Ar Dating Research Articles

Comparison of 3D‐Var and 4D‐Var data assimilation in an NWP‐based system for precipitation nowcasting at the Met Office

A high‐resolution demonstration Numerical Weather Prediction (NWP) nowcasting system was developed and run in real time at the Met Office for the prediction of precipitation on a domain covering southern England and Wales. The hourly cycling nowcasting system, known as NDP (Nowcasting Demonstration Project), combined a 3 km resolution 4D‐Variational data assimilation (4D‐Var) and a 1.5 km resolution version of the Met Office Unified Model (UM) to provide hourly NWP analyses and forecasts for a period of 0–6 h. Central to the NDP was the rapid updating cycling and its timely delivery of analyses and forecasts using the latest conventional and sub‐hourly novel observations. In this article the benefits of using 4D‐Var assimilation compared to First Guess at Appropriate Time (FGAT) 3D‐Variational assimilation (3D‐Var) on forecasts of precipitation are considered by comparing model forecasts with radar‐derived hourly surface accumulations for the period of June 2012, using an objective, scale‐dependent verification scheme. It is shown that 4D‐Var assimilation has a positive impact on precipitation forecasting skills compared to the corresponding 3D‐Var assimilation for the whole nowcast period [T + 0, T + 6]. The 4D‐Var assimilation system produced forecasts with greater spatial accuracy and longer lead times with acceptable skill. By comparing 3D‐Var FGAT using sub‐hourly observations with 3D‐Var FGAT using only the observations closest to the analysis time, the sub‐hourly observations are seen to be beneficial in the hourly cycling convective‐scale system in both 3D‐Var FGAT and 4D‐Var. The forecast clearly benefitted from the use of extra, higher time frequency observations in both 3D‐Var and 4D‐Var.

Timing and implications for the late Mesozoic geodynamic settings of eastern North China Craton: Evidences from K–Ar dating age and sedimentary–structural characteristics records of Lingshan Island, Shandong Province

The Lingshan Island in Shandong Province in the eastern North China Craton, well known for the Late Mesozoic multi-scale slide-slump structures is related to paleo-earthquake. Terrigenous clastic rocks, volcanic clastic rocks and volcanic lavas are extensively exposed in the Lingshan Island and its adjacent regions of the Shandong Province, which led to fierce debates on their ages, sedimentary characteristics and tectono-sedimentary evolution. In this contribution, we present the characteristics of the Late Mesozoic stratigraphy in the Lingshan Island. Whole-rock K–Ar dating of dyke at Beilaishi and rhyolites at Laohuzui of the Lingshan Island yielded ages of 159 Ma and \(106\hbox {–}92\hbox { Ma}\) which coincides with the Laiyang Period rifting and the Qingshan Period rifting in the Jiaolai Basin, respectively. On the basis of the analysis to the Late Mesozoic sedimentary environment of ‘flysch’ and ‘molasse’-like formations as well as tectonic stress fields reconstruction, four episodes of the tectono-sedimentary evolution were established in the Lingshan Island and its adjacent regions in the eastern North China Craton. They consist of two episodes of extensional events for the syn-rift, and two episodes of compression events for the inversion of the post-rift. The entire episodes can be summarized as follows: (1) the first syn-rift NW–SE extension in Laiyang Period can be identified by the ‘flysch’ formation (Unit 1) and by emplacement of the NE-trending dyke in the Laiyang Group. This syn-rift episode can be related to the NW–SE post-orogenic extension resulted from the gravity collapse of the thickened lithosphere along the Sulu Orogen. (2) The first post-rift NW–SE inversion, which was caused by the NW-directed subduction of Izanaqi Plate, can be well documented by the ‘X’ type conjugate joints as well as slide slump folds in Unit 1. (3) The second syn-rift NW–SE extension in Qingshan Period is characterized by rhyolite rocks (Unit 2). This syn-rift episode can be considered to be associated with lithospheric delamination of the thickened lithosphere in the eastern North China Craton. And finally, (4) the second post-rift NW–SE inversion which resulted from the subduction of the Pacific Plate under the eastern North China Craton in the NW direction at the end of the Qingshan Period is recorded by ‘molasse’-like formation (Unit 3).

Constraining the timing of brittle deformation and sedimentation in southern Finland: Implications for Neoproterozoic evolution of the eastern Fennoscandian shield

Sheared sedimentary clay was found in a faulted fracture in crystalline bedrock in a tunnelling site at 60 m depth in southern Finland. Brittle faults are numerous in the Fennoscandian Palaeoproterozoic bedrock, but only some of them have relative age constraints, while absolute ages are nearly lacking. Sedimentary rocks altogether are uncommon in Finland and only sparsely dated by micropaleontological studies. This study reports K–Ar data of fresh, non-weathered authigenic illite and constrains a time framework for the local faulting and sedimentation. The Neoproterozoic Tonian to Cryogenian ages derived from the grains in diminishing grain-size order are c. 967, 947, 809 and 697 Ma. Results indicate that the formation of the extension fracture is related to the collapse of Sveconorwegian orogeny c. 1000 Ma; clay and mature quartz sand were deposited in this extension fracture in shallow water in an intracratonic basin followed by early diagenetic processes and neocrystallization of illite around 967–947 Ma. The Neoproterozoic 1000–700 Ma sedimentation documented in this study is rare in the Fennoscandian shield as a whole. Neocrystallization of authigenic illite in the finest 0.4 and <0.1 µm fractions c. 809–697 Ma ago is interpreted as resulting from reactivation in the fault due to the continental break-up on the western side of the craton as documented by arenite crosscutting relationships. The younger ages may also be attributed to a Caledonian thermal overprint ca. 410 Ma ago that would influence the 967 Ma age if sufficient thermal energy had been present.

On the history of the early meteoritic bombardment of the Moon: Was there a terminal lunar cataclysm?

This work revisits the hypothesis of the so-called ‘lunar terminal cataclysm’ suggested by Tera et al. (1973, 1974) as a strong peak in the meteorite bombardment of the Moon around 3.9 Ga ago. According to the hypothesis, most of the impact craters observed on the lunar highlands formed during this short time period and thus formed the majority of the lunar highland impact breccias and melts. The hypothesis arose from the observation that the ages of highland samples from all the lunar missions are mostly grouped around 3.9–4.0 Ga. Since those missions, however, radiometric dating techniques have progressed and many samples, both old and new, have been re-analyzed. Nevertheless, the debate over whether there was a terminal cataclysm persists. To progress in this problem we summarized results of 269 K–Ar datings (mostly made using the 40Ar–39Ar technique) of highland rocks represented by the Apollo 14, 15, 16, 17 and Luna 20 samples and 94 datings of clasts of the highland rocks from 23 lunar meteorites representing 21 localities on the lunar surface, and considered them jointly with the results of our modelling of the cumulative effect of the impact gardening process on the presence of impact melt of different ages at the near-surface of the Moon.The considered results of K–Ar dating of the Apollo-Luna samples of lunar highland rocks confirmed a presence of strong peak centered at 3.87 Ga. But since the time when the hypothesis of terminal cataclysm was suggested, it has become clear that this peak could be a result of sampling bias: it is the only prominent feature at the sites with an apparent domination of Imbrium basin ejecta (Apollo 14 and 15) and the age pattern is more complicated for the sites influenced not only by Imbrium ejecta but also that of other basins (Nectaris at the Apollo 16 site and Serenitatis at the Apollo 17 site). Our modelling shows that the cataclysm, if it occurred, should produce a strong peak in the measured age values but we see in the considered histograms and relative probability plots not only the 3.87 Ga peak (due to Imbrium basin), but also several secondary peaks caused by the formation of other basins distributed between 3.87 and 4.25 Ga.The lunar terminal cataclysm hypothesis is in disagreement with the distribution of K–Ar ages for the highland rocks of the lunar meteorites. The population of lunar meteorites representing localities randomly distributed over the lunar surface, and thus free from the mentioned sampling bias, shows no ∼3.9 Ga peak as it should, if the cataclysm did occur.We conclude that the statistics of sample ages contradict the terminal cataclysm scenario in the bombardment of the Moon. We also see evidence for the formation of several impact basins between 3.87 and 4.25 Ga which is likewise incompatible with the hypothesis of a short interval cataclysm. There remain other basins, including the largest South Pole – Aitken, the ages of which should be determined in future studies to further clarify the impact history. Sample-return missions targeted to date several key basins need to be planned, and the continued study of lunar meteorites may also bring new details to the general view of the impact bombardment of the Moon.

Mayer Kangri metamorphic complexes in Central Qiangtang (Tibet, western China): implications for the Triassic–early Jurassic tectonics associated with the Paleo-Tethys Ocean

The Mesozoic orogeny in Central Qiangtang Metamorphic Belt, northern Tibet, provides important insights into the geological evolution of the Paleo-Tethys Ocean. However, the Triassic–early Jurassic tectonics, particularly those associated with the continental collisionstage, remains poorly constrained. Here we present results from geological mapping, structural analysis, P–T data, and Ar–Ar geochronology of the Mayer Kangri metamorphic complex. Our data reveal an E–W-trending, ~2 km wide dome-like structure associated with four successive tectonic events during the Middle Triassic and Early Jurassic. Field observations indicate that amphibolite and phengite schist complexes in this complex are separated from the overlying lower greenschist melange by normal faulting with an evident dextral shearing component. Open antiform-like S2 foliation of the footwall phengite schist truncates the approximately north-dipping structures of the overlying melange. Microtextures and mineral chemistry of amphibole reveal three stages of growth: Geothermobarometric estimates yield temperatures and pressures of 524 °C and 0.88 GPa for pargasite cores, 386 °C and 0.34 GPa for actinolite mantles, and ~404 °C and 0.76 GPa for winchite rims. Peak blueschist metamorphism in the phengite schist occurred at ~0.7–1.1 GPa and ~400 °C. Our Ar–Ar dating of amphibole reveals rim-ward decreasing in age bands, including ~242.4–241.2 Ma, ≥202.6–196.8, and 192.9–189.8 Ma. The results provide evidence for four distinct phases of Mesozoic tectonic evolution in Central Qiangtang: (1) northward oceanic subduction beneath North Qiangtang (~244–220 Ma); (2) syn-collisional slab-break off (223–202 Ma); (3) early collisional extension driven by buoyant extrusion flow from depth (~202.6–197 Ma); and (4) post-collision contraction and reburial (195.6–188.7 Ma).

Slab-derived components in the subcontinental lithospheric mantle beneath Chilean Patagonia: Geochemistry and Sr–Nd–Pb isotopes of mantle xenoliths and host basalt

In subduction zones, ultramafic xenoliths hosted in alkaline basalts can yield significant information about the role of potential slab-derived components in the subcontinental lithospheric mantle (SCLM). Chemical and isotopic heterogeneities in such xenoliths are usually interpreted to reflect melt extraction followed by metasomatic re-enrichment. Here we report new whole-rock major, trace element and isotopic (Sr–Nd–Pb) data for a Proterozoic suite of 17 anhydrous spinel-lherzolites and Eocene (new K–Ar data) host alkaline basalt found near Coyhaique (~46°S), Aysén Region, Chile. These Patagonian nodules are located in a current back-arc position, ~100km east of the present day volcanic arc and ~320km from the Chile Trench.The mantle xenoliths consist of coarse- to medium-grained spinel-lherzolites with trace element compositions characteristic of a subduction zone setting, such as pronounced negative Nb, Ta and Ti anomalies coupled with significant enrichment of LILEs (e.g., U) and chalcophile elements (W, Pb and Sn). Most of them are characterized by flat to depleted light-rare earth element (LREE) patterns (Ce/YbN=0.6–1.1) coupled with less radiogenic Sr–Pb (87Sr/86Sr=0.702422–0.703479; 206Pb/204Pb=18.212–18.539) and more radiogenic Nd isotopic compositions (143Nd/144Nd=0.512994–0.513242), similar to the depleted mantle component (DMM or PREMA). In contrast, samples with slight LREE enrichment (Ce/YbN=1.3–1.8) show more radiogenic Sr–Pb (87Sr/86Sr=0.703791–0.704239; 206Pb/204Pb=18.572–18.703) and less radiogenic Nd isotopic compositions (143Nd/144Nd=0.512859–0.512934), similar to the EM-2 reservoir. These new geochemical and isotope data suggest that the Coyhaique spinel-lherzolites are derived from a heterogeneous SCLM resulting from mixing between a depleted mantle component and up to 10% of slab-derived components. The enriched component added to the SCLM represents variable extents of melts of both subducted Chile Trench sediments and modified oceanic crust throughout the initial stages of the Farallón–Aluk ridge collision during Paleocene to Eocene time. However, based on the tectonic evolution of southern South America, we cannot exclude the influence of long-lived subduction events beneath south Patagonia. Although we believe that the studied samples were brought to the surface in this geodynamic context, there is no evidence that ocean island basalt (OIB)–like melts related to the Farallón–Aluk asthenospheric slab window affected the peridotite composition.The host alkaline basalt is a single unit with a HIMU-like OIB signature characterized by marked positive Nb–Ta anomalies coupled with negative anomalies in highly incompatible and fluid-mobile elements (Rb, K, Pb, and Sr). The compositional similarity between the HIMU-like OIB mantle source and the host basalt is also evident from trace element ratios [(Ba–Th–K–La–Zr)/Nb] as well as by the low 87Sr/86Sri (0.703039–0.703058) and relatively high 143Nd/144Ndi (0.512880–0.512874) and 206Pb/204Pb (19.333–19.389) isotopic ratios. The low 206Pb/204Pb ratios compared to end-member HIMU lavas (e.g., Sta. Helena and the Cook-Austral Islands) suggest that this region was modified by processes associated with a prolonged period of subduction related to the Andean orogenesis and the recycling of several oceanic plates beneath the continent, following the Mesozoic breakup of Gondwana or an even older subduction-related event with young recycling ages (<2Ga).

Biostratigraphy, geochronology and sedimentation rates of the upper Miocene Pisco Formation at two important marine vertebrate fossil-bearing sites of southern Peru

The Mio – Pliocene Pisco Formation, Peru, is a world renowned Konservat-Lagerstatte. Beside its potential in preserving important information on the history of the Pacific Ocean during the Miocene, the Pisco Fm. provides palaeontologists with the unique opportunity to study in detail the evolution of marine vertebrate fauna during the Neogene. Because diatomites are one of the main lithologies of the Pisco Fm., previous authors have hypothesised exceptionally high sedimentation rates related to the deposition of diatom frustules as the main cause leading to the formation of the Lagerstatte. However, sedimentation rates were never calculated. With the perspective to build a solid stratigraphic framework for the whole Pisco Fm., we started to investigate two important fossiliferous sites: Cerro Colorado and Cerro Los Quesos. Within these two areas, measurement of multiple stratigraphic sections combined with extensive field mapping of a number of distinct marker beds was carried out, which allowed a high-resolution correlation and creation of a local lithostratigraphic framework. Integrated tephrostratigraphy,40Ar/39 Ar dating and diatom biostratigraphy permitted an accurate chronostratigraphy to be established. Both biostratigraphic and radiometric ages converge to indicate a late Miocene age for the two sites, amending previous attribution of Cerro Colorado and Cerro Los Quesos to the middle Miocene. Age models for the site of Cerro Los Quesos reveal that sedimentation rates are high but in line with those of high-productivity environments, thus invalidating the hypothesis of exceptional sedimentation rates as the main cause for the formation of the Lagerstatte. These results are extremely important also in the perspective of building a complete stratigraphic framework for the Pisco Fm.

Upper Paleocene ultramafic igneous rocks offshore mid-Norway: Reinterpretation of the Vestbrona Formation as a sill complex

Continental breakup between northwest Europe and Greenland (approximately 56 Ma) was associated with widespread magmatism. Silica undersaturated alkaline porphyritic igneous rocks of a similar age have previously been dredged near the mid-Norwegian coast. These igneous rocks of the Vestbrona Formation have previously been interpreted as either igneous plugs or volcanic flows. New 3D seismic data indicate that relatively small sill complexes are abundant in the same region. In total, 36 sills with a size of [Formula: see text] have been mapped. In addition, 10 seismic horizons were interpreted and tied to nearby wells to obtain a robust stratigraphic framework. The sills mainly intrude Cretaceous and Paleocene sequences; however, one sill is also identified in the pre-Cretaceous sequences. The sills locally form erosional remnants on the seabed due to massive uplifting and erosion of the continental margin. Vintage igneous and sedimentary dredge samples have been reanalyzed, including petrography, geochemistry (X-ray fluorescence [XRF], X-ray diffraction [XRD]), biostratigraphy, and Ar-Ar geochronology. The new Ar-Ar data suggest that the sills are 1–2 Ma older than breakup (approximately 57–58 Ma). Furthermore, the biostratigraphy and petrography of two sediment samples suggest that the samples were collected from near in situ subcrops and not of an ice rafted origin. The sediment samples are of Danian age and are strongly metamorphosed, most likely by contact metamorphism resulting from heating during sill emplacement. The newly identified sills have implications for the petroleum prospectivity of the study area including source rock maturation within thermal aureoles and the long-term alteration of fluid migration pathways.

Estimating carbon dioxide residence time scales through noble gas and stable isotope diffusion profiles

The study of natural carbon dioxide reservoirs provides fundamental insight into processes involved in carbon capture and storage. However, the calculations of process rates such as dissolution of CO2 into formation water remain uncertain due to indirectly determined ages of the CO2 influx. The proposed ages for the Bravo Dome gas field in New Mexico, USA, vary from 56 ka to 1.5 Ma. Here we demonstrate that residence times can be estimated from simple modeling of noble gas and stable isotope diffusion profiles from the gas-water contact through the gas column. The Bravo Dome gas field shows a gradient in noble gas concentrations and isotopic ratios from east to west across the 70-km-wide field. A mantle-like end member with a 3He/4He (R/RA) ratio of up to 4.7 is found in the west in contrast to a groundwater end member with high concentrations of air- and crustal-derived noble gases in the east. The air- and crustal-derived noble gases decrease gradually toward the west. Stable isotope compositions (C and O) also vary across the field. Diffusion modeling of He, Ne, Ar, Kr, Xe, and δ13C data yield residence times for the CO2 between 14.1 ± 0.2 ka and 16.9 +1.1/–0.5 ka. This is far less than the previous estimates of 1.2–1.5 Ma based on apatite (U-Th)/He thermochronology, leading to a dissolution rate of 29,900 +11,800/–10,700 t/a to 35,900 ± 12,300 t/a, implying that 28% of the total emplaced CO2 dissolved. This new method can be applied to a wide variety of gas fields with variation in the concentration of groundwater-derived noble gases and allow a better assessment of the time scale of other diffusive fluid-fluid interactions.

Further evidence for an impact origin of the Tsenkher structure in the Gobi-Altai, Mongolia: geology of a 3.7 km crater with a well-preserved ejecta blanket

AbstractThe Tsenkher structure in the Gobi-Altai, Mongolia is ac. 3.7 km diameter crater with a well-preserved ejecta blanket. It has been hypothesized to be either of impact or volcanic origin in our previous work. Observations during our 2007 expedition and related sample analyses give further support for an impact origin. The evidence includes the presence of a structurally uplifted near-circular rim surrounded by an ejecta blanket, and abundant breccias, some of which are melt- and millimetre-scale spherule-bearing. Planar deformation features (PDFs) were found in one quartz grain in a breccia sample. Fe-rich grains are found in a vesicular melt sample that is also characterized by elevated platinum group element (PGE) abundances with respect to the sedimentary bedrock of the area (approximately an order of magnitude). Noble gas analysis of one breccia sample yielded an elevated3He/4He value of (5.0±0.2) × 10−6. Although not conclusive alone, these geochemical results are consistent with a contribution of meteoritic components. A volcanic origin, in particular a maar formation, would require explanations for the unusual conditions associated with Tsenkher, including its large size occurring in isolation, the structurally uplifted rim and the lack of a bedded base surge deposit. A pronounced rampart structure observed at the eastern ejecta is also unusual for any volcanic origin.40Ar–39Ar dating of a vesicular melt sample gives an age of the Tsenkher structure of 4.9±0.9 Ma. The rampart structure could provide insights into the formation of similar ejecta morphologies associated with numerous impact craters on Mars.

Laser Ablation ICP-MS U-Pb and 40Ar-39Ar age constraints on Neoarchean to Paleoproterozoic magmatic and tectono-metamorphic evolution of the link between Hope Bay and Elu greenstone belts, northeast Slave craton, NWT, Canada

The Elu Link connecting the Neoarchean Hope Bay and Elu granite-greenstone belts in the Bathurst block of the northeast Slave craton consists of volcano-sedimentary and gabbro–granite rocks. Laser ablation ICP-MS zircon and titanite U-Pb dating was combined with mineral 40Ar-39Ar age data to date the Elu Link rocks and D1–D3 tectono-metamorphic events, as well as characterize the response to the Thelon Orogeny in the area. The volcanic rocks are correlated with the ca. 2716Ma Flake Lake suite in the adjacent Hope Bay belt, whereas the sedimentary units have a minimum depositional age of 2684±11Ma nearly coinciding with the onset of D1 deformation. Most gabbro–granite bodies were emplaced between 2651±14Ma and 2577±13Ma coeval with the D2 deformation whose accompanying metamorphism culminated at 2632±7Ma. Older ages (up to 3042±22Ma) retained by either the gabbro–granite or their hosts are indicative of crustal contamination. A few gabbro–granites were also emplaced during the 2577±13Ma to 2494±21Ma D3 deformation event. However, the 2494±21Ma intrusions are unusual and mark the Archean–Paleoproterozoic transition. The D3 deformation was followed by erosional exhumation and localized thermal pulses associated with the emplacement of diabase dikes at 2228±8Ma, 2128±11Ma, and 802±75Ma. The Thelon Orogeny imprint, inferred from the 2054±11Ma to 1919±5Ma biotite ages, corresponds to nearly isobaric, low-temperature (<350°C) metamorphic re-equilibration.

An Early Ordovician 40Ar-39Ar age for the ∼50 km Carswell impact structure, Canada

The formation age of the large (∼50 km) Carswell impact structure, Canada, has been a matter of debate ever since its discovery five decades ago, with proposed ages ranging from Mesoproterozoic to Early Cretaceous. Here, we present new 40Ar-39Ar data for aliquots of euhedral adularia, separated from vesicles in an impact melt rock from the central uplift of the structure. The analyses of the adularia yielded a statistically robust Early Ordovician crystallization age of 481.5 ± 0.8 Ma (2σ, mean square of weighted deviates = 1.06, P = 0.30). The most plausible explanation for the formation of vesicle-filling adularia is through low-temperature mineral precipitation during residual hydrothermal circulation that followed the impact, as no other known major intrusive, extrusive, or thermal events have occurred in the Carswell region in the Phanerozoic. The new age of the Carswell impact structure overlaps within uncertainty with the most precise Ar-Ar ages proposed for the L-chondrite parent body breakup event, but not with the age of the stratigraphic sequence from which the meteorites and micrometeorites from this event were recovered. Thus, either the Carswell impact represents a separate, unrelated impact event, or the dynamic evolution of the L-chondrite parent body breakup is more complicated than presently understood, and Carswell represents one of the earliest and largest known impacts of this event on Earth. (Less)

Geology and Timing of Ore Formation in the Willow Creek Gold District, Talkeetna Mountains, Southern Alaska

The Willow Creek mining district was the third-largest lode gold district in Alaska in the 20 th century, having produced 19 metric tons (t) of gold from vein deposits and another 2 t of gold from associated placer deposits. The district is located in the southern Talkeetna Mountains, north of the Castle Mountain strike-slip fault system. Most gold occurs in widespread mineralized quartz veins hosted by granitic rocks of the Willow Creek pluton. A geochronologic study of granitic rocks, alteration minerals, and adjacent metamorphic rocks was conducted in order to better understand the timing of magmatism, thermochronologic evolution, and age of gold mineralization in the district. New U-Pb zircon dates from the Talkeetna batholith, including the Willow Creek pluton, range from 72.1 to 71.3 Ma. The U-Pb dates are similar to reported U-Late Cretaceous Pb and K-Ar dates from elsewhere in the study area, and tightly constrain the age of plutonic rocks that host the Willow Creek lode gold deposits. Granitic rocks that intrude the Willow Creek pluton along its northwestern margin yield U-Pb dates of 75.7 and 70.8 Ma. The 40 Ar/ 39 Ar thermochronologic data for the Willow Creek pluton and other granitoids reveal rapid postemplacement cooling through hornblende, muscovite, and biotite closure temperatures. Thermochronologic data from K-feldspars document slower cooling rates through lower temperatures, which may be related to slow uplift and exhumation during the Paleocene-Eocene. The data are compatible with a simple tectonothermal history following emplacement of the plutonic rocks, with essentially no evidence of significant thermal disturbance of the rocks after about 64 Ma. Uranium-thorium-lead dating of hydrothermal phosphate minerals (monazite and xenotime) and 40 Ar/ 39 Ar dates from hydrothermal sericite from wall rocks and veins indicate that the main stage of gold mineralization occurred at about 67 to 65 Ma. Combining U-Th-Pb data from monazites from the Independence, Talkeetna, Gold Bullion, and Fern deposits gave an average of 65.4 ± 2.3 Ma; hydrothermal xenotime from the Independence vein also yields a statistically indistinguishable age of 64.5 ± 2.3 Ma. These ages overlap 40 Ar/ 39 Ar dates of 67 to 66 Ma obtained from hydrothermal sericite associated with formation of the gold-bearing veins, and the age of a muscovite from a weakly mineralized pegamatite that cuts the Willow Creek pluton. In contrast to previous studies that suggested that at least some mineralization was Eocene in age, we found no compelling evidence for significant gold mineralization within the district at this time. The 40 Ar/ 39 Ar and K-Ar dates from graphitic white micas from schist south of the Hatcher Pass fault zone yield ages of 61 to 50 Ma that are probably related to uplift and cooling of these Late Cretaceous metasedimentary rocks. The absence of evidence for postemplacement thermal events affecting the plutonic rocks of the Willow Creek district to the north indicates that the schists and granitoids do not share a similar tectonothermal history, and that they have been juxtaposed by subsequent faulting along the Hatcher Pass fault zone.

Characteristics and 40Ar/39Ar Geochronology of the Erdenet Cu-Mo Deposit, Mongolia

The Early to Middle Triassic Erdenet porphyry Cu-Mo deposit, in northern Mongolia, developed in a continent-continent arc collision zone, within the Central Asian orogenic belt. The porphyry system is related to multiple intrusions of crystal-crowded biotite granodiorite porphyry, which formed a composite stock about 900 m in diameter, with multiple porphyritic microgranodiorite dikes. Wall rocks are Late Permian to Early Triassic, medium-grained granodiorite, with similar whole-rock geochemistry, mineralogy, and composition to the granodiorite porphyry. Whole-rock analysis of the granodiorite porphyry and wall rocks shows that these rocks cannot be discriminated, but both have depleted middle heavy rare earth elements and Y, typical of fertile porphyry magmatic suites. At the current pit level (1,250 m elev), early porphyry-style quartz veins (A and B type) are locally infilled by pyrite-chalcopyrite, with subordinate bornite, but most of the chalcopyrite occurs in D veins that constitute more than 50% of the Cu grade (~0.5 wt % Cu). The 0.3 wt % Cu shell resembles a molar tooth, enveloping the granodiorite porphyry, with deeper roots extending down the wal-rock contacts. Molybdenite occurs in monomineralic veins, and in finely laminated to massive quartz-molybdenite veins. The most important alteration is quartz-muscovite, which occurs as relatively coarse (100–500 μ m) alteration selvages (1–5 cm) that envelop D veins. The D veins cut illite ± kaolinite-smectite (or intermediate argillic) alteration. Intermediate argillic alteration, together with abundant pink anhydrite (commonly hydrated to gypsum), extends from at least 1,300- to 900-m elevation in the deepest drill holes, and has overprinted early potassic alteration, or relatively unaltered red granodiorite. Meter-wide zones of kaolinite cut the anhydrite-gypsum at all levels. There is an abrupt transition outward from the intermediate argillic alteration to chlorite-epidote (propylitic) alteration, at 50 to 200 m from the granodiorite porphyry contact, although D veins (and chalcopyrite) extend outward to the propylitic zone. The Erdenet porphyry system, was overprinted by advanced argillic alteration, which outcrops 2 km northwest of the pit, and forms a lithocap that extends over 10 × 2.5 km. It is characterized by residual quartz, andalusite, Na-Ca and K-alunite, diaspore, pyrophyllite, zunyite, topaz, dickite, and kaolinite. The upper part of the porphyry Cu-Mo deposit (removed by mining), comprised a bornite-chalcocite enriched zone up to 300 m thick with an average grade of 0.7 wt % Cu and up to 5 wt % Cu locally. Based on hypogene bornite-chalcocite mineral textures and high-sulfidation state mineralogy, the enriched zone is inferred to be of hypogene origin, but modified by supergene processes. Consequently, it may be related to formation of the lithocap. Previous Re-Os dates of 240.4 and 240.7 ± 0.8 Ma for molybdenite in quartz veins are comparable to new 40 Ar/ 39 Ar dates of 239.7 ± 1.6 and 240 ± 2 Ma for muscovite that envelops D veins. One 40 Ar/ 39 Ar date on K-alunite from the lithocap of 223.5 ± 1.9 Ma suggests that it may be about 16 m.y. younger than Erdenet, but this result needs to be verified by further dating.

Timing and deformation conditions of the Tallulah Falls dome, NE Georgia: Implications for the Alleghanian orogeny

Structural domes composed of crystalline rocks metamorphosed to midcrustal conditions are present in every major orogen and record deformation of the deep interior of mountain ranges. The Tallulah Falls dome in northeast Georgia is a shear zone−bounded body of metasedimentary rocks that is defined by a foliation pattern that crosscuts the regional fabric of the Eastern Blue Ridge crystalline nappe in the southern Appalachian Mountains. Previously, the Tallulah Falls dome was interpreted as the expression of duplexed underlying slices of Laurentian margin sedimentary rocks imbricated during Alleghanian emplacement of the allochthonous Blue Ridge thrust sheet under greenschist-facies conditions. We provide new observations of quartz and feldspar recrystallization fabrics and new 40 Ar/ 39 Ar dates from muscovite that complement existing thermochronometric ages to determine the timing and conditions of doming within the Tallulah Falls dome. Our microstructural observations are consistent with deformation mechanisms active dominantly at amphibolite-facies conditions, with some samples recording temperatures in excess of 650 °C. Thermochronometric ages define a period of relatively rapid cooling from ∼550 °C to 340 °C within and around the Tallulah Falls dome between 321 and 317 Ma, with differential cooling across the northwestern dome-bounding shear zone lasting until at least 313 Ma. Our observations reveal a period of early Alleghanian cooling from amphibolite- to greenschist-facies conditions associated with displacement along a shear zone bounding the Tallulah Falls dome. Across the Blue Ridge, the Alleghanian orogeny has long been characterized only by thrusting at greenschist-facies conditions. Similarly, dome formation has been interpreted as the result of duplexing within the thin-skinned Blue Ridge crystalline thrust sheet as it was emplaced upon the Laurentian margin. Both of these interpretations are inconsistent with the new data presented here, which suggest more complex Alleghanian deformation over a broader range in temperatures, and perhaps a diverse suite of structural styles. Our results are best explained by doming of the footwall of a normal-displacement shear zone at amphibolite-facies conditions during the earliest Alleghanian period.

Quantitative Potassium Measurements with Laser-Induced Breakdown Spectroscopy Using Low-Energy Lasers: Application to In Situ K-Ar Geochronology for Planetary Exploration.

In situ radiogenic isotope measurements to obtain the absolute age of geologic events on planets are of great scientific value. In particular, K-Ar isochrons are useful because of their relatively high technical readiness and high accuracy. Because this isochron method involves spot-by-spot K measurements using laser-induced breakdown spectroscopy (LIBS) and simultaneous Ar measurements with mass spectrometry, LIBS measurements are conducted under a high vacuum condition in which emission intensity decreases significantly. Furthermore, using a laser power used in previous planetary missions is preferable to examine the technical feasibility of this approach. However, there have been few LIBS measurements for K under such conditions. In this study, we measured K contents in rock samples using 30 mJ and 15 mJ energy lasers under a vacuum condition (10-3 Pa) to assess the feasibility of in situ K-Ar dating with lasers comparable to those used in NASA's Curiosity and Mars 2020 missions. We obtained various calibration curves for K using internal normalization with the oxygen line at 777 nm and continuum emission from the laser-induced plasma. Experimental results indicate that when K2O < 1.1 wt%, a calibration curve using the intensity of the K emission line at 769 nm normalized with that of the oxygen line yields the best results for the 30 mJ laser energy, with a detection limit of 88 ppm and 20% of error at 2400 ppm of K2O. Futhermore, the calibration curve based on the K 769 nm line intensity normalized with continuum emission yielded the best result for the 15 mJ laser, giving a detection limit of 140 ppm and 20% error at 3400 ppm K2O. Error assessments using obtained calibration models indicate that a 4 Ga rock with 3000 ppm K2O would be measured with 8% (30 mJ) and 10% (15 mJ) of precision in age when combined with mass spectrometry of 40Ar with 10% of uncertainty. These results strongly suggest that high precision in situ isochron K-Ar dating is feasible with a laser used in previous and upcoming Mars rover missions.

Mineral characterization, clay quantification and Ar–Ar dating of faulted schists in the Carboneras and Palomares Faults (Betic Cordillera, SE Spain)

Two major clay-rich fault zones (Betic Cordillera, SE Spain) exhumed under arid conditions were studied to establish the origin and nature of phyllosilicates in deformed mica schists and the role of fault zones in the genesis of rocks rich in clay minerals. Both areas contain wide zones (≈200 m across) of alternating fault gouge and protolith creating a complex arrangement of sheared lenses along fault strand cores. Minerals in schists were characterized by X-ray diffractometry and scanning and transmission electron microscopy, and two samples were selected for 40 Ar/ 39 Ar dating of four clay grain-size fractions each. Quartz, chlorite and K-micas are the principal phases in all samples (crystalline protolith and faulted schists). Paragonite and Fe and Ti-oxides are present in some samples and, additionally, most of the damaged samples have kaolinite, illite and gypsum. Backscattered electron images show irregularly oriented stacks of phyllosilicates with curved shapes in the fault rocks and quartz grains deformed by brittle fracturing. Clay fabrics in the damaged rocks are poorly developed. Lattice fringe images of faulted damaged samples revealed defect-free dioctahedral mica and Fe-rich chlorite packets which were generally with no more than several tens of nm thickness, although chlorite grains were usually thicker than micas. Chlorite showed more obvious deformation features. The calculated ages of the authigenic clay component in the two samples are 6.3 ± 0.8 Ma and 6.2 ± 0.4 Ma, while the ages calculated for the protolith clays are 22.8 ± 1.9 Ma and 11.7 ± 1.3 Ma. The age of the authigenic clay overlaps the range of ages from the nearby Cabo de Gata volcanic series (SE Spain). Textural and chemical results point toward that although the fault gouge is mostly a product of mechanical crushing of the protolith, the presence of quartz provided a strong and fragile behaviour to the faulted rocks increasing their permeability, and creating permeable paths through which low-temperature hydrothermal fluids circulated producing an intense leaching of the parent material and promoting crystallization of new authigenic minerals (illite, kaolinite, smectite and gypsum).

Phanerozoic low-temperature evolution of the Uruguayan Shield along the South American passive margin

The crystalline basement of Uruguay was assembled during the Brasiliano Orogeny in the Neoproterozoic Era and was later affected by discrete tectonic activity. A new multi-method low-temperature dataset including (U–Th)/He ages from both zircon and apatite, T – t modelling and K–Ar dating of fine sericite fractions and fault gouge reveal a detailed post-orogenic geological history spanning the Phanerozoic Eon. The juxtaposition of the terranes that compose the area was achieved in the Ediacaran Period, and post-collision was marked by intense exhumation, in which the crystalline basement reached near-surface conditions by the early to mid-Palaeozoic. Regional subsidence promoted sedimentation in the Paraná Basin until the Permian, covering and reheating much of the basement that is at present exposed. Afterwards, deposition and volcanism were mostly confined to its current limits. Regional exhumation of the shield during the Permo-Triassic exposed much of the northern portion of the basement, and the south was further affected by the opening of the South Atlantic Ocean during the Mesozoic. Little exhumation affected the Uruguayan Shield during the Cenozoic, as reflected in its modest topography. The reactivation of inherited Neoproterozoic structures influenced the development of Mesozoic basins and the present-day landscape. Supplementary material: Supplementary data (sample locations, (U-Th)/He data and K-Ar data) are available at https://doi.org/10.6084/m9.figshare.c.3702043

The age of salinic deformation constrained by 40 Ar/ 39 Ar dating of multiple cleavage domains: Bathurst Supergroup, New Brunswick Appalachians

In the New Brunswick Appalachians, polydeformed felsic volcanic rocks of the Bathurst Supergroup record four cleavage-forming tectonic events, of which D1 and D2 record subduction-related underplating of buoyant elements of the Tetagouche backarc basin and subsequent collision between composite Laurentia and the Gander margin, respectively, during the Salinic orogenic cycle. Here we present an integrated approach to dating multiple cleavage domains in which we performed step heat 40Ar/39Ar analyses, microstructural observations and mineral-chemistry analysis on a suite of samples, as well as in situ40Ar/39Ar analysis on two samples from the suite with clear S1 and S2 cleavage relationships. We use this dataset to characterize the complex relationships between S1 and S2 white mica between samples at different structural settings across the supergroup. We refine the timing of S1 white mica growth, and hence M1-D1 of the Salinic cycle in the Bathurst Supergroup to ca. 452 to 437 Ma, and S2 white mica growth (also D2) to ca. 427 to 418 Ma. New and published data indicate local thermal resetting of white mica at ca. 411 Ma, which we interpret to indicate a buried intrusion, likely a component of the Central plutonic belt. Delineation of the two white mica age components, particularly within a single sample, was informed by both spatially-controlled analysis (in situ40Ar/39Ar), and the higher age precision of step heat 40Ar/39Ar analyses.

Devonian alkaline magmatism in South Qinling, China: evidence from the Taohekou Formation, Northern Daba Mountain

ABSTRACTThe Langao area in South Qinling consists of a poorly dated sequence of volcano-sedimentary units (Taohekou Formation). This study focuses on volcanic edifice of Taohekou Formation. Detailed field mapping, section facies analysis, geochemistry, and Ar–Ar geochronology is presented here to develop a more accurate understanding of the tectonic evolution of the Qinling Orogen. Detailed field mapping and section facies analysis reveals the presence of a seamount volcanic edifice in western Xiaozhen area. Ar–Ar dating of alkaline basalts in the sequence indicates an Early Devonian magmatic age. Whole rock geochemistry of these alkaline basalts indicates that both potassic and sodic basalts were erupted respectively in the eastern and western areas. Both types of samples are typically characterized by high concentrations of TiO2, Nb, Ta, and Zr. Ocean-island basalt (OIB)-like rare earth element distribution patterns, trace element spider diagrams, and similar Sr and Nd isotopic compositions (i.e. relatively low Sr and moderate Nd isotopic ratios) likely indicate that the samples were comagmatic but have experienced varying degrees of fractional crystallization. In addition, the samples appear to have all originated from the asthenospheric mantle and undergone mantle metasomatism, ultimately resulting in an OIB-type provenance. Most importantly, in combination with its regional tectonic evolution, the Taohekou Formation is constrained to a seamount environment. This formation does not represent a plume but, rather, an extensional fracture zone related to subduction. Thus, this geologic interpretation generates pivotal evidence for accurately investigating the properties and temporal constraints of the Mianlue oceanic basin.