Volatile-rich melts as markers of the asthenospheric influx prior to rifting events: the case of the alkaline-carbonatitic lamprophyres of the Dolomitic Area (Southern Alps, Italy)

No abstract availabl

The alkaline lamprophyres of the Dolomitic Area (Southern Alps, Italy): markers of the Late Triassic change from orogenic-like to anorogenic magmatism

We present the first complete petrological, geochemical and geochronological characterization of the oldest lamprophyric rocks in Italy, which crop out around Predazzo (Dolomitic Area), with the aim of deciphering their relationship with Triassic magmatic events across the whole of the Southern Alps. Their Mg# of between 37 and 70, together with their trace element contents, suggests that fractional crystallization was the main process responsible for their differentiation, together with small-scale mixing, as evidenced by some complex amphibole textures. Moreover, the occurrence of primary carbonate ocelli suggests an intimate association between the alkaline lamprophyric magmas and a carbonatitic melt. 40Ar/39Ar data show that the lamprophyres were emplaced at 219·22 ± 0·73 Ma (2σ; full systematic uncertainties), around 20 Myr after the high-K calc-alkaline to shoshonitic, short-lived, Ladinian (237–238 Ma) magmatic event of the Dolomitic Area. Their trace element and Sr–Nd isotopic signatures (87Sr/86Sri = 0·7033–0·7040; 143Nd/144Ndi = 0·51260–0·51265) are probably related to a garnet–amphibole-bearing lithosphere interacting with an asthenospheric component, significantly more depleted than the mantle source of the high-K calc-alkaline to shoshonitic magmas. These features suggest that the Predazzo lamprophyres belong to the same alkaline–carbonatitic magmatic event that intruded the mantle beneath the Southern Alps (e.g. Finero peridotite) between 190 and 225 Ma. In this scenario, the Predazzo lamprophyres cannot be considered as a late-stage pulse of the orogenic-like Ladinian magmatism of the Dolomitic Area, but most probably represent a petrological bridge to the opening of the Alpine Tethys

Future directions and priorities for Arctic bryophyte research

The development of evidence-based international strategies for the conservation and management of Arctic ecosystems in the face of climate change is hindered by critical knowledge gaps in Arctic floristic diversity and evolution. Particularly poorly studied are the bryophytes, which dominant the vegetation across vast areas of the Arctic, and consequently, play an important role in global biogeochemical cycles. Currently, much of what is known about Arctic floristic evolution is based on studies of vascular plants. Bryophytes, however, possess a number of features, such as poikilohydry, totipotency, several reproductive strategies, and the ability to disperse through microscopic diaspores, which may cause their responses to Arctic environments to differ from those of the vascular plants. Here we discuss several priority areas identified in the Arctic Council's ‘Arctic Biodiversity Assessment’ that are necessary to illuminate patterns of Arctic bryophyte evolution and diversity, including dispersal, glacial refugia, local adaptation, and ecological interactions within bryophyte-associated microbiomes. A survey of digitally available herbarium data archived in the largest online aggregate, GBIF, across the Arctic to boreal zones, indicates that sampling coverage of mosses is heterogeneous, and relatively sparse in the Arctic sensu stricostricto. A coordinated international effort across the Arctic will be necessary to address knowledge gaps in Arctic bryophyte diversity and evolution in the context of ongoing climate change

The Variscan subduction inheritance in the Southern Alps Sub-Continental Lithospheric Mantle: clues from the Middle Triassic shoshonitic magmatism of the Dolomites (NE Italy)

Although often speculated, the link between the Middle Triassic shoshonitic magmatism at the NE margin of the Adria plate and the subduction-related metasomatism of the Southern Alps Sub-Continental Lithospheric Mantle (SCLM) has never been constrained. In this paper, a detailed geochemical and petrological characterization of the lavas, dykes and ultramafic cumulates belonging to the shoshonitic magmatic event that shaped the Dolomites (Southern Alps) was used to model the composition and evolution of the underlying SCLM in the time comprised between the Variscan subduction and the opening of the Alpine Tethys. Geochemical models and numerical simulations enabled us to define that 5–7% partial melting of an amphibole + phlogopite-bearing spinel lherzolite, similar to the Finero phlogopite peridotite, can account for the composition of the primitive Mid-Triassic SiO2-saturated to -undersaturated melts with shoshonitic affinity (87Sr/86Sri = 0.7032–0.7058; 143Nd/144Ndi = 0.51219–0.51235; Mg # ~ 70; ~1.1 wt% H2O). By taking into account the H2O content documented in mineral phases from the Finero phlogopite peridotite, it is suggested that the Mid-Triassic SCLM source was able to preserve a significant enrichment and volatile content (600–800 ppm H2O) for more than 50 Ma, i.e. since the slab-related metasomatism connected to the Variscan subduction. The partial melting of a Finero-like SCLM represents the exhaustion of the subduction-related signature in the Southern Alps lithosphere that predated the Late Triassic-Early Jurassic asthenospheric upwelling related to the opening of the Alpine Tethys

Production of 40Ar by an overlooked mode of 40K decay with implications for K-Ar geochronology

The decay of 40K to the stable isotopes 40Ca and 40Ar is used as a measure of time for both the K-Ca and KAr geochronometers, the latter of which is most generally utilized by the variant 40Ar/ 39Ar system. The increasing precision of geochronology has forced practitioners to deal with the systematic uncertainties rooted in all radioisotope dating methods. A major component of these systematic uncertainties for the K-Ar and 40Ar/ 39Ar techniques is imprecisely determined decay constants and an incomplete knowledge of the decay scheme of 40K. Recent geochronology studies question whether 40K can decay to 40Ar via an electron capture directly to ground state (ECground), citing the lack of experimental verification as reasoning for its omission. In this study, we (1) provide a theoretical argument in favor of the presence of this decay mode and (2) evaluate the magnitude of this decay mode by calculating the electron capture to positron ratio (ECground/β+) and comparing calculated ratios to previously published calculations, which yield ECground/β+ between 150–212. We provide support for this calculation through comparison of the experimentally verified ECground/β+ ratio of 22Na with our calculation using the theory of β decay. When combined with measured values of β + and β − decay rates, the best estimate for the calculated ECground/β+ for 40K yields a partial decay constant for 40K direct to ground-state 40Ar of 11.6±1.5×10−13 a −1 (2σ). We calculate a partial decay constant of 40K to 40Ar of 0.592 ± 0.014 × 10−10 a −1 and a total decay constant of 5.475 ± 0.107 × 10−10 a −1 (2σ), and we conclude that although omission of this decay mode can be significant for K-Ar dating, it is minor for 40Ar/ 39Ar geochronology and is therefore unlikely to have significantly biased published measurements

Evidence for ground-state electron capture of 40^{40}K

Potassium-40 is a widespread isotope whose radioactivity impacts estimated geological ages spanning billions of years, nuclear structure theory, and subatomic rare-event searches - including those for dark matter and neutrinoless double-beta decay. The decays of this long-lived isotope must be precisely known for its use as a geochronometer, and to account for its presence in low-background experiments. There are several known decay modes for $^{40}$K, but a predicted electron-capture decay directly to the ground state of argon-40 has never been observed, while theoretical predictions span an order of magnitude. The KDK Collaboration reports on the first observation of this rare decay, obtained using a novel combination of a low-threshold X-ray detector surrounded by a tonne-scale, high-efficiency $\gamma$-ray tagger at Oak Ridge National Laboratory. A blinded analysis reveals a distinctly nonzero ratio of intensities of ground-state electron-captures ($I_{\text{EC}^0}$) over excited-state ones ($I_{\text{EC}^*}$) of $I_{\text{EC}^0} / I_{\text{EC}^*}=0.0095\stackrel{\text{stat}}{\pm}0.0022\stackrel{\text{sys}}{\pm}0.0010$ (68% CL), with the null hypothesis rejected at 4$\sigma$ [Stukel et al., DOI:10.1103/PhysRevLett.131.052503]. This unambiguous signal yields a branching ratio of $I_{\text{EC}^0}=0.098\%\stackrel{\text{stat}}{\pm}0.023\%\stackrel{\text{sys}}{\pm}0.010$, roughly half of the commonly used prediction. This first observation of a third-forbidden unique electron capture improves understanding of low-energy backgrounds in dark-matter searches and has implications for nuclear-structure calculations. A shell-model based theoretical estimate for the $0\nu\beta\beta$ decay half-life of calcium-48 is increased by a factor of $7^{+3}_{-2}$. Our nonzero measurement shifts geochronological ages by up to a percent; implications are illustrated for Earth and solar system chronologies.Comment: This is a companion submission to Stukel et al (KDK collaboration) "Rare $^{40}$K decay with implications for fundamental physics and geochronology" [arXiv:2211.10319; DOI: 10.1103/PhysRevLett.131.052503]. As such, both texts share some figures and portions of text. This version updates the text following its review and production proces

Diamondites: evidence for a distinct tectono-thermal diamond-forming event beneath the Kaapvaal craton

The petrogenesis and relationship of diamondite to well-studied monocrystalline and fibrous diamonds are poorly understood yet would potentially reveal new aspects of how diamond-forming fluids are transported through the lithosphere and equilibrate with surrounding silicates. Of 22 silicate- and oxide-bearing diamondites investigated, most yielded garnet intergrowths (n = 15) with major element geochemistry (i.e. Ca–Cr) classifying these samples as low-Ca websteritic or eclogitic. The garnet REE patterns fit an equilibrium model suggesting the diamond-forming fluid shares an affinity with high-density fluids (HDF) observed in fibrous diamonds, specifically on the join between the saline–carbonate end-members. The δ13C values for the diamonds range from − 5.27 to − 22.48‰ (V-PDB) with δ18O values for websteritic garnets ranging from + 7.6 to + 5.9‰ (V-SMOW). The combined C–O stable isotope data support a model for a hydrothermally altered and organic carbon-bearing subducted crustal source(s) for the diamond- and garnet-forming media. The nitrogen aggregation states of the diamonds require that diamondite-formation event(s) pre-dates fibrous diamond-formation and post-dates most of the gem monocrystalline diamond-formation events at Orapa. The modelled fluid compositions responsible for the precipitation of diamondites match the fluid-poor and fluid-rich (fibrous) monocrystalline diamonds, where all grow from HDFs within the saline-silicic-carbonatitic ternary system. However, while the nature of the parental fluid(s) share a common lithophile element geochemical affinity, the origin(s) of the saline, silicic, and/or carbonatitic components of these HDFs do not always share a common origin. Therefore, it is wholly conceivable that the diamondites are evidence of a distinct and temporally unconstrained tectono-thermal diamond-forming event beneath the Kaapvaal craton

Repeat-sequence turnover shifts fundamentally in species with large genomes

Given the 2,400-fold range of genome sizes (0.06–148.9 Gbp (gigabase pair)) of seed plants (angiosperms and gymnosperms) with a broadly similar gene content (amounting to approximately 0.03 Gbp), the repeat-sequence content of the genome might be expected to increase with genome size, resulting in the largest genomes consisting almost entirely of repetitive sequences. Here we test this prediction, using the same bioinformatic approach for 101 species to ensure consistency in what constitutes a repeat. We reveal a fundamental change in repeat turnover in genomes above around 10 Gbp, such that species with the largest genomes are only about 55% repetitive. Given that genome size influences many plant traits, habits and life strategies, this fundamental shift in repeat dynamics is likely to affect the evolutionary trajectory of species lineages.We thank Natural Environment Research Council (NE/G020256/1), the Czech Academy of Sciences (RVO:60077344) and Ramón y Cajal Fellowship (RYC-2017-2274) funded by the Ministerio de Ciencia y Tecnología (Gobierno de España) for support. We also thank Natural Environment Research Council for funding a studentship to S.D. and the China Scholarship Council for funding W.W.Abstract Main Methods Data availability Code availability References Acknowledgements Author information Ethics declarations Additional information Extended data Supplementary information Rights and permissions About this article Further readin

Measurement of the 92,93,94,100Mo(γ,n) reactions by Coulomb Dissociation

The Coulomb Dissociation (CD) cross sections of the stable isotopes 92,94,100Mo and of the unstable isotope 93Mo were measured at the LAND/R3B setup at GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, Germany. Experimental data on these isotopes may help to explain the problem of the underproduction of 92,94Mo and 96,98Ru in the models of p-process nucleosynthesis. The CD cross sections obtained for the stable Mo isotopes are in good agreement with experiments performed with real photons, thus validating the method of Coulomb Dissociation. The result for the reaction 93Mo(γ,n) is especially important since the corresponding cross section has not been measured before. A preliminary integral Coulomb Dissociation cross section of the 94Mo(γ,n) reaction is presented. Further analysis will complete the experimental database for the (γ,n) production chain of the p-isotopes of molybdenum