Large Zircon Research Articles

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.

Eo-Paleoarchean detrital zircon in the Winnipeg River terrane, Western Superior Province: Provenance and implications

Metasedimentary rocks within Archean cratons can preserve a record of early cratonic crustal evolution and later metamorphic events. However, interpreting this record is difficult where the metasedimentary rocks have undergone amphibolite-granulite facies metamorphic overprinting and migmatization. In the Western Superior Province, migmatitic metasedimentary rocks represent a dominant supracrustal component in several terranes, including, from north to south, the English River, Winnipeg River, Wabigoon and Quetico terranes. Here we present zircon U-Pb-Hf isotope data for three metasedimentary rocks and one augen gneiss from the Mystery Lake dome in the Winnipeg River plutonic-gneiss terrane using in-situ techniques and large zircon populations. The augen gneiss contains zircon that has overlapping U-Pb-Hf isotope systematics with the ca. 3250 Ma gneissic basement of the terrane. The detrital zircon grains from the metasedimentary rocks have age populations at ca. 3750 Ma, 3600 Ma, 3500 Ma, 3400 Ma, 3300–3100 Ma, 3050 Ma, 2950–2900 Ma, 2850–2800 Ma, and 2800–2745 Ma, with metamorphic zircon between approximately 2745 Ma and 2550 Ma. The paucity of exposed Eo-Paleoarchean rocks in adjacent terranes suggests that the source rocks were derived from within the Winnipeg River terrane. The age and Hf isotope systematics of Eo-Paleoarchean detrital zircons from the Winnipeg River terrane overlap with zircon data from the Hudson Bay and Minnesota River Valley terranes on the northern and southern margins of the craton, respectively. The evidence suggests all three plutonic-gneiss terranes have preserved similar Eo-Paleoarchean geological histories, supporting the hypothesis that they were derived from a contiguous proto-craton.

Distribution of trace elements controlled by sector and growth zonings in zircon from a miaskite pegmatite of the Vishnegorsky massif, the Southern Urals

Data on the content and distribution of trace and rare-earth elements (SIMS method) in sectors and growth zones of a large zircon crystal from miaskite pegmatites of the Vishnegogorsky massif are presented. The morphology of the zircon crystal is a combination of a dipyramid {111} and prism {010}. It has been established that the growth sector of dipyramid {111} is characterized by almost one order of magnitude higher contents of Y, Nb, REE, Th; higher Th/U and Eu/Eu* values; REE distribution spectra are flatter compared to prism {010} growth sector. A regular decrease in the content of trace and rare-earth elements in the direction from the central zone to the marginal zone of crystal growth was revealed. A smooth regression of zircon crystallization temperature of zircon from 960 °C in the central zone to 740 °C in the marginal zone of the dipyramid sector and 700-650 °C in the prism sector has been revealed, which may be a reflection of thermal evolution of the crystallization process. It is assumed that crystallization of the central zone of zircon occurred at early stages from a relatively trace-еlement-rich melt. The crystallization was completed at lower temperatures, probably, simultaneously with the formation of REE-concentrating minerals, which resulted in natural decrease of content of trace and rare-earth elements in the melt and, consequently, in zircon crystallizing from it.

Large zircon age spans record multi-stage history of batholith assembly: Insights from the Late Triassic Dongcuo batholith in the eastern Tibetan Plateau

Large and homogeneous plutons can assemble incrementally over millions of years, yielding large age spans. Determination of the age span is crucial to constrain the timing and nature of the batholith assembly. This paper reports comprehensive whole-rock geochemical data, LA-ICP-MS zircon U-Pb age, and zircon Hf isotopic data to constrain the timing and assembly process of the Late Triassic Dongcuo batholith, the largest batholith in the Yidun Terrane, eastern Tibetan Plateau. Samples from the batholith consist of metaluminous to weakly peraluminous granitoids (SiO2 = 69.91–79.85 wt%). These granitoids show large variation of zircon Hf isotopic compositions with εHf(t) of -17.15 to +0.13. The multiple zircon populations in single hand specimens revealed by kernel density estimation spread over 20 Myr (ca. 225–204 Ma), which is also confirmed by core-rim ages of single zircon grains, suggesting prolonged magmatism of the Dongcuo batholith. Our data indicate that the assembly of the batholith resulted from emplacement of discrete magma batches which derived from spatially varied sources and underwent variable differentiation. The assembly mainly involved three stages, including an early episode of magmatism (ca. 228 Ma), a high-flux magmatism between ca. 220 Ma and ca. 216 Ma, and a late-stage thermal event (ca. 211–204 Ma) possibly related to high temperature and localized remelting of the batholith. This study highlights the complexity of zircon populations in composite granitic batholith, showing that the search for single crystallization ages for a sample is fraught with difficulties and may mask rather than elucidate the crystallization history of the magmatic body.

PHASE ANALYSIS AND PHYSICAL PROPERTIES OF B2O3-ADDED ZIRCON CERAMICS SINTERED AT 1300 °C

This study was aimed to know the effect of B2O3(boria) addition on the phase composition and physical properties of zircon ceramics.The raw zircon powder used in the study was a purified natural zircon sand from Kereng Pangi, Central Kalimantan, Indonesia. The zircon ceramics were prepared by a solid state reaction method with variation of B2O3 addition of 3 wt%, 6 wt% and 9 wt% and sintered at 1300 °C for 5h. The phase composition, density and microstructure were characterized using X-ray diffraction (XRD), densimeter and Scanning Electron Microscope (SEM), respectively. Vickers Hardness measurement was perfomed at the polished surface of the ceramics. Results showed that all samples contained pure zircon phase, i.e. there was no effect of B2O3 addition on the phase composition. In general, the density and hardness increased with increasing B2O3 addition, but addition up to 9 wt% is not optimum to achieve ultra-dense zircon ceramics. Furthermore, the SEM image also showed no significant difference in average grain size. The crystallite size has grown nearly eight times (325 nm) of its original powder. The Vickers hardness of the ceramics is not significantly influenced by the addition of boria. It appears that the boria failure to increase densification also results in the extent of contact between grains which then produces relatively large zircon grains.

Distribution of Trace Elements Controlled by Sector and Growth Zonings in Zircon from Feldspathic Pegmatites (Ilmen Mountains, the Southern Urals)

The present study contains the detailed ion microprobe data on trace and rare earth elements distribution in the large zircon crystal about 10 × 6 mm in size with distinct growth and sector zonings from Ilmen Mountains feldspathic pegmatite. The zircon crystal morphology is a combination of a prism {110} and a dipyramid {111}. It is found out that the growth sector of the prism {110} generally contains higher concentrations of Th, U, REE, Y, and Nb and exhibits a more gently sloping HREE distribution pattern and a steeper LREE distribution pattern, in contrast to zircon from the growth sector of the dipyramid {111} development. Such a sector zoning pattern was formed at a late stage in crystal growth, when the prism {110} began to prevail over the dipyramid {111}. The zircon studied displays the growth zoning formed of alternating bands in back-scattered electron (BSE) image: wide dark and thin light bands. The last ones contain elevated Th, U, REE, Y, Nb, and Ti concentrations, Th/U ratio and Ce/Ce*. This growth zoning is most probably due to simultaneous crystallization of other minerals that concentrate trace elements, e.g., apatite and monazite, and the lack of equilibrium between zircon and fluid (melt).

Zircon survival in shallow asthenosphere and deep lithosphere

Abstract Zircon (ZrSiO4) is the most frequently used geochronometer of terrestrial and extraterrestrial processes. To shed light on question of zircon survival in the Earth's shallow asthenosphere, high-temperature experiments of zircon dissolution in natural mid-ocean ridge basaltic (MORB) and synthetic haplobasaltic melts have been performed at temperatures of 1250–1300 °C and pressures from 0.1 MPa to 0.7 GPa. Zirconium measurements were made in situ by electron probe microanalyses (EPMA) at high current. Taking into account secondary fluorescence effects in zircon-glass pairs during EPMA, a zirconium diffusion coefficient of 2.87E-08 cm2/s was determined at 1300 °C and 0.5 GPa. When applied to the question of zircon survival in asthenospheric melts of tholeiitic basalt composition, the data are used to infer that typical 100 mm zircon crystals dissolve rapidly (~10 h) and congruently upon reaction with basaltic melt at pressures of 0.2–0.7 GPa. We observed incongruent (to crystal ZrO2 and SiO2 in melt) dissolution of zircon in natural mid-ocean ridge the basaltic melt at low pressures <0.2 GPa and in the haplobasaltic melt at 0.7 GPa pressure. Our experimental data raise questions about the origin of zircon crystals in mafic and ultramafic rocks, in particular, in shallow oceanic asthenosphere and deep lithosphere, as well as the meaning of the zircon-based ages estimated from these minerals. The origin of zircon in shallow (ultra-) mafic chambers is likely related to the crystallization of intercumulus liquid. Large zircon megacrysts in kimberlites, peridotites, alkali basalts, and carbonatite magmas suggest fast transport and short interaction durations between zircon and melt. The origin of zircon megacrysts is likely related to metasomatic addition of Zr into the mantle as an episode of mantle melting should eliminate them on geologically short timescales.

U‐Pb SIMS ages of Apollo 14 zircon: Identifying distinct magmatic episodes

AbstractU‐Pb ages of zircon in four different Apollo 14 breccias (14305, 14306, 14314, and 14321) were obtained by secondary ion mass spectrometry. Some of the analyzed grains occur as cogenetic, poikilitic zircon grains in lithic clasts, revealing magmatic events at ~4286 Ma, ~4200–4220 Ma, and ~4150 Ma. The age distribution of the crystal clasts in the breccias exhibits a minor peak at ~4210 Ma, which can be attributed to a magmatic event, as recorded in zircon grains located in noritic clasts. An age peak at ~4335 Ma is present in all four breccias, as well as zircon grains from different Apollo landing sites, enhancing the confidence that these grains recorded a global zircon‐forming event. The overall age distribution among the four breccias exhibits minor differences between the breccias collected farther away from the Cone Crater and the ones collected within the continuous ejecta blanket of the Cone Crater. A granular zircon grain yielded a 207Pb/206Pb age of 3936 ± 8 Ma, which is interpreted as an impact event. A similar age of 3941 ± 5 Ma (n = 17, MSWD = 0.89, P = 0.58) was obtained for a large zircon grain (~430 × 340 μm in size). This grain might have crystallized in the same impact melt sheet which formed the granular zircon or the age is representative of the final extrusion of KREEP magma. The majority of zircon grains, however, occur as isolated crystal clasts within the matrix and their ages cannot be correlated with any real events (impact or magmatic) nor can the possibility be excluded that these ages represent partial resetting of the U‐Pb system.

New Zr–Hf Geothermometer for Magmatic Zircons

A geothermometer equation $$T = \frac{{1531}} {{\ln K_d + 0.883}}$$ , where $$K_{\dot d} = \frac{{X_{Zr}^S X_{Hf}^m }} {{X_{Zr}^m X_{Hf}^s }}$$ [X is the concentration (in ppm) of component i in phase j] is the Zr and Hf distribution coefficient between melt and zircon, and T is temperature in K, was derived by thermodynamic processing of literature experimental data on Zr and Hf distribution between acid melts (m) and zircon (s) and on the solubility of zircon and hafnon in the melts with variable silica content. In calculations with this equations, we assumed the Zr concentration in zircon to be constant: 480000 ppm. It is shown that the commonly observed increase in Hf concentration from the cores to margins of magmatic zircon crystals is caused by the fractional crystallization of zircon. For differentiated acid magmatic series, the initial crystallization temperature of zircon in the least silicic cumulates should be evaluated using the cores of large zircon grains with the highest Zr/Hf ratio. Application of the geothermometer for mafic and intermediate rocks may be hampered due to simultaneous crystallization of zircon with some other ore and mafic minerals relatively enriched in Zr and Hf. The newly derived geothermometer has some advantages over other indicators of the crystallization temperature of magmatic zircon based on the zircon saturation index (Watson and Harrison, 1983; Boehnke et al., 2013) and on Ti concentration in this mineral (Ferry and Watson, 2007) as it does not depend on the major-oxide melt composition and on the accuracy of the estimated SiO2 and TiO2 activities in the melts. Calculations of the Zr and Hf fractionation trends in the course of zircon crystallization in granitoid melts allow one to evaluate the temperature at which more evolved melt portions were segregated.

Listwaenite in the Sartohay ophiolitic mélange (Xinjiang, China): A genetic model based on petrology, U-Pb chronology and trace element geochemistry

Listwaenite lenses in the Sartohay ophiolitic mélange (Xinjiang, China) were formed via reactions between serpentinite and metasomatic fluids. First, serpentinite changed into talc schist via the reaction of serpentine + CO2 → talc + magnesite + H2O. Second, talc schist changed into listwaenite via the reaction of talc + CO2 → magnesite + quartz + H2O. Magnetite was progressively destroyed during transformation from serpentinite to talc schist, and completely consumed in listwaenite. Zircon crystals 30–100 μm long, disseminating in talc schist, undeformed listwaenite and mylonitized listwaenite, coexist with talc, quartz and magnesite, while micron-sized zircon grains (<5 μm in length) occur along the shearing foliation in the weakly deformed listwaenite and mylonitized listwaenite. We postulate that these micron-sized zircon crystals may have grown in-situ from medium-temperature hydrothermal fluids. Concentrations of most trace elements including high field strength elements (HFSE) increase from the undeformed, through the weakly deformed, to the mylonitized listwaenite, showing a positive correlation with the degrees of deformation and proportions of micron-size zircon, apatite, rutile and monazite. The large zircon crystals recovered from talc schist, undeformed listwaenite and mylonitized listwaenite yield similar weighted mean U-Pb ages (302.9 ± 6.8 Ma, 299.7 ± 5.5 Ma and 296.5 ± 3.5 Ma), and are thought to represent the age of formation of the talc schist and listwaenite. These ages are indistinguishable within errors and suggest a rapid transformation from talc schist to listwaenite. Some zircon rims in samples of the undeformed listwaenite and mylonitized listwaenite give much younger apparent U-Pb ages (280–277 Ma), which could be interpreted as a recrystallization age reflecting late-stage shearing in the Sartohay ophiolitic mélange.

3.1 Ga crystallization age for magnesian and ferroan gabbro lithologies in the Northwest Africa 773 clan of lunar meteorites

The Northwest Africa (NWA) 773 clan of meteorites is a group of paired and/or petrogenetically related stones that contain at least six different lithologies: magnesian gabbro, ferroan gabbro, anorthositic gabbro, olivine phyric basalt, regolith breccia, and polymict breccia. Uranium-lead dates of baddeleyite in the magnesian gabbro, ferroan gabbro, and components within breccia lithologies of paired lunar meteorites NWA 773, NWA 3170, NWA 6950, and NWA 7007 indicate a chronologic link among the meteorites and their components. A total of 50 baddeleyite grains were analyzed and yielded weighted average 207Pb-206Pb dates of 3119.4±9.4 (n=27), 3108±20 (n=13), and 3113±15 (n=10) Ma for the magnesian gabbro, ferroan gabbro, and polymict breccia lithologies, respectively. A weighted average date of 3115.6±6.8Ma (n=47/50) was calculated from the baddeleyite dates for all lithologies. A single large zircon grain found in a lithic clast in the polymict breccia of NWA 773 yielded a U-Pb concordia date of 3953±18Ma, indicating a much more ancient source for some of the components within the breccia. A U-Pb concordia date of apatite and merrillite grains from the magnesian gabbro and polymict breccia lithologies in NWA 773 is 3112±33Ma, identical to the baddeleyite dates. Magnesian and ferroan gabbros, as well as the dated baddeleyite and Ca-phosphate-bearing detritus in the breccia lithologies, formed during the same igneous event at about 3115Ma. These data also strengthen proposed petrogenetic connections between magnesian and ferroan gabbro lithologies, which represent some of the youngest igneous rocks known from the Moon.

From the Neoproterozoic mafic rock to the Silurian high-grade metamorphic rock: Evidence from zircon U-Pb geochronological, bulk-rock geochemical and mineral EPMA studies of Longyou garnet amphibolite in SE China

The Longyou garnet amphibolite is exposed on the southeast of the Neoproterozoic Jiangshan-Shaoxing fault zone of South China. Some recent studies propose that the Longyou garnet amphibolite was retrograded from an Early Paleozoic collision-related eclogite, which is inconsistent with previous numerous studies. Here we carry out a systematic study including field investigation, petrography, mineral chemistry, zircon geochronology and bulk rock geochemistry to ascertain its genesis. Our results indicate that a small amount of plagioclase inclusions found in garnet and clinopyroxene are anorthite, while plagioclase that largely distribute in the outer part of garnet and clinopyroxene porphyroblasts are andesine and labradorite (with An values of 44–66). No sodium-rich plagioclase (albite or oligoclase) which is indicative of omphacite symplectite has been found. The zircon CL images and Th/U ratios indicate that, apart from some newborn-metamorphic small zircons, a number of large zircon crystals exhibit obvious core-rim structures. Zircon U-Pb dating results indicate that the zircon cores yield concordant weighted average age of 879±11Ma and the zircon metamorphic rims yield weighted average age of 446±5Ma. Furthermore, the bulk rock geochemical features of garnet amphibolite suggest that its protolith is basic igneous rock (the SiO2 content within 46.8–48.2%), and its trace elements exhibit E-MORB geochemical affinity. This study also constrains the P-T pseudosection for Longyou garnet amphibolite (LGA), which revealing that the LGA at least experienced three stages of metamorphism in Silurian: the prograde metamorphism stage with P-T condition of 0.67–0.75GPa/685–719°C (M1), the peak-metamorphism with P-T condition of 0.9–1.1GPa/760–778°C (M2), and the retrograde metamorphism stage with P-T condition of ∼0.68GPa/∼767°C (M3). In combination with regional geological setting, this study constrains the formation process for the LGA.

Age and geochemistry of zircon from the oldest metamorphic rocks of the Omolon Massif (Northeast Russia)

This study provides SHRIMP-RG data on zircons from garnet gedritites, the products of retrograde metamorphism of eclogite-like rocks constituting belonging to the basement of the Omolon Massif. The earliest episode recorded by oscillatory-zoned cores having high HREE and Ti contents occurred at 3.25–3.22 Ga (Paleoarchean) and is interpreted to represent an upper age limit of a metamorphic or magmatic protolith. One zircon core with a pronounced negative Eu anomaly yielded a concordant age of 2.6 Ga, which is interpreted to mark a Neoarchean episode of granite formation. The studied population of zircons provides the most distinct record of a Paleoproterozoic (1.9 Ga) event, which is marked by formation of garnet gedritites under amphibolite-facies conditions. This event is recorded by transparent recrystallization rims of preexisting large zircon grains and small newly-formed grains, which are characterized, compared with their cores, by lower crystallization temperatures and one order of magnitude lower concentrations of U, Th, and HREE, and the presence of garnet micro-inclusions.

Mechanisms of radon loss from zircon: Microstructural controls on emanation and diffusion

Understanding how radon escapes from minerals is important for many fields in Earth science, yet few studies have focused on the mechanisms for radon escape. We measured radon emanation rate and radon loss upon heating for crushed aliquots of three large zircon crystals from three localities: Mud Tank (Australia), Bancroft (Canada), and Malawi (Africa). Our study, in conjunction with published data, shows that the room temperature radon emanation coefficient (REC) varies over 5 orders of magnitude in zircon. For low U zircon, Mud Tank, there are variations in REC that appear to be related to annealing at different temperatures, possibly due to annealing of fission tracks, however, all REC values for Mud Tank zircon are within error of one another. Bancroft and Malawi zircons have higher U content and do not show any systematic relationship of REC to annealing temperature. Results from Mud Tank zircon suggest that partial annealing of fission tracks decreases REC, but when all fission tracks are annealed REC reaches a maximum. REC in zircons with high U content, Bancroft and Malawi, is slightly higher than in zircon with lower U, although results are within error. Results of measurements of radon loss upon heating suggest that radon diffusion is slow, ∼30% of the radon is lost during heating at 975°C for 48h. Samples heated a second time yield less fractional radon loss, ∼10%, suggesting that diffusion parameters are changed during heating at temperatures ⩾975°C, which is likely the result of annealing of radiation damage. Diffusion parameters calculated from the fractional loss experiments reflect diffusion in highly radiation damaged or metamict zircons. Our results indicate that internal microstructures in zircon, such as fission tracks and alpha-radiation damage, influence radon escape for diffusion and recoil mechanisms, and hence if these effects can be further characterized, measurements of 222Rn escape have the potential to be useful for probing the internal structure of zircon, and other U-bearing minerals.

Syn-collisional granitoids in the Qilian Block on the Northern Tibetan Plateau: A long-lasting magmatism since continental collision through slab steepening

In this paper we present a new model that can explain the large zircon age spectrum of ~510 – 420Ma within a single sample from the Gangcha (Gcha) biotite granodiorite and the Huangyuan (HY) two-mica monzogranite on the northern Tibetan Plateau. The large age spread recorded in zircons is characteristic of granitoid samples from the studied region, which is best explained by the long-lasting magmatism since the onset of continental collision at ~500Ma, followed by slab steepening and the ultimate slab break-off at ~450Ma. These granitoids have a large major and trace element compositional variation, but limited initial Sr (ISr[450]=0.709 to 0.715), Nd (ԐNd[450]=−6.5 to −3.7), Hf (ԐHf[450]=−4.3 to 1.5) and Pb (206Pb/204Pb[450]=17.70 to 17.17; 207Pb/204Pb[450]=15.60 to 15.69; 208Pb/204Pb[450]=38.04 to 38.73) isotopic variation. The small negative whole rock ԐNd[450] and ԐHf[450] values are most consistent with the granitoid source being dominated by subducted seafloor materials. The inherited zircons with large negative ԐHf[450] values (e.g. −50) are indicative of input from the lower continental crust and subducted sediments. The correlated variations among major elements, trace elements and radiogenic isotopes are best interpreted as reflecting melting-induced mixing of a compositionally heterogeneous source with superimposed effect of varying extent of fractional crystallization and crustal assimilation. The inherited zircons of Palaeo-Proterozoic age and the Archean crustal model ages signify the involvement of ancient basement rocks.

U–Pb detrital zircon dates and source provenance analysis of Phanerozoic sequences of the Congo Basin, central Gondwana

The Congo Basin (CB) is the largest sediment sink of central Gondwana, built on a mosaic of Precambrian crustal blocks amalgamated during the mid-Paleoproterozoic (Eburnian; 2.1–1.8Ga), late Mesoproterozoic (Kibaran; 1.4–1.0Ga), and late Neoproterozoic–early Cambrian (Pan African; 750–500Ma). Sporadic uplift, tilting and erosion of these Precambrian terrains form the source regions for the sedimentary sequences that fill the CB. We investigate the Phanerozoic successions in the field and along four historic deep boreholes drilled in the center of the basin, and date detrital zircons from the main stratigraphic groups to characterize their provenance ages and reconstruct the paleogeographic evolution of the CB during amalgamation and break-up of the Gondwana supercontinent. Sedimentological data show that the oldest, upper Neoproterozoic–lower Paleozoic Redbeds (the Inkisi, Aruwimi and Biano Groups) were derived from the north. Zircons from these sequences have two dominant age-populations of 1100–950Ma and 800–600Ma, likely sourced from Kibaran and Pan African terrains within the Oubanguides (e.g. the Central Saharan Belt) and parts of the North African Shield (e.g. Darfour). The overlying Carboniferous–Permian glacial and deglaciation sequences (the Lukuga Group) have similar peaks, as well as abundant zircons of 2.05–1.85Ga and a subordinate number dated at 1.42–1.37Ga. The latter are from Eburnian and Kibaran sources in east-central Africa, consistent with west-facing glacial paleo-valleys preserved along the eastern margin of the CB. The succeeding Triassic (the Haute–Lueki Group) and Jurassic–Cretaceous (the Kwango Group) fluvial and aeolian red sandstones were again derived from the north. Their range of zircon dates has two main peaks at 1000Ma and 600Ma, but also contain small younger grains of 290–240Ma and 200–190Ma. We interpret these younger zircons to be derived from volcanic dust that originated during late Paleozoic–Jurassic magmatism of the Choiyoi and Chon Aike Provinces flanking the Andean subduction margin of Gondwana. By contrast, the uppermost Cenozoic alluviums of the CB (the Kalahari Group) contain diamond concentrates and large zircon fragments dated at 3–2.5Ga, derived from the Kasai and Cuango Cratons, to the south, which host Cretaceous diamondiferous kimberlites.

Major and trace elements of zircons from basaltic eucrites: Implications for the formation of zircons on the eucrite parent body

The major and trace elements in zircons obtained from basaltic eucrites Yamato [Y]-75011, Y-792510, Y-82082, Asuka [A]-881467, Juvinas, and Stannern were determined using an electron microprobe analyzer and a sensitive high-resolution ion microprobe, and the formation of zircons on the parent body was discussed. The maximum sizes of zircon grains in each sample are likely to be related to its metamorphic grade. The least-metamorphosed basaltic eucrite Y-75011 contains zircons with sizes of a few μm. Conversely, the highly metamorphosed basaltic eucrites Y-792510 and A-881467 include zircons larger than 20 μm. The relationship between the maximum grain size and metamorphic grade suggests that the formation process of zircons is related to metamorphic events. The Zr/Hf ratios of the zircons found in this study show a wide variation (31.6–71.7), as compared to the chondritic values (Zr/Hf = 32.8–34.3) and bulk basaltic eucrites (34.0–34.7). The Zr/Hf ratios of the zircons from Y-792510 and A-881467 are relatively constant, whereas those of the zircons from Y-82082, Juvinas, and Stannern show large variations. The rare earth element (REE) content in the zircons from Y-792510 and A-881467 is given by La=0.1×CI and Lu = 1000 × CI. On the other hand, the zircons from Stannern show higher REE content (La=0.1–1×CI and Lu=1000–10000×CI) than those from Y-792510 and A-881467. The most reliable REE data of a large zircon from A-881467 show no Ce anomaly and a distinct negative Eu anomaly. Therefore, it is presumed that the zircons formed under reducing condition in which Ce3+ was stable. The melt compositions coexisting with the studied zircons suggest that the zircons from highly metamorphosed eucrites might react with the REE-enriched melt derived from partial melting of the mesostasis region during metamorphism.

Zircon‐bearing, crystallized melt inclusions in peritectic garnet from the western Adirondack Mountains, New York State, USA

AbstractCrystallized melt inclusions (MI) occur in garnet from metapelitic gneiss extracted from a hydroelectric plant on the Black River at Port Leyden, NY (western Adirondack Highlands). The garnet host grains are small (0.5–1.0 mm), euhedral neoblasts that formed peritectically during biotite melting accompanying the Ottawan phase of the Grenville Orogeny. The crystallized MI are typically 10–25 μm in diameter and occur randomly in garnet. Nearly all of the crystallized MI observed contain a relatively large (3–15 μm), often euhedral, zircon grain surrounded by finer‐grained (some &lt;1 μm) phases with energy dispersive X‐ray spectra consistent with biotite, K‐feldspar, quartz, and albite. The inclusions are similar in size and composition to those described in garnet from other migmatites and are similarly interpreted as crystallized anatectic melt. Low Zr solubility in silicate melt indicates that the large zircon grain in each inclusion must be a trapped solid. It is inferred that micrometer‐scale droplets of anatectic melt chemically adhered to occluded zircon grains in biotite while peritectic garnet grew around them.

Synexhumation anatexis of ultrahigh-pressure metamorphic rocks: Petrological evidence from granitic gneiss in the Sulu orogen

Petrological evidence is provided for partial melting of ultrahigh-pressure (UHP) metamorphic granitic gneiss in the Sulu orogen. Petrographic observations show the occurrence of elongated, highly cuspate feldspars in grain boundaries, interstitial cuspate feldspars in triple junctions, felsic veinlets mainly consisting of K-feldspar+quartz, and feldspar crystal faces against quartz. These features indicate that the feldspar and quartz would have grown from anatectic melts in the granitic gneiss. Zircon domains grown from these melts were identified based on CL images, mineral inclusions and REE patterns. Some large zircon grains (>100μm) contain small relict domains of magmatic origin, suggesting nearly complete dissolution of the protolith zircon during the anatexis. All newly grown zircon domains are categorized into two groups based on the presence or absence of coesite inclusions. One group of domains contain no coesite inclusion and exhibit high U contents but low Th/U ratios (<0.1), steep REE patterns with strong negative Eu anomalies, and UPb ages of 217±2 to 224±2Ma. The other group of domains contain coesite inclusions and exhibit low U contents and very low Th/U ratios (<0.01), steep REE patterns with strong negative Eu anomalies, and UPb ages of 221±5 to 226±3Ma. The two groups of zircon domains are thus interpreted as growing from the anatectic melts at different pressures because they exhibit marked negative Eu anomalies that are absent for metamorphic zircons grown from aqueous fluids. The zircon UPb ages of 217±2 to 226±3Ma are close to, but slightly younger than, known ages for major UHP metamorphism in the Sulu orogen. Therefore, the UHP gneiss experienced incipient melting during the initial exhumation subsequent to the peak UHP metamorphism and extensive anatexis later at lower pressures. Muscovite relicts coexist with cuspate feldspars, suggesting that the anatectic melts originate from dehydration melting due to decompression breakdown of phengitic muscovite in the UHP granitic gneiss. As the degree of partial melting increases with temperature, significant fractions of the anatectic melts would be produced in the regional gneiss provided that the UHP rocks experienced “hot” exhumation. Such melts can be gathered together, being eventually emplaced as synexhumation granitic intrusions.

Brittle-ductile microfabrics in naturally deformed zircon: Deformation mechanisms and consequences for U-Pb dating

We present an electron backscatter diffraction, cathodoluminescence, and radiogenic U-Pb dating study of large zircon grains (0.8-1.5 mm) that show evidence of intracrystalline deformation, fracturing, grain size reduction and a large spread in U-Pb ages. The samples are from an amphibolite facies deformation zone within granulite facies anorthositic rocks (Bergen Arc, Norway). Large zircon grains show three main lattice distortion types: (I) distortions with rotations around &lt;001&gt; and an orientation change of ~0.3 °/μm subparallel to (100); (II) highly distorted, half circular shaped zones located at grain edges with at least 0.8-1°/μm distortions; and (III) low-angle boundary networks forming deformation zones up to 100 μm wide. Types II and III distortions exhibit significant disturbances of the otherwise homogeneous CL signature. Crystal plastic deformation with the slip system [010](100) resulted in type I distortions. Stress concentrations at grain contacts between rheologically hard grains caused localized crystal plastic deformation with minor amount of microfracturing forming type II distortions. Type III distortions formed by crystal plastic deformation often associated with inclusions using several slip systems. Distortions of types I and II show minor and moderate resetting of the original ca. 900 Ma zircon grains, respectively, due to enhanced pipe diffusion along dislocation walls. In type II distortions, accelerated lattice diffusion through the highly distorted crystal lattice, combined with exceptionally high boundary to volume ratio, caused significant chemical disturbance and age resetting to 410 Ma. Fine-grained aggregates contain grains with low internal deformation and an oscillatory zoned CL signature (Z-grains) or high internal deformation and a disturbed CL signature (D-grains). Z- and Dgrains are interpreted to have formed by heterogeneous nucleation and growth, and fracturing along strain-hardened low-angle boundaries present within types I and II, respectively. Z-grains show a clustered chemical signature with a 437 ± 11 Ma age interpreted to directly date the Caledonian amphibolite facies reworking.