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In situ238U‐230Th dating was conducted using an ICP‐mass spectrometer coupled with a laser ablation sampling system (LA‐ICP‐MS). To minimise the contribution of mass spectrometric interferences on 230Th, a kinetic energy discrimination technique using a collision/reaction cell (CRC) system was employed. The major problem associated with the CRC‐ICP‐MS system is the lower transmission efficiencies of ions than sector field mass spectrometers. To improve the signal‐to‐background ratio for analytes, a combination of a multiple‐spot laser ablation (msLA) system together with a sensitivity‐enhanced dry plasma cone was employed. Analytical accuracy was evaluated through analyses of three zircon reference materials (GJ‐1, OD‐3 and Plešovice) and the resulting 230Th/238U ratios for GJ‐1 and Plešovice zircons agreed with the secular equilibrium value. The developed technique was applied to Quaternary volcanic zircon and monazite samples from Japan. The resulting ages are 113.5 +5.4‐5.2 ka for the Toya tephra, 105.1 +6.4‐6.1 ka for the Sambe Kisuki tephra, 55.2 +7.7‐7.2 ka for the Hikageyama lava and 16.2 +4.2‐4.1 ka for the Susaki pumice (95% confidence levels). The resulting ages are consistent with previous chronological constraints including zircon fission track ages. The data obtained here demonstrate clearly that the combination of the CRC‐ICP‐MS system and the msLA protocol can become a powerful analytical tool for in situ U‐Th disequilibrium dating. [ABSTRACT FROM AUTHOR]

The Nemuro and Saroma Groups and Yusenkyo Formation occur in eastern Hokkaido and are considered to be forearc or intra‐arc basin sediments of the Paleo‐Kuril arc (PKA) deposited during the Late Cretaceous to middle Eocene. To further clarify the origin of the PKA, we examined the U–Pb ages of detrital zircons within these sandstones and acidic tuff beds; based on our results, we drew the following conclusions. (1) The PKA originated from an oceanic island arc on the oceanic Izanagi Plate around 85 Ma, to which the Nikoro Group greenstone complex was accreted between 81–80 Ma; the Lowest Unit of the Saroma Group covered the surface of the Nikoro accretional greenstone complex. (2) The PKA then first collided with NE Asia around the beginning of the deposition of the Hamanaka Formation (~70 Ma) and transitioned to a continental arc adjacent to NE Asia. This collision generated giant slump beds during deposition of the Akkeshi Formation. During deposition of the Kiritappu Formation, the entire Nemuro Peninsula was uplifted, supplying large volumes of clastic sediments. (3) Although we do not have data directly bearing on why the North American Plate was established in the edge of NE Asia, we speculate that it separated from the Eurasian continent around ~70 Ma when NE Asia first collided with the PKA. Subsequently, the PKA has behaved as part of the North American Plate. The Paleo‐Japan arc (or East Sikhote–Alin arc) and the PKA became joined via the Hidaka Belt. Around 40 Ma, during the deposition of coarse conglomerate beds of the Urahoro Group, the PKA was uplifted and bent clockwise due to a second collision with NE Asia. (4) The modern Kuril arc was established around 36 Ma (late Eocene–early Oligocene), and the Kuril backarc basin opened into the present tectonic setting in the late Oligocene–early Miocene. [ABSTRACT FROM AUTHOR]

[Display omitted] • REE contents of the Paleoproterozoic dolostones were determined using LA-ICP-MS. • The obtained Y/Ho values indicate that the dolostones were marine sediments. • Its upper unit shows a positive correlation between Mn contents and Ce anomalies. • The Fe-Mn oxides leave traces on the dolostones of upper unit even after diagenesis. The Paleoproterozoic Francevillian Group in the eastern Gabonese Republic has been recently attracting increasing attention because it includes distinctive macroscopic structures interpreted as eukaryotic fossils. Therefore, its depositional setting and associated redox conditions need to be clearly understood. To clarify these, petrological studies and rare earth element (REE) abundances of carbonate rocks of the Francevillian Group in the Lastoursville basin were determined with ICP-MS coupled with a laser ablation sampling technique. Detailed microscopic observations indicate that dolostones in this basin underwent the complicated diagenetic history, including sparitization, decomposition of organic matter, reduction of ferromanganese oxides, silicification, and later calcification. The dolostones of lower Francevillian B (FB) Formation show heterogeneous textures formed by alterations during diagenesis. By the alterations, changes of REE abundances and degrees of positive Y anomalies appear to have occurred. This indicates that REE signatures in carbonate rocks can vary during diagenesis. Characteristically high Mn contents in the dolostones of upper FB Formation were derived from the reduction of ferromanganese oxides in sediments. The signatures of REE abundance in dolostones can be explained by the mixing of seawater and ferromanganese oxide components. In that sense, the dolostones inherited the trace element characteristics of precursor ferromanganese oxides even after the diagenesis. Y/Ho values of the most primary parts of each sample exceed 33, the newly determined threshold value for marine carbonate rocks based on our compilation. The magnitude of the La anomaly values also falls within the range of that of Holocene reefal microbialites. These indicate that a precursor of the dolostone of the Francevillian Group was deposited in a marine environment. In addition, presence of Ce anomalies in the upper FB Formation suggests that the Paleoproterozoic ocean was oxidized to such an extent as for Ce to be preferentially absorbed by ferromanganese oxides. [ABSTRACT FROM AUTHOR]

Greenstone bodies emplaced upon or into clastic sediments crop out ubiquitously in the Hidaka belt (early Paleogene accretionary and collisional complexes exposed in the central part of northern Hokkaido, NE Japan), but the timing and setting of their emplacement has remained poorly constrained. Here, we report new zircon U–Pb ages for the sedimentary complexes surrounding these greenstones. The Hidaka Supergroup in the northern Hidaka belt is divided into four zones from west to east: zones S, U, and R, which contain in situ greenstones; and zone Y, which does not. Detrital zircons in zones S, U, and R have early Eocene U–Pb ages (55–47 Ma) and these strata are intruded by early Eocene granites (46–45 Ma), indicating that they were deposited between 55 and 46 Ma. Therefore, in situ greenstones in the northern Hidaka belt can only be explained by the subduction of the Izanagi–Pacific Ridge during 55–47 Ma. In contrast, the deposition of zone Y (the Yubetsu Group, younging to the west) began by 73–71 Ma, indicating that the accretionary prism in front of the paleo‐Kuril arc formed at the same time as that in the Idonnappu zone and grew continuously until 48 Ma. The plutonic rocks that intruded the Hidaka belt are roughly divided into three stages: (1) early Eocene granites intruded the northern Hidaka belt at 46–45 Ma, during subduction of the Izanagi–Pacific Ridge; (2) the upper sequence of the Hidaka metamorphic zone was metamorphosed by magmatism at 40–37 Ma associated with the collision of the paleo‐Kuril arc and NE Asia; and (3) younger granites intruded the entire Hidaka belt at 20–17 Ma in association with asthenospheric upwelling caused by back‐arc expansion. [ABSTRACT FROM AUTHOR]

We present a new LA–ICP–MS system for zircon fission‐track (FT) and U–Pb double dating, whereby a femtosecond laser combined with galvanometric optics simultaneously ablates multiple spots to measure average surface U contents. The U contents of zircon measured by LA–ICP–MS and standardized with the NIST SRM610 glass are comparable to those measured by the induced FT method, and have smaller analytical errors. LA–ICP–MS FT dating of seven zircon samples including three IUGS age standards is as accurate as the external detector method, but can give a higher‐precision age depending on the counting statistics of the U content measurement. Double dating of the IUGS age standards gives FT and U–Pb ages that are in agreement. A chip of the Nancy 91500 zircon has a homogeneous U content of 84 ppm, suggesting the possibility of using this zircon as a matrix‐matched U‐standard for FT dating. When using the Nancy 91500 zircon as a U‐standard, a zeta calibration value of 42–43 year cm2 for LA–ICP–MS FT dating is obtained. While this value is strictly valid only for the particular session, it can serve as a reference for other studies. [ABSTRACT FROM AUTHOR]

Anorthositic series apatites of the Duluth Complex, Minnesota, USA, have high spontaneous fission‐track densities of up to ~107 cm–2 and a homogeneous age of ~900 Ma, allowing high‐precision fission‐track dating based on LA–ICP–MS U analysis. Absolute fission‐track dating, track‐length measurement and chemical composition analysis were performed to evaluate a cooling history, which is essential for age reference materials. Preliminary inverse modelling for a sample with a shortened track‐length distribution yielded a monotonic cooling history from ~100°C at 925 Ma. The apatites incur an over‐etching problem when employing the commonly used etching protocol involving 5.5 M HNO3. [ABSTRACT FROM AUTHOR]

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We have measured 238 U– 206 Pb, 235 U– 207 Pb, and 232 Th– 208 Pb ages on Quaternary zircons by laser ablation, single-collector, magnetic sector inductively coupled plasma mass spectrometry (LA-ICP-MS). To obtain reliable ages for Quaternary zircons, corrections for initial disequilibrium associated with deficits and excesses of both 230 Th and 231 Pa relative to secular equilibrium resulting from differential partitioning during zircon crystallization or source melting must be made. In contrast, the 232 Th– 208 Pb decay system is clearly advantageous for samples affected by disequilibrium because the 232 Th decay system lacks long-lived intermediate daughter isotopes. Conventionally, the initial disequilibrium for the 238 U and 235 U decay series has been determined by the distribution ratio between the melt and zircon (i.e., ƒ Th/U = (Th/U) Zircon /(Th/U) Melt and ƒ Pa/U = (Pa/U) Zircon /(Pa/U) Melt ). In our study, these correction factors were determined from comparison of the measured 238 U– 206 Pb and 235 U– 207 Pb ages with 232 Th– 208 Pb ages obtained for three zircons of known eruption and, in some cases, zircon crystallization ages (Kirigamine Rhyolite, Bishop Tuff, and Toga Pumice). The resulting correction factors are ƒ Th/U = 0.19 ± 0.14 and ƒ Pa/U = 3.66 ± 0.89 (Kirigamine Rhyolite), ƒ Th/U = 0.24 ± 0.20 and ƒ Pa/U = 3.1 ± 1.2 (Bishop Tuff), and ƒ Th/U = 0.28 ± 0.17 and ƒ Pa/U = 3.04 ± 0.99 (Toga Pumice). Although the uncertainties of these f values are relatively large, our results support the adequacy of the conventional approach for correction of initial disequilibrium. A recent study published results that apparently show zircon crystallization ages are younger than the eruption age of Bishop Tuff. It seems to be difficult to eliminate these discrepancies, even if the Th/U partitioning and disequilibrium generated during partial melting are taken into account for recalculation of its zircon age. However, magma chamber process and history of Bishop Tuff are too complex to obtain accurate zircon ages by U–Pb method. To overcome this, therefore, the Th–Pb zircon dating method is a key technique for understanding complex, pre-eruptive magma processes, and further efforts to improve its precision and accuracy are desirable. [ABSTRACT FROM AUTHOR]

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Abstract: The Neoproterozoic and Cambrian were two of the most dramatic periods in the history of the Earth, because large multi-cellular animals first appeared then in the so-called “Cambrian Explosion”. To better understand this event, many paleontological and geochemical studies now focus on rocks in South China, because of the fossiliferous succession and good exposure. Since the recognition of the Yangtze Gorges and Chengjiang area as type localities of the Sinian (Ediacaran) and Meishucunian (Early Cambrian) Systems, both sections have been intensively investigated. In order to decipher the relationships between the evolution of life and surface environmental changes, it is necessary to understand their paleontological, geochemical and geo-chronological constraints. This study presents new chronological constraints for the Cryogenian to Cambrian rocks in the Three Gorges, Weng'an and Chengjiang areas, South China. We discovered two tuff layers, one at the base of the Shuijingtuo Fm at Three Gorges and the other at the bottom of the Dahai Member in Chengjiang. In addition, we collected sandstones from Neoproterozoic strata in the Three Gorges, Chengjiang and Weng'an areas for provenance analysis. Zircons, separated from the tuff layers, provide new Nano-SIMS U–Pb ages of 526.4±5.4Ma at the base of the Shuijingtuo Fm, and 523.9±6.7Ma at the bottom of the Dahai Member. The tuffaceous beds occur at an unconformity, and nodules are common in the Three Gorges, Meishucun and Taoying sections, indicating that major and relatively wide-scale volcanic and sedimentological events occurred at ca. 525Ma. Moreover, carbonate carbon isotope chemostratigraphies at Morocco, Siberia, Three Gorges and Meishucun display different characteristics during the Tommotian. One possibility is that the South China Ocean was separated from an outer ocean at that time. Detrital zircons in sandstones have age populations at ca. 2.7, 1.8, 1.6, 1.0 and from 0.9 to 0.75Ga. indicating that the paleo-hinterland of the Nanhua and Kangdian rift basins was geologically complex. Despite the lack of ca. 1.6Ga rocks in the Yangtze and Cathaysia Blocks, these data are nevertheless interpreted to indicate derivation of the zircons from basement rocks in the Yangtze craton. [Copyright &y& Elsevier]

This paper evaluates the analytical precision, accuracy and long-term reliability of the U-Pb age data obtained using inductively coupled plasma–mass spectrometry (ICP-MS) with a frequency quintupled Nd-YAG (λ = 213nm) laser ablation system. The U-Pb age data for seven standard zircons of various ages, from 28 Ma to 2400 Ma (FCT, SL13, 91500, AS3, FC1, QGNG and PMA7) were obtained with an ablation pit size of 30 μm diameter. For 207Pb/206Pb ratio measurement, the mean isotopic ratio obtained on National Institute of Standards and Technology (NIST) SRM610 over 4 months was 0.9105 ± 0.0014 ( n = 280, 95% confidence), which agrees well with the published value of 0.9096. The time-profile of Pb/U ratios during single spot ablation showed no significant difference in shape from NIST SRM610 and 91500 zircon standards. These results encouraged the use of the glass standard as a calibration standard for the Pb/U ratio determination for zircons with shorter wavelength (λ = 213 nm) laser ablation. But 206Pb/238U and 207Pb/235U ages obtained by this method for seven zircon standards are systematically younger than the published U-Pb ages obtained by both isotope dilution–thermal ionization mass spectrometry (ID-TIMS) and sensitive high-resolution ion-microprobe (SHRIMP). Greater discrepancies (3–4% younger ages) were found for the 206Pb/238U ages for SL13, AS3 and 91500 zircons. The origin of the differences could be heterogeneity in Pb/U ratio on SRM610 between the different disks, but a matrix effect accuracy either in the ICP ion source or in the ablation-transport processes of the sample aerosols cannot be neglected. When the 206Pb/238U (= 0.2302) newly defined in the present study is used, the measured 206Pb/238U and 207Pb/235U ages for the seven zircon standards are in good agreement with those from ID-TIMS and SHRIMP within ±2%. This suggests that SRM610 glass standard is suitable for ICP-MS with laser ablation sampling (LA-ICP-MS) zircon analysis, but it is necessary to determine the correction factor for 206Pb/238U by measuring several zircon standards in individual laboratories. [ABSTRACT FROM AUTHOR]

The Shishimuta-Pink (Ss-Pnk) tephra erupted from the Shishimuta caldera on Kyushu Island, Japan, at 1 Ma, and it is distributed extensively over the Japanese Islands. We quantitatively analyzed 58 elements in volcanic glass shards of this tephra, which sampled at 9 points, using laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS). The nine samples have been correlated as Ss-Pnk tephra in previous studies, based on various geologic and petrographic properties. The element patterns of the tephra samples are similar, except for that of the Kuratama–Rindo O7 sample obtained in the Kazusa Group at the Boso Peninsula, located ∼900 km from the source volcano. The results of the Tukey–Kramer multiple comparison test suggest that 62%–71% of the concentrations of the 58 elements in the Kuratama–Rindo O7 sample differ significantly from the element concentrations in the other eight Ss-Pnk samples, whereas only 2%–31% of the element concentrations of the eight Ss-Pnk samples vary significantly. The element pattern of the sample of the Osaka Group Pink (OGPK), which is treated as being representative of the Ss-Pnk tephra, can be clearly distinguished from five other widespread marker tephras erupted from Kyushu Island. These results strongly suggest that the multi-element analysis of volcanic glass shards from tephras can be used as a basis for the identification and correlation of widespread Quaternary tephras. [ABSTRACT FROM AUTHOR]

The abundances of 58 elements from lithium to uranium of volcanic glass shards in 11 Quaternary widespread tephras of Japan and those of 4 tephras of North America have been analyzed using the laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) technique. The analytical values of the major components (SiO 2 , TiO 2 , Al 2 O 3 , FeO, MnO, MgO, CaO, and K 2 O) were systematically lower (<10%) than those of analyzed using electron microanalysis techniques in previous researches, whereas that Na 2 O tend to be ∼20% higher than those of previous researches using the electron-beam microanalysis technique. The higher values of Na 2 O obtained by LA-ICP-MS may result from the absence of the Na-loss phenomenon associated with the electron-beam analytical techniques. Most elements were analyzed with less than 20% of the relative standard deviations (RSDs), whereas some trace elements such as beryllium and some transition metals exhibit higher RSDs (up to ∼120%) in 5-repeated analyses on the single glass shard. The LA-ICP-MS technique can be reasonably available for quantitative analyses of the major elements in volcanic glass shards, in addition to the trace elements. The volcanic glass can be clearly distinguished from each other by the patterns of the element abundances. The element abundances and the patterns of the elements of the volcanic glass shards are helpful to more precise identification and correlation of tephra samples. [ABSTRACT FROM AUTHOR]

The Ballantrae Complex (at Bennane Lea in SW Scotland) contains important ocean plate stratigraphy (basalt, chert, mudstone, sandstone) in an accretionary prism that is associated with a classic Ordovician ophiolite. We used the ocean plate stratigraphy to sub-divide the prism into 11 tectonic units. To determine the depositional age of bedded cherts, zircons were separated from 9 tuff beds from 6 different units. All the tuffs have early to middle Ordovician ages, even though their present positions are mutually distant. These ages are consistent with microfossil records of radiolaria and graptolites. The stratigraphic–structural relationships demonstrate that the ocean plate stratigraphy has been repeated by bedding-parallel thrusts; this is typical of a modern accretionary duplex. We calculated the sedimentation rate of Early to Middle Ordovician bedded cherts at Bennane Lea on the basis of U–Pb zircon ages obtained from several tuff beds; the data indicate that the depositional rate (0.6–3 m/myr) was as slow as that of Mesozoic–Cenozoic equivalents defined by radiolaria. The age spectra of detrital zircons from Ballantrae sandstones show prominent single peaks at ca. 467 and 478 Ma, and a lack of Precambrian zircons. Integration of our new zircon ages with published isotopic data and palaeo-geographic maps indicates that the sandstones were deposited near an intra-oceanic arc and far from any continent containing Precambrian rocks. The pelagic-to-clastic sediments at Bennane Lea were deposited in the closing Iapetus Ocean from ca. 477 Ma to ca. 464 Ma, when they were accreted with the intra-oceanic arc of Ballantrae. [ABSTRACT FROM AUTHOR]

The Gwna Group in Anglesey island and Lleyn peninsula, Wales consists of a latest Proterozoic volcano-sedimentary trench mélange, which has a complicated accretionary structure, and is poorly constrained by isotopic ages. The mélange contains oceanic-trench rocks including pillow basalts, cherts, mudstones and sandstones, which have not previously been interpreted as ocean plate stratigraphy (OPS). We reconstructed imbricated OPS at 5 localities in the coastal Lleyn peninsula. In order to constrain the depositional U–Pb age of the upper clastic sediments, detrital zircons, separated from 9 clastic sediments, were analyzed with a Nu AttoM single-collector inductively-coupled plasma-mass spectrometer. The ages indicate that there are two Gwna Groups (maximum depositional ages of: 1 at 608–601 Ma, and 2 at 564–539 Ma) that were deposited between the late Neoproterozoic and the Middle Cambrian contemporaneously with dated calc-alkaline arc magmatism and regional metamorphism in the Anglesey–Lleyn complex. The age spectra of the detrital zircons show a prominent peak at ca. 650–600 Ma, and several Proterozoic and Archean ages. To account for the older ages, we integrated our new isotopic data with published radiometric and fossil ages, and conclude that the clastic sediments at the top of the OPS were deposited in a trench on the western active margin of Avalonia when it was close to the Amazonian craton, and that the Gwna Group OPS began to be incorporated into an accretionary wedge in an active subduction zone in the latest Proterozoic. [ABSTRACT FROM AUTHOR]

Abstract: The petro-chemical characteristics of the arc lower-crust, important for understanding continental growth, have been rarely obtained because of their scarcity at the surface of the Earth. To constrain the formation age of the arc lower-crust, U–Pb zircon dating was applied to mafic enclaves in tonalites of the Tanzawa Tonalitic Pluton (TTP), which is regarded as the exposed middle crust of the former Izu–Bonin–Mariana arc, using a laser ablation-inductively coupled plasma–mass spectrometer (LA-ICP-MS). The texture and shape of mafic enclaves indicate an injection of mafic magma into tonalitic magma at the mid-crustal level. While 44 zircon grains from a host tonalite show a narrow-age distribution with a mean age of 4.6±0.2Ma, 301 zircon grains from 9 mafic enclaves show wide-age distributions from ca. 5 to 43Ma. This study is the first to reveal a U–Pb age older than previously reported for the rock materials that compose the TTP, now identified to be 18Ma compared with an age range from 4Ma to 9Ma. Because there are no other components of the TTP yet identified to be older than 17Ma, the zircons separated from the TTP in this study which dated to be 18–43Ma are interpreted to be xenocrysts derived from the arc lower-crust beneath the TTP. The oldest zircon age obtained from the mafic enclaves indicates that the formation of the arc lower-crust beneath TTP took place before 42.9±8.6Ma, consistent with the history of the Izu–Bonin–Mariana arc. This is the first time that the formation age of the lower crust has been estimated using zircons from mafic enclaves. This study shows that the zircon U–Pb dating from mafic enclaves in granites can yield significant information about the age of the continental lower crust. [Copyright &y& Elsevier]