Formation (Schilfsandstein)—Witness of Tephra Fallout in the Central European Basin and New Constraints on the Mid-Carnian Episode

The Carnian Stuttgart-Formation (Schilfsandstein) of the Central European Basin contains relics of Triassic volcanic detritus in form of euhedral zircon grains and authigenic analcime. Multiple LA-ICP-MS spot analyses of single zircon crystals from an outcrop near Heilbronn (SW Germany) yielded weighted average 206Pb/238U ages between 250 and 230 Ma, providing evidence for tephra fallout in the southern part of the Central European Basin related to Olenekian, Anisian–Ladinian and Carnian volcanic activity. The tephra was probably transported by monsoonal circulations from volcanic centres of the NW Tethys to the Central European Basin. The four youngest zircon crystals gave a weighted average 206Pb/238U age of 231.1 ± 1.6 Ma (10 analyses), which is interpreted to date syn-depositional tephra fallout into the fluvial Lower Schilfsandstein Member of the Stuttgart Formation. This new maximum depositional age provides the first evidence that deposition of the Stuttgart Formation, which represents the type-example of the mid-Carnian episode, a global episode of enhanced flux of siliciclastic detritus and related environmental perturbations, occurred during the Tuvalian 2 substage at ca. 231 Ma, about 3 million years later than suggested by previous correlations. Zircon grains with weighted average 206Pb/238U ages of 236.0 ± 1.2 Ma (n = 17) and 238.6 ± 1.5 Ma (n = 6) and 206Pb/238U ages between 241 ± 6 and 250 ± 3 Ma point to the presence of tephra in early Carnian to Olenekian strata of the Keuper to Buntsandstein Groups. Traces of these reworked tephra were incorporated into the Stuttgart Formation due to fluvial erosion in the southern Central European Basin and at its margins.

Optical emission of shocked magnesium oxide single crystals: Heat-conduction and orientation effects

To investigate light emission properties of shocked high-conductivity single crystals during plastic deformation, [100] and [110] magnesium oxide (MgO) single crystals are shock-compressed up to 64 GPa via a planar impact. [111] gadolinium–gallium garnet (GGG) and [100] Y-stabilized zirconate (YSZ) single crystals, respectively, with intermediate and low conductivities are also examined for comparison. Emission spectral radiance and velocity histories are simultaneously measured with a fast, high-sensitivity pyrometer and a Doppler pin system. While velocity histories of [100] MgO, [111] GGG, and [100] YSZ show typical two-wave structures at elastic–plastic transition, abnormal three-wave structures are observed for [110] MgO, consistent with literature reports. Correspondingly, spectral radiance histories of [100] MgO, [111] GGG, and [100] YSZ under each tested stress demonstrate a typical “single-wave” structure, while a “two-wave” structure is observed for [110] MgO shocked to 33–37 GPa. The peak intensities of spectral radiance for both [100] and [110] MgO are much lower than those of GGG and YSZ shocked to similar plastic strains as a result of different heat conductivities. A model, taking into account heat conduction from a shear band into its surrounding bulk areas and two-zone emission, is thus developed and describes well the measurements. Our observations make a concrete connection between shear banding and localized heating/cooling as proposed previously.

LOWER AGE BOUNDARY OF THE FORMATION OF METATERRIGENOUS ROCKS OF THE VYSOKOPILLYA GREENSTONE STRUCTURE, MIDDLE!DNIEPER REGION OF THE UKRAINIAN SHIELD

The stratigraphic relationships of meta-terrigenous rocks of the Kryvyi Rih Series belonging to the Kryvyi Rih-Kremenchuk structure with metasediments of similar composition belonging to the upper part of the greenstone belt section (the Bilozerka Series) in the Middle Dnieper Domain remain unexplored. The adjacent “strip-like” Vysokopillya, Skhidno- Hannivska and Zhovta Richka structures are of particular interest; in these structures, metasediments and ferruginoussiliceous rocks overlie metabasites of the Mesoarchean greenstone belts. The Vysokopillya greenstone belt (GB), which is adjacent to the Likhmanivka syncline of the Kryvyi Rih structure, is a steep-bedding monocline. The northern part of the belt comprises a thick homogeneous meta-tholeiite sequence (the Sura Suite). The Chertomlyk Suite is located higher in the stratigraphic section and includes meta-andesites with subordinate rhyodacites and tholeiitic basalts. They form the southern part of this structure. Subvolcanic rhyolite-dacite bodies (the Solenivka Suite) occur as crosscutting intrusions and dykes. The middle and southern parts of the Vysokopillya GB consist of a heterogeneous mass of orthogneisses and paragneisses, which developed after sedimentary (graywackes, sub-graywackes and melawackes) and volcanic (tuff sandstones, lava breccias, meta-dacites and meta-andesites with a subordinate amount of meta-tholeiites) rocks were deposited. Graywackes host thin beds of quartz-magnetite quartzites and thick (up to 100 m) beds of quartzmagnetite- cummingtonite shales with garnet. We applied LА-ICP-MS for U-Pb dating of 102 detrital zircons from biotite-quartz meta-sandstones and determined their Hf isotope compositions. Among them, four crystals have 207Pb/206Pb age over 3400 (up to 3800) Ma. A single zircon crystal was dated at 2930 Ma. The remaining 97 crystals have ages in the range of 3000-3360 Ma, with distinct peaks at 3065 (15 grains), 3185 (21 grains), 3260 (16 grains), and 3320 Ma (22 grains). Hafnium isotope composition varies over a wide range. The oldest grain dated at 3800 Ma, has eHf = –0,8, whereas the next two younger grains have positive eHf values (+1,2 at 3645 Ma, and +2,2 at 3535 Ma). Crystals dated at ca. 3320 Ma demonstrate wide variations in eHf — from +1,8 to -6,1; those dated at ca. 3260 Ma — from +4,5 to –2,5, those dated at ca. 3185 Ma — from +3,3 to –4,3, and those dated at ca. 3065 Ma — from +6,2 to –3,9. According to the obtained data, metasedimentary rocks of the Vysokopillya GB started to accumulate in the Neoarchean. The same age was established for the metamorphosed sedimentary rocks of the Skhidno-Hannivska and Krasnoyarsk-Alferivka structures, as well as for the Bilozerka Series of the Bilozerka GB. Thus, sedimentary rocks in these structures were formed during the denudation of the Mesoarchaean craton. The source rocks were predominantly represented by the TTG of the Sura complex (3.1-2.96 Ga). For the first time, a significant number (38 grains) of Paleoarchean (3.3 Ga) detrital zircons were discovered in the Middle Dnieper Domain, which indicates the presence of Paleoarchean rocks in the denudation area. Data on Hf isotope systematics indicate the possible presence of Hadean (older than 4.0 Ga) material.

Thermochronology and exhumation history of the basement and sediments of the NNE border of the Paraná basin, Brazil

Combined zircon fission-track (FT) and U–Pb dating were used to obtain correlated cooling ages and tectonic processes in the eastern border of the Phanerozoic Paraná Basin, a large cratonic sedimentary basin in South America deposited over Precambrian basement, which is represented mainly by the Neoproterozoic Brasília Orogenic Belt. Zircon ages obtained for clastic sediments suggest that the provenance history of the Paraná Basin was derived chiefly from the proximal basement sources. Twelve zircon samples were collected, from which 194 grains were analyzed and simultaneously dated by the FT and U–Pb methods, and 146 zircon grains were additionally dated only through the fission-track method (FT). The standard sample of Fish Canyon Tuff (FCT) was also analyzed through FT and U–Pb, and the age results of 26.6 ± 1.1 Ma, and 28.33 ± 0.68 Ma, respectively, are consistent with the literature data. The fission-track ages, obtained for the Araxá (~330 Ma) and Canastra (~500 Ma) Groups, and Piumhi Massif (~700 Ma) pertaining to the Brasília Belt, in average increase towards the São Francisco Craton. This fission-track age pattern can be linked to the temperature and exhumation of each one of these units. The U–Pb detrital zircon ages obtained for the basement units range from 1.6 to 3.2 Ga and indicate different provenance sources in the evolution of the Brasília Belt. However, it is clear that the main sources of metasedimentary rocks are derived from the São Francisco-Congo Craton. For samples collected in the NNE border of the Paraná Basin, FT ages were obtained for 127 zircon grains with ages ranging between 100 and 600 Ma. Forty of these grains dated simultaneously with FT and U–Pb bear mean age populations of 0.6 and 1.0 Ga, and subordinately between 1.5 and 2.5 Ga. These results suggest that the units that represent the São Francisco Craton's Neoproterozoic passive margin were being formed with depositional sediments 1.0 Ga ago and their recycling was the main source for the zircon derivation recorded in the Paraná Basin sediments. An important methodological result of this work is that U–Pb radiometric dating was done in the same areas of single zircon crystals employed to determine the FT ages, which improved the concordance in the U–Pb ages. As such, U–Pb dating on zircon micro areas that have undergone leaching, metamictization or another type of lattice damage was avoided.

Integrated U–Pb, Lu–Hf and (U–Th)/He analysis of zircon from the Banxi Sb deposit and its implications for the low-temperature mineralization in South China

Low-temperature Sb (Au–Hg) deposits in South China account for more than 50% of the world’s Sb reserves, however, their genesis remains controversial. Here we report the first study that integrates U–Pb and Lu–Hf analysis by LA-(MC)-ICPMS and conventional (U–Th)/He analysis, all applied to single zircon crystals, in an attempt to constrain the origin and timing of world-class Sb (Au–Hg) deposits in Banxi (South China). Zircon separated from a quartz-stibnite ore and an altered country rock samples revealed similar U–Pb age spectra defining two major populations – Paleoproterozoic (~1900–2500 ​Ma) and Neoproterozoic (~770 ​Ma), which are characterized by variable εHf(t) values (–10.7 to 9.1 and –16.5 to 11.2, respectively) and Hf crustal model ages (TDMC) (2.48 to 3.24 ​Ga and 0.97 to 2.71 ​Ga, respectively). The U–Pb age and Hf isotopic features of the zircons are consistent with the Banxi Group in the region, indicating that the zircons involved in the low-temperature hydrothermal system were originally from the Banxi Group country rocks. Thirty-three mineralization-related zircon crystals yielded a mean (U–Th)/He age of 123.8 ​± ​3.8 ​Ma, which is interpreted to represent the timing of the latest low-temperature mineralization stage of the Banxi Sb deposit. The combined U–Pb, Lu–Hf and (U–Th)/He data suggest that Precambrian basement rocks were the major contributors to the low-temperature mineralization, and that Early Cretaceous (130–120 ​Ma) could be the most important ore-forming epoch for the Sb deposits in South China. This study also demonstrates the analytical feasibility of integrated U–Pb - Lu–Hf - (U–Th)/He “triple-dating”, all applied to single zircon crystals. This approach reveals the full evolution of zircon, from its origin of the magmatic source, through its crystallization and low-temperature cooling. Although this study demonstrates the usefulness of this integrated approach in dating low-temperature mineralization, it has great potential for zircon provenance and other studies that may benefit from the large amount of information that can be extracted from single zircon crystals.

LKZ-1: A New Zircon Working Standard for the In Situ Determination of U–Pb Age, O–Hf Isotopes, and Trace Element Composition

This study introduces a new zircon reference material, LKZ-1, for the in situ U–Pb dating and O–Hf isotopic and trace element analyses. The secondary ion mass spectrometric analyses for this gem-quality single-crystal zircon yielded a weighted mean 206Pb/238U age of 572.6 ± 2.0 Ma (2σ, n = 22, MSWD = 0.90), with moderately high U concentrations (619 ± 21 ppm, 1 SD), restricted Th/U ratios (0.146 ± 0.002, 1 SD), and negligible common Pb content (206Pbc < 0.2%). A comparable 206Pb/238U age (570.0 ± 2.5 Ma, 2σ) was produced by the isotope dilution-thermal ionization mass spectrometry. The secondary ion mass spectrometric and laser ablation-assisted multiple collector inductively coupled plasma mass spectrometer analyses respectively showed that LKZ-1 had little variation in O (δ18OV-SMOW = 10.65 ± 0.14‰; laser fluorination value = 10.72 ± 0.02‰; 1 SD) and Hf (176Hf/177Hf = 0.281794 ± 0.000016, 1 SD) isotopic compositions. LKZ-1 was also fairly homogeneous in its chemical composition (RSD of laser ablation ICPMS data ≤ 10%), displaying a relatively uniform chondrite-normalized rare earth element pattern ((Lu/Gd)N = 31 ± 3, Eu/Eu* = 0.43 ± 0.17, Ce/Ce* = 44 ± 32; 1 SD). These consistencies suggest that the LKZ-1 zircon is a suitable working standard for geochronological and geochemical analyses.

Intrusive reawakening of El Chichón volcano prior to its Holocene eruptive hyperactivity

El Chichón volcano in southeastern Mexico (Chiapas) has produced compositionally homogeneous trachyandesite throughout its ca. 372 kyr eruptive history, but crystal-scale heterogeneity reveals a complex interplay between magmas generated and stored at different crustal levels. High-sensitivity ion microprobe 238U-230Th zircon dating from the last two eruptions of El Chichón (1982 and 550 BP) demonstrates that most zircon crystallized between 0 and ca. 40 ka. Few zircon crystals date back to ca. 130 kyr, and none to the Middle Pleistocene phase of eruptions. Trace elements and oxygen isotope variations in zircon indicate fairly stable conditions in the magma chamber between ca. 20 and 130 ka, with average Ti-in-zircon temperatures estimated at 712 °C and δ18O values of +6.6‰. However, excursions to high Ti-in-zircon temperatures and high δ18O values in zircon occurred at ca. 24 ka. These excursions also mark the onset of subsequent progressive changes to occasionally higher Ti-in-zircon temperatures, lower δ18O values, and more primitive magma compositions (e.g., high Eu/Eu*, Zr/Hf and Th/U). Systematic rim-ward decrease in δ18O is recorded by juvenile zircon domains which quasi-continuously crystallized for up to 30 kyr prior to eruption. Textural relationships and O-isotopic variations within single zircon crystals are thus evidence for magma recharge causing initial resorption of xenocrystic zircon, rapid zircon re-saturation driven by local supersaturation, and effective magma mixing with sediment-contaminated high-δ18O melts from a boundary layer at the contact of the magma reservoir to country rock. Initial thermal and compositional defrosting of an intermediate boundary layer and its crystal cargo may have been rapid due to the brief duration of high-temperature recharge events, as evidenced by the preservation of zircon xenocrysts. The delay of at least 12 kyr between major recharge episodes recorded by zircon and the onset of Holocene eruptions at brief recurrence intervals suggests that frequent eruptions only started when trachybasaltic magma recharge continued or intensified during the Holocene.

238U/235U measurement in single-zircon crystals: implications for the Hadean environment, magmatic differentiation and geochronology

We present novel analytical methods allowing the precise determination of the uranium isotopic composition (238U/235U) of single zircon crystals.