Isotopic Evolution Research Articles

The sulfur isotope evolution of the Duobuza Cu-Au porphyry deposit in the Duolong district, Central Tibet, China

The sulfur isotope evolution of the Duobuza Cu-Au porphyry deposit in the Duolong district, Central Tibet, China

Evolution of the Hydrothermal Fluids Inferred from the Occurrence and Isotope Characteristics of the Carbonate Minerals at the Pogo Gold Deposit, Alaska, USA

Pogo is identified as a deep-seated, intrusion-related gold deposit. Carbonate minerals have a close spatial relationship to hydrothermal gold mineralization in all of its principal ore zones. The carbon and oxygen isotopic ratios of carbonate minerals (siderite, ankerite, and calcite) present within the deposit illustrate the isotopic evolution of the ore-forming fluid. The initial hydrothermal fluid phase is interpreted to be magmatic in origin. The fluid evolution was characterized by a gradual decrease in δ18O and a slight increase in δ13C with decreasing temperature. The dominant carbon-bearing species was CO2, with methane introduced sporadically. Siderite is associated with early-stage mineralization and occurs with ankerite in main-stage ore assemblages. Calcite is recognized in the later stages of mineralization. Gold in the Pogo deposit occurs as native gold, Au-Bi-Te minerals, inclusions in sulfide minerals, or as “invisible gold”. The latter is found in pyrite, chalcopyrite, arsenopyrite, and quartz, based on ion microprobe analysis. The presence of invisible gold in these minerals has significant metallurgical implications for gold processing at the Pogo mine.

A theoretical study of even-even 162-17870Yb isotopes using interacting vector boson model

This theoretical study investigates the properties of even-even 162-17870Yb isotopes using the interacting vector boson model (IVBM). Our study focuses on the ground state band and negative parity band energy-level patterns, which provide insights into the shapes and symmetries of these nuclei. Furthermore, we investigate the collective properties of these isotopes, such as rotational and vibrational motion, as well as their interplay. The results of our theoretical analysis shed light on the structural evolution of ytterbium isotopes with increasing neutron numbers. The comparison of our theoretical predictions with experimental data will provide valuable insights into the nuclear structure of these isotopes and help validate the IVBM model's effectiveness in describing collective phenomena. This theoretical study employs the IVBM to investigate the dynamic symmetry of even-even 162-17870Yb isotopes. By conducting tests such as the ratio, backbending, and staggering analyses, we aim to determine the underlying symmetries governing the behavior of these isotopes. These results indicate that 16270Yb possess O(6) symmetry, 164-16670Yb have transition O(6) - SU(3) symmetry, and 168-17870Yb possess SU(3) symmetry. The study's outcomes show that the IVBM is dependable and useful for nuclear physics research because it aligns well with the corresponding experimental data.

Isotope Evolution of the Depleted Mantle

The depleted mantle reservoir is that part of Earth's mantle from which crust has been extracted, leaving the remaining mantle depleted in incompatible elements. Knowing how and when it formed is essential for understanding the chemical evolution of Earth, including formation of continental crust. The best-constrained Hf isotope data presented here indicate that the mantle does not become significantly depleted until as late as 700 million years after Earth's accretion. This onset of mantle depletion coincides with the first appearance of substantial volumes of continental crust in the geological record. These data compel a revision to the reference depleted mantle parameters used in Hf isotope studies of planetary evolution. This new reference line follows chondritic evolution until 3.8 Ga and then describes a linear trajectory to a present-day depleted mid-ocean ridge basalt source mantle composition (εHf = +18). We infer that stabilization of continental crust only occurred in earnest on Earth after 3.8 Ga. ▪ Hf isotopes show that Earth's mantle does not become significantly depleted until 700 million years after planetary accretion. ▪ Most of Earth's oldest rocks formed from mantle sources that had radiogenic isotope compositions similar to those of chondritic meteorites. ▪ Isotope evidence shows that Hadean (>4.0-billion-year-old) crust was not essential for formation of younger crust in Archean terranes. ▪ Growth of Earth's continents only began in earnest after 3.8 Ga.

Kinetic Isotopic Degassing of CO2 During the 2021 Fagradalsfjall Eruption and the δ13C Signature of the Icelandic Mantle

AbstractCO2 is the first volatile to exsolve in magmatic systems and plays a crucial role in driving magma ascent and volcanic eruptions. Carbon stable isotopes serve as valuable tracers for understanding the transfer of CO2 from the melt to the gas phase during passive degassing or active eruptions. In this study, we present δ13C measurements from the 2021 Fagradalsfjall eruption, obtained from (a) volcanic gases emitted during the eruption and collected via unmanned aerial systems (UAS), and (b) a series of mineral‐hosted melt inclusions from the corresponding tephra deposits. These data sets jointly track the carbon isotopic evolution of the melt and gas phases during the last 10 km of magma ascent. The isotopic evolution of both phases indicates that kinetic degassing, a process previously only identified in mid‐ocean ridge basalts, took place in the 10 to 1 km depth range, followed by equilibrium degassing at near‐surface conditions in the last kilometer. Postulating that the melt was first saturated with CO2 at 27 km depth and that degassing from then to 10 km depth took place via equilibrium isotopic fractionation, the melt inclusion data constrain the initial δ13C signature of the Icelandic mantle to −6.5 ± 2.5‰ but also show indications of possible isotopic heterogeneity in the mantle source.

Reconstructing Tarim within Rodinia: Constraints from early Tonian accretionary and collisional records in the Altyn belt

The position of the Tarim craton within Rodinia has long been debated, with competing models varying from internal to external ones. The Altyn belt in the southeast Tarim margin preserves extensive early Neoproterozoic magmatic successions, which are possibly related to the Rodinia assembly. Thus, we here investigated the granitoids in the South Altyn belt (SAB), and present field geology, zircon U–Pb–Hf–O isotopes and H2O contents, and whole rock geochemistry data from these granitoids. The Tula granite is dated at 914 ± 3.9 Ma, whereas the Kuoshi granites are dated at 919 ± 5.2 Ma and 927.7 ± 5.0 Ma. The Tula and Kalaqiaoka granite samples display high ACNK values that are typical of S-type granitoids, consistent with the occurrence of garnet and muscovite. In addition, the Tula granite has higher zircon δ18O (7.62 to 10.85 ‰) and lower εHf(t) (−2.4 to +0.1) values, along with lower zircon H2O content (medium values at 102 ppmw), indicating a primary water-deficient magma generated from recycled ancient crust, with minor juvenile contribution. On the other hand, the Kuoshi granite shows high Sr (169–259 ppm), Sr/Y (17.85–19.33) and (La/Yb)N (30–49) ratios. The Kuoshi granites are also characterized by higher MgO, lower zircon δ18O (4.15 to 9.81 ‰) and εHf(t) (−4.0 to −0.4) values, higher zircon H2O content (medium values at 255 ppmw), as well as subduction related chemical features. These signatures suggest that the Kuoshi granitic magma might have been formed by partial melting of recycled ancient crust and juvenile crust in a subduction setting. Thus, these granitoids across the SAB reflect a transformation from water-enriched subduction setting to water-deficient syn-collisional setting at ca. 930–920 Ma. Moreover, the zircon Hf isotope evolution trend across the SAB also suggest a possible transformation from slab retreat to syn-collision at 930–920 Ma. Furthermore, the end Mesoproterozoic and early Neoproterozoic metamorphic and magmatic events in the Tarim Craton, North Qaidam-Kunlun Block, Australia and India blocks display a close affinity. Therefore, we conclude a position of the Tarim craton between QKB, Australia and North India blocks in the periphery of Rodinia, consistent with earlier paleomagnetic data.

Uncovering Below Cloud Rain‐Vapor Interactions During Tropical Rain Events Through Simultaneous and Continuous Real‐Time Monitoring of Rain and Vapor Isotopes

AbstractDue to limited water vapor measurements, vapor isotopes have been traditionally estimated under the assumption of isotopic equilibrium between rain and vapor below cloud base. However, recent advancements in analytical instruments allow more vapor isotopic measurements that have challenged this assumption. To enhance our understanding of rain‐vapor interactions below cloud base in tropical regions, we established an automated system to measure rain and vapor isotopes simultaneously and continuously in real time at minute intervals in Singapore. Among 324 rain events monitored from 2016 to 2019, 81% exhibited a substantial departure of rain and vapor isotopes from the expected equilibrium. This departure suggests that raindrop evaporation plays a larger role in determining their isotopes. The conclusion is supported by the generally lower slopes of the local meteoric water line. Seasonal variations in rain event characteristics indicate changing influences of rain‐vapor interactions: during monsoons, more frequent heavy rainfall maintains relatively high humidity below cloud base, favoring rain‐vapor isotopic equilibrium, whereas during inter‐monsoons, more light rain events lead to pronounced rain evaporation and larger isotopic differences. Furthermore, rain‐vapor interactions below cloud base significantly modulated their isotope evolution during individual events. As events progressed, reduced humidity favored evaporation, increasing rain isotope values and decreasing its d‐excess, whereas vapor isotope values decreased and its d‐excess increased. Our study introduces a new approach to capturing real‐time high‐resolution rain and vapor isotopes at minute intervals to understand the dynamics of rain‐vapor interactions below cloud base. Findings underscore the crucial role of these interactions in influencing rain and vapor isotopes during tropical rain events.

Magmatic maturation of Archean continental crust via a three-step crustal reworking, western Singhbhum Craton

The western part of the Singhbhum Craton preserves Paleo-Mesoarchean mafic greenstone lava flows, felsic tonalite-trondhjemite-granodiorite (TTG)-granite associations, and high-K granite and volcanic suites, similar to other Archean cratonic blocks. These successions are crucial components of early continental crust, and unravelling their respective petrogenetic relations is important for understanding the evolution from mafic to felsic crust. This study presents detailed investigations of zircon UPb age and Hf isotope data from the Bonai TTG/gneiss-granite Suite, and the overlying Tamperkola high-K granite and rhyolite Suite. Our results indicate concurrent crystallization of the Bonai TTG gneiss (3316 ± 9 Ma), associated porphyritic high-K granite (3299 ± 9 Ma), and their amphibolite enclaves (3325 ± 9 Ma) with older, inherited zircon grains intercepting at 3586 ± 25 Ma. The entire Bonai Suite yields an overall juvenile Hf isotope composition (ɛHf(t) = −1.7 to +4.6, 95 % ɛHf(t) > 0). Combined with the mantle-like Hf isotope signatures of the inherited zircons grains (ɛHf(t) = +1.7 to +6.2), this indicates a Hf isotope evolution array with a mafic crustal 176Lu/177Hf ≈ 0.022. Considering that these grains represent the source of the TTGs, this implies lower crustal residence of ca. 300 Myr of the mafic precursor rocks. The Tamperkola high-K magmatic suite yields a crystallization age of 2810 ± 8 Ma with subchondritic Hf isotope composition (ɛHf(t) = −3.2 to −0.6). This Tamperkola Suite plots on the Hf isotope evolutionary array defined by the Bonai Suite and its mafic precursor, suggesting remelting of the Bonai (transitional) TTGs to produce these high-K granitoids in an internal reworking process. Our new and published data yield a threefold crustal evolution with (i) initial formation of the mafic crust at ca. 3586 Ma, (ii) subsequent residence for ca. 300 Myr and crustal reworking at ca. 3316–3299 Ma to form TTGs and (iii) their melting at ca. 2810 Ma to form high-K magmas. This succession of re-melting of igneous rocks drove the transition from mafic to felsic continental crust in the Singhbhum Craton. Given the consistent lithological sequence of predominantly mafic greenstone rocks, TTG-granite suites, and high-K granites observed across global cratons, this Paleo-Mesoarchean process likely reflects the order of crustal maturation in the Archean continental crust.

The earliest phases of CNO enrichment in galaxies

Context. The recent detection of super-solar carbon-to-oxygen and nitrogen-to-oxygen abundance ratios in a group of metal-poor galaxies at high redshift by the James Webb Space Telescope has sparked renewed interest in exploring the chemical evolution of carbon, nitrogen, and oxygen (the CNO elements) at early times and prompted fresh inquiries into their origins. Aims. The main goal of this paper is to shed light onto the early evolution of the main CNO isotopes in the Galaxy and in young distant systems, such as GN-z11 at ɀ = 10.6 and GS-zl2 at ɀ = 12.5. Methods. To this aim, we incorporated a stochastic star formation component into a chemical evolution model calibrated with high-quality Milky Way (MW) data while focusing on the contribution of Population III (Pop III) stars to the early chemical enrichment. Results. By comparing the model predictions with CNO abundance measurements from high-resolution spectroscopy of an homogeneous sample of Galactic halo stars, we first demonstrate that the scatter observed in the metallicity range −4.5 ≤ [Fe/H] ≤ −1.5 can be explained by pre-enrichment from Pop III stars that explode as supernovae (SNe) with different initial masses and energies. Then, by exploiting the chemical evolution model, we provide testable predictions for log(C/N), log(N/O), and log(C/O) versus log(O/H)+12 in MW-like galaxies observed at different cosmic epochs (redshifts). Finally, by calibrating the chemical evolution model to replicate the observed properties of GN-z11 and GS-z12, we provide an alternative interpretation of their high N/O and C/O abundance ratios, demonstrating that an anomalously high N or C content can be reproduced through enrichment from faint Pop III SNe. Conclusions. Stochastic chemical enrichment from primordial stars explains both the observed scatter in CNO abundances in MW halo stars and the exceptionally high C/O and N/O ratios in some distant galaxies. These findings emphasize the critical role of Pop III stars in shaping early chemical evolution.

Development of fuel depletion code for molten salt reactor with very deep burnup

A liquid-fueled molten salt reactor (MSR) can reach a deep burnup based on online reprocessing and continuously refueling, which requires significantly different burnup calculation methods for MSRs compared with those for the traditional reactors. To address the unique burnup features and consider the fidelity of isotopic evolution in an MSR, a fuel depletion code ThorMCB is developed based on the OpenMC coupled with a specific depletion code, MODEC. Furthermore, to lower the computational cost of acquiring the equilibrium state through the time evolution step by step for an MSR, an equilibrium burnup calculation code ThorMCB-eq based on the OpenMC and MODEC is developed, which can obtain the equilibrium burnup efficiently. A single fuel lattice of MSR and an a Molten Salt Fast Reactor (MSFR) benchmark are applied for verifying the correctness of the ThorMCB and ThorMCB-eq codes. Compared with a neutron transport calculation code KENO-VI coupled with MODEC, the maximum deviation of the dominant heavy nuclides (HNs) at equilibrium state by ThorMCB is less than 10%, and that of the total mass of fission products (FPs) is less than 3%. For the MSFR benchmark, the neutronic parameters including temperature reactivity coefficient, the mass evolution of main HNs and FPs and breeding ratio (BR) from ThorMCB agree with the references. The equilibrium behavior can be quickly obtained with ThorMCB-eq, and the relative mass deviations of most nuclides keep around 2% in comparison with the results of step-by-step burnup evolution with ThorMCB. Furthermore, the same fuel contents and micro one-group cross sections at equilibrium are obtained with two different types of start-up fuels and a constant power density and fuel reprocessing scheme. In conclusion, the verified results indicate that ThorMCB and ThorMCB-eq can both provide reliable simulation for depletion evolution and equilibrium burnup for MSR fuel cycle.

Carbon isotopic record of a platform-to-basin transect through the Permian Reef Complex (Guadalupian) in the Delaware Basin of Texas and New Mexico

The Guadalupian Permian Reef Complex of the Delaware Basin is one of the most studied carbonate reef systems of the Paleozoic. Despite extensive work on the carbonate sedimentology, sequence stratigraphy, and diagenetic history of the Delaware Basin, a high-resolution carbonate carbon isotope record along a platform to basin transect for the Capitanian (264.3–259.5 Ma, the youngest age of the Guadalupian Epoch) does not yet exist. The carbon isotopic record of the Delaware Basin is important because 1) it allows us to test hypotheses about controls on the carbon isotope proxy, 2) it provides constraints on how well modern carbonate platforms like the Great Bahama Banks serve as analogues for ancient carbonate settings, and 3) these types of restricted basins likely played an important role in Permian carbon cycling and the Capitanian extinctions.In this study we present 493 new Capitanian carbonate carbon isotopic values paired with a detailed sedimentological and sequence stratigraphic framework from the platform, slope, toe of slope, and deep basin of the Delaware Basin. The bulk of the new δ13C values fall within the range of previously reported unaltered carbonates from the basin, suggesting that these results record primary environmental processes and were not significantly altered by diagenetic overprinting. With this dataset, we test hypotheses about sources of carbon isotopic variability in shallow carbonate platforms. Our results indicate that in the Delaware Basin there are no systematic and resolvable depth or lateral gradients in carbon isotopic values, that δ13C values do not vary as a function of grain type, and that there is no resolvable relationship between carbon isotopic composition and sea level change. However, we do document statistically significant differences in δ13C distributions among facies associations which we attribute to the isotopic evolution of an upwelling water mass due to direct precipitation of mud along the slope. Our results support the idea that increasing carbon isotopic values through the Capitanian were driven by increased organic carbon burial in restricted basins.

Hypogene sulfide precipitation during phyllic alteration: insights from copper isotopic evolution of the Dexing porphyry Cu–Mo–Au deposit, South China

Hypogene sulfide precipitation during phyllic alteration: insights from copper isotopic evolution of the Dexing porphyry Cu–Mo–Au deposit, South China

Spatial Distribution of Critical Metals and Chemostratigraphy in Co‐Rich Ferromanganese Nodules in the Northwestern Pacific Ocean

AbstractFerromanganese nodules are important marine storehouses for critical metals and windows for changing oceans. Although advanced in situ analytical techniques have been applied to visualize the elemental distribution in the nodule cross‐sections, their spatial distribution remains largely uncertain. This study addresses this gap by employing micro X‐ray fluorescence mapping of parallel nodule cross‐sections to delineate the spatial distributions of critical metals (Mn, Fe, Co, Ti, Ni, and Cu) in three Co‐rich ferromanganese nodules from the northwestern Pacific Ocean. The 10‐layer Os isotopic compositions of one nodule closely align with the well‐documented marine Os isotope evolution of seawater, providing a chronological framework and a maximum age of ∼36 Ma for these nodules. Three concentric chemostratigraphic layers, labeled L1, L2, and L3, were identified from the inside out, based on microscopic structures and the distributions of critical metals. The early growth stage was marked by Mn‐rich, Si‐rich, and high Mn/Fe ratios, suggesting a diagenetic‐driven process attributed to high paleoproductivity conditions because of the low latitude of the study area at that time. The subsequent growth stages are all hydrogenetic in origin to be rich in Fe, Co, and Ti with low Mn/Fe ratios. The apparent detritus present during the second growth stage of the nodules may correspond to the stronger bottom currents in the early Miocene. The final mineralization stage indicates a more stable environment with diminished bottom current activity, leading to the formation of a dense, laminar hydrogenetic layer.

LOMU Type Alkali Basalts in East Asia Sourced from Subduction Recycling of Multiple Ancient Crustal Components

Abstract The origin of alkali basalts with distinctive time-integrated low U/Pb (low μ, LOMU) from East Asia is controversial due to the complex geochemical and tectonic signatures reported from this region. We report new data on the petrology and geochemistry of the Higashi-Matsuura and Kita-Matsuura alkali basalts from Southwest Japan, which confirm the presence of a LOMU-type mantle component below the Japanese Islands, similar to East Asian intraplate volcanoes. We use whole rock geochemistry and mineral chemistry to determine mantle melting, magma evolution conditions and isotopic evolution of the LOMU mantle source. Petrological studies show that the Higashi-Matsuura alkali basalts (~3 Ma) were derived from a hydrous mantle source with ~880 μg/g H2O, at a pressure of 1.8 to 2.3 GPa at a mantle potential temperature of ~1300°C. Trace element modelling concludes that these basalts may have been generated through the hydrous upwelling and partial melting of the asthenospheric mantle containing ~15% dehydrated eclogitic component. Higashi-Matsuura alkali basalts show 206Pb/204Pb values of 17.72 to 18.04 which are among the lowest values from Southwest Japan. Relatively older (6–8 Ma) alkali basalts from the Kita-Matsuura area showing similar physicochemical characteristics, do not show LOMU-type isotopic trends. Trace element and Pb–Sr–Nd isotopic data indicate that the Higashi-Matsuura mantle component is similar to the extreme LOMU components reported from the Northeast China alkali basalts, as well as the recently discovered Petit Spot volcanoes on the Pacific Plate. We model the origin of the LOMU signature from the lowest reported Pb isotope ratios in East Asia, from intraplate alkali basalts in Northeast China. Our model suggests that at least two separate subduction events of marine sediments, at 1.8 and 2.2 Ga, are required to explain the observed Pb isotopic variation in the East Asian region. Other LOMU type basalts from East Asia, including Southwest Japan and Petit Spot, define a linear trend between the Xiaogulihe basalts and lithospheric mantle xenoliths, with 40% to 60% Pb in Higashi-Matsuura basalts being derived from the lithospheric mantle during upwelling. This suggests that the LOMU array in East Asia may have been formed by mixing between multiple ancient, subducted sediment components accumulated at the mantle transition zone for about 2 billion years, and its recent upwelling due to dehydration of the stagnant Pacific slab and related melting of the metasomatised asthenospheric mantle.

Stable carbon isotope evolution of formaldehyde on early Mars

Organic matter in the Martian sediments may provide a key to understanding the prebiotic chemistry and habitability of early Mars. The Curiosity rover has measured highly variable and 13C-depleted carbon isotopic values in early Martian organic matter whose origin is uncertain. One hypothesis suggests the deposition of simple organic molecules generated from 13C-depleted CO derived from CO2 photochemical reduction in the atmosphere. Here, we present a coupled photochemistry-climate evolution model incorporating carbon isotope fractionation processes induced by CO2 photolysis, carbon escape, and volcanic outgassing in an early Martian atmosphere of 0.5–2 bar, composed mainly of CO2, CO, and H2 to track the evolution of the carbon isotopic composition of C-bearing species. The calculated carbon isotopic ratio in formaldehyde (H2CO) can be highly depleted in 13C due to CO2-photolysis-induced fractionation and is variable with changes in atmospheric CO/CO2 ratio, surface pressure, albedo, and H2 outgassing rate. Conversely, CO2 becomes enriched in 13C, as estimated from the carbonates preserved in ALH84001 meteorite. Complex organic matter formed by the polymerization of such H2CO could explain the strong depletion in 13C observed in the Martian organic matter. Mixing with other sources of organic matter would account for its unique variable carbon isotopic values.

Evaporite sequences as archives for Mg isotope compositions of seawater - Evidence from a Tethys marginal shelf basin in the Anisian

Evaporites have recently been suggested as a potential archive for recording the Mg isotope compositions (δ26Mg) of coeval seawater. However, episodic dolomitization during the deposition of massive evaporites could cause considerable Mg removal and isotopic fractionation. To constrain the hydrological changes and influence of dolomitization on ambient brine δ26Mg, we present petrographic and mineralogical features, as well as Mg-C-O-Sr isotope data extracted from carbonate phases of a middle Triassic (ca. 247 Myr) marine anhydrite-dolostone sequence from a drill core in eastern China. The drilled lithologies are characterized by massive dolostone layers in the lower part, followed by an upward decline in dolomite contents accompanied by a rise in anhydrite. Multiple lines of evidence consistently point to a syn-depositional origin for the dolostones in a marginal basin of the Tethys Ocean and a lack of diagenetic alteration since deposition. We reconstructed the dynamic changes of δ26Mg values of basin waters based on Mg isotope compositions in dolomite leachates. According to our findings, the δ26Mg values of the basin waters were remarkably high (about 0.38 ± 0.05‰) at the onset of evaporation, indicating a significant Mg sink of the massive dolomitization. The δ26Mg of brine in the basin then changed towards the value of coeval seawater (about −0.32 ± 0.05‰) starting with the deposition of evaporites. Concurrently, 87Sr/86Sr ratios of dolomites shift from radiogenic values towards contemporaneous seawater composition. Our results demonstrate that the evaporite basin was not strictly restricted, and water exchange with the open ocean never ceased. Modeling calculation reveals that, even when the seawater exchange rate is far below the average ocean circulation, δ26Mg of brine in the basin will reach the value of open ocean within 1 Myr, completely removing the influence of early dolomitization. We suggested that massive marine evaporite sequences have the potential to record the long-term evolution of seawater Mg isotopes.

Post-depositional modification on seasonal-to-interannual timescales alters the deuterium-excess signals in summer snow layers in Greenland

Abstract. We document the isotopic evolution of near-surface snow at the East Greenland Ice Core Project (EastGRIP) ice core site in northeast Greenland using a time-resolved array of 1 m deep isotope (δ18O, δD) profiles. The snow profiles were taken from May–August during the 2017–2019 summer seasons. An age–depth model was developed and applied to each profile, mitigating the impacts of stratigraphic noise on isotope signals. Significant changes in deuterium excess (d) are observed in surface snow and near-surface snow as the snow ages. Decreases in d of up to 5 ‰ occur during summer seasons after deposition during two of the three summer seasons observed. The d always experiences a 3 ‰–5 ‰ increase after aging 1 year in the snow due to a broadening of the autumn d maximum. Models of idealized scenarios coupled with prior work indicate that the summertime post-depositional changes in d (Δd) can be explained by a combination of surface sublimation, forced ventilation of the near-surface snow down to 20–30 cm, and isotope-gradient-driven diffusion throughout the column. The interannual Δd is also partly explained with isotope-gradient-driven diffusion, but other mechanisms are at work that leave a bias in the d record. Thus, d does not just carry information about source-region conditions and transport history as is commonly assumed, but also integrates local conditions into summer snow layers as the snow ages through metamorphic processes. Finally, we observe a dramatic increase in the seasonal isotope-to-temperature sensitivity, which can be explained solely by isotope-gradient-driven diffusion. Our results are dependent on the site characteristics (e.g., wind, temperature, accumulation rate, snow properties) but indicate that more process-based research is necessary to understand water isotopes as climate proxies. Recommendations for monitoring and physical modeling are given, with special attention to the d parameter.

Garnet Pyroxenite Cumulates from Cretaceous Alkaline Intraplate Magmas Underplate the Zealandia Mantle Lithosphere

Abstract The elemental and isotopic properties of garnet pyroxenites can yield information on lithospheric mantle composition, thermal state, and evolution. The 34Ma Kakanui Mineral Breccia in New Zealand contains spectacular but little-studied mantle peridotite and pyroxenite xenoliths that yield new insights into the evolution of a portion of the underlying mantle lithosphere of a former Gondwana margin. The moderately depleted and metasomatized spinel peridotites, as judged from spinel and olivine compositions and bulk rock major and platinum group element abundances, give mineral equilibration temperatures <1020°C and are derived from the middle to shallow (~35 to 50 km) lithospheric mantle when projected onto a 70 mW∙m−2 geotherm. These residues have low Re/Os and Re-depletion 187Os/188Os model ages that range from Eocene (0.05 Ga) to Paleoproterozoic (1.9 Ga), consistent with extraction from a lithospheric mantle comprising fragments with complex depletion histories. Although the peridotites have restricted δ18O (olivine +5.2 to 6.2), evidence for an isotopically heterogeneous mantle column in addition to the 187Os/188Os is seen in clinopyroxene 87Sr/86Sr (0.70244 to 0.70292), εNd (+4.1 to 18.8), 206Pb/204Pb (17.8 to 20.3), and εHf (+10 to +101). Higher metamorphic equilibrium temperatures of the garnet pyroxenites (Fe–Mg exchange of >1150°C) compared to the peridotites indicate their Eocene extraction was from towards the base of this isotopically heterogeneous mantle lithosphere. Pyroxenite bulk compositions point to cumulate origins, and the mineral isotope ratios of 87Sr/86Sr (0.70282 to 0.70294), εNd (+5.5 to 8.0) and 206Pb/204Pb (18.1 to 19.3) match many of the Zealandia metasomatized mantle peridotite xenoliths as well as the primitive intraplate basalts but not the garnet pyroxenite host magmas. In contrast to many global pyroxenite studies, the garnet pyroxenite 87Sr/86Sr and δ18O (+5.2 to 5.8) data provide no evidence for subducted crustal material in the primary magma source region, and Sm–Nd and Lu–Hf isotope data yield mid-Cenozoic ages that are probably related to isotope closure during eruption. An exception is one sample that yields a Lu–Hf isochron age of 111.9 ± 9.1 Ma, which corresponds to the convergence of the Lu–Hf isotope evolution curves of three other samples. Liquids calculated to have been in equilibrium with these cumulates have trace element compositions comparable to primitive alkaline intraplate basalts like those found at the surface of Zealandia. The new data, therefore, indicate that a pulse of intraplate magmatism occurred during or directly after the cessation of long-lived subduction on the former Zealandia Early Cretaceous forearc Gondwana margin, despite any volcanic surface exposure having been long eroded away. The lower lithospheric mantle emplacement of the garnet pyroxenites suggests that the source of the alkaline parent magmas was probably the convecting mantle, which supports conclusions that intraplate magmas in Zealandia have asthenospheric and lithospheric mantle sources.

Dissolved Cu isotope compositions in hydrothermal plumes over back-arc volcanoes in the Northeast Lau Basin, Southwest Pacific Ocean

Seafloor hydrothermal venting may be an important source of marine Cu and affect the biogeochemical cycling of Cu in the oceans. The distribution of Cu and its isotope compositions (δ65Cu) can provide insight into seafloor hydrothermal processes and their role in the mass balance of global Cu. To date, there are no published Cu isotope data for hydrothermal plumes and very few reports on Cu concentration distributions. This study presents both Cu concentrations and dissolved Cu isotope (δ65Cu) in hydrothermal plumes from back-arc volcanoes in the Northeast Lau Basin. The dissolved Cu (dCu) concentrations range from 2.14 to 5.66 nM most of which are higher than the deep seawater concentrations. However, the concentrations for some plume samples were lower than the deep seawater dCu concentrations due to adsorption onto particles in the plumes. The δ65Cu of hydrothermal plumes vary from 0.25 to 1.05 ‰. As plumes dispersed, the Cu isotopes from high-temperature Mata Fitu and Mata Ua vents shifted towards light values. In contrast, the δ65Cu in plumes from low-temperature East Mata and West Mata vents show increasingly higher values with plume dispersal. Rayleigh distillation models are built based on the adsorption of dCu onto Fe particles and complexation with organic ligands to describe the dCu isotope evolution in hydrothermal plumes. The results suggest that the adsorption and organic complexation may be the likely explanations for the observed dCu isotope compositions in plumes from high- and low-temperature venting, separately, besides the mixing with background seawater. Our measured δ65Cu in three fluid samples (0.08, 0.09 and 0.20 ‰) are lower than that of the characterized sinks (∼0.3 ‰) in the oceans, indicating that hydrothermal fluids might be a source of light Cu isotopes. However, the δ65Cu (0.53 ‰ on average) in plume samples are higher than 0.3 ‰ and these seafloor hydrothermal plumes do not seem to be a source of light Cu isotope of dCu, at least near the venting sites. Our study first reveals the Cu isotope compositions and evolution in hydrothermal plumes and provides new hints about the impact of seafloor hydrothermal venting on the modern oceanic Cu isotope budget.

Analysis of initial core and time dependent fuel burnup for high temperature testing reactors (HTTRs)

Abstract HTTRs are high temperature testing reactors which are considered generation IV nuclear power reactors. The reactor core startup and fuel burnup time dependent are simulated and analyzed. For the present computer model core startup, fuel loading with approach to criticality, control rod critical positions, scram reactivity and control rod worth, different core excess reactivity are evaluated. The results are compared with both typical measurements and other previously published methods. Moreover, the time dependent core model and fuel burn up are simulated to calculate the core multiplication factor, cycle length, discharged burnup, core isotopic evolution with time and burnable poisons behavior.