Zircon Crystals Research Articles

High-precision zircon age spectra record the dynamics and evolution of large open-system silicic magma reservoirs

The emplacement history and thermal evolution of subvolcanic magma reservoirs determine their longevity, size, and ability to feed volcanic eruptions. As zircon saturation is dependent on melt temperature and composition, quantitative analysis of zircon age distributions provides insight into the timing and magnitude of intensive parameter variations during the lifetime of a magma reservoir. Here we present chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon crystallization ages and in-situ zircon trace-element geochemistry for a suite of caldera-related, coeval plutonic and volcanic units of the Permian Sesia Magmatic System (northern Italy). This dataset documents the protracted growth and evolution (∼1 Myr) of a voluminous (>1000 km3), upper crustal (3.5–2.0 kbar) silicic magma reservoir. Systematic changes in zircon composition with time reveal episodic intrusions of new magma into a single, progressively differentiating reservoir which was dominantly kept at high crystallinity conditions (>60 vol.%). The volcanic and plutonic units both show dispersed and heterogeneous CA-ID-TIMS age distributions. A stochastic, thermodynamics-based zircon saturation model accounting for fractional crystallization, recharge and thermal rejuvenation effects on zircon growth and stability in a rhyolitic magma body reproduces the observed distributions of zircon U-Pb ages. Model results suggest that zircon compositional variability and crystallization age heterogeneity, characteristic of the Sesia and other large silicic systems, distinguish open- and closed-system magmatic processes. Specifically, an increase of the mass of crystallized zircons over time is the hallmark of thermally and chemically mature magma reservoirs, capable of producing voluminous eruptions upon changes in the thermal and mechanical state of the system. Conversely, a decrease in crystallized zircon mass over time characterizes systems where crystallization dominates over magma mobility, hindering substantial mass release during eruptive events. We suggest that quantitative analyses of dispersed zircon crystallization age distributions obtained with high-precision techniques can be applied to the plutonic and volcanic record to identify mature silicic magma reservoirs whose properties and storage conditions allowed for catastrophic caldera-forming eruptions. This work provides the petrologic community with a new tool to investigate the genesis and evolution of zircon-bearing intermediate to silicic intrusive magmatism and caldera-forming volcanism through time and across tectonic settings.

Trace elements in ilmenite, titanomagnetite and apatite unravel the petrogenesis of Fe-Ti-P-(+/-Zr) rich rocks and associated nelsonite from the Raftsund intrusion, Vesterålen-Lofoten AMCG suite, Northern Norway

The formation of Fe-Ti-P- (+/− Zr)-rich rocks and nelsonite (Fe-Ti oxide-apatite +/− zircon-rich rocks mostly devoid of silicates) remains poorly understood. Accumulation of mafic minerals during the evolution of magma by fractional crystallization and silicate-liquid immiscibility are the two main processes considered. However, the latter process is difficult to identify in intrusive rocks as melt compositions are rarely preserved. In this study, we analyzed the trace element content of ilmenite, titanomagnetite, and apatite from the Raftsund intrusion, a 1800 Ma monzonitic to syenitic batholith belonging to the Lofoten-Vesterålen anorthosite-mangerite-charnockite-granite suite in order to test whether liquid-immiscibility was responsible for the formation of Fe-Ti-P-rich rocks. In addition, we extend the study to a new nelsonite occurrence. Detailed microtextural characterization of apatite in the monzonite shows evidence of metasomatic alteration in the form of thin rims of allanite, whereas apatite in the nelsonite contains small monazite inclusion along fractures. Ilmenite and titanomagnetite, not affected by later events, record enrichments in elements that should have behaved compatibly (Mg, Sc, Co, Cu, V) and incompatibly (Zr, Mn, Zn) in a fractionating system where zircon and sphalerite are absent from the mineral assemblage. This observation supports the liquid immiscibility hypothesis. Similar enrichments are observed in other parts of the system, including the nelsonite. However, the high field strength elements are affected by the stability and crystallization of abundant zircon and ilmenite, resulting in lower concentration of these element in the Fe-Ti oxides. The Fe-Ti oxides in the nelsonite record enrichments that are more pronounced than the Fe-Ti oxides in the surrounding rocks, which indicate that not only liquid immiscibility is responsible for the genesis of the nelsonite, but that the Fe-rich melt from which the Fe-Ti oxides crystallized was more enriched in trace elements than the Fe-Ti-P-rich rocks in the rest of the intrusion. Enrichment in transition metals (Co, V, Cu) is related to mingling with a less evolved monzodioritic magma, which most likely triggered the liquid immiscibility that formed the nelsonite. Fractionation and segregation of Fe-rich silicates, such as augite and fayalite, are necessary to explain the formation of the nelsonite.

The Ferrar Continental Flood Basalt: A ∼1.6 Ma long duration evidenced by high-precision 40Ar/39Ar ages suggest a potential role in the Pliensbachian-Toarcian extinction event

One of the most impassioned topics in large igneous province (LIP) research is how prolonged the duration of these large-scale magmatic events are, as LIP magmatism has considerable impact on models of associated reconstructions, of climate variability or tectonic events. High-precision geochronology is pivotal to LIP basalt emplacement rate, and thus to unravel the role these enormous magmatic events have throughout Earths geological and environmental history. Four high-precision 40Ar/39Ar plagioclase plateau ages for the Tasmanian dolerites (Ferrar) indicate ∼1.6 ± 0.4 Ma of resolvable, continuous magmatic activity; 184.27 ± 0.24 to 182.69 ± 0.54 Ma (2σ). The 40Ar/39Ar results provide evidence of distinctly older intrusions and a more prolonged duration than the observed 182.4-182.9 Ma age range and duration indicated by the main zircon record. Moreover, the precision of our 40Ar/39Ar results coupled with secondary electron microscopy analyses provide evidence of plagioclase crystal inheritance from slightly older magmatism entrained into younger magmatic pulses by exploiting pre-existing conduits. Numerical diffusion models, calculated for a theoretical age spectrum resulting from two slightly different plagioclase ages, provide an excellent match for measured data. Coupling geochemical data to the new age data indicates a silica and incompatible element evolution of the Ferrar magmatic system through time. The older generation of intrusions (ca. Zr: 92 ppm, SiO2: 53.67 wt.%) are seemingly less enriched in incompatible elements and silica than the youngest generation (ca. Zr: 147 ppm, SiO2: 56.5 wt.%). Here, we suggest that the magma chambers differentiated to more incompatible/silica-rich compositions saturating zircon only at evolved magmatic stages. This implies that plagioclase dates the full duration of magmatic Ferrar LIP activity of ca. 1.6 Myrs whilst zircon ages might be naturally biased and restricted to post-Zr saturation stage. The extended duration of Ferrar magmatism indicates that it is coeval with the Pliensbachian-Toarcian boundary. Therefore, we speculate that Ferrar (±Karoo) magmatism triggered the Pliensbachian-Toarcian extinction event and contributed to the Toarcian oceanic anoxic event, from which the environment did not begin to recover until only after the waning and cessation of Ferrar magmatic activity at ∼182 Ma, with zircon crystals recording the final flux of magma.

A shifty Toba magma reservoir: Improved eruption chronology and petrochronological evidence for lateral growth of a giant magma body

Polycyclic caldera complexes hold clues to understanding why some magmatic systems develop into supersized magma bodies and how they can recover to produce several caldera-forming eruptions. However, the geologic records of the transitions between successive caldera events are very often inaccessible due to limited preservation of eruptive products of inter-caldera activity, prompting the search for alternative archives of magma evolution such as accessory minerals. Here we applied multiple geochemical tools to study one of the most active caldera centres of the Quaternary, the Toba caldera complex in Sumatra (Indonesia), which produced at least four caldera-forming eruptions in the last 1.6 My, including the iconic Youngest Toba Tuff at 74 ka. We combined feldspar 40Ar/39Ar and zircon U–Pb geochronology of proximal pyroclastic deposits with glass and mineral chemistry of both the tuffs and distal marine tephra to revise the eruption chronology of Toba, obtaining new eruption ages of 1417 -31/+14 ka (zircon) or 1339 ± 39/39 ka (plagioclase, internal/full external 2σ uncertainty) for the Haranggaol Dacite Tuff, 783.81 ± 0.85/1.32 ka (sanidine) for Oldest Toba Tuff, and 503.61 ± 1.36/1.50 ka (sanidine) for Middle Toba Tuff. Isotope dilution thermal ionisation mass spectrometry (ID-TIMS) U–Pb crystallisation ages, trace element contents and Hf isotopic ratios of zircons illuminate changes in the shallow magma reservoir which saw near-continuous zircon crystallisation over 1.6 My. Prolonged build-ups to each eruption with highly scattered zircon trace element compositions reflect a complex, heterogeneous character of the shallow reservoir, without a clear temporal trend or indications of the eruption trigger. In contrast, hafnium isotopes in zircon display a pronounced shift towards unradiogenic values immediately after the OTT caldera collapse, followed by a gradual recovery to a baseline value of εHf = -7 at the time of YTT eruption, interpreted as a reflection of the shift in magma reservoir position corresponding to change in the character of assimilated crust. We can show in unprecedented detail how a large caldera collapse affects magma geochemistry; however, identification of patterns in the behaviour of the Toba system and making geochemistry-based predictions about its future development remain a challenge.

AgeSpectraAnalyst: A MATLAB based package to model zircon age distributions in silicic magmatic systems

In the last decade, improvements in the analytical precision achievable by zircon U-Pb geochronological techniques have allowed to resolve complexities of zircon crystallization histories in magmatic rocks to an unprecedented level. A number of studies have strived to link resolvable dispersion in zircon age spectra of samples from fossil magmatic systems to the physical parameters of their parent magma bodies. However, the methodologies developed have so far been limited to reproduce the effect of simple thermal histories on the final distribution of zircon ages. In this work we take a more nuanced approach, fine-tuning a thermodynamics-based zircon saturation model to predict the relative distribution of zircon ages in samples from silicic magma reservoirs experiencing open-system processes (e.g. heat/mass addition, mechanical mixing). Employing the MATLAB package (AgeSpectraAnalyst) presented in this contribution:•Users can forward model the effect that diverse thermal histories and mechanical mixing processes characteristic of silicic magma bodies have on zircon age distributions as measured by high-precision, chemical abrasion thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb geochronology.•Zircon CA-ID-TIMS datasets from silicic magmatic systems can be easily compared with model output to gain semi-quantitative information on thermo-mechanical history of the system of interest.•We demonstrated (Tavazzani et al., in press) that distribution of high-precision zircon ages in crystallized remnants of shallow (∼ 250 MPa), silicic magma reservoirs can discriminate between systems that experienced catastrophic, caldera-forming eruptions and systems that underwent monotonic cooling histories.

Linking Significant Zr Isotopic Fractionation in Magmatic Zircons With Petrographic Textures

AbstractThe origin of crystal‐scale Zr isotopic variation remains enigmatic. Here, we report high‐precision Zr isotopic compositions of thin‐sectioned and hand‐picked zircon crystals from monzodiorites and K‐feldspar granites, which represent the mafic and felsic products of a differentiating magma. Our results reveal that zircons from both rock associations are dominated by a rim‐ward increase in the 94Zr/90Zr ratio and accompanied by ∼1‰ intracrystalline and intercrystalline variations, confirming the preferential incorporation of light Zr isotopes and thus kinetic isotopic fractionation during zircon growth. Notably, zircons enclosed by K‐feldspar in monzodiorites generally yield lower 94Zr/90Zr and higher Zr/Hf ratios than that of separated crystals, while in K‐feldspar granites, tiny zircons intergrown with apatite obtain higher 94Zr/90Zr but lower Zr/Hf ratios relative to that of sizable separates. The diversity in petrographic textures and sizes of zircon crystals, corresponding to their crystallization sequence, potentially accounts for some of the heterogeneity of Zr isotopes at the mineral scale.

Does Zircon Shape Retain Petrogenetic Information?

AbstractZircon shape is commonly reported during geochronology and geochemistry analyses of igneous, metamorphic, and sedimentary rocks, but the relationship of zircon shape to primary growth environmental conditions remains poorly constrained. Current models for the control on igneous zircon shape focus on the relative growth of crystal prisms and pyramids, which are not discernible in the imaging techniques used for rapid quantification of zircon shape in geochronology sample mounts. We model the relationship between whole‐rock composition and zircon 2D shape in mineral separates from 45 mafic to felsic igneous samples, representative of Archean and Proterozoic crust in Western Australia. Shape parameters are derived from semi‐automated measurement of photomicrographs of polished zircon crystals in epoxy resin mounts. Whole‐rock composition shows a statistically significant relationship to median magmatic zircon crystal area and mathematically defined “roundness.” Zircon populations show reduced median area and increased median roundness as whole‐rock silica decreases. Phase equilibrium modeling based on whole‐rock composition, and automated electron microscopy mineral maps, indicates that the compositional predisposition of zircon shape is influenced by fundamentally different physical growth environments in mafic versus felsic melts. Specifically, influential factors that differ between mafic and felsic liquids include crystallization sequence and duration—which influence unconstrained growth space—and the potential for absorption/exsolution of zirconium from the accompanying mineral assemblage. We present quantitative, explanatory models for the relationship between zircon 2D shape and whole‐rock silica and demonstrate that the relationships are adhered to across a broad spectrum of whole‐rock compositions.

Himalayan-like Crustal Melting and Differentiation in the Southern North American Cordilleran Anatectic Belt during the Laramide Orogeny: Coyote Mountains, Arizona

Abstract The southern US and northern Mexican Cordillera experienced crustal melting during the Laramide orogeny (c. 80–40 Ma). The metamorphic sources of melt are not exposed at the surface; however, anatectic granites are present throughout the region, providing an opportunity to investigate the metamorphic processes associated with this orogeny. A detailed geochemical and petrochronological analysis of the Pan Tak Granite from the Coyote Mountains core complex in southern Arizona suggests that prograde metamorphism, melting, and melt crystallization occurred here from 62 to 42 Ma. Ti-in-zircon temperatures (TTi-zr) correlate with changes in zircon rare earth elements (REE) concentrations, and indicate prograde heating, mineral breakdown, and melt generation took place from 62 to 53 Ma. TTi-zr increases from ~650 to 850 °C during this interval. A prominent gap in zircon ages is observed from 53 to 51 Ma and is interpreted to reflect the timing of peak metamorphism and melting, which caused zircon dissolution. The age gap is an inflection point in several geochemical-temporal trends that suggest crystallization and cooling dominated afterward, from 51 to 42 Ma. Supporting this interpretation is an increase in zircon U/Th and Hf, a decrease in TTi-zr, increasing zircon (Dy/Yb)n, and textural evidence for coupled dissolution–reprecipitation processes that resulted in zircon (re)crystallization. In addition, whole rock REE, large ion lithophile elements, and major elements suggest that the Pan Tak Granite experienced advanced fractional crystallization during this time. High-silica, muscovite± garnet leucogranite dikes that crosscut two-mica granite represent more evolved residual melt compositions. The Pan Tak Granite was formed by fluid-deficient melting and biotite dehydration melting of meta-igneous protoliths, including Jurassic arc rocks and the Proterozoic Oracle Granite. The most likely causes of melting are interpreted to be a combination of (1) radiogenic heating and relaxation of isotherms associated with crustal thickening under a plateau environment, (2) heat and fluid transfer related to the Laramide continental arc, and (3) shear and viscous heating related to the deformation of the deep lithosphere. The characteristics and petrologic processes that created the Pan Tak Granite are strikingly similar to intrusive suites in the Himalayan leucogranite belt and further support the association between the North American Cordilleran anatectic belt and a major orogenic and thermal event during the Laramide orogeny.

Zircon double-dating, trace element and O isotope analysis to decipher late Pleistocene explosive-effusive eruptions from a zoned ocean-island magma system, Ascension Island

In this first detailed study of zircon from Ascension Island, South Atlantic, we take a novel approach combining trace element and O isotope compositional data with double-dating (disequilibrium 238U–230Th and (U–Th)/He) to decipher timescales and dynamics of magmatic processes. The Echo Canyon (EC) sequence comprises small-volume explosive-effusive eruptions of trachyte that tapped a compositionally zoned magma system. Associated volcanic hazards may be constrained from the age of volcanism, duration of magma storage, and magma source and plumbing system character. Zircon U–Th–Pb dating of lithic lava clasts has revealed recurrent evolved volcanism at 1.34 and 0.6 Ma, and 95 ka. The (U–Th)/He zircon cooling ages indicate that most of the EC explosive-effusive sequence erupted in a brief episode at ca. 95 ka. Additionally, uniform 238U–230Th zircon crystallisation ages suggest moderately protracted magma storage with melt present at depth for at most 103–104 years before eruption. The enriched character of zircon trace element compositions, relative to MORB, in the absence of a continental crustal signature in the oxygen isotope values (δ18O range 2.67–5.63‰), suggests the presence of an enriched component in the EC magma source. Furthermore, low δ18O zircon compositions imply assimilation of high temperature hydrothermally altered country rock by the source magma. The mineral assemblage in crystal-poor pumices indicates equilibrium storage conditions: zircon saturation and Ti-in-zircon crystallisation temperatures are consistent with alkali feldspar-melt temperatures. Significantly, zircon crystals were preserved both as macrocryst inclusions and in the groundmass of EC explosive and effusive deposits. These rocks preserve evidence of magma evolution by fractional crystallisation. This process led to pre-eruptive compositional stratification, which is evidenced in the range of whole-rock major and trace element compositions and zircon Zr/Hf values. Notably, zircon crystallisation and cooling ages derived from pumice, lava, and accidental lithic lava clasts in highly explosive pyroclastic deposits, have revealed episodes of evolved magmatism that would otherwise have gone undetected. In addition, the zircon trace element and isotope compositions, in combination with the range of crystallisation ages, evidence progressively deeper tapping of less evolved magma stored in discrete lenses. Thus, a combined zircon geochronological-geochemical approach can place constraints on the ca. 0.6 Ma recurrence of past explosive-effusive pulses of millennial to decamillennial duration and their enriched magma sources. This information is relevant for assessing hazards and informing monitoring and forecasting efforts to assist in managing associated risks for small ocean island volcanoes with particularly vulnerable populations and infrastructure.

Sources, evolution and ages of A-type granites from the post-orogenic Itu batholith, SE Brazil: Inferences from zircon U–Pb dating, Lu–Hf isotope ratios and trace-element geochemistry

A combination of U–Pb dating by SHRIMP and Lu–Hf isotope and trace-element analyses by LA-ICPMS in zircon crystals from representative samples of the Itu Granitic Batholith, allowed to determine timing, sources and intensive parameters of crystallization (T, fO2) of the plutons that generated this large (>300 km2) occurrence of A-type granites that locally marks the end of the Neoproterozoic orogeny in SE Brazil. The main episode of batholith construction is dated at 578 ± 4 Ma, with possible minor manifestations as young as ∼560 Ma. Negative zircon εHf(t) as low as −19 to −17 in the Indaiatuba pluton requires a significant crustal input, but a component from enriched mantle (which has εHf(t) as low as −10, reflecting contamination from previous subduction) is evident in all plutons by the presence of small mafic bodies and frequent mafic microgranular enclaves showing evidence of mixing with the host granites. Distinct sources are implied by large contrasts in redox state, estimated qualitatively from negative Eu anomalies and quantitatively from Ce anomalies in zircon, with samples from the Itupeva and Salto plutons showing fO2 ∼FMQ+2 and a sample from the Cabreúva pluton closer to FMQ. Significant intra-sample variations of εHf(t) and trace-elements (e.g., Th, U) in zircon are also indicative that multiple sources have contributed to individual magma batches and were probably preserved by fast crystallization related to magma mixing processes.

Pb diffusion in perfect and defective zircon for thermo/petro-chronological investigations: An atomistic approach

Factors that affect the diffusion of lead (Pb) in zircon are still a debatable issue amongst geochronologists. These are anisotropy, chemical composition, metamictization, radiation damage, and defects such as vacancy and Frenkel pairs. Careful investigations of such effects require a detailed description at atomic levels. This study focuses on the details of Pb diffusion pathways (anisotropy) at the atomic scale level in perfect zircon lattices for the better understanding of thermo/petro-chronological problems in geosciences. The applications of Density Functional Theory (DFT) combined with both the Nudged Elastic Band (NEB) method and Transition State Theory (TST) together with Kinetic Monte Carlo (KMC) simulations provide a reasonable estimate of the activation energy and diffusion of lead (Pb) in a perfect zircon lattice. Results of the tested methodology give the diffusion co-efficient of anisotropic nature in perfect zircon i.e. the activation energies and frequency factors (D0) along the z-axis [001] is 687 kJ/mol and 4.16×10−3cm2/sec, along zx-axis [101]/[011] is 1331 kJ/mol and 8.57×10−4cm2/sec and along the x-axis is 5810 kJ/mol and 8.57×10−4cm2/sec respectively. The Closure temperatures for Pb in perfect zircon lattices are reasonably high for an effective grain size of 60 μm and a cooling history of 10 °C/Ma. This study also demonstrates that the closure temperature is affected not only by the geometry of a given grain, but also by defects and radiation damage. Furthermore, the calculated closure temperatures show poor calibrations with previous experiments of Pb diffusion in natural zircon, implying that radiation damage and vacancies are causing the reduction in Pb diffusion. Therefore, the gap between the computed and experimental results may probably be due to the change in the chemistry of each zircon crystal used in the experiments. This study is useful in characterizing the effect of individual entities on Pb diffusion in zircon, showing good agreement with experiments, and indicating the applicability of DFT + NEB+TST + KMC methods for characterization of Pb diffusion in perfect zircon.

Hydrothermal alteration, not metamictization, is the main trigger for modifying zircon in highly evolved granites

The question of whether the high U and Th concentrations in zircon are primary or secondary is often difficult to resolve, and a clear understanding of the modification processes of secondary U- and Th-rich zircon is lacking. Zircon crystals from two well-studied, highly evolved granites, the Jiangjunshan muscovite granite in the Chinese Altai Mountains and the Cuonadong leucogranite in the Eastern Tethyan Himalaya, have been investigated and classified into two types. Type I exhibits typical igneous growth zoning, and type II has “diffuse” or “spongy” internal structures. These textures, along with compositional data, indicate that the type II zircon crystals formed through hydrothermal modification of magmatic zircon (type I) by infiltrating hydrothermal fluids. During hydrothermal modification, the U and Th concentrations increase from type I to type II in the Jiangjunshan muscovite granite but decrease from type I to type II in the Cuonadong leucogranite. The Raman spectra of type II zircon crystals from Jiangjunshan muscovite granite have broader peaks (i.e., measured as the full width at half maximum, FWHM) with decreased intensities than their type I counterparts, which indicates that the former are affected by significant accumulated radiation damage. However, the preserved radiation damage in both the type I and II zircon measured by Raman spectroscopy is less than that expected from the total dose of alpha particles calculated from the U and Th contents, which indicates variable degrees of annealing. We propose that late magmatic-hydrothermal alteration was responsible for the modification of magmatic zircon grains in highly evolved granites and resulted in the enrichment or redistribution of U and Th. The calculated radiation dose of the Cuonadong leucogranite zircon is far below that required for metamictization, which indicates that metamictization is not always responsible for diffuse and spongy textures.

A LONG-LASTING EVOLUTION OF THE ORE-MAGMATIC SYSTEM AT THE MURUNTAU GOLD DEPOSIT (WESTERN UZBEKISTAN, TIEN SHAN): THE EVIDENCE FROM ISOTOPIC U-PB ZIRCON AGE (LA-ICP-MS METHOD) OF THE GRANITOIDS OF THE SARDARA (SARYKTY) PLUTON

The paper presents the first data of the isotopic zircon U–Pb study (LA–ICP–MS method) on the granodiorite-granite from the Sardara (Sarykty) pluton in the district of the giant Muruntau Au deposit in the Western Tien Shan (Uzbekistan). In a single sample, three age groups of zircon crystals identified, with their concordant U–Pb age being 322.0 ± 3.7 Ma (MSWD = 3.1, 4 zircon grains), 301.6 ± 2.1 Ma (MSWD = 0.17, 11 zircon grains), and 289.1 ± 4.9 Ma (MSWD = 0.98, 2 zircon grains), respectively. The scatter of the isotope age data obtained can be interpreted using the model of subsequent crystallization of various zircon generations in variably-deep magmatic batches, followed by zircon capturing during the magma differentiation and crystallization. Notable is a quite close coincidence of the two (the younger) age intervals to the concordant U–Pb zircon ages, which were published before for the granitoid rocks found directly at the Muruntau deposit. The latter thus can represent the dike “splits” occurred at the respective stages of the magma evolution (progressing differentiation) in the deeper batches of the granitoid magma, with its larger intrusions represented by the Sardara (Sarykty) and other plutons outcropping at some distance from the deposit. Overall, the older isotopic U–Pb zircon data of the Sardara (Sarykty) pluton (in the order of 322 Ма and 302 Ма, respectively) correspond closer to the subduction event, whereas the younger U–Pb zircon data (in the order of 289.5 Ma) are quite relevant to the post-collisional stage occurred in the region. Therefore, the initiation and initial development of the magma batches in the Muruntau deposit district could occur in the subduction environment but the final granitoid magma differentiation and crystallization were completed already at the post-collisional stage.

Microprobe Analysis of Titanium in Zircon: an Estimation of Secondary Fluorescence

Microprobe study of the effects of secondary fluorescence from high-titania glass (TiO2 ≈ 16 wt %) on apparent titanium content in Ti-free zircon was conducted. It was demonstrated that small crystals (10 µm) are not suitable for analysis. In boundary parts of large zircon crystals (20–30 µm) the error in Ti estimation may reach 10–23%. However, in central parts of large crystals ( 30 µm) the microprobe analysis gives practically undistorted titanium contents.

Paleo–Mesoarchean sedimentary record in the Dharwar Craton, India: Implications for Archean ocean oxygenation

The early Archean oceans were marked by significant redox changes which have subsequently shaped the Earth’s biosphere. Archean chemical sediments of banded Iron and Manganese formations provide important geochemical proxies for these historical shifts in the redox conditions and to trace the ancient sedimentation patterns and protoliths. In this study, we investigate the proto-ore of the Archean Mn-formations of the Sandur, Chitradurga and Shimoga greenstone belts of Dharwar Craton of southern Peninsular India, which is geochemically characterised as quartz arenites, Mn-arenites, Fe-arenites, Mn-argillites and Fe-argillites. The geochemical systematics suggest their deposition in shallow to deeper shelf in the Archean proto-ocean. The detrital zircon U-Pb systematics of Mn arenites and argillites indicate their maximum depositional age of 3230 ± 52 Ma representing the oldest onset of sedimentation during the Paleo–Mesoarchean timeframe in the Chitradurga Group of Dharwar Supergroup. The detrital influx proxies suggest variations in sedimentation rates associated with the Archean transgressive–regressive cycles and fluctuating hydrodynamic conditions, together reflecting an increasing trend in the contributions of recycled sediment from Sandur to Chitradurga and Shimoga greenstone belts. The available detrital zircon ages of the Mn arenites and argillites from these greenstone belts indicate a ∼ 600 Ma prolonged period of Mn deposition for which high-T hydrothermal fluids from Archean mid-oceanic ridges supplied the manganese. The trace element compositions of the concordant detrital zircons suggest 3.3–3.1 Ga Dharwar basement TTG/granitoid source which is corroborated by the zircon crystallization temperatures of 690–820 °C. The source-normalised α-dose rates of the detrital zircons signify greater degrees of sediment transport and multi-cycle nature which correspond to the earliest episode of crustal growth in the Indian sub-continent associated with the Mesoarchean Ur supercontinent. The clastic-chemogenic sedimentation attained through concomitant detrital sediment–seawater-metalliferous hydrothermal fluid mixing at an epicontinental passive margin resulted in the deposition of Mn-arenites and argillites closer to the higher Eh shore, while the Fe-rich sediments formed at a relatively deeper shelf characterised by comparatively lower Eh and more alkaline conditions. The comprehensive geochemical and geochronological data of the Archean Mn arenite-argillite sequences reveal the significance of regional episodes of ocean oxygenation at the shallow shelves of Archean oceans prior to great oxygenation event (GOE) that was mediated by the prolific growth of ancient microbiota which transformed the Earth to a more habitable planet.

Synthesis, characterization, mechanical and magnetic characteristics of Gd3+ /PO4 3 - substituted zircon for application in hard tissue replacements.

The study reports on the use of sol-gel technique to yield zircon type [Zr(1-0.1-x) GdxTi0.1 ] [(SiO4 )1-x (PO4 )x ] solid solution. Titanium has been used as a mineralizer to trigger zircon formation while equimolar concentrations of Gd3+ and PO4 3- were added to determine their accommodation limits in the zircon structure. The crystallization of t-ZrO2 as a dominant phase alongside the crystallization of m-ZrO2 and zircon were detected at 1200°C while their further annealing revealed the formation of zircon as a major phase at 1300°C. Heat treatment at 1400°C revealed the formation of zircon-type solid solution [Zr(1-0.1-x) Gdx Ti0.1 ][(SiO4 )1-x (PO4 )x ] comprising the accommodation of 10 mol.% of Gd3+ /PO4 3- at the zircon lattice. Beyond 10 mol.% of Gd3+ /PO4 3- , the crystallization of GdPO4 as a secondary phase is noticed. Structural analysis revealed the expansion of zircon lattice due to the simultaneous occupancy of Gd3+ /PO4 3- for the corresponding Zr4+ /SiO4 4- sites. The mechanical strength of single-phase zircon solid solution was higher in comparison to that of multiphase materials, namely in the presence of GdPO4 formed as a secondary phase in samples with added equimolar Gd3+ /PO4 3- contents beyond 10 mol.%. Nevertheless, the paramagnetic behavior of the samples demonstrated a steady surge as a function of enhanced Gd3+ content.

Geochemistry and geochronology of the Imbicuí Complex, Western Dom Feliciano Belt, Brazil: Tonian adakite-like subduction-related rocks

Dioritic-tonalitic-trondhjemitic metamorphic rocks represent the Imbicuí Complex in Western Dom Feliciano Belt, São Gabriel Terrane, Lavras do Sul region, Brazil. This complex presents rocks with an adakite-like signature showing high Sr contents (573.5–1337 ppm), Sr/Y ratio higher than 64.08, low Y content (< 33.7 ppm), (La/Yb)N > 10, and YbN < 16 being classified as Type 1, high silica and high-Al2O3 adakites. The adakites present a low-K calc-alkaline affinity, with a U–Pb LA-ICP-MS in zircon crystallization age of 828 ± 8 Ma, the product of an eclogitic melting source. Subordinate, metagabbroic rocks occur with tholeiitic affinity, presenting a U–Pb LA-ICP-MS in zircon crystallization age of 845 ± 5 Ma. The adakitic rocks showed εNd(828) (+ 8.2 to + 7.5), model ages Nd-TDM of 0.8 and 0.9 Ga, similar to the age of crystallization, suggesting that these magmas represent juvenile rocks generated in an orogenic environment mixed with reworked rocks (ratios 206Pb/204Pb and 207Pb/204Pb, 17.70 to 18.40 and 15.49 and 15.55) during an oceanic slab melting associated to the closure of Charrua Ocean throughout Tonian in the Western Dom Feliciano Belt.

The youngest lunar zircon reveals an extremely fractionated nature of Chang’e-5 basalt

The U(Pb)-Pb age of zircon is commonly used to represent the crystallization age for igneous rocks due to its high closure temperature and robust resistance to impact disturbance. However, no zircon crystallization age has yet been reported for Chang'e-5 (CE-5) basalt due to their limited occurrence in mare basalt. In this study, rare zircon grains from CE-5 lunar samples were investigated by the in-situ Pb isotopic analysis, and a precise zircon crystallization age of 2036 ± 19 Ma was determined from Pb-Pb isochron. This is hitherto the youngest reported crystallization age of lunar zircon, similar to the ages of CE-5 lunar basalts obtained by zirconium (Zr)-bearing minerals such as baddeleyite, tranquillityite, and zirconolite. Petrographic evidence and rare-earth element geochemistry indicate that zircon in the CE-5 lunar basalts were formed by the reaction of early-formed baddeleyite with SiO2 melt within the latest residue of extreme fractionation of a non-KREEP (an acronym for potassium, REE, and phosphorus) basaltic magma. In contrast to the prevailing view that lunar zircon have formed in late-stage enriched melts resulting from extensive fractional crystallization of the Lunar Magma Ocean, this study shows that zircon could be derived from extreme fractionation of non-KREEP basaltic magma unrelated to Lunar Magma Ocean.

Early Neoproterozoic magmatism and Caledonian metamorphism recorded by the Mårma terrane, Seve Nappe Complex, northern Swedish Caledonides

Petrology, geochronology and bulk-rock chemistry are combined to investigate the early Neoproterozoic magmatism and Cambrian–Ordovician metamorphism in the northern Swedish Caledonides. This work includes several lithologies of the Mårma terrane in the Seve Nappe Complex exposed in the Kebnekaise region. U–Pb zircon geochronology yielded crystallization ages of 835 ± 8 Ma for a mylonitic orthogneiss, 864 ± 3 Ma for the Vistas Granite and 840 ± 7 Ma for an intruded granitic dyke, whereas a gabbro and a granodiorite intrusion gave U–Pb zircon crystallization ages of 856 ± 3 Ma and 850 ± 1 Ma, respectively. U–Pb monazite dating of the mylonitic orthogneiss gave an upper intercept age of 841 ± 7 Ma and a lower intercept age of 443 ± 20 Ma. Pressure–temperature estimates derived for the mylonitic orthogneiss reveal metamorphic peak-pressure and peak-temperature of 10.5–11.75 kbar and 550–610°C and 7.4–8.1 kbar at 615–675°C, respectively. Metamorphic pressure–temperature estimates for the Vistas Granite yield 6.5–7.5 kbar at 600–625°C. Whole-rock chemistry coupled with U–Pb geochronology indicates that bimodal magmatism was related to attempted rifting of Rodinia between 870 and 840 Ma. Supplementary material: Supplementary figures and tables are available at https://doi.org/10.6084/m9.figshare.c.6675075 Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/topic/collections/the-caledonian-wilson-cycle

Petrology, geochemistry and U–Pb zircon geochronology of Fe–Ti oxide ore‐bearing mafic sill from Saltora‐Mejia area of Chotanagpur Granite Gneissic Complex, eastern India: Implication for late tectonic emplacement of mafic rock of E‐MORB nature in an orogenic belt

An east–west‐trending medium‐grained mafic sill containing co‐genetic Fe–Ti oxide ore lenses is found disposed within granite gneisses around Saltora‐Mejia area in the eastern part of the Chotanagpur Granite Gneissic Complex (CGGC) of eastern India. CGGC is considered as a Proterozoic mobile belt as it witnessed multiple phases of deformation and high‐ grade metamorphism during 1.8–0.8 Ga. Occurrence of such Fe–Ti oxide ore‐bearing mafic sill is unique in the entire CGGC which is a vast Proterozoic orogenic belt and has witnessed many phases of voluminous mafic and felsic magmatisms. The mafic rock is of gabbronorite composition which contains plagioclase, clinopyroxene, orthopyroxene as major constituent primary minerals and amphibole as late magmatic mineral. The rock shows sub‐ophitic, intergranular, mosaic and poikilitic texture (defined by larger pargasitic grain). The gabbronorite shows iron enriched tholeiitic character, low Mg#, low abundances of Ni and Cr, slight enrichment in LILE, LREE and slight depletion in HFSE like Nb and Ti. The computed melt in equilibrium with the studied gabbronorite shows transitional orogenic to anorogenic, within‐plate and E‐MORB‐like geochemical character. In this study, the U–Pb zircon crystallization age (~960 Ma) of the Saltora‐Mejia gabbronorite is reported for the first time which coincides with the late tectonic stage of the most pervasive orogenic activity in the CGGC around 1.2–0.9 Ga. Transitional orogenic to anorogenic geochemical character, late tectonic evolution and other field and laboratory evidences together suggest evolution of the Saltora‐Mejia gabbronorite sill in a late tectonic extensional environment which might have been facilitated by delamination of a subducted plate and upwelling of asthenospheric mantle during the waning stage of a major orogeny in the CGGC.