238U 206Pb Research Articles

A simplified isotope dilution approach for the U–Pb dating of speleogenic and other low-<sup>232</sup>Th carbonates by multi-collector ICP-MS

Abstract. We describe a new method for the measurement of U/Pb ratios by isotope dilution multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) for the dating of geologically young clean carbonates, particularly speleothems. The method is intended for materials containing little or no initial 232Th. We illustrate and validate the method with four examples ranging from 0.57 to 20 Ma. The new method is capable of applying the 235U–207Pb and 238U–234U–206Pb chronometers, common Pb and quantifiable residual 234U/238U disequilibrium permitting. These provide an alternative to the more widely used 238U–206Pb chronometer, which can be highly inaccurate for samples that are < ca. 20 million years old, owing to uncertainties in the excess initial 234U (hence, excess radiogenic 206Pb) commonly observed in speleothems.

238U–206Pb dating of U-series disequilibrium zircons by secondary ion mass spectrometry

Theoretical and analytical methods for 238U–206Pb dating of U-series disequilibrium zircons by secondary ion mass spectrometry are developed. U–Pb dates improve age precision 3 times better than U–Th dating for zircon >150 ka.

Late Silurian zircon U–Pb ages from the Ludlow and Downton bone beds, Welsh Basin, UK

The Ludlow Bone Bed (Welsh Basin) is a critical stratigraphic horizon and contains a rich assemblage of fish scales. Units above provide insights into the early evolution of animal and plant life. The bed has not yet been radioisotopically dated. Here, we report 207 secondary ion mass spectrometry (SIMS) ages from 102 zircon (ZrSiO 4 ) grains from the Ludlow ( n = 2) and stratigraphically higher Downton ( n = 1) bone beds. SIMS ages are middle Ordovician (471.6 ± 20.7 Ma) to late Devonian (375.7 ± 14.6 Ma, 238 U– 206 Pb, ±1σ analytical uncertainty). Cathodoluminescence images show that the youngest ages appear affected by alteration. Chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) U–Pb geochronology was utilized to improve precision. Detrital zircon grains from Downton yield 424.91 ± 0.34/0.42/0.63 Ma and from Ludlow 424.85 ± 0.32/0.41/0.62 Ma ( n = 5 each, 238 U– 206 Pb, ±2σ analytical, tracer or systematic uncertainty). These ages provide a maximum deposition age. Results overlap the basal Přídolí age (423.0 ± 2.3 Ma) in its stratotype (Požáry Section, Reporyje, Prague, Czech Republic). The Ludlow Bone Bed marks the base of the local Downton Group, which has previously been correlated with the base of the Přídolí Series. The CA-ID-TIMS ages are older than those for other land arthropod-bearing sediments, such as the Cowie Harbour Fish Bed and Rhynie Chert. Supplementary material: An Excel file containing detailed information on the SIMS analyses, a figure showing calibration curves for AS3 standards sputtered over sessions 1 and 2, and a figure showing CA-ID-TIMS U–Pb age data (concordia and weighted mean plots) are available at https://doi.org/10.6084/m9.figshare.c.5087031

Multi-chronometer dating of the Souter Head complex: rapid exhumation terminates the Grampian Event of the Caledonian Orogeny

ABSTRACTThe Souter Head sub-volcanic complex (Aberdeenshire, Scotland) intruded the high-grade metamorphic core of the Grampian Orogen at 469.1 ± 0.6 Ma (uranium-238–lead-206 (238U–206Pb) zircon). It follows closely peak metamorphism and deformation in the Grampian Terrane and tightly constrains the end of the Grampian Event of the Caledonian Orogeny. Temporally coincident U–Pb and argon/argon (40Ar/39Ar) data show the complex cooled quickly with temperatures decreasing from ca.800 °C to less than 200 °C within 1 Ma. Younger rhenium–osmium (Re–Os) ages are due to post-emplacement alteration of molybdenite to powellite. The U–Pb and Ar/Ar data combined with existing geochronological data show that D2/D3 deformation, peak metamorphism (Barrovian and Buchan style) and basic magmatism in NE Scotland were synchronous at ca.470 Ma and are associated with rapid uplift (5–10 km Ma−1) of the orogen, which, by ca.469 Ma, had removed the cover to the metamorphic pile. Rapid uplift resulted in decompressional melting and the generation of mafic and felsic magmatism. Shallow slab break-off (50–100 km) is invoked to explain the synchroneity of these events. This interpretation implies that peak metamorphism and D2/D3 ductile deformation were associated with extension. Similarities in the nature and timing of orogenic events in Connemara, western Ireland, with NE Scotland suggest that shallow slab break-off occurred in both localities.

Zircon U–Pb dating of gabbro and diorite from the Bato pluton, northeast Japan

AbstractTo constrain the timing of the tectonothermal events and formation process of a plutonic suite, U–Pb dating was carried out by laser ablation inductively coupled plasma mass spectrometry combined with cathodoluminescence imaging on zircon grains extracted from the Bato pluton, northern Yamizo Mountains, Japan. The Bato pluton consists of gabbro and diorite. Zircon grains separated from a gabbro sample had a unimodal 238U–206Pb age (105.7 ±1.0 Ma). It was interpreted as the solidification age of the gabbro. Cathodoluminescence observation showed that the zircon grains from a diorite sample were characterized by anhedral cores, oscillatory zoned mantles, and dark rims. The 238U–206Pb age of the anhedral cores ranged from 2 165 Ma to 161 Ma, indicating the assimilation of surrounding sedimentary rocks. The 238U–206Pb ages of the oscillatory zoned mantles and dark rims are 109.0 ±1.3 Ma and 107.7 ±1.3 Ma, respectively. Observation under polarizing microscopy suggests that the anhedral cores occurred before plagioclase and hornblende, and the oscillatory zones around the anhedral cores had crystallized at the same time as the crystallization of biotite. Moreover, the dark rims formed at the same time as the crystallization of quartz and K‐feldspar. The formation process of the gabbro‐diorite complex in the Bato pluton was inferred as follows. (i) A mafic initial magma intruded into Mesozoic sedimentary rocks, and the assimilation of these sedimentary rocks led to geochemical variation yielding a dioritic composition. Subsequently, plagioclase and hornblende of the diorite were crystallized before 109.0 ±1.3 Ma. (ii) Biotite crystallized in the middle stage around 109.0 ±1.3 Ma. (iii) Quartz and K‐feldspar of the diorite were crystallized at 107.7 ±1.3 Ma. The gabbroic magma solidified (105.7 ±1.0 Ma) after solidification of the diorite.

A new approach for constraining the magnitude of initial disequilibrium in Quaternary zircons by coupled uranium and thorium decay series dating

Abstract We have measured 238U–206Pb, 235U–207Pb, and 232Th–208Pb 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 230Th and 231Pa relative to secular equilibrium resulting from differential partitioning during zircon crystallization or source melting must be made. In contrast, the 232Th–208Pb decay system is clearly advantageous for samples affected by disequilibrium because the 232Th decay system lacks long-lived intermediate daughter isotopes. Conventionally, the initial disequilibrium for the 238U and 235U decay series has been determined by the distribution ratio between the melt and zircon (i.e., ƒTh/U = (Th/U)Zircon/(Th/U)Melt and ƒPa/U = (Pa/U)Zircon/(Pa/U)Melt). In our study, these correction factors were determined from comparison of the measured 238U–206Pb and 235U–207Pb ages with 232Th–208Pb 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 ƒTh/U = 0.19 ± 0.14 and ƒPa/U = 3.66 ± 0.89 (Kirigamine Rhyolite), ƒTh/U = 0.24 ± 0.20 and ƒPa/U = 3.1 ± 1.2 (Bishop Tuff), and ƒTh/U = 0.28 ± 0.17 and ƒPa/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.

Age and emplacement of the Permian–Jurassic Menghai batholith, Western Yunnan, China

ABSTRACTThe Menghai batholith (Yunnan Province, China) is the southern extension of the ~370 km long Lincang granite body that syntectonically intruded the collisional zone between Gondwana (Baoshan block) and Laurasia (Simao block) terranes during closure of the Palaeo-Tethyan Ocean. Eight Menghai granodiorites were analysed across an ~45 km E–W transect from the pluton’s central region to eastern perimeter. Each rock was imaged in cathodoluminescence and geochemically analysed for major and trace elements. A minimum 30 zircons per sample were dated using laser ablation inductively coupled plasma–mass spectrometry. Samples are peraluminous to strongly peraluminous, magnesian, calcic or calc-alkalic granodiorites. Trace element suggest a high pressure (12–15 kbar) low clay source with >20–30% volume interaction with basalt. Crustal anatexis was likely related to post-collisional lithosphere delamination and upwelling of hot asthenosphere, forming large-volume melts. Zircon ages (207Pb–206Pb and 238U–206Pb) range from 3234 ± 42 to 171.7 ± 5.4 Ma (±2σ). Inherited zircon ages include the Palaeoarchaean–Neoarchaean (average 2938 ± 27 Ma, n = 8 ages), Lüliang (2254 ± 38 Ma, n = 7), Changcheng–Jixianian (1274 ± 47 Ma, n = 33), Qinbaikou (963 ± 29 Ma, n = 7), Nanhua (787 ± 24 Ma, n = 7), Sinian (595.4 ± 12.2 Ma, n = 14), Qilian (452.2 ± 8.7 Ma, n = 24) and Tienshan (358.9 ± 12.4 Ma, n = 5). The presence of these ages decrease from the batholith’s central portion (>50% ages) to eastern perimeter (2–16% ages), as the rocks appear progressively metamorphosed. The distribution of U/Th ratio suggests inherited zircons are Carboniferous (317.6 ± 5.7 Ma) and older and crystallization ages span the Permian to Early Jurassic. The average and youngest zircon age per sample decreases from the centre of the batholith to its eastern perimeter, from 226.8 ± 8.8 and 210.7 ± 3.3 to 211.8 ± 5.7 and 171.0 ± 5.4 Ma, respectively. If recorded by syntectonic zircon crystallization, collision and closure of a branch of the Palaeo-Tethyan Ocean occurred here over an ~100 million years time period from the Permian (281.0 ± 13.0 Ma) to Jurassic (171.5 ± 5.4 Ma).

In-situ U–Pb dating and Nd isotopic analysis of perovskite from a rodingite blackwall associated with UHP serpentinite from southwestern Tianshan, China

Metamorphic perovskite associated with diopside+chlorite occurs in blackwalls surrounding rodingitized mafic dikes that are included in ultrahigh-pressure (UHP) serpentinized ultramafic rocks from Changawuzi, Chinese southwestern Tianshan. Perovskite forms discrete crystals in the chlorite-rich matrix or appears at the rim of ilmenite. The metamorphic perovskites from the blackwalls are characterized by high concentrations of REE, Nb, Ta, Th and U. The primitive mantle normalized patterns of perovskite are two to three orders of magnitude higher but parallel to the bulk rock patterns, demonstrating that perovskite is a major host of these incompatible trace elements in the ultramafic rocks. The bulk-rock major and trace element compositions of rodingite dikes and associated blackwalls display variable patterns, consistent with different mafic protoliths of the rodingites.Perovskites from three samples were analysed for U–Pb by in-situ secondary ion mass spectrometry. Two samples contained essentially common Pb whereas one sample had enough radiogenic Pb to define a discordia in the Tera–Wasserburg diagram with an age intercept of 317±11Ma. If all analyses are pooled together, a 238U–206Pb age of 308±5 Ma is obtained. These ages are in agreement with the age of peak UHP metamorphism in Chinese southwestern Tianshan. In-situ Nd isotopes of perovskite from the three samples were obtained by laser ablation multi-collector inductively coupled plasma mass spectrometry. The ɛNd(t)310 of two samples are 8.37±0.14 and 7.88±0.25, respectively, which suggests that they have similar protoliths. These samples plot close to a 310Ma Sm–Nd isochron. In contrast, the third sample has a different ɛNd(t)310 of 0.90±0.51, indicating a completely different protolith. Thus, in-situ analyses of perovskite can provide important information on the protolith as well as the metamorphic evolution of reaction zones between mafic and ultramafic rocks.

Stratigraphic and tectonic settings of Early Paleolithic of North-West Armenia

The Quaternary stratigraphy and tectonic development of the southern Javakheti Upland and the adjacent Upper Akhurian and Lori uplifted basins in NW Armenia (Lesser Caucasus) and geological position of the early and middle Acheulian lithic industries were studied using a multi-method approach. Studies of sedimentary sections, petrological and geochemical correlation of lavas and tuffs, K–Ar dating of volcanic rocks and SIMS 238U–206Pb dating of tuffs, examination of obtained fossils and pollen analysis, and determination of remanent magnetic polarity of volcanic rocks and clastic sediments have been used to compile the stratigraphic scheme and to estimate the age of units.The low-mountain topography was differentiated to ridges and basins to the beginning of Quaternary. The eruptions of basalts and basaltic andesites evened topography of the Upper Akhurian and Lori basins in the Gelasian. The lava flows spread along big river valleys at tens of kilometers. At the late Gelasian, the eruptions of basaltic trachyandesites, trachyandesites and dacites replaced the basaltic eruptions. The latter dammed the Akhurian River flowing to the south and the upper Akhurian River found the flowing to the east via the valley-like depression of the Karakhach Pass to the Dzoraghet-Debed valley. The course-grained tuffaceous-clastic Karakhach unit was deposited during the Olduvai subchron (not earlier than 1.9–1.85 Ma) and the earliest Calabrian. The water transit between the Upper Akhurian and Lori basins was interrupted later because of rise of the Karakhach Pass. Volcanic activity renewed for a short time in the early Calabrian (∼1.7 and 1.5–1.4 Ma). The end Calabrian and earliest Middle Pleistocene sedimentation (∼1–0.5 Ma) occurred in stagnant water, partly lacustrine conditions. This was expressed by formation of the relatively fine-grained Kurtan unit. During the last ∼0.5 Ma, the region underwent flexure-fault deformation and tectonic uplift at 350–800 m.The epoch of formation of the Karakhach unit was characterized by middle mountain topography and humid climate. Not later than 1.85 Ma, the region was occupied by the earliest hominines producing lithic industries of the Early Acheulian aspect. They contained crude hand-axes and other macro-tools, made of local dacite and basalt (sites of Karakhach, Muradovo and Agvorik). Early appearance of these industries might be caused by natural parting of dacite and basalt to tabulated fragments that gave a possibility to make such macro-tools. The Middle Acheulian artifacts were found in the Kurtan I section of the Kurtan unit.

Determination of U–Pb Ages for Young Zircons using Laser Ablation‐ICP‐Mass Spectrometry Coupled with an Ion Detection Attenuator Device

We have developed new analytical procedures to measure precise and accurate 238U–206Pb and 235U–207Pb ages for young (~ 1 Ma) zircons using laser ablation‐ICP‐mass spectrometry. For young zircons, both careful correction for the background counts and analysis of very small Pb/U ratios (i.e., 206Pb/238U < 0.00016 and 207Pb/235U < 0.0001 for 1 Ma zircons) are highly desired. For the correction of the background, the contribution of the background signal intensities for the analytes, especially for the residual signal intensities for 206Pb and 207Pb, was defined through laser ablation of synthesised zircons (ablation blank) containing negligible Pb. The measured signal intensities for 202Hg, 206Pb and 207Pb signals obtained by the ablation blank were slightly higher than those obtained by data acquisition without laser ablation (gas blank). For the wider dynamic range measurements on Pb/U ratios, an attenuator device for the ion detection system was employed to extend the capability to monitor high‐intensity signals (i.e., > 3 Mcps). Through the attenuator device, the ion currents were reduced to 1/450 of the signal intensity without the attenuator. Because the switching time for the attenuator was shorter than 1 ms, signal intensities for only specific isotopes could be reduced. With attenuation of the 238U signal, counting statistics on 206Pb and 207Pb isotopes could be improved and counting loss on the 238U signal could be minimised. To demonstrate the reliability of this new analytical technique, 238U–206Pb and 235U–207Pb ages for three young zircon samples (collected from Osaka Group Pink Volcanic Ash, Kirigamine and Bishop Tuff) were measured. The data presented here demonstrate clearly that the present technique could become a major analytical tool for in situ U–Pb age determination of young zircons (~ 1 Ma).

Zircon U–Th–Pb dating using LA-ICP-MS: Simultaneous U–Pb and U–Th dating on the 0.1 Ma Toya Tephra, Japan

Simultaneous U–Pb and U–Th zircon dating using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was performed on the ~0.1Ma Toya Tephra, Hokkaido, Japan. The method employs the concurrent measurement of 206Pb, 230Th, and 238U signals, which enables the determination of 238U–206Pb (U–Pb) and 238U–230Th (U–Th) ages simultaneously. The U–Pb ages revealed that the Toya Tephra contains zircons with ages that cluster at ~0.1Ma (0.11±0.01Ma: error shown as 95% confidence level) and xenocrysts with ages >2Ma. Excluding zircons >2Ma, the U–Th method also gave an isochron age of ~0.1Ma (108±19ka or 0.11±0.02Ma; error shown as 2σ). Therefore, both U–Pb and U–Th methods result in similar ages of ~0.1Ma for the Toya Tephra, in agreement with the quartz TL age (104±30 to 118±30ka) of Ganzawa and Ike (2011). It was revealed that the simultaneous U–Pb and U–Th zircon dating technique using LA-ICP-MS is easy to apply and is useful for Quaternary tephras in that it can give age information on the tephra itself and xenocrystic zircons in a quick and cost-effective manner. The double dating technique employed here has the merit that zircons yielding Quaternary U–Pb ages can be immediately cross-checked by the U–Th method. It was also revealed that the Toya Tephra magma crystallized zircons and experienced eruptions instantaneously at ~0.1Ma, which is an important implication to decipher magmatic and eruptive processes for caldera-forming large volcanic systems.

Quaternary plutonic magma activities in the southern Hachimantai geothermal area (Japan) inferred from zircon LA-ICP-MS U–Th–Pb dating method

In order to constrain the timing of plutonic magma emplacements relevant to current geothermal activity in the Hachimantai geothermal area, northeast Japan, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U–Th–Pb analyses were performed on zircons from two different origins. One is from the Quaternary Kakkonda Granite in the Kakkonda geothermal field, whose present temperature at ~3700m depth is 500°C, and the other is from a clastic dike in the Matsukawa geothermal field. Zircons from the Kakkonda Granite were obtained at a depth of 1998m and their 238U–206Pb and U–Th ages showed ~0.1Ma, which probably indicates that the Kakkonda Granite is the world's youngest emplaced granitic magma body ever confirmed. The clastic dike in the Matsukawa geothermal field contains dioritic to tonalitic xenoliths; therefore zircons extracted from the dike should also be of plutonic origin. 238U–206Pb age of 1.08±0.08Ma (after correction for initial 230Th disequilibrium; error shown as 95% confidence level) was obtained from the zircons. Therefore it was found that there were at least two Quaternary plutonic magma activities (~1.1Ma and ~0.1Ma) in the Hachimantai geothermal area. Through this study, it was also found that U–Th–Pb dating using a quadrupole type ICP-MS coupled with excimer laser is applicable to Quaternary zircons as young as 0.1Ma and Fish Canyon Tuff zircon is usable as a standard material for U–Th–Pb dating method.

Zeta equivalent fission‐track dating using LA‐ICP‐MS and examples with simultaneous U–Pb dating

AbstractThe use of laser ablation‐inductively coupled plasma–mass spectrometry (LA‐ICP‐MS) in the measurement of uranium concentration for fission‐track (FT) dating can provide additional geochronological information by simultaneously acquiring data for U–Pb dating. To circumvent the use of several calibration factors with uncertainties in the FT age equation (i.e. the decay constant, values related to neutron irradiation, and observation efficiencies), the zeta calibration, which is based on the analysis of age‐known standard samples, is recommended internationally. In this report, procedures to acquire LA‐ICP‐MS data for simultaneous dating by FT and U–Pb methods, the age equation with zeta‐equivalent calibration, and the statistical treatment of FT age data, are proposed. An excimer laser beam with 20 μm diameter was used for sampling. We empirically determined zeta calibration constants against NIST SRM610 for FT zircon and apatite dating by analyzing several age standards. The data agreed with the calculated values, which can be obtained using reasonable constants in the equation. The correction factor in 238U–206Pb age determination with NIST SRM610 as an external standard was defined as 0.920 ± 0.034 by analyzing several standards (FCT, Plešovice, Duluth, GJ‐1). Based on this calibration system, zircons from the Nisatai Dacite, Japan, which is a secondary age standard commonly used in the Japanese FT community, were simultaneously dated to be 22 Ma by FT and U–Pb methods on the same grains.

An inter‐laboratory evaluation of OD‐3 zircon for use as a secondary U–Pb dating standard

AbstractZircon OD‐3 from the Paleogene Kawamoto Granodiorite (Mihara body) in Japan has been identified as a potential multi‐grain secondary standard for U–Pb dating. We have carried out an inter‐laboratory evaluation in order to evaluate possible heterogeneity amongst the OD‐3 zircon grains. U–Pb ages were obtained using two analytical techniques (a sensitive high‐resolution ion microprobe and a laser ablation–inductively coupled plasma–mass spectrometry) in eight laboratories. All the 238U–206Pb ages show good agreement, with an overall weighted average 238U–206Pb age of 33.0 ± 0.1 Ma (2σ). The U–Pb age results revealed no significant variation or heterogeneity in the U–Pb ages of the OD‐3 grains. Twelve fission‐track (FT) ages from three laboratories are also reported, and have a weighted average of 32.6 ± 0.6 Ma (2σ). Despite the different closure and annealing temperatures of the U–Pb and FT chronometers, respectively, the FT age is in good agreement with the U–Pb age. This suggests that the OD‐3 zircon had a relatively fast cooling history and has not experienced later thermal annealing. The chronological dataset reported here clearly demonstrates that the OD‐3 zircon could be a useful and reliable secondary standard for use during U–Pb dating studies of Cenozoic zircons.

Some footnotes to the optimization-based calibration of the 40 Ar/ 39 Ar system

Abstract A recent calibration of the 40 Ar/ 39 Ar geochronometer is based on an optimization analysis of 40 K activity data, isotopic data for the Fish Canyon sanidine (FCs) standard, and pairs of 40 Ar/ 39 Ar+ 238 U– 206 Pb data from selected samples meeting well-documented quality criteria. Inclusion of 238 U– 206 Pb data in the calibration incorporates the precisely known 238 U decay constant. Thus, this calibration is inherently consistent with the U–Pb chronometer. Initial presentation of the calibration included an inappropriate datum and should be eschewed in preference to a revision. Compared with previous calibrations, including those focusing mainly on the age of a standard (e.g. astronomical calibrations), the optimization calibration provides superior accuracy in the sense of propagated age uncertainty, particularly for ages much older than the FCs. Recent literature reveals that the optimization-based calibration has been misused and misrepresented in some cases; discussion of these cases clarifies the correct usage of the approach. Apparent conflict between 40 Ar/ 39 Ar and 238 U– 206 Pb ages for a Quaternary tuff do not appear to be a result of error in one of the three parameters determined by the optimization approach for the 40 Ar/ 39 Ar system. The optimization approach easily accommodates new constraints, but rigorous quality control is needed to maintain accuracy.

Petrogenesis of the Bao'anzhai granite and associated Mo mineralization, western Dabie orogen, east-central China: Constraints from zircon U–Pb and molybdenite Re–Os dating, whole-rock geochemistry, and Sr–Nd–Pb–Hf isotopes

Recently, many Mo deposits genetically related to emplacement of Early Cretaceous granites have been found in the Dabie–Qinling belt. A typical intrusion that combines magmatism and metallogenesis, the Bao'anzhai granite, yields a zircon 238U–206Pb age of 123.2 ± 1.1 Ma and a molybdenite Re–Os isochron age of 122.5 ± 2.7 Ma. This granite is characterized by high silica and alkali, but low Mg, Fe, and Ca. It is enriched with light rare earth elements (REEs) and large-ion lithophile elements (LILEs, Rb, K, Th, U) but depleted of heavy REEs, high field strength elements (HFSEs, Nb, Ta, Ti, and Y), and Sr. This high-K granite has medium initial 87Sr/86Sr ratios (0.706518–0.707116) and low initial Pb isotopic ratios [(206Pb/204Pb)i, 16.423–16.699; (207Pb/204Pb)i, 15.285–15.345; (208Pb/204Pb)i, 37.335–37.633], and is characterized by low ϵNd(t) and ϵHf(t) values (−14.92 to −14.22 and −21.67 to −19.19, respectively). These data indicate that this pluton is a high-K calc-alkaline fractionated I-type granitite. It was generated by partial melting of the Yangtze lower crust, which is probably similar to Neoproterozoic TTG-like magmatic rocks at the north Yangtze Block under a non-thickened lower crust environment (<35 km). The ores also have low radiogenic Pb isotopes (206Pb/204Pb, 16.592–17.674; 207Pb/204Pb, 15.300–15.476; 208Pb/204Pb, 37.419–37.911) and low Re content in molybdenite (5.693–10.970 ppm), suggesting a crustal magmatic source for the metallogenic minerals in the Mo deposit.

Constraints on fluid evolution during metamorphism from U–Th–Pb systematics in Alpine hydrothermal monazite

The timing and duration of hydrothermal activity during orogenesis are difficult to constrain, because such systems are open and multistage. Using mm-sized monazite crystals from two Alpine clefts from the Central Alps (Switzerland), we demonstrate that combined U–Th–Pb isotopic systematics of hydrothermal monazite can constrain both timing and fluid evolution during crystallization. Our data highlight four major characteristics of the cleft monazites: (i) extreme Th/U ratios (ii) significant common Pb in the U system (up to 39% 206Pb), (iii) excess 206Pb (up to 54%), and (iv) precise and reliable Th–Pb ages.Comparison of our results with literature data indicates that Th/U in monazite is a remarkable discriminant of geological conditions, capable of distinguishing metasedimentary, magmatic and hydrothermal origin. Hydrothermal monazite crystals are characterized by Th/U ratios up to 629, among the highest values ever reported. Extreme Th/U ratios in monazite enhance incorporation of 230Th, a short-lived intermediate product in the 238U–206Pb decay chain, generating 206Pbexcess which, if not accounted for, results in 206Pb/238U ages that are too old, for example in the samples investigated here by as much as 300%.For the two zoned monazite crystals studied in detail, 208Pb/232Th ages are reliable for the different compositional domains visible in back-scattered electron (BSE) images. 232Th/208Pb ages for the older domains (15.2±0.3Ma and 14.1±0.3Ma, respectively) are consistent with the structural relationships of the hydrothermal veins, indicating early retrograde genesis. The youngest rim age at 13.5±0.4Ma for the Blauberg crystal marks termination of monazite precipitation. The resolvable age difference is interpreted to reflect pulsed monazite growth over an extended period of hydrothermal activity.In both crystals, the outer rims have the highest 206Pbexcess, confirming a two-stage crystallization. Two scenarios are envisaged to account for the 206Pbexcess evolution. In the first, increasing 206Pbexcess is caused by a modification of the Th/U fractionation between monazite and fluid due to an evolution of the fluid towards more oxidizing conditions that favor partitioning of hexavalent U into the fluid. Alternatively, it is possible that 230Th increased with time in the fluid. In this second case, monazite growth would have occurred in a closed system from a fluid that was initially in disequilibrium with the 238U–206Pb decay chain and progressively equilibrated (t<0.5Ma).

Origin of zircon in jadeitite from the Nishisonogi metamorphic rocks, Kyushu, Japan

On the basis of internal structures, laser ablation U–Pb ages and trace element compositions, the origin of zircon in jadeitite in the Nishisonogi metamorphic rocks was examined. The zircon comprises euhedral zoned cores overgrown by euhedral rims. The cores contain inclusions of muscovite, quartz, albite and possibly K-feldspar, yield 238U–206Pb ages of 126 ± 6 Ma (±2 SD, n = 45, MSWD = 1.0), and have Th/U ratios of 0.48–1.64. The rims contain inclusions of jadeite, yield 238U–206Pb ages of 84 ± 6 Ma (±2 SD, n = 14, MSWD = 1.1), and have Th/U ratios of <0.06. The cores are richer in Y, Th, Ti and rare earth elements (REEs), but the rims are richer in Hf and U. Chondrite-normalized REE patterns of the cores indicate higher SmN/LaN ratios, lower YbN/GdN ratios and larger positive Ce anomalies compared with those of the rims. Thus, the cores and rims have different 238U–206Pb ages and trace element compositions, suggesting two stages of zircon growth. Although the 238U–206Pb ages of the rims are consistent with the reported 40Ar/39Ar spot-fusion ages of matrix muscovite in the jadeitite, the 238U–206Pb ages of the cores are older. The mineral inclusions and high Th/U ratios in the cores are best explained by crystallization from felsic magma. Therefore, the cores are considered relicts from igneous precursor rocks. The rims surrounding the inherited cores possibly precipitated from aqueous fluids during jadeitite formation. The elevated U concentrations in the rims suggest that infiltration of external fluids was responsible for the precipitation. This study provides an example of jadeitite formation by metasomatic replacement of a protolith.

Mantle convection and evolution with growing continents

We present a three‐dimensional spherical shell numerical model for chemical differentiation and redistribution of incompatible elements in a convective Earth's mantle heated mostly from within by U, Th, and K and slightly from below. The evolution‐model equations guarantee conservation of mass, momentum, energy, angular momentum, and four sums of the number of atoms of the pairs 238U–206Pb, 235U–207Pb, 232Th–208Pb, and 40K–40Ar. The pressure‐ and temperature‐dependent viscosity is supplemented by a viscoplastic yield stress, σy. The lithospheric viscosity is partly imposed to mimic its increase by dehydration of oceanic lithosphere and other effects. Also, the asthenosphere is generated not only by the distribution of temperature and melting temperature, but essentially by the profiles of water solubility and water abundance. Therefore we introduced a radial viscosity profile factor describing that behavior. However, the focus of this paper is the episodic growth of continents and oceanic plateaus. As a complement, the differentiation generates the depleted MORB mantle (DMM) which predominates immediately beneath the lithosphere. Our continents are not artificially imposed on the surface of the spherical shell, but instead they evolve by the interplay between chemical differentiation and convection/mixing. No restrictions are imposed regarding number, size, form, and distribution of continents. However, oceanic plateaus that impinge upon a continent have to be united with it. This mimics the accretion of terranes. The numerical results show an episodic growth of the total mass of the continents and display a plausible time history for the laterally averaged surface heat flow, qob, and the Rayleigh number, Ra. We use our model to explore a moderate region of Ra–σy parameter space. We find Earth‐like continent distributions in a central part of the Ra–σy space we explored. We identified a Ra–σy region where the calculated total continental volume is very close to the observed value; another Ra–σy region where the Urey number, Ur, is close to the accepted value; a third Ra–σy area where surface heat flow is very close to the present‐day observed mean global heat flow using typical abundances of the heat‐producing elements. It is remarkable that these different acceptable Ra–σy regions share a common overlap area, where Earth‐like behavior is simultaneously fulfilled.Although the convective flow patterns and the chemical differentiation of oceanic plateaus are coupled, the evolution of time‐dependent Rayleigh number, Rat, is relatively well predictable and the stochastic parts of the Rat(t) curves are small. Regarding the time distribution of juvenile growth rates of the total mass of the continents, predictions are possible only in the first epoch of the evolution, presumed that the initial conditions are given. Later on, the distribution of the continental growth episodes is increasingly stochastic. Independent of the varying individual runs, our model shows that the total mass of the present‐day continents is not generated in a single process at the beginning of the thermal evolution of the Earth but in episodically distributed processes. This is in accord with observation.

A precise U–Pb age on cassiterite from the Xianghualing tin-polymetallic deposit (Hunan, South China)

We report the first precise U–Pb isotope data on cassiterite from the large Xianghualing tin-polymetallic deposit in the central Nanling district, South China. The results show that four separates from sample XF-51 have a relatively narrow range of 206Pb/238U apparent ages, varying from 152 to 157 Ma, and the three 206Pb/238U apparent ages yield a weighted average value of 156 ± 4 Ma (MSWD = 0.32). Separates from two other cassiterite samples do not have sufficient radiogenic Pb to generate a reliable 206Pb/238U age. Seven separates from the above three cassiterite samples define a well-constrained 238U–206Pb isochron corresponding to an age of 157 ± 6 Ma (MSWD = 34). A comparison of the U–Pb cassiterite ages with published Ar–Ar dates on muscovite from this deposit and K–Ar age data on biotite from the pluton genetically related to the tin mineralization in this area demonstrates that the U–Pb isotope system of cassiterite is a potential geochronometer. Combined with the Ar–Ar dates of muscovite from this deposit, we can constrain the absolute age of tin-polymetallic mineralization in Xianghualing at 154–157 Ma. The dates obtained in this study, consistent with the published geochronological results from other important deposits in this region, reveal that the large-scale tungsten–tin mineralization in the central Nanling region was predominantly emplaced during 150–161 Ma.