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³⁹Ar dating with small samples provides new key constraints on ocean ventilation.

ABSTRACT: Ocean ventilation is the integrated effect of various processes that exchange surface properties with the ocean interior and is essential for oxygen supply, storage of anthropogenic carbon and the heat budget of the ocean, for instance. Current observational methods utilise transient tracers, e.g. tritium, SF_6, CFCs and ¹⁴C. However, their dating ranges are not ideal to resolve the centennial-dynamics of the deep ocean, a gap filled by the noble gas isotope ³⁹Ar with a half-life of 269 years. Its broad application has been hindered by its very low abundance, requiring 1000 L of water for dating. Here we show successful ³⁹Ar dating with 5 L of water based on the atom-optical technique Atom Trap Trace Analysis. Our data reveal previously not quantifiable ventilation patterns in the Tropical Atlantic, where we find that advection is more important for the ventilation of the intermediate depth range than previously assumed. Now, the demonstrated analytical capabilities allow for a global collection of ³⁹Ar data, which will have significant impact on our ability to quantify ocean ventilation.

SUBMITTER: Ebser S

PROVIDER: S-EPMC6261945 | biostudies-literature |

REPOSITORIES: biostudies-literature

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40Ar/39Ar dating of Glacial Termination VI: constraints on the duration of Marine Isotopic Stage 13.

Project description:We present four new 40Ar/39Ar ages of tephra layers from an aggradational succession (Valle Giulia Formation) near the mouth of the Tiber Valley in Rome that was deposited in response to sea-level rise during Marine Isotopic Stage (MIS) 13. These new ages, integrated with seven previously determined ages, provide the only extant independent, radioisotopic age constraint on glacial termination VI and on the duration of MIS 13 sea-level rise. The new geochronologic constraints suggest a long duration for the period of sea-level rise (533 ± 2 through 498 ± 2 ka) encompassing two consecutive positive peaks of the δ18O curve (substages 13.3 and 13.1). Consistently, the litho-stratigraphic features of the sedimentary record account for two aggradational phases separated by an intervening erosional phase. Moreover, the ages obtained for this study give us the opportunity to compare the timing of the sea-level fluctuations inferred from the stratigraphic record and that provided by the astrochronologic calibration of the Oxygen isotopic curves, and to assess the calibrations of 40Ar/39Ar standards. Results of this comparison indicate that the best match is for an age of 1.186 Ma for the Alder Creek Rhyolite sanidine and 28.201 Ma for the Fish Canyon Tuff sanidine.

| S-EPMC5566960 | biostudies-literature

Dating of polyhalite: a difficult 40Ar/39Ar dating tool of diagenetic to very low-grade metamorphic processes.

Project description:Halite already deforms at surface temperatures. A valuable universal dating tool to constrain the timing of sedimentary, diagenetic, or deformational structures is still missing. The evaporite mineral polyhalite can be dated by the 40Ar/39Ar method. On the example of the extremely deformed halite deposits of the Eastern Alps, polyhalite was tested to date early diagenetic stages of the deposits. The sedimentological investigation of the present study indicates that some of the macrostructures of polyhalite had a syn-depositional origin during the late Permian. It is supposed that polyhalite originated during reflux of brines. All samples selected for age dating represent characteristic microfabric types of euhedral to subhedral polyhalite crystals. Intact macro- and non-recrystallized looking microstructures of polyhalite can be expected to give plateau ages. However, nearly all measurements produced overdispersed data that do not define an age. The oldest age steps thus represent only minimum ages. A closer look revealed grain boundary migration, subgrain rotation recrystallization, twinning, and fluid-supported grain size increase. These recovery processes obscured the original ages and/or reflect the origin of new polyhalite in place of the original individuals. Based on these microstructures, the age data are supposed to reflect the circulation of aqueous fluids. Just extremely careful separation of individual crystals or in situ age dating under the microscope will be successful in dating polyhalite. Nevertheless, polyhalite can potentially serve to date deformational events of halite deposits due to its easy recrystallization property.Supplementary informationThe online version contains supplementary material available at 10.1007/s00531-022-02219-9.

| S-EPMC9334404 | biostudies-literature

In Situ and Step-Heating 40Ar/39Ar Dating of White Mica in Low-Temperature Shear Zones (Tenda Massif, Alpine Corsica, France).

Project description:In order to clarify the link between 40Ar/39Ar record in white mica and deformation, we performed in situ and bulkwise 40Ar/39Ar dating over the East Tenda Shear Zone (Tenda massif, Alpine Corsica). White micas from 11 samples were selected and extensively analyzed using in situ techniques across nested scales of strain-intensity gradients developed at the expense of a late-Variscan protolith. 40Ar/39Ar systematics are unaffected by inherited Ar and directly linked to deformation with little or no Ar lattice (volume) diffusion. Extensive sampling allows constraining the end of deformation related to burial and exhumation, respectively, at ~34 and ~22 Ma, bracketing the duration of regional extensional shear to ~12 Myr. Results also highlight a regional strain localization toward the upper contact of the unit with smaller-scale localization in specific lithologies, notably meta-aplites. Second-order complications exist, such as local ill-defined correlations between ages and finite-strain microstructures. Thus, the use of a strain gradient as a proxy for strain localization in time is regionally valid but sometimes locally too complex to track or resolve strain partitioning/localization trends at the meter (outcrop) scale and below. Age mixing and incomplete isotopic homogenization by dissolution/precipitation are identified as the main causes of local discrepancies that complicate the link between age and microstructure and the derivation of strain localization rates. Tracking temporal trends in shear distribution across regional-scale deformation gradients in such settings is possible but requires a multi-scale approach as implemented here to reveal younging patterns associated to strain localization.

| S-EPMC7756386 | biostudies-literature

The 40Ar/39Ar dating of quartz: new insights into the metallogenic chronology of the Jinchang gold deposit and its geological significance.

Project description:The Jinchang gold deposit has been extensively studied, but precise dates for its formation are debated. Native gold mainly occurs as inclusions within pyrite and quartz. In this study, we analysed quartz crystals coeval with gold precipitation from two different types of mineralization using the ArgusVI multi-collector noble gas mass spectrometer by the stepwise crushing technique to resolve the timing and genesis of gold mineralization. 40Ar/39Ar dating of quartz samples (J12Q) from breccia ore yields a plateau age of 109.87 ± 0.86 Ma, and an inverse isochron age of 109.87 ± 0.88 Ma. Quartz samples (J18Q) from vein ore yields a slightly younger plateau age of 107.76 ± 0.85 Ma, with an inverse isochron age of 107.76 ± 0.92 Ma. These dates place the ore-forming age of the Jinchang gold deposit at 107~110 Ma, much younger than previously published radiometric ages, suggesting the gold mineralization is spatio-temporally associated with the granite porphyry. The formation of the Jinchang gold deposit is consistent with the regional late Mesozoic porphyry-epithermal gold mineralization event in the Yanbian-Dongning area. Finally, our study shows that 40Ar/39Ar of quartz can be used as a powerful tool to date the formation ages of hydrothermal ore deposits.

| S-EPMC6141584 | biostudies-literature

Progressive crushing 40Ar/39Ar dating of a gold-bearing quartz vein from the Liaotun Carlin-type gold deposit, Guangxi, southern China.

Project description:Carlin-type gold deposits are among the largest hydrothermal gold deposits in the world. However, direct dating the metallogenic age of these deposits is difficult, because not all deposits provide material suitable for conventional radiometric methods. Syn-mineralization stage quartz veins from these deposits usually contain abundant fluid inclusions, which allow fluid inclusion 40Ar/39Ar dating. In this study, progressive crushing 40Ar/39Ar dating has been performed on a gold-bearing quartz vein from the Liaotun Carlin-type gold deposit in northwestern Guangxi, China. Argon isotopes liberated from the later steps yielded an isochron age of 200.7 ± 2.1 Ma. We infer that Ar-bearing gas was extracted from the primary fluid inclusions, and that the age of ca. 200.7 Ma reflects the timing of gold mineralization. The initial 40Ar/36Ar ratio corresponding to the isochron is 298.0 ± 4.3, which is statistically indistinguishable from the value for air, indicating that the ore-forming fluids probably mainly derived from gravitational pressure flow in the basin of air-saturated water. Our preliminary study shows the feasibility and great potential of 40Ar/39Ar dating of gases from fluid inclusions by progressive crushing of quartz veins to date the mineralization age and decipher the fluid origins of Carlin-type gold deposits.

| S-EPMC9329373 | biostudies-literature

Refining lunar impact chronology through high spatial resolution (40)Ar/(39)Ar dating of impact melts.

Project description:Quantitative constraints on the ages of melt-forming impact events on the Moon are based primarily on isotope geochronology of returned samples. However, interpreting the results of such studies can often be difficult because the provenance region of any sample returned from the lunar surface may have experienced multiple impact events over the course of billions of years of bombardment. We illustrate this problem with new laser microprobe (40)Ar/(39)Ar data for two Apollo 17 impact melt breccias. Whereas one sample yields a straightforward result, indicating a single melt-forming event at ca. 3.83 Ga, data from the other sample document multiple impact melt-forming events between ca. 3.81 Ga and at least as young as ca. 3.27 Ga. Notably, published zircon U/Pb data indicate the existence of even older melt products in the same sample. The revelation of multiple impact events through (40)Ar/(39)Ar geochronology is likely not to have been possible using standard incremental heating methods alone, demonstrating the complementarity of the laser microprobe technique. Evidence for 3.83 Ga to 3.81 Ga melt components in these samples reinforces emerging interpretations that Apollo 17 impact breccia samples include a significant component of ejecta from the Imbrium basin impact. Collectively, our results underscore the need to quantitatively resolve the ages of different melt generations from multiple samples to improve our current understanding of the lunar impact record, and to establish the absolute ages of important impact structures encountered during future exploration missions in the inner Solar System.

| S-EPMC4644078 | biostudies-literature

High-precision 14C and 40Ar/39Ar dating of the Campanian Ignimbrite (Y-5) reconciles the time-scales of climatic-cultural processes at 40 ka.

Project description:The Late Pleistocene Campanian Ignimbrite (CI) super-eruption (Southern Italy) is the largest known volcanic event in the Mediterranean area. The CI tephra is widely dispersed through western Eurasia and occurs in close stratigraphic association with significant palaeoclimatic and Palaeolithic cultural events. Here we present new high-precision 14C (34.29 ± 0.09 14C kyr BP, 1σ) and 40Ar/39Ar (39.85 ± 0.14 ka, 95% confidence level) dating results for the age of the CI eruption, which substantially improve upon or augment previous age determinations and permit fuller exploitation of the chronological potential of the CI tephra marker. These results provide a robust pair of 14C and 40Ar/39Ar ages for refining both the radiocarbon calibration curve and the Late Pleistocene time-scale at ca. 40 ka. In addition, these new age constraints provide compelling chronological evidence for the significance of the combined influence of the CI eruption and Heinrich Event 4 on European climate and potentially evolutionary processes of the Early Upper Palaeolithic.

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New radiometric 40Ar-39Ar dates and faunistic analyses refine evolutionary dynamics of Neogene vertebrate assemblages in southern South America.

Project description:The vertebrate fossil record of the Pampean Region of Argentina occupies an important place in South American vertebrate paleontology. An abundance of localities has long been the main basis for constructing the chronostratigraphical/geochronological scale for the late Neogene-Quaternary of South America, as well as for understanding major patterns of vertebrate evolution, including the Great American Biotic Interchange. However, few independently-derived dates are available for constraining this record. In this contribution, we present new 40Ar/39Ar dates on escorias (likely the product of meteoric impacts) from the Argentinean Atlantic coast and statistically-based biochronological analyses that help to calibrate Late Miocene-Pliocene Pampean faunal successions. For the type areas of the Montehermosan and Chapadmalalan Ages/Stages, our results delimit their age ranges to 4.7-3.7 Ma and ca. 3.74-3.04 Ma, respectively. Additionally, from Buenos Aires Province, dates of 5.17 Ma and 4.33 Ma were recovered for "Huayquerian" and Montehermosan faunas. This information helps to better calibrate important first appearances of allochthonous taxa in South America, including one of the oldest records for procyonids (7.24-5.95 Ma), cricetids (6.95-5.46 Ma), and tayassuids (> 3.74 Ma, oldest high-confidence record). These results also constrain to ca. 3 Ma the last appearances of the autochthonous sparassodonts, as well as terror birds of large/middle body size in South America. South American faunal turnover during the late Neogene, including Late Pliocene extinctions, is interpreted as a consequence of knock-on effects from global climatic changes and initiation of the icehouse climate regime.

| S-EPMC8110973 | biostudies-literature

Timescale of a magmatic-hydrothermal system revealed by 40Ar-39Ar geochronology: the Mio-Pliocene Campiglia Marittima system (Tuscany, Italy).

Project description:Petrology and timing of magmatic-hydrothermal systems and the linkage between plutonic and volcanic domains are central topics in geosciences, because of broad implications for natural hazards and exploitation of natural resources. We investigated by the 40Ar-39Ar method the timescale of a well-characterized natural example, the Mio-Pliocene Campiglia Marittima magmatic-hydrothermal system (Tuscany, Italy). 40Ar-39Ar data from pristine and homogeneous trioctahedral micas and sanidine from the plutonic-hydrothermal-subvolcanic-volcanic sequence (from the Botro ai Marmi Granite to the San Vincenzo Rhyolite) record crystallization ages and define a temporal sequence lasting 973 ± 43 ka, starting from 5.409 ± 0.043 Ma. K-feldspar from mafic and felsic porphyries, unlike micas, are affected by submillimetre, micropore laden, alteration domains consisting of secondary K-feldspar and albite, and yielded staircase-shaped age spectra, compatible with a ternary mixing. Results document that the San Vincenzo Rhyolite consists of two diachronous batches, the first emplaced at 5.0024 ± 0.0062 Ma, closely following emplacement of mafic porphyries, the second at 4.4359 ± 0.0045 Ma. Bulk of hydrothermal deposits, consisting of skarns and associated Zn-Pb(-Ag) mineralization predating Fe-Cu ore, formed within the first ~ 400-ka lifetime of the whole sequence and was closely followed by the first eruption which should have run out most of the ore-forming potential of the system.

| S-EPMC9065031 | biostudies-literature

Major element data, 40Ar/39Ar step-heating and step-crushing data for anorthoclase megacrysts from the Newer Volcanic Province, south-eastern Australia.

Project description:We provide the dataset associated with the research article "40Ar/39Ar ages of alkali feldspar xenocrysts constrain the timing of intraplate basaltic volcanism" Matchan et al. [1]. This dataset contains major element data for 15 large anorthoclase xenocrysts ('megacrysts') collected from six Pleistocene eruption centres (Mount Leura, Mount Shadwell, Mount Noorat, Mount Franklin, Lake Keilambete and The Anakies (East Cone)) in the basaltic Newer Volcanic Province of south-eastern Australia. It also contains multi-collector (Argus VI) 40Ar/39Ar step-heating for 13 of these anorthoclase megacrysts. 40Ar/39Ar vacuo step-crushing experiment data is also provided for three of these megacrysts.

| S-EPMC6141373 | biostudies-literature