Open-system Behavior Research Articles

Comment on “190Pt-186Os geochronometer reveals open system behaviour of 190Pt-4He isotope system” by Luguet et al. (2019)

Comment on “190Pt-186Os geochronometer reveals open system behaviour of 190Pt-4He isotope system” by Luguet et al. (2019)

Geochemical, isotopic and U-Pb geochronological investigation of the late Cretaceous Karaçayır carbonatite (Sivas, Turkey): Insights into mantle sources within a post-collisional tectonic setting

The Karaçayır carbonatite is located within the northeastern edge of the Central Anatolian Crystalline Complex, and represents one of the significant rare earth element (REE) mineralizations in Turkey. The carbonatite intrudes the associated syenite as dikes and veins, and is mainly composed of medium- to coarse-grained calcite, and lesser amounts of apatite, mica, amphibole, pyroxene, and magnetite. This study reports new and combined geochemical, radiogenic (Sr, Pb), stable (O and C), and for the first time B and Nd isotopic ratios and in-situ U-Pb age data from the Karaçayır carbonatite. The carbonate separates from the Karaçayır carbonatite are characterized by high Ba (30 to 484 ppm), Sr (613 to 9106 ppm) and total rare earth element (TREE) concentrations (172 to 2892 ppm), which are consistent with geochemical signatures for mantle-derived carbonate melts. The combined δ18OV-SMOW (+10.7 to +19.0 ‰), δ13CV-PDB (−2.9 to +1.8 ‰) and δ11B (−13.9 to −2.9 ‰) data for a majority of the carbonate separates investigated here in contrast indicate that they are the result of open system behavior involving contamination by the country rock (marble), and/or sub-solidus, low temperature alteration. Based on the combined stable and radiogenic isotopic signatures for the least altered/contaminated samples, the carbonatite magma at Karaçayır most likely represents a low-degree partial melt of an isotopically heterogeneous (metasomatized) mantle source containing EM (enriched mantle)-like components and recycled crustal carbon; the latter finding is consistent with those recorded for young (<300 Ma old) carbonatites worldwide. A weighted mean 206Pb/238U age of 69.3 ± 1.7 Ma is obtained for apatite from the Karaçayır carbonatite, and indicates its emplacement within a post-collisional tectonic setting during the late Cretaceous.

Open-system behaviour of detrital zircon during weathering: an example from the Palaeoproterozoic Pretoria Group, South Africa

AbstractDetrital zircon in six surface samples of sandstone and contact metamorphic quartzite of the Magaliesberg and Rayton formations of the Pretoria Group (depositional age c. 2.20–2.06 Ga) show a major age fraction at 2.35–2.20 Ga, and minor early Palaeoproterozoic – Neoarchaean fractions. Trace-element concentrations vary widely, with Ti, Y and light rare earth elements (LREEs) spanning over three orders of magnitude. REE distribution patterns range from typical zircon patterns (LREE depletion, heavy REE enrichment, well-developed positive Ce and negative Eu anomalies) to patterns that are flat to concave downwards, with indistinct Ce and Eu anomalies. The change in REE pattern correlates with increases in alteration-sensitive parameters such as Ti concentration and (Dy/Sm) + (Dy/Nd), U–Pb discordance and content of common lead, and with a gradual washing-out of oscillatory zoning in cathodoluminescence images. U and Th concentrations also increase, but Th/U behaves erratically. Discordant zircon scatters along lead-loss lines to zero-age lower intercepts, suggesting that the isotopic and chemical variations are the results of disturbance long after deposition. The rocks sampled have been in a surface-near position (at least) since Late Cretaceous time, and exposed to deep weathering under intermittently hot and humid conditions. In this environment, even elements commonly considered as relatively insoluble could be mobilized locally, and taken up by radiation-damaged zircon. Such secondary alteration effects on U–Pb and trace elements can be expected in zircon in any ancient sedimentary rock that has been exposed to tropical–subtropical weathering, which needs to be considered when interpreting detrital zircon data.

Dating martian mafic crust; microstructurally constrained baddeleyite geochronology of enriched shergottites Northwest Africa (NWA) 7257, NWA 8679 and Zagami

Baddeleyite (monoclinic; m-ZrO2) is a widespread accessory phase within shergottites. However, the effects of shock loading on baddeleyite U-Pb isotopic systematics, and therefore its reliability as a geochronometer within highly shocked lithologies, are less well constrained. To investigate the effects of shock metamorphism on baddeleyite U-Pb chronology, we have conducted high-resolution microstructural analysis and in-situ U-Pb isotopic measurements for baddeleyite within enriched basaltic shergottites Northwest Africa (NWA) 7257, NWA 8679 and Zagami. Electron backscatter diffraction (EBSD) analyses of baddeleyite reveal significant microstructural heterogeneity within individual thin sections, recording widespread partial to complete reversion from high-pressure (≥3.3 GPa) orthorhombic zirconia polymorphs. We define a continuum of baddeleyite microstructures into four groupings on the basis of microstructural characteristics, including rare grains that retain magmatic twin relationships. Uncorrected U-Pb isotopic measurements form Tera-Wasserburg discordia, yielding new 238U-206Pb discordia ages of 195 ± 15 Ma (n = 17) for NWA 7257 and 220 ± 23 Ma (n = 10) for NWA 8679. Critically, there is no resolvable link between baddeleyite microstructure and U-Pb isotope systematics, indicating negligible open-system behaviour of U-Pb during zirconia phase transformations. Instead, we confirm that high post-shock temperatures exert the greatest control on Pb mobility within shocked baddeleyite; in the absence of high post-shock temperatures, baddeleyite yield robust U-Pb isotope systematics and date the age of magmatic crystallization. Low bulk post-shock temperatures recorded within Zagami (≤220 °C), and suggested within NWA 7257 and NWA 8679 by baddeleyite microstructure and other petrological constraints, confirm that the previously derived baddeleyite age of Zagami records magmatic crystallization, and provide greater age diversity to 225 Ma to 160 Ma enriched shergottites. While our data yield no resolvable link between microstructure and U-Pb isotopic composition, we strongly recommend that microstructural analyses should represent an essential step of baddeleyite U-Pb chronology within planetary (e.g., martian, lunar, asteroidal) and shocked terrestrial samples, allowing full contextualisation prior to destructive isotopic techniques. Microstructurally constrained in-situ U-Pb analyses of baddeleyite thus define new opportunities for the absolute chronology of martian meteorites and, more broadly, shocked planetary materials.

New constraints on mid-Proterozoic ocean redox from stable thallium isotope systematics of black shales

Stable thallium (Tl) isotope data from organic-rich siliciclastic sedimentary rocks have the potential to track ocean redox state on a broad scale. Here, we report new Tl isotope data from the Mesoproterozoic Velkerri Formation (Roper Group) and the Paleoproterozoic Wollogorang Formation (Tawallah Group), McArthur Basin, Northern Territory, Australia, and interpret these in the context of rhenium-osmium (Re-Os) geochronometry on the same sample suite. Previous work has shown that marine black shales from the Velkerri Formation provide evidence for closed-system Re-Os systematics, yielding a precise isochron with an age of 1361 ± 21 Ma that agrees well with independent age constraints for the unit. The isotopic composition of authigenic Tl in euxinic black shales from the upper Velkerri Formation (ε205Tl = −2.4 ± 0.8, 2SD) indicates that the Tl isotope composition of local seawater at 1.36 Ga was within a plausible range for Tl inputs to the ocean. Isotope mass-balance modeling of the Tl isotope system within a Monte Carlo framework suggests that the Tl isotopic composition of seawater at 1.36 Ga was homogenous on a global scale and that the burial of Mn-oxides exerted minimal isotopic leverage on the Tl isotope composition of seawater at 1.36 Ga. Taken together with existing Mo, Cr, and U isotope data from the same samples, these observations are consistent with a low-O2 ocean-atmosphere system during this period of the Mesoproterozoic.Previous work has shown that the Re-Os systematics of black shales from the older (1.73 Ga) Wollogorang Formation are scattered and yield an erroneously young isochron age of 1359 ± 150 Ma, which has been attributed to post-depositional hydrothermal alteration at ∼1640 Ma. We observe no systematic relationship between stable Tl isotope compositions and the extent of alteration as gauged by open-system Re-Os behavior (−4.7 ± 1.4 for the upper Wollogorang Formation and −4.8 ± 0.4 for the lower Wollogorang Formation), in marked contrast to previous observations for the molybdenum (Mo) and uranium (U) isotope systems. The invariant signature of the Tl isotope data suggests the Tl isotope system was largely unperturbed during hydrothermal alteration. However, it remains difficult to definitively rule out the possibility that authigenic Tl isotope signatures have been overprinted by later localized hydrothermal fluid alteration in the Wollogorang Formation shales. These observations highlight the potential insights afforded by evaluating open-system behavior via radiogenic isotope systems together with other stable isotope tracers in efforts to reconstruct the redox landscape of Earth’s oceans over time.

Multi-Proxy Study of a Holocene Soil Profile from Romania and Its Relevance for Speleothem Based Paleoenvironmental Reconstructions

In this study, we describe a sedimentary deposit situated above Ascunsă Cave (SW Romania) that should be in depositional connection with coeval stalagmites from the cave. We excavated a 2.5 m deep soil profile and took contiguous bulk samples every 5 cm. Soil samples were analyzed for clay mineralogy, grain size, chemical composition, magnetic susceptibility, and stable carbon isotopes. Radiocarbon dating revealed that the soil is of Holocene age, and presents a depositional hiatus between 5.4 and 2.3 thousand years before the present. Due to the open system behavior of soils and mobility of organic matter, a few hundred years of uncertainty should be considered for the duration of this hiatus. The fine fraction is dominated by silt, while the clay mineralogical association is made of illite, chlorite, kaolinite, vermiculite, and illite–vermiculite and illite–chlorite mixed layered minerals. The sediment source of this soil is represented by a nearby mélange complex, as well as by an underlying terra rossa-type soil. As this latter type of soil is widespread in our study area, we performed luminescence dating on two samples from a representative location close to our site and it appears that this type of Mediterranean soil was formed during the Last Interglacial period. δ13C variability in soil organic matter and a stalagmite from the cave are comparing well, and could help future studies identify modifications in isotopic fractionation processes within the cave.

A global compilation of U-series-dated fossil coral sea-level indicators for the Last Interglacial period (Marine Isotope Stage 5e)

Abstract. This dataset is a comprehensive, global compilation of published uranium–thorium (U-series) dated fossil coral records from ∼ 150 000–110 000 years ago, as well as associated elevation measurements and sample metadata. In total, 1312 U-series measurements from 994 unique coral colonies are included in the current version of the dataset, all of which have been normalized and recalculated using the same decay constant values. Of these measurements, 444 analyses from 330 colonies are relative sea-level indicators, whereas 15 analyses from 13 colonies are marine limiting. Two example geochemical screening criteria have been included to assist users with identifying altered fossil corals that display geochemical open-system behavior, and the originally published interpretations on age quality have been preserved within the sample metadata. Additionally, a clear distinction has been made between coral colonies that are in primary growth position, which may be used for relative sea-level reconstructions, and colonies that have been transported/reworked, which cannot be used for these purposes. Future research efforts involving fossil coral sea-level reconstructions should emphasize an “integrated” and holistic approach that combines careful assessment of U-series age quality with high-precision surveying techniques and detailed facies/stratigraphic observations. This database is available at https://doi.org/10.5281/zenodo.4309796 (Chutcharavan and Dutton, 2020).

The genesis and age of the Grunehogna Granite and Rb–Sr and Sm–Nd chemistry of the Annandagstoppane Granite, Ahlmanryggen, Dronning Maud Land, Antarctica

Interpretations of Rb–Sr and Sm–Nd data from the Grunehogna Granite demonstrate that anatexis of and mixing with Høgfonna Formation sedimentary rocks, caused by hot basic sills of ~1100 Ma Borgmassivet Suite, are consistent, with the granite εNdεSr100Ma data, partially overlapping Borgmassivet Suite data and are marginally displaced toward published data from the Høgfonna Formation. Upper intercept LA-ICPMS U–Pb data from the Grunehogna Granite and Borgmassivet Suite zircons are within error at 1088 ± 17Ma and 1088 ± 68Ma, and are consistent with with unpublished data for the gabbro of 1107Ma. Interpretations of syn-depositional anatexis of semi-consolidated sediments under, high pH2O, are consistent with deposition at ~1130-1107Ma for the Høgfonna Formation. Sm–Nd data define an age of 3058 ± 116Ma for the Annandagstoppane Granite, consistent with a reported U–Pb zircon age of 3067± 8Ma. Rb–Sr data for the Annandagstoppane Granite define an errorchron of 2252 ± 82Ma. Differences between Rb–Sr and Sm–Nd data suggest that Rb–Sr systematics in many samples from all units have experienced open system behaviour in contrast to Sm–Nd systematics from which no disturbance is apparent. Localised tonalitic partial melts of gabbro yield an upper intercept LA-ICPMS U–Pb age from zircon of 1114 ± 47Ma.

Pervasive fluid-rock interaction in subducted oceanic crust revealed by oxygen isotope zoning in garnet

Dehydration reactions in the subducting slab liberate fluids causing major changes in rock density, volume and permeability. Although it is well known that the fluids can migrate and interact with the surrounding rocks, fluid pathways remain challenging to track and the consequences of fluid-rock interaction processes are often overlooked. In this study, we investigate pervasive fluid-rock interaction in a sequence of schists and mafic felses exposed in the Theodul Glacier Unit (TGU), Western Alps. This unit is embedded within metaophiolites of the Zermatt-Saas Zone and reached eclogite-facies conditions during Alpine convergence. Chemical mapping and in situ oxygen isotope analyses of garnet from the schists reveal a sharp chemical zoning between a xenomorphic core and a euhedral rim, associated to a drop of ~ 8‰ in δ18O. Thermodynamic and δ18O models show that the large amount of low δ18O H2O required to change the reactive bulk δ18O composition cannot be produced by dehydration of the mafic fels from the TGU only, and requires a large contribution of the surrounding serpentinites. The calculated time-integrated fluid flux across the TGU rocks is 1.1 × 105 cm3/cm2, which is above the open-system behaviour threshold and argues for pervasive fluid flow at kilometre-scale under high-pressure conditions. The transient rock volume variations caused by lawsonite breakdown is identified as a possible trigger for the pervasive fluid influx. The calculated schist permeability at eclogite-facies conditions (~ 2 × 10–20 m2) is comparable to the permeability determined experimentally for blueschist and serpentinites.

The Distribution Coefficients of Major and Minor Elements in Coral Skeletons Under Variable Calcium Seawater Concentrations

Coral skeletons are one of the best archives for past ocean seawater (SW) chemistry and isotopes. However, the distribution coefficients of major and minor elements in coral skeletons are not well determined. In this study, we launched an experiment to determine the distribution coefficients of multiple elements in corals’ skeletons by changing Ca concentrations in SW (CaSW). Two scleractinian corals, Pocillopora damicornis and Acropora cervicornis were cultured in modified Gulf of Eilat water (Red-Sea) with CaSW of approximately 10, 15, 20, and 25 mM. After almost three months, the newly grown skeletons were analyzed for the following elements: Li, Na, Mg, K, Sr, and Ba. Their ratios to Ca in the coral skeleton (El/Cacoral) increased linearly with El/CaSW (with R2 values above 0.98), crossing the origin and thus indicating constant distribution coefficient for each element over the experimental range of El/CaSW. The values of DEl were in good agreement with values reported for corals collected in natural seawater. However, differences were observed between the two species, and both were slightly deviating from inorganic aragonite D values. These deviations are well explained by Rayleigh fractionation process in the calcifying fluid (assuming it is mainly seawater). This was observed both for elements with D &amp;gt; 1 (Ba and Sr) and D &amp;lt; 1 (Li, Mg, Na, and K). P. damicornis showed open system behavior (∼20% of its Ca utilized) while A. cervicornis showed more closed calcifying reservoir (∼50% of its Ca utilized). The finding that the distribution coefficients of the six minor and trace elements are constant for a given species, should help in the reconstruction of past seawater chemistry based on multielement measurements in fossil corals. In particular, Na/Cacoral can be used to reconstruct past ocean Ca concentrations and with El/Cacoral ratios for other elements, their concentrations for the Cenozoic can be reconstructed.

High temperature hydrothermal alteration and amphibole formation in Gakkel Ridge abyssal peridotites

Alteration mineral assemblages in abyssal peridotites offer insights into the temperature and pressure conditions during hydrothermal fluid circulation in the oceanic lithosphere. Abundant hydrothermal venting has been documented along the ultraslow spreading Gakkel Ridge and peridotites have been extensively sampled from the ridge, yet these peridotites have not been systematically evaluated for the nature and extent of their alteration. Here, we present an analysis of the alteration of 40 Gakkel peridotites from 19 dredges spanning ~600 km of the ridge within the Sparsely Magmatic and Eastern Volcanic Zones. We use a petrographic-based alteration scoring system to assign a rank to each sample of 1 (unaltered) to 5 (altered) in 0.5 step intervals, based on the alteration scale of Birner et al. (2016). Gakkel peridotites cover the full range of alteration, from essentially unaltered to completely serpentinized, but are generally moderately to highly altered (average score of 3.5). The extent of alteration is independent of peridotite lithology and is not systematically different between the two zones. Serpentine is present in all samples, magnetite occurs in all samples with alteration scores >2.5, and 27% of samples contain carbonate veins that cross-cut all other minerals. In 20% of samples, all with alteration scores >2.5, tremolite + talc occurs as rims around pyroxene, while chlorite occurs as haloes around spinel. A gabbro-veined lherzolite also contains pargasite replacing pyroxene, with later overprinting by tremolite-talc intergrowths.The abundance of tremolite, chlorite, and talc in Gakkel samples is much lower than reported for talc-tremolite-chlorite schists from oceanic core complexes, where fluid flow through the lower crust leads to open-system behavior. At Gakkel, our petrological observations suggest that alteration was nearer to closed-system conditions, which can be explained by the absence of crust along some sections of the ridge. Pseudosection modeling with Perple_X, which assumes closed-system behavior, indicates that pyroxene reacts to form tremolite + chlorite + talc at temperatures >500 °C, corresponding to fluid flow to depths >20 km for the Gakkel Ridge. The sample with pargasitic amphibole requires temperatures >750 °C to form. Overall, our results suggest that hydrothermal alteration of Gakkel peridotites, and more generally of the lithospheric mantle in regions that lack significant crust, results in different alteration mineral abundances and relationships than in regions where the mafic crust is involved in alteration.

Age disequilibrium between zircon and their granitoid hosts caused by intracrustal reworking: Nd-Hf-Ar isotope evidence of Archaean Granitoids from Barberton Mountain Land (Kaapvaal craton, South Africa)

The degree of early Archaean terrestrial differentiation into mantle and crust is long debated. A number of studies rely on bulk rock Hf and Nd isotope data to gain further insight. Here we present Hf-Nd data from two igneous basements suites of Barberton Mountain Land, namely a tonalite-trondhjemite-granodiorite (TTG) and granodiorite-granite-monzogranite (GGM) suite to further decipher crust formation and its timing in the Archaean. TTG show high initial ε(Hf) and ε(Nd) if corrected using their respective U/Pb zircon ages that can be interpreted in two ways: (i) early differentiation of bulk silicate Earth or (ii) overcorrection of these data caused by decoupling of zircon from the ‘zircon-free’ bulk rock due to metamorphic overprint. Indeed, regional metamorphism affected the early TTG ca. 250 Ma after their emplacement and disturbed their bulk rock Hf and Nd systematics. Correction of bulk rock Hf and Nd data to the age of metamorphism yields values between +0.9 and − 6.2 and from −1.3 to −4.3, respectively, thus indicating open-system behaviour. Lu-Hf and Sm-Nd mineral isochron ages of two TTG plutons are significantly younger than their corresponding U/Pb zircon age, but agree within uncertainty perfectly with external age constraints for the age of the metamorphic overprint. The same accounts for the 40Ar-39Ar systematics of the investigated plutons. Amphibole samples consist of a mixture of heterochemical generations that yield ages that are both older and younger than the metamorphic overprint at 3.2 Ga. The metamorphic minerals in each plutons provide a highly divergent age information from the coexisting zircon. Hence, the positive ε(Nd) and ε(Hf)-values that resulted from age-correction to the respective U/Pb zircon age are most probably over-corrected. Finally, if all data is age-corrected to the metamorphic overprint all samples agree with models for the Nd-Hf isotopic evolution in the mantle that do not require a large-scale early differentiation of Earth into enriched and depleted reservoirs. Mantle extraction ages (TDM) of TTG are interpreted as minimum ages for the generation of felsic Archaean crust whereas TDM of GGM are interpreted as maximum ages for the last major crust growth episode in the Barberton Mountain Land.

Re-Os isotope geochemistry of the Palaeoproterozoic Sakatti Cu-Ni-PGE sulphide deposit in northern Finland

The Sakatti Cu-Ni-PGE sulphide deposit is located in the Central Lapland greenstone belt in northern Finland. It comprises three mineralised olivine cumulate units called the main, north-east and south-west bodies. Sulphides form massive, semi-massive, disseminated and vein-type ores in these ultramafic rocks. The ore mineral assemblage is typical for magmatic sulphide deposits, including chalcopyrite, pyrrhotite, pentlandite, pyrite and magnetite. We present Re-Os concentration and isotope data for whole-rock olivine cumulate samples and mineral separates (chromite, olivine) as well as for massive sulphides and associated oxides. Chromite separates contain 2.49–115.4 ppb Os and 0.03–2.85 ppb Re while olivine separates have 3.13–42.1 ppb Os and 0.43–3.89 ppb Re. Their host olivine cumulates show Os and Re concentrations of 1.17–23.7 ppb and 0.04–0.93 ppb, respectively. The sulphide and magnetite separates show a considerable variation and overlap in the Os and Re concentrations, with the total range being 0.69–393.5 ppb for Os and 4.44–246.9 ppb for Re. Among the sulphide separates, the highest Re and Os concentrations were obtained for pyrrhotite and pyrite, respectively. On average, Re/Os ratios increase in the following order: chromite, whole-rock olivine cumulate, olivine, pyrite, pentlandite, chalcopyrite, pyrrhotite, and magnetite. Excluding the most disturbed samples, 30 analyses yielded a Re-Os isochron age of 2063 ± 35 Ma for the Sakatti deposit, being similar to the U-Pb zircon age of 2058 ± 4 Ma determined for the Kevitsa Ni-Cu sulphide ore-bearing intrusion, which is located ~20 km NE of Sakatti. Re-Os dating of massive sulphide samples yields variable Re-Os isochron ages from 1852 ± 200 Ma to 2045 ± 300 Ma, with all sulphide samples together giving an age of 1928 ± 16 Ma. This indicates resetting and open-system behaviour of some sulphide samples, potentially related to Svecofennian orogenic events. In spite of this resetting, the low Re/Os ratios and high Os concentrations of chromite and several sulphide separates allow us to determine their initial γOs values, which turned out to be very close to chondritic for chromite and moderately radiogenic (γOs from +21 to +56) for sulphides, indicating that sulphide saturation took place after chromite crystallisation.

Controls on the scales of equilibrium during granulite facies metamorphism

AbstractPhase equilibrium modelling is commonly employed to constrain the pressure–temperature (P–T) evolution of granulite facies rocks, from which their geodynamic setting may be inferred. However, defining a suitable equilibrium volume in such rocks is non‐trivial due to heterogeneities in protolith composition and open system behaviour, including melt loss or gain and deformation. Consequently, equilibrium volumes and the mineral assemblages they contain may vary both temporally and spatially within a single rock. Additionally, despite the extreme temperatures they attained, granulites commonly contain microtextures indicating gradients in chemical potential after the metamorphic peak. This study assesses the processes that control compositional heterogeneity between equilibrium volumes in a suprasolidus granulite and the consequences for the development of a range of microstructures. An approach combining phase equilibrium forward modelling and chemical potential diagrams is used to simulate the suprasolidus evolution of two adjacent equilibrium volumes in an Mg‐rich metapelite. Assuming equilibrium within the two compositional domains, evolving granulite facies assemblages vary along a high T/P (125°C/kbar) clockwise P–T path representative of high T/P metamorphic terranes. Many retrograde microtextures in metapelitic rocks can be reproduced by considering chemical potential (µ) gradients in µMgO, µFeO and µCaO between phases, assuming (a) the presence of melt on grain boundaries, (b) that Al2O3 is perfectly immobile and (c) that K2O and Na2O are perfectly mobile. Additionally, documented microtextures in granulites imply local SiO2‐undersaturation, despite the presence of matrix quartz in the rock. Preserving such chemical potential gradients requires that solid phases are chemically and physically isolated from melt. Efficient prograde melt drainage may lead to a loss in melt interconnectivity at high temperature (&gt;900°C), meaning that equilibration of centimetre‐scale compositional domains is controlled by solid‐state diffusion. The presence of isolated pockets of viscous melt allows the formation of discontinuous intergranular reaction microtextures.

Formation of the model of the open production-and-economic system at the construction enterprise

The research is dedicated to the multi-structural model of the complex system at the construction enterprise as well as the model of structurally functional open production-and-economic system at the construction enterprise. The models are created for the acceptance of the justified managerial decisions and carrying out the applied research aimed at the efficiency of the construction enterprises functioning in the conditions of uncertainty, instability of the internal and external environment and heterogeneity of the indicators, which characterize the enterprise’s activity. The offered models are developed on the basis of the probability theory and the information theory; they are based on the principles of self-organization, openness of the complex systems, synergy and information-and-statistical approach. The concept of the open productive-and-economic system at the construction enterprise is considered, the content of the functions of the complex system at the construction enterprise is described, their stochastic and quasi-determined communications in the uniform information-and-statistical field of functioning are presented. Bit measurement for the whole set of functions, allowing to receive an adequate assessment of the complex open system behavior, is offered for further scientific and applied research, including the development of more exact forecasts and improvement of statistical heterogeneity of indicators.

U-Th dating of lake sediments: Lessons from the 700 ka sediment record of Lake Junín, Peru

Deep sediment cores from long-lived lake basins are fundamental records of paleoenvironmental history, but the power of these reconstructions has been often limited by poor age control. Uranium-thorium (U-Th) dating has the potential to fill a gap in current geochronological tools available for such sediment archives. We present our systematic approach to U-Th date carbonate-rich sediments from the ∼100 m drill core from Lake Junín, Peru. The results form the foundation of an age-depth model spanning ∼700 kyrs. High uranium concentrations (0.3–4 ppm) of these sediments allow us to date smaller amounts of material, giving us the opportunity to improve sample selection by avoiding detrital contamination, the greatest factor limiting the success of previous U-Th dating efforts in other lake basins. Despite this advantage, the dates from 174 analyses on 55 bulk carbonate samples reveal significant scatter that cannot be resolved with traditional isochrons, suggesting that at least some of the sediments have not remained closed systems. To understand the source of noise in the geochronological data, we first apply threshold criteria that screen samples by their U/Th ratio, reproducibility, and δ234Uinitial value. We then compare these results with facies types, trace element concentrations, carbonate and total organic carbon content, color reflectance, mineralogy, and ostracode shell color to investigate the causes of open system behavior. Alongside simulations of the isotopic evolution of our samples, we find that the greatest impediment to U-Th dating of these sediments is not detrital contamination, but rather post-depositional remobilization of uranium. Examining U-Th data in these contexts, we identify samples that have likely experienced the least amount of alteration, and use dates from those samples as constraints for the age-depth model. Our work has several lessons for future attempts to U-Th date lake sediments, namely that geologic context is equally as important as the accuracy and precision of analytical measurements. In addition, we caution that significant geologic scatter may remain undetected if not for labor intensive tests of reproducibility achieved through replication. As a result of this work, the deep sediment core from Lake Junín is the only continuous record in the tropical Andes spanning multiple glacial cycles that is constrained entirely by independent radiometric dates.

U-Pb, Hf isotope and REE constraints on high-pressure acid migmatites from the Cabo Ortegal Complex (NW Spain): New evidence of short-duration metamorphism in a Variscan subduction channel

Paired LA-ICP-MS UPb and LuHf isotopic data and trace element data are reported on zircon in thin section from poorly known acid migmatites exposed in the Cabo Ortegal Complex, Western Variscides, NW Spain. The migmatites are part of the high-pressure granulite unit, tectonically placed between the eclogite and ultramafic units. Large oscillatory zoned zircon cores date the felsic igneous protolith at 459 ± 7 Ma as a minimum age reflecting a protracted basic magma differentiation in a Cambro-Ordovician arc setting. Lower Th/U, weakly- to un-zoned overgrowth rims are attributed to partial melting at 392 ± 5 Ma during high-pressure metamorphism. The distinctly lower epsilon Hf (+4.0) of the rim relative to the core (+8.2) implies an open-system behavior metamorphism and mass transfer between different units in deep parts of a subduction channel during the Variscan Orogeny. Tiny xenomorphic zircon crystals devoid of internal microstructures dated at 355 ± 4 Ma and with nearly flat REE patterns are interpreted as product of metamorphic overprint driven by hot fluids. The latter is envisaged as a subduction channel short-duration metamorphic phenomenon.

U/Th dating and open system behavior: implications for travertines based on the study of Süttő (Hungary) and Ballik (Turkey) sites

Reliable dating is essential in order to put palaeoclimatological, palaeoenvironmental and archeological information from continental carbonates in the correct time frame. This study evaluates U/Th dating of travertines based on two case studies, i.e., in the Ballik (Denizli, SW Turkey) and Suttő (N Hungary) areas. Previous petrographic, mineralogical and elemental characterization of the travertines enabled sampling of, purely calcitic, micrite or spar dominant, zones that are non-, or solely weak, cathodoluminescent. Despite adapted sampling and detrital thorium corrections, U/Th ages of the Ballik and the Suttő travertines are inconsistent with their stratigraphic position. Based on the observation of wavy extinction affecting micritic fabrics and ghost structures, the studied travertines behaved as an open system for uranium after deposition through recrystallisation. Their U/Th ages, consequently, can only be considered as minimum ages. The Ballik travertines precipitated earlier than 0.55-1.31 Ma ago, when late-stage cementation along fractures and in cavities took place, and form the oldest travertines deposited in the Denizli basin. The formation of the Suttő travertine area started at the earliest around 2.5 Ma with precipitation of the Cukor travertine. With some short interruptions, active travertine formation in the area took place maximally until 120 ka. The decreasing ages of the Suttő travertines towards the Danube, prove the complex nature of travertines both during deposition as well as during diagenesis, which leads to anomalies with regard to the law of superposition. The diagenetic susceptibility of continental carbonate deposits poses problems in terms of accurate and precise dating. U/Th dating of the intrinsically heterogeneous fabrics of travertines and tufa, as presently applied, should be ceased. Travertines and tufa up to ~100 ka might be the exception due to the limited extent of recrystallisation. Sparite cements and veins can be dated more accurately but, even with multiple samples from the same study site and inclusion of coeval samples, give only minimum ages for the whole deposits. To better constrain the time frame of the whole bodies, results from independent dating methods and from the surrounding deposits are necessary. In addition, it is important to keep in mind that the open-system behavior of continental carbonates has implications for diagenetic, palaeoenvironmental and palaeoclimatological studies based on geochemical proxies.

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb carbonate geochronology: strategies, progress, and limitations

Abstract. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb geochronology of carbonate minerals, calcite in particular, is rapidly gaining popularity as an absolute dating method. The high spatial resolution of LA-ICP-MS U–Pb carbonate geochronology has benefits over traditional isotope dilution methods, particularly for diagenetic and hydrothermal calcite, because uranium and lead are heterogeneously distributed on the sub-millimetre scale. At the same time, this can provide limitations to the method, as locating zones of radiogenic lead can be time-consuming and “hit or miss”. Here, we present strategies for dating carbonates with in situ techniques, through imaging and petrographic techniques to data interpretation; our examples are drawn from the dating of fracture-filling calcite, but our discussion is relevant to all carbonate applications. We review several limitations to the method, including open-system behaviour, variable initial-lead compositions, and U–daughter disequilibrium. We also discuss two approaches to data collection: traditional spot analyses guided by petrographic and elemental imaging and image-based dating that utilises LA-ICP-MS elemental and isotopic map data.

U-series dating at Nerja cave reveal open system. Questioning the Neanderthal origin of Spanish rock art

U/Th data obtained on CaCO3 layers covering rock art at Nerja Cave (Spain) evidence erroneous ages and an inverse relation between uranium concentration and apparent ages. This open system behavior could be due to a mechanism causing uranium mobility, resulting in apparent ages being too old with respect to their real age. This article also questions recently published U/Th data on samples from Ardales, Maltravieso and La Pasiega caves (Spain) where a few U/Th ages older than 40 ka BP, suggested that cave art could be attributed to Middle Palaeolithic population (Hoffmann et al., 2018). For these caves, U/Th data also display an inverse relation between U content and ages, indicating possible uranium mobility and erroneous ages.