Zircon Samples Research Articles

Unusually high thermal stability of the Uranium-Xenon isotopic system in nonmetamict zircons

Modern isotopic geology requires new approaches to geothermochronological problems for deciphering temperature evolution of magmatic and sedimentary rocks and related geological processes. One of the future research avenues in this field is the simultaneous application of genetically interrelated isotopic geochronological systems of noble gases: not only U‐Th‐He [1], but also U–Xe s , U–Xe n , U–Kr s , and U–Kr n [2, 3] in combination with the U‐Th‐Pb isotopic system (Xe and Kr are the products of spontaneous and neutroninduced fission [4]). This will maximally increase the geochronological reliability of the obtained ages and give opportunities unique for geothermogeochronology owing to a common mechanism of migration of different radiogenic isotopes having different kinetic parameters of migration. In order to make this approach applicable to practice, it is necessary to carry out extensive studies of migration characteristics of He, Xe, and Kr and the stability of the Xe/U, He/U, and Kr/U isotopic systems in mineral geochronometers, primarily, zircons. In this work, we experimentally determined the migration parameters of radiogenic xenon and energetic characteristics of the U‐Xe isotopic system in the nonmetamict zircons with perfect crystalline structure; demonstrated for the first time the unique stability of the uranium‐xenon isotopic system in the nonmetamict zircon samples; studied and compared the state of genetically linked isotopic systems: U‐Xe and U‐Pb (using local methods of ion microprobe). The study objects were thoroughly selected zircons with perfect structure, without signs of metamictization: a large fragment of a gem-quality zircon crystal from Sri Lanka (Ceylon Island); a zircon from Nigeria that had a nonmetamict undisturbed structure of grains; two samples from plagiogranites (enderbites), MacMaxon Island, Antarctica: sample 11v32-b represented by opaque lilac‐yellow grains with fused crystal faces; and sample 11g-1/32 from plagiogranite (enderbite) from MacMaxon Island, Antarctica, consisting of almost transparent long-prismatic crystals with very rare inclusions.

The effect of chemical etching on LA–ICP-MS analysis in determining uranium concentration for fission-track chronometry

Abstract LA–ICP-MS (laser ablation–inductively coupled plasma-mass spectrometry) has the potential to measure uranium concentration for fission-track (FT) chronometry as an alternative to thermal neutron-induced fission of 235 U. This study examines the effect that chemical etching, required to reveal spontaneous fission tracks of 238 U, has upon LA–ICP-MS analyses. Uranium concentrations were measured before and after etching for six large gem-quality apatite crystals and six zircon samples – three large crystals and three FT age standards. Comparison of the results shows no significant difference in 238 U concentrations measured on the etched and unetched mineral surfaces. The 238 U concentrations determined by the LA–ICP-MS provide reasonable FT ages for the zircon age standards, which, with the previously reported LA–ICP-MS apatite FT results, promotes the use of the LA–ICP-MS for FT chronometry.

Geothermochronology based on noble gases: I. Stability of the U-Xe isotopic system in nonmetamict zircons

The paper presents data on persistent tendencies and relations in the migration of noble gases in U-bearing minerals of various composition: uraninite, pitchblende, metamict zircon, khlopinite, samarskite, betafite, and ampangabeite from various regions worldwide. The escape curves of all noble gases (starting with radiogenic He, Kr, and Xe and ending with nucleogenic 38Ar) during annealing in the laboratory are demonstrated to include three or four extrema, which suggest a change (increase or decrease) in the escape of rare gases at certain temperatures. This, in turn, implies that noble gas atoms are contained in the structures of minerals in various energy states and that all calculations of migration parameters of noble gases according to the classic diffusion model based on Fick’s laws are inaccurate. The paper presents a brief description of an alternative mechanism underlain by an exponential dependence of the migration kinetics on temperature and time. A principle is proposed for the isotopic dating of minerals based on the application of the neutron-induction U-Xe techniques. The paper presents a technique for studying the escape of noble gases from minerals. Experimental data are obtained on the escape kinetics of radiogenic and neutron-induced (in a nuclear reactor) Xe components from four nonmetamict zircon samples. One of them (zircon from Sri Lanka) is characterized by a particularly homogeneous crystal structure. Its optical examination indicates that this transparent gem-quality zircon contains no inclusions or structural flaws or defects. Its cathodoluminescence images show only thin, weakly contrasting zoning of the magmatic type. Mass spectrometric SHRIMP II data reveal a very homogeneous distribution of U and Th in the zircon grain. Nevertheless, the curve of Xe escape kinetics for this sample shows well pronounced extrema, which suggest that Xe atoms are characterized by different settings in the zircon structure: they can be accommodated at defects and structural zones of various types (crystalline, amorphous, and metamict). According to literature data, such zones are nanometer-sized. For each of the detected extrema, which correspond to the discrete energy settings of Xe atoms in the structure, migration parameters are calculated: the activation energy and frequency factor, which are proved to be strongly correlated. The stability of radiogenic Xe in the structure of crystalline (nonmetamict) zircons calculated from these parameters was demonstrated to be extremely high.

Micro‐Raman spectroscopy and SEM/EDX applied to improve the zircon fission track method used for dating geological formations

AbstractThe zircon mineral is widely studied in geochronology. In the case of the fission track method (FTM), the age is determined by the density of fission tracks at the zircon surface, which can be observed with an optical microscope after an appropriate chemical treatment (etching). The etching must be isotropic at the zircon grain surface to be used in the FTM, which leads those zircon grains whose etching is anisotropic to be discarded. The only reason for this discarding is the nonuniform morphology of the surface grain seen by optical microscopy, that is, no further physicochemical analysis is performed. In this work, combining micro‐Raman and scanning electron microscopy (SEM) to study the etching anisotropy, it was shown that zircon grains that present at least one area at the surface where the density of fission track is uniform can be used in the FTM. The micro‐Raman showed characteristic spectra of the standard zircon sample either from the areas where there are tracks or from where there are not. The only difference found was in the Raman bandwidths, which were broader for the areas with higher density of fission tracks. This suggests simply a decrease in the relative percentage of the crystalline/amorphous phases at these areas. The SEM/energy dispersive spectrometry (EDX) showed that there were no significant differences in the principal chemical composition at the areas with and without fission tracks. Copyright © 2008 John Wiley & Sons, Ltd.

Raman study of radiation-damaged zircon under hydrostatic compression

Pressure-induced changes of Raman band parameters of four natural, gem-quality zircon samples with different degrees of self-irradiation damage, and synthetic ZrSiO4 without radiation damage, have been studied under hydrostatic compression in a diamond anvil cell up to ~10 GPa. Radiation-damaged zircon shows similar up-shifts of internal SiO4 stretching modes at elevated pressures as non-damaged ZrSiO4. Only minor changes of band-widths were observed in all cases. This makes it possible to estimate the degree of radiation damage from the width of the ν3(SiO4) band of zircon inclusions in situ, almost independent from potential “fossilized pressures” or compressive strain acting on the inclusions. An application is the non-destructive analysis of gemstones such as corundum or spinel: broadened Raman bands are a reliable indicator of self-irradiation damage in zircon inclusions, whose presence allows one to exclude artificial color enhancement by high-temperature treatment of the specimen.

Crustal‐scale structural architecture, shortening, and exhumation of an active, eroding orogenic wedge (Chugach/St Elias Range, southern Alaska)

Active mountain building associated with the accretion of the Yakutat microplate (YT) in southern Alaska is characterized through the combination of a new balanced cross section and new low‐temperature cooling ages. This analysis constrains the amount and timing of shortening, the spatial and temporal trends of exhumation, and the interplay between structural development and exhumation. A fold‐and‐thrust belt comprising three principal thrust sheets (the Hope Creek, Sullivan, and an offshore thrust sheet from north to south) characterizes the YT internal structure, which has absorbed a minimum of ∼82 km shortening. Assuming shortening and foreland basin development occurred contemporaneously, the shortening rate across the YT is ∼13–14 mm/a after 5.6 Ma. Detrital apatite fission track ages, from south to north, are unreset along the southern edge of the Sullivan thrust sheet at the coast, are reset and have ages younger than 6.3 Ma within the thrust belt, and have reset cooling ages of ∼13 Ma in the North American upper plate. Only the zircon samples from the northern, internal YT are potentially reset. Exhumation rates within the thrust belt vary from 0.3 ± 0.1 mm/a to 4 ± 1.8 mm/a. Combining the thermochronometric and structural data indicate that wedge exhumation since ∼6 Ma is 500 km2 and that particle trajectories have larger horizontal than vertical components. Whereas exhumation and shortening have been focused on the windward side of the Chugach/St. Elias Range since ∼6 Ma, the cooling ages do not uniquely distinguish between orographically versus tectonically controlled erosion.

Development of mullite/zirconia composites from a mixture of aluminum dross and zircon

In order to obtain mullite/zirconia composites, mixtures of aluminum dross and zircon were sintered. Aluminum dross was collected and purified by a milling, sieving and washing process. Stoichiometric mixtures of aluminum dross and zircon were sintered at several temperatures (1400, 1450 and 1500 °C) for several periods of time (2, 4 and 6 h). After the purifying treatment the dross contained mainly Al 2O 3, AlN, MgAl 2O 4, SiO 2 and metallic Al. A homogeneous mullite matrix with small zirconia particles was obtained by sintering the aluminum dross–zircon samples at 1500 °C for 6 h.

Micro‐Analytical Zircon and Monazite U‐Pb Isotope Dating by Laser Ablation‐Inductively Coupled Plasma‐Quadrupole Mass Spectrometry

In this study we evaluated the capability of a 213 nm laser ablation system coupled to a quadrupole‐based ICP‐MS in delivering accurate and precise U‐Pb ages on zircons and monazites. Four zircon samples (ca. 50 Ma to ca. 600 Ma) and four monazite samples (ca. 30 Ma to ca. 1390 Ma) of known ages were analysed utilising laser ablation pits with diameters of 20 μm and 60 μm. Instrument mass bias and laser induced time‐dependent elemental fractionation were corrected for by calibration against a matrix‐matched reference material. Tera‐Wasserburg plots of the calculated U‐Pb data were employed to assess, and correct for, common Pb contributions. The results indicated that the LA‐ICP‐MS technique employed in this study allowed precise and accurate U‐Pb isotope dating of zircon and monazite on sample areas 20 μm in diameter. At this spot size, the precisions achieved for single spot 206Pb/238U ages, were better than 5% (2s) for monazites and zircons with ages down to 30 Ma and 50 Ma, respectively. The precisions reported are comparable to those generally reported in SIMS and LA‐MC‐ICP‐MS U‐Pb isotope determinations.

Gangdese retroarc thrust belt and foreland basin deposits in the Damxung area, southern Tibet

Geologic mapping and U–Pb detrital zircon geochronologic studies of (meta)sedimentary rocks in the Damxung area (∼90 km north of Lhasa) of the southern Lhasa terrane in Tibet provide new insights into the history of deformation and clastic sedimentation prior to late Cenozoic extension. Cretaceous nonmarine clastic rocks ∼10 km southeast of Damxung are exposed as structural windows in the footwall of a thrust fault (the Damxung thrust) that carries Paleozoic strata in the hanging wall. To the north of Damxung in the southern part of the northern Nyainqentanglha Range (NNQTL), metaclastic rocks of previously inferred Paleozoic age are shown to range in depositional age from Late Cretaceous to Eocene. The metaclastic rocks regionally dip southward and are interpreted to have been structurally buried in the footwall of the Damxung thrust prior to being tectonized during late Cenozoic transtension. Along the northern flank of the NNQTL, Lower Eocene syncontractional redbeds were deposited in a triangle zone structural setting. All detrital zircon samples of Cretaceous–Eocene strata in the Damxung area include Early Cretaceous grains that were likely sourced from the Gangdese arc to the south. We suggest that the that newly recognized Late Cretaceous to Early Eocene (meta)clastic deposits and thrust faults represent the frontal and youngest part of a northward directed and propagating Gangdese retroarc thrust belt and foreland basin system that led to significant crustal thickening and elevation gain in southern Tibet prior to India-Asian collision.

Laser ablation coupled with ICP-MS applied to U–Pb zircon geochronology: A review of recent advances

Improvements in the technology of laser ablation and ICP-MS instruments make LA-MC-ICPMS a rapid, precise and accurate method for U–Pb zircon geochronology. In this review we describe the main stages of the evolution of this in situ approach from the early 1990s to the present time. Some key points have been progressively improved. The crater size has been reduced to achieve real in situ measurements. The laser wavelength has been reduced as well as the duration of each pulse in order to lower inter-element fractionation. The blank from the gas has to be lowered as far as possible. Double focusing instruments and magnetic field sectors allow flat-topped peaks required for precise isotope ratio measurement to be obtained. The use of a multi-ion counting system significantly improves the sensitivity of the method and the static mode of integration favours the precision of measurement of the transient signal originating from a noisy laser ablated particle beam. Combining the use of a 213 nm UV laser and a MC-ICPMS equipped with a multi-ion counting system operating in static mode, the common precisions achieved for the key ratios 207Pb/ 206Pb and 206Pb/ 238U are better than 1% and 3% (2 σ) respectively, including error propagation associated with standard normalization. Until now, the use of a zircon standard has remained necessary to ensure the accuracy of the calculated age. A strategy for common-Pb correction is proposed according to the age of the zircon and according to the Th/U ratio of the grains. After recording sixteen to twenty spot analyses the precision usually achieved on the age is about 1% and even significantly better for Proterozoic samples. In order to show the performance achieved by modern LA-MC-ICPMS geochronology, we tested four zircon samples covering a wide age range from 290 to 2440 Ma. These new age determinations can be compared in term of precision and accuracy since they have already been dated by reference methods (ID-TIMS and SHRIMP). Further developments in the technology of ion counters equipping modern MC-ICPMS and in laser systems will certainly be applied to a large field of geochronology studies in the near future as an alternative to SIMS for in situ age determination.

Thermochronology of non-volcanic hydrothermal activity in the Kii Peninsula, Southwest Japan: Evidence from fission track dating and helium isotopes in paleo-hydrothermal fluids

Although there is no indication of volcanism during the Pliocene and Quaternary in the Kii Peninsula, Southwest Japan, the peninsula has long been recognized to be unusual and atypical of non-volcanic regions, with anomalous high-temperature hot springs and high 3He content in hot spring gases. Recently, it has been suggested that aqueous fluids generated by dehydration of the subducting Philippine Sea plate carry mantle helium from the subcrustal lithosphere to terrestrial water, resulting in the emanation of high 3He gases at high-temperature hot springs [e.g., Matsumoto T., Kawabata T., Matsuda J., Yamamoto K. and Mimura K. 2003. 3He/ 4He ratios in well gases in the Kinki district, SW Japan surface appearance of slab-derived fluids in a non-volcanic area in Kii Peninsula. Earth Planet. Sci. Lett. 216, 221–230]. Fission track dating of zircon and apatite samples from the altered host rocks around three hot springs (Yunomine Hot Spring, Kamiyu Hot Spring and Tosenji Hot spring) was carried out in order to elucidate the thermochronology of the hydrothermal system in this non-volcanic region. Additionally, new helium isotope data from paleo-fluids trapped in fluid inclusions in hydrothermal minerals were obtained for comparison with the present-day hydrothermal fluids from the above hot springs. The apatite FT ages from the altered host rocks around the hot springs ranged from 2.5 Ma to about 5 Ma, which are significantly younger than the 12 Ma age of the non-altered host rocks far from the hot springs. In the southern Kii Peninsula, therefore, it is obvious that the non-volcanic hydrothermal activity already has been ongoing since Pliocene or early Pleistocene time. The 3He/ 4He ratios of quartz veins and pyrite disseminated in the altered host rock around the hot springs ranged from 1.1 to 3.6 R A , generally in agreement with those of the present-day hydrothermal fluids. Almost constant values of 3He/ 4He ratios over time indicate that no important changes have occurred in deep source gases, at least during the last several million years.

Pure zircon process for removing radionuclides from zircon concentrates

The presence of U and Th as impurities in zircon makes it radioactive. Provided the U and Th levels are below 500 ppm, or 70 Bq g-1 activity, the limits that allow transportation of the material as a non-radioactive substance, it is acceptable as a commercial product. However, some zircon sands contain radioactivity levels well above this limit. A new process (the pure zircon process) is described for the removal of radioactivity from zircon. Tests to identify the best flux addition, calcination temperature and time, and leaching conditions indicated that by grinding a zircon sample containing 1059 ppm U + Th to a particle size of about 15–20 μm, calcining it with 15 wt-% calcium borate (colemanite) at 1200°C for 4 h and leaching the calcine with up to 1M HCl at 80°C for 30 min gave a product with <500 ppm U + Th. While the grade of Zr of the final product was slightly increased, the levels of major impurities, Al, Fe, Ti and P were decreased. Operating costs for the pure zircon process using the optimum conditions were estimated to be A$243/t leach product.

U-Pb geochronologic evidence for the evolution of the Gondwanan margin of the north-central Andes

We investigated the Neoproterozoic–early Paleozoic evolution of the Gondwanan margin of the north-central Andes by employing U-Pb zircon geochronology in the Eastern Cordilleras of Peru and Ecuador using a combination of laser-ablation–inductively coupled plasma–mass spectrometry detrital zircon analysis and dating of syn- and post-tectonic intrusive rocks by thermal ionization mass spectrometry and ion microprobe. The majority of detrital zircon samples exhibits prominent peaks in the ranges 0.45–0.65 Ga and 0.9–1.3 Ga, with minimal older detritus from the Amazonian craton. These data imply that the Famatinian-Pampean and Grenville (= Sunsas) orogenies were available to supply detritus to the Paleozoic sequences of the north-central Andes, and these orogenic belts are interpreted to be either buried underneath the present-day Andean chain or adjacent foreland sediments. There is evidence of a subduction-related magmatic belt (474–442 Ma) in the Eastern Cordillera of Peru and regional orogenic events that pre- and postdate this phase of magmatism. These are confirmed by ion-microprobe dating of zircon overgrowths from amphibolite-facies schists, which reveals metamorphic events at ca. 478 and ca. 312 Ma and refutes the previously assumed Neoproterozoic age for orogeny in the Peruvian Eastern Cordillera. The presence of an Ordovician magmatic and metamorphic belt in the north-central Andes demonstrates that Famatinian metamorphism and subduction-related magmatism were continuous from Patagonia through northern Argentina to Venezuela. The evolution of this extremely long Ordovician active margin on western Gondwana is very similar to the Taconic orogenic cycle of the eastern margin of Laurentia, and our findings support models that show these two active margins facing each other during the Ordovician.

Uranium-lead dating method at the Pará-Iso isotope geology laboratory, UFPA, Belém - Brazil

Analytical procedures for U-Pb isotope dilution analyses at the Pará-Iso isotope geology laboratory of the Federal University of Pará (UFPA) are described in detail. The procedures are applied to zircon, titanite, rutile, apatite, columbite-tantalite and whole rock. Reagent preparation and chemical processing are done in clean-room conditions. Samples are dissolved using Teflon microcapsules in steel jacket Teflon Parr Instrument bomb or Teflon screw cap containers. U and Pb are separated using anion exchange AG 1 x 8 resin columns. Typical blanks for mineral sample amounts of 0.01 to 1.0 mg are less than 1 pg U and 20-30 pg Pb. Isotope analysis of the U and Pb from the same filament are carried out using a Finnigan MAT 262 mass-spectrometer in static and dynamic modes. The current analytical level is demonstrated on analyses of international standard zircon 91500 with three different 235U-205Pb and 235U-208Pb isotope tracers and whole rock standards. Results of analyses of two zircon samples are also presented.

Application of inductively coupled plasma quadrupole mass spectrometry for the determination of monazite ages by lead isotope ratios

In order to evaluate the applicability of inductively coupled plasma quadrupole mass spectrometry to the determination of Pb/Pb, U/Pb and Th/Pb ages of monazite, studies were carried out initially applying lead atom ratio reference standards (NIST SRM 981 and 982). Further, the optimized methodology was applied to monazite sands from three different sites, Sugar Loaf Hill (Rio de Janeiro city), Buena (Rio de Janeiro state) and Black Sands Beach (Guarapari, Espirito Santo state); the obtained mean ages, (581 ± 21)Myears, (552 ± 32)Myears and (535 ± 3)Myears, respectively, are in agreement with the literature values for zircon and monazite samples from the same sites. Single grain Th/Pb ages were also determined for the monazite sample from Black Sands beach, nine grains were analyzed and the mean value, (530 ± 26)Myears, is in agreement with the value obtained with bulk samples. Finally, the method was applied to a thorianite sample from Amapá state and the observed Th-Pb, U-Pb and Pb-Pb ages obtained were (2.15 ± 0.05)Gyears, (2.03 ± 0.01)Gyears anda (2.044 ± 0.006)Gyears respectively, with a mean value of (2.08 ± 0.07)Gyears. This value is coherent with the (2.08 ± 0.02)Gyears age of the Bacuri complex, Amapá, reported in the literature.

Autoclave breakdown of zircon with ammonium fluorides

Autoclave breakdown of zircon with ammonium fluorides was studied. DOI: 10.1134/S1070427206110036 Zircon is one of the main mineral sources of zirconium materials, but the technology of its breakdown and processing is still labor-consuming, and much efforts are made to improve it. The classical industrial methods of breaking down zirconium concentrates are sintering with fluosilicates and chlorination. Numerous studies summarized in [1] were devoted to zircon fluorination; however, it did not gain industrial application. The fluoride technology allows the scheme of the zirconium product processing to be reduced considerably, but high cost and corrosive behavior of HF and F2, and also SiF4 liberation restrain industrial application of these reagents. In this study we examined the autoclave breakdown of zircon with ammonium fluorides. Unlike fluorine and hydrogen fluoride, ammonium fluorides are very convenient to use, as these are solid crystalline substances under normal conditions. Ammonium fluorides can be obtained from metal fluorides by the action of ammonia water, which is the basis for conversion and recycling of ammonium fluorides. Ammonium fluoride is known to react slowly with zircon [2, 3]. At temperatures below 240 C, the reaction rate is negligible and the degree of breakdown is unsatisfactory for the industrial use, whereas at higher temperatures ammonium fluoride evaporates with decomposition. Here we report new data on the zircon breakdown with ammonium fluoride and hydrofluoride. The essence of the method consists in the reaction of zircon with ammonium fluoride or hydrofluoride under isochoric conditions (in an autoclave), after which the reaction products are separated by sublimation to give zirconium tetrafluoride and dioxide. The advantage of ammonium fluorides over other fluorinating agents consists in their convenient physicochemical properties allowing their regeneration and recycling. The experiments were carried out with zircon samples from Tugan deposit of Tomsk oblast. The reactions of zircon with ammonium fluoride and hydrofluoride were studied under isochoric conditions in a 100 cm3 stainless steel autoclave. A weighed portion of zircon with a grain size of 0.074 mm was placed in the autoclave together with a weighed portion of ammonium fluoride taken in a 20% excess in relation to the stoichiometric amount calculated for the reaction ZrSiO4 + 13NH4F = (NH4)3ZrF7 + (NH4)2SiF6 + 8NH3 + 4H2O. The autoclave was heated to 150, 200, 250, 300, 350, and 400 C and kept at these temperatures for 0.5, 1, 2, and 4 h. After opening the autoclave, its content was transferred into a crucible and calcined for 30 min at 400 C to remove by sublimation unchanged NH4F and formed (NH4)2SiF6. The residue in the crucible was a mixture of unchanged ZrSiO4 and NH4ZrF5, to which 5 ml of concentrated sulfuric acid was added, and the mixture was kept until H2SO4 vapor disappeared. Zirconium in the form of ZrOSO4 solution was leached with water from the formed cake, converted to zirconium dioxide, and weighed to determine the degree of conversion. The same procedure was used to study the reaction of ammonium hydrofluoride with zircon 2ZrSiO4 + 13NH4F HF = 2(NH4)3ZrF7 + 2(NH4)2SiF6 + 3NH3 + 8H2O.

Dating of prograde metamorphic events deciphered from episodic zircon growth in rocks of the Dabie–Sulu UHP complex, China

The timing of ultra-high pressure (UHP) metamorphism has been difficult to determine because of a lack of age constraints on crucial events, especially those occurring on the prograde path. New Sensitive High-Resolution Ion Microprobe (SHRIMP) U–Pb age and rare-earth element (REE) data of zircon are presented for UHP metamorphic rocks (eclogite, garnet peridotite, garnet pyroxenite, jadeite quartzite and garnet gneiss) along the Dabie–Sulu UHP complex of China. With multiphase metamorphic textures and index mineral inclusions within zircon, the Dabie data define three episodes of eclogite-facies metamorphism, best estimated at 242.1 ± 0.4 Ma, 227.2 ± 0.8 Ma and 219.8 ± 0.8 Ma. Eclogite-facies zircons of the Sulu UHP complex grew during two major episodes at 242.7 ± 1.2 and 227.5 ± 1.3 Ma, which are indistinguishable from corresponding events in the Dabie UHP complex. A pre-eclogite metamorphic phase at 244.0 ± 2.6 Ma was obtained from two Sulu zircon samples which contain low pressure–temperature (plagioclase, stable below the quartz/Ab transformation) and hydrous (e.g., amphibole, stable below ∼ 2.5 Gpa) mineral inclusions. In terms of Fe–Mg exchange of trapped garnet–clinopyroxene pairs within zircon domains, we are able to determine the Pressure–Temperature ( PT) conditions for a specific episode of metamorphic zircon growth. We suggest that mineral phase transformations and associated dehydration led to episodic eclogite-facies zircon growth during UHP metamorphism (∼ 2.7 Gpa) began at 242.2 ± 0.4 Ma ( n = 74, pooling the Dabie–Sulu data), followed by peak UHP metamorphism (> ∼ 4 Gpa) at 227.3 ± 0.7 Ma ( n = 72), before exhumation (< ∼ 220 Ma) to quartz stability (~ 1.8 Gpa). The Dabie–Sulu UHP metamorphism lasted for about 15 Ma, equivalent to a minimum subduction rate of 6 mm/year for the descending continental crust.

Thermal histories of the Cretaceous Pungam and Yeongdong basins, Korea based on apatite and zircon fission track analysis

Apatite and zircon fission track (FT) analyses were carried out to reconstruct the thermal histories of the Cretaceous Pungam and Yeongdong basins, Korea. These basins were formed along the sinistral strike-slip faults in the Early Cretaceous and were compressed in the Late Cretaceous by transpressional stresses due to the change in subduction direction of the Kula/Pacific Plate. In the Pungam and Yeongdong basins, apatite samples have consistent FT ages of ca. 50 Ma and ca. 63 Ma, respectively, much younger than their stratigraphic ages. In contrast, the zircon FT ages of both basins show relatively wide ranges, i.e., from 89 to 70 Ma in the Pungam Basin, and from 83 to 64 Ma in the Yeongdong Basin. Zircon single-grain age spectra also show multiple age populations. Co-existence of both the older and younger FT ages in comparison to the depositional age (Pungam Basin: ∼70 Ma, Yeongdong Basin: ∼100 Ma) indicates that the zircon samples from both basins were partially annealed. The Pungam Basin was heated into the zircon partial annealing zone (ZPAZ) by burial, volcanic activity and associated hydrothermal fluid, then cooled below the apatite closure temperature atca. 50 Ma. The Yeongdong Basin was also heated into the ZPAZ after deposition by burial and volcanic activity, then cooled down below the apatite closure temperature atca. 63 Ma, and was uplifted to the present surface. Comparing these data with those of the Gyeongsang Basin, the response to transpressional stresses seems not to be controlled by the distance of the basin from the active continental margin. Further studies are needed to clarify such tectonic inversion of the sedimentary basins in the active continental margin.

U‐Pb dating of zircon by LA‐ICP‐MS

In this study we used LA‐ICP‐MS (laser ablation–inductively coupled plasma–mass spectrometry) to determine U‐Pb ages of 5 zircon samples of known age (∼1800 Ma to ∼50 Ma) in order to determine the reproducibility, precision, and accuracy of this geochronologic technique. This work was performed using a ThermoFinnigan Element2 magnetic sector double‐focusing ICP‐MS coupled with a New Wave Research UP‐213 laser system. The laser ablation pit sizes ranged from 30 to 40 μm in diameter. Laser‐induced time‐dependent fractionation is corrected by normalizing measured ratios in both standards and samples to the beginning of the analysis using the intercept method. Static fractionation, including those caused during laser ablation and due to instrumental discrimination, is corrected using external zircon standards. Total uncertainty for each laser analysis of an unknown is combined quadratically from the uncertainty in the measured isotope ratios of the unknown and the uncertainty in the fractionation factors calculated from the measurement of standards. For individual analyses we estimate that the accuracy and precision are better than 4% at the 2 sigma level, with the largest contribution in uncertainty from the measurement of the standards. Accuracy of age determinations in this study is on the order of 1% on the basis of comparing the weighted average of the LA‐ICP‐MS determinations to the TIMS ages. Due to unresolved contributions to uncertainty from the lack of a common Pb correction and from potential matrix effects between standards and unknowns, however, this estimate cannot be universally applied to all unknowns. Nevertheless, the results of this study provide an example of the type of precision and accuracy that may be possible with this technique under ideal conditions. In summary, the laser ablation technique, using a magnetic sector ICP‐MS, can be used for the U‐Pb dating of zircons with a wide range of ages and is a useful complement to the established TIMS and SHRIMP techniques. This technique is especially well suited to reconnaissance geochronologic and detrital zircon studies.

Fission track evidence for the Mesozoic-Cenozoic tectonic uplift of Mt. Bogda, Xinjiang, northwest China

Fission-track dating evidence from 5 apatite samples and 4 zircon samples, and modeled time-temperature thermal history indicate that since Late Jurassic-Cretaceous (150 – 106 Ma), the uplift process of Mt. Bogda can be divided into four stages of thermal evolution: 150 – 106, 75 – 65, 44 – 24 and 13 – 9 Ma. Before 44 – 24 Ma, the cooling rate and uplifting rate of the southern and northern segments of Mt. Bogda are almost the same, showing that the uplifting of Mt. Bogda is an overall process. Since 44 – 24 Ma, the uplifting of the southern and northern segments of Mt. Bogda has shown differences. During 42 – 11 Ma, the northern segment of Mt. Bogda was at a steady stage, with the cooling rate being 0.03°C/Ma and the uplifting rate being 0.001 mm/a. From 11 Ma to the present, the northern segment of Mt. Bogda was at a rapid cooling and uplifting stage, with the cooling rate being 5.72°C/Ma and the uplifting rate being 0.19 mm/a. However, the southern segment of Mt. Bogda has been at a rapid cooling and uplifting stage since 26 Ma, with the cooling rate being 1.24°C/Ma and the uplifting rate being 0.041 mm/a during 26 – 9 Ma; 4.88°C/Ma and 0.163 mm/a from 9 Ma till now.