Heavy Isotopes Research Articles

New heavy uranium isotope made

New heavy uranium isotope made

Identification of atmospheric particulate matter derived from coal and biomass burning and from non-exhaust traffic emissions using zinc isotope signatures

Improving urban air quality is a global challenge. To implement successful abatement measures that reduce atmospheric particulate matter (APM) and associated metal concentrations, precise source apportionment is needed. For this, apportioning contributions from coal and biomass burning and differentiating these from non-exhaust traffic emissions in urban APM is critical. Recent studies characterising the metal isotope composition of urban APM, and potential source materials suggested that non-traditional isotope systems could prove unique fingerprinting tools. Zinc isotopes should be able to separate APM derived from uncontrolled combustion (fly ash, isotopically heavy) from non-exhaust traffic sources (tyre and brake wear, intermediate) and from controlled industrial emissions (flue gas, light). To test this hypothesis, we determined zinc isotope ratios of APM (TSP, PM2.5, PM1) in Beijing (coal combustion for residential heating) and Varanasi (biomass burning in pre-monsoon periods). In Beijing, δ66ZnLyon values of PM2.5 ranged from −0.41 to +1.01‰ in 2015 (avg = +0.25 ± 0.50‰, n = 19). Aerosols (including TSP, PM2.5 and PM1 samples) from the heating period were significantly (t-test, p < 0.001) heavier (avg = +0.90 ± 0.12‰, n = 7) than those from the non-heating period (avg = +0.14 ± 0.36‰, n = 23). Average δ66ZnLyon values of PM2.5 in Varanasi in spring 2015 were +0.82 ± 0.11‰ (n = 4). Extent and direction of isotope fractionation is in line with that expected from theoretical models and the isotope signatures observed agree with previously determined ratios of source materials. Our study links for the first time comprehensively the heavy zinc isotope compositions in APM to coal and biomass burning and shows that zinc isotope compositions of aerosols can discriminate between non-exhaust traffic and combustion sources.

Petrology of the Kureika Graphite Deposit in the Roof of the First-Rapid Trap Intrusion (Siberian Platform)

Abstract —The leading role of lithostratigraphic, structural/tectonic, magmatic, metamorphic, and fluid geologic factors in the formation of a graphite deposit is substantiated. We present the petrographic and chemical compositions of rocks and minerals of igneous rocks, the δ13С values of graphites and calcitites, the δ18O values of carbonates, the δ34S values of sulfides, and the 87Sr/86Sr and 87Rb/86Sr ratios in calcitites and microdolerite injections. The thermal effect of magmatic melt on coal led to its degassing and graphitization. The steaming of microdolerite injections with hydrocarbon fluids was probably accompanied by the removal of iron from them and the crystallization of low-Fe clinopyroxene, and the interaction of fluids with basaltic melt in the upper endocontact zone of the intrusion led to the segregation of the melt and the formation of a globular structure of rocks. The presence of a calcitite lens and a sulfide vein between the roof of the intrusion and the graphite bed is due to the earlier intrusion of sulfate salt melt and its subsequent interaction with hydrocarbon fluids, which resulted in sulfate reduction and the formation of calcitites. The sulfate reduction terminated with iron sulfurization in the basaltic melt and the formation of a sulfide vein. Sulfate salt melt was a source of heavy sulfur isotope (δ34S = 14.9–17.5‰) for sulfides and of calcium for calcitites. The similar isotope compositions of calcitites (δ13СPDB = –22.5 to –23.5‰), coals (–22.6 to –25.5‰, n = 25), and graphites (–23.5 to –25.0) suggest that coal is a source of carbon for calcitites. In the isotopic compositions of strontium (87Sr/86Sr ≈ 0.70834–0.70885) and oxygen δ18O ≈ 15‰) the calcitites are similar to the Devonian evaporites of the Siberian Platform. The isotope data (δ34S, δ18O, δ13С, and 87Rb/86Sr) point to the participation of the coal seam and anhydrite as sources of substance in the formation of calcitite and sulfide mineralization.

Tracer or toxicant: Does stable isotope labeling affect central processes in aquatic food webs?

Stable isotope analysis (SIA) is an elementary technique in food web ecology, but its insights become increasingly ambiguous in complex systems. One approach to elevate the utility of SIA in such systems is the use of heavy isotope tracers (i.e., labeling). However, the fundamental assumption that the addition of such tracers does not affect in situ conditions has been challenged. This study tests if labeling is suitable for autotrophy-based and detritus-based aquatic food webs. For the former, the survival and reproduction of Daphnia magna fed with phytoplankton cultured at different levels of 15N addition were assessed. For the latter, the microbial decomposition of leaf litter was assessed at the same tracer levels. While no significant differences were observed, effect patterns were comparable to a previous study, supporting the isotopic redundancy hypothesis that postulates discrete quantum mechanical states at which the reaction speeds of metabolic processes are altered. Although physiology (reproduction) and activity (microbial decomposition) might not be altered to an ecologically significant level, labeling with heavy stable isotopes could potentially affect isotopic fractionation in biochemical processes and bias conclusions drawn from resulting SI ratios.

Formation of ferrihydrite induced by low pe+pH in paddy soil reduces Cd uptake by rice: Evidence from Cd isotope fractionation

Ferrihydrite (Fh) is an important iron mineral in paddy soil and is prone to phase transition during dynamic redox condition, which affects Cd distribution and induces Cd isotope fractionation across soil to rice. Here, we conducted rice culture experiments with or not Fh application under different irrigation regimes to study the relationship between Fe species and Cd availability, as well as the isotope ratio of Cd in different Cd pools in paddy soil-rice system. Fh addition under continuous flooding (FL) with the decrease of pe + pH from 9.36 to 3.44 promoted the formation of amorphous Fe oxides as increased by 120.1% and facilitated Cd immobilization along with the increase of Fe/Mn oxides bound Cd by 25.3%, compared with continuous drying (DY) treatment. The isotopically heavy Cd were preferentially enriched from soil to extractable Cd (Δ114/110Cdextractable Cd-soil = 0.39–0.62‰) and from soil to grain (Δ114/110Cdgrain-soil = 0.40–0.66‰) particularly at low pe + pH and with Fh addition, while light Cd were enriched in Fe/Mn oxides (Δ114/110CdFe/Mn oxides bound Cd-extractable Cd = −0.65 ∼ −0.14‰). Besides, the expression of transporters involved in Cd transport in rice like OsNRAMP1, OsNRAMP1, OsHMA3, OsHMA2 and OsLCT1 were suppressed under low pe + pH condition. These findings indicated that low pe + pH facilitated Cd stabilization by the existence of more amorphous iron oxides, which induced the enrichment of heavy Cd isotope in liquid phase and light in Fe/Mn (oxy)hydroxides, respectively.

Tissue-variation of iron stable isotopes in marine fish coupled with speciation analysis using X-ray absorption fine structure

The Fe stable isotope ratio (δ56Fe) in tissues is a potential parameter for examining the Fe metabolism in marine fish. Although the effect of ferritin storage has been proposed as a possible cause of heavy isotope (56Fe) enrichment in the liver, no speciation and stable isotope ratio coupling data are available. Here, we report the δ56Fe values measured by multicollector ICP-MS and the result of Fe K-edge X-ray absorption near-edge structure (XANES) analysis of multiple tissues obtained from a skipjack tuna (Katsuwonus pelamis) and a chub mackerel (Scomber japonicus). Apparent isotopic fractionation between the liver and the muscle samples (Δ56FeL–M = δ56Feliver − δ56Femuscle) from these species was observed (0.85 ‰ and 0.57 ‰, respectively). The dominant Fe species in the muscle was heme Fe (the sum of methemoglobin, oxyhemoglobin, and deoxyhemoglobin), while ferritin was not detected according to the linear combination fitting of the XANES spectra. In the liver, ferritin contribution was ca. 28 %–54 % of the total Fe content. The apparent difference in δ56Fe between heme Fe and ferritin was estimated to range from 1.41 ‰ to 1.52 ‰ based on the tissue-specific δ56Fe values and the XANES results. These results indicate that the Fe storage as ferritin does not induce the lowering of δ56Fe in the muscle, considering the low contribution of the liver Fe to the total Fe content in the body.

Zinc Stable Isotope Fractionation Mechanisms during Adsorption on and Substitution in Iron (Hydr)oxides.

The Zn isotope fingerprint is widely used as a proxy of various environmental geochemical processes, so it is crucial to determine which are the mechanisms responsible for isotopic fractionation. Iron (Fe) (hydr)oxides greatly control the cycling and fate and thus isotope fractionation factors of Zn in terrestrial environments. Here, Zn isotope fractionation and related mechanisms during adsorption on and substitution in three FeOOH polymorphs are explored. Results demonstrate that heavy Zn isotopes are preferentially enriched onto solids, with almost similar isotopic offsets (Δ66/64Znsolid-solution = 0.25-0.36‰) for goethite, lepidocrocite, and feroxyhyte. This is consistent with the same average Zn-O bond lengths for adsorbed Zn on these solids as revealed by Zn K-edge X-ray absorption fine structure spectroscopy. In contrast, at an initial Zn/Fe molar ratio of 0.02, incorporation of Zn into goethite and lepidocrocite by substituting for lattice Fe preferentially sequesters light Zn isotopes with Δ66/64Znsubstituted-stocksolution of -1.52 ± 0.09‰ and -1.18 ± 0.15‰, while Zn-substituted feroxyhyte (0.06 ± 0.11‰) indicates almost no isotope fractionation. This is closely related to the different crystal nucleation and growth rates during the Zn-doped FeOOH formation processes. These results provide direct experimental evidence of incorporation of isotopically light Zn into Fe (hydr)oxides and improve our understanding of Zn isotope fractionation mechanisms during mineral-solution interface processes.

Effect of thermal maturation on the isotopic compositions of volatile organic sulfur compounds released from a kerogen by hydrous and anhydrous pyrolysis

Volatile organic sulfur compounds (VOSCs) may provide an alternative and improved approach in the evaluation of source and thermal maturity of natural gas. However, the role of water in the generation and decomposition of VOSCs is still ambiguous. Hydrous pyrolysis experiments were conducted on the isolated type Ⅱ-S kerogen of a source rock collected from the Ghareb Formation, Israel at 200–360 ℃ for 72 h (Easy%Ro = 0.32–1.20). The evolved gases were quantified for C1-C5 hydrocarbons, CO2, H2S and VOSCs. Compound specific isotope analysis were conducted both on the VOSCs and H2S (δ34S), and also on the C1-C5 hydrocarbons and CO2 (δ13C, δ2H). The results show that thiols concentration increased from 200 ℃ to 300 ℃ and then decreased from 300 ℃ to 360 ℃. The formed thiols isotopically co-vary with H2S from 200 ℃ to 340 ℃ and present the opposite trend with H2S from 340 ℃ to 360 ℃. The δ34S values of thiols are usually 3–7‰ lighter than the parent kerogen and 0.5–4‰ heavier than H2S. The concentrations of thiophenes generally increased with thermal stress with peak generation at 360 ℃, the δ34S values of thiophenes are mostly < 2–4‰ lighter than the parent kerogen and 2–8‰ heavier than H2S through the experiments. The δ13C and δ2H values of gaseous hydrocarbons are gradually enriched in heavy isotopes with increased thermal maturity. VOSCs dominated by thiols and thiophenes are generated by two ways: 1. the interactions between H2S and hydrocarbon molecules (synthetic pathway), 2. thermal cracking of OMs including kerogen and associated bitumen primary cracking and hydrocarbon secondary cracking (thermal cracking pathway). The synthetic pathway played the major role in the generation of thiols at the immature and early mature stages (200–300 ℃, Easy%Ro = 0.32–0.71), the cracking pathway has the dominant effect on the occurrence of thiols at higher thermal maturity levels (300–360 ℃, Easy%Ro = 0.71–1.20). The main pathway of thiophenes formation throughout the experiments is by the thermal cracking pathway. In the hydrous experiments, water promotes the synthetic pathway that generated thiols and retarded the thermal cracking pathway that forms thiophenes, but had no significant impact on δ34S values of individual VOSCs, δ13C and δ2H values of gaseous hydrocarbons. Therefore, combining with carbon and hydrogen isotopic compositions of gaseous hydrocarbons, molecular and sulfur isotopic compositions of individual VOSCs in the gas phase can provide valuable clues for petroleum systems in sedimentary basins including gas sources, generation pathways, thermal maturity levels and petroleum secondary transformations, such as thermochemical sulfate reduction in the subsurface reservoirs.

A fresh look at thermal cycling absorption process for hydrogen isotopes separation by dynamic simulation: Initial feeding process and total reflux mode

Thermal Cycling Absorption Process (TCAP) is an important method to separate hydrogen isotopes for the production and engineering application of tritium. There are so many influencing factors that it is difficult to fully understand the influence mechanisms only through experiments. A new simulation model of TCAP was developed. Using the simulation model, the influencing trend and optimized value of parameters of initial feeding process and total reflux mode in TCAP are obtained as follows. (1) The temperatures of initial feeding and the cold cycle are not the lower the better as commonly assumed but exists an optimal value of 30 °C in initial feeding and -20 °C in the cold cycle. (2) The heating rate and cooling rate also exist optimal values of 45 °C/min and 55 °C/min (3) Lower feeding rate is conducive to a better separation effect of hydrogen isotopes. (4) Increasing the reflux ratio benefits the enrichment of heavy hydrogen isotope at the product side but not the light hydrogen isotope at the tail side. (5) It needs more cycles to achieve the same isotope abundance in D/T separation than H/D separation and the separation degree of hydrogen isotopes exists a maximum value after multiple cycles. The findings can provide references for the process optimization and the device design of TCAP, and also can shed new light on the understanding of the basic physical and chemical process of TCAP.

Lithium Isotope Fractionation During Intensive Felsic Magmatic Differentiation

AbstractThe Xihuashan and Yaogangxian granitic plutons in South China comprise highly evolved multiphase Li‐rich granites and host quartz‐vein‐type tungsten deposits. The δ7Li values of Phase A (early stage), B (middle stage), and C (late stage) from the Xihuashan pluton are 1.0–1.2‰, 1.1–3.0‰, and 2.4–2.8‰ respectively, increasing through chemical evolution. The granites from the Yaogangxian pluton also display gradually enriched in heavy Li isotopes in a later stage, although systematically lighter than those of the Xihuashan pluton. In both plutons, the δ7Li shows good correlations with SiO2 and Li concentrations as well as Rb/Sr, Nb/Ta, and Zr/Hf ratios, indicating Li isotopic fractionation most likely caused by magmatic differentiation. In situ analyses show that the minerals of Xihuashan pluton record a continuous elemental spectrum, reflecting the results of progressive magmatic differentiation. The δ7Li values of quartz, feldspar, mica, and zircon all correlate well with the chemical evolutions of granitic magma, systematically elevated in Phases B and C relative to Phase A. The Li isotope data of the mineral separates further document that the enrichment of 7Li in the residual melt was most likely due to the equilibrium fractionation between the mineral and melts. The data are interpreted to reflect that intense magmatic differentiation was responsible for Li isotopic variations coupled with the enrichment in the Li, F, P, and rare metals in the late‐phase granites of the Xihuashan pluton. The lithium isotope behavior documented in this study provides new insights into magmatic differentiation and associated rare‐metal mineralization.

Molybdenum isotopes record recycling of subducting sediment in active continental margin, Northeast China

Highly variable molybdenum (Mo) isotope compositions have been reported for mafic arc volcanics above oceanic subduction zones. The heavy Mo isotope signatures of mafic arc magmas are generally attributed to addition of high δ98Mo oceanic crust-derived fluids to the subarc mantle sources. However, subduction zone fluids at subarc depths may occur in the forms of aqueous solution and hydrous melts, and seafloor sediments would also be subducted to subarc depths to produce hydrous melts. If such sediment-derived melts were incorporated into the mantle wedge, it is intriguing what is their effect on the Mo isotope composition of mafic arc magmas. Here we report Mo isotope data for Early Jurassic mafic igneous rocks from Northeast China, which was an active continental margin during subduction of the Paleo-Pacific Plate in the Mesozoic. These rocks have δ98Mo values of −0.26‰ to 0.17‰, which are mostly higher than the normal mantle value of −0.20‰ ± 0.01‰. They also exhibit arc-like trace element compositions, moderate (87Sr/86Sr)i ratios of 0.7042 to 0.7047, and neutral εNd(t) values of −2.8 to 2.2. The high δ98Mo values are associated with low Mo/Ce ratios, enriched SrNd isotopes, and high melt-mobile element contents in the mafic igneous rocks. These geochemical features indicate that the heavy Mo isotope signatures in these rocks were mainly inherited from subducting sediment-derived melts. Subducting Paleo-Pacific oceanic crust-derived aqueous solutions and seafloor sediment-derived hydrous melts would metasomatize the overlying mantle wedge beneath the northeastern China continent, and the sediment-derived melts would have contributed the heavy Mo isotope signatures to the mantle sources of mafic arc magmas in the active continental margin. Therefore, the Mo isotope study of mafic igneous rocks is a powerful means to decipher the recycling of crustal components at convergent plate margins.

Zinc Supply Affects Cadmium Uptake and Translocation in the Hyperaccumulator Sedum Plumbizincicola as Evidenced by Isotope Fractionation.

This study employs stable isotope analysis to investigate the mechanisms of cadmium (Cd) and zinc (Zn) interaction in the metal hyperaccumulating plant species Sedum plumbizincicola. To this end, the Cd and Zn isotope compositions of root, stem, leaf, and xylem sap samples were determined during metal uptake and translocation at different Cd and Zn concentrations. The enrichment of light isotopes of both elements in plants during uptake was less pronounced at low metal supply levels, likely reflecting the switch from a low-affinity to a high-affinity transport system at lower levels of external metal supply. The lower δ114/110Cd values of xylem sap when treated with a metabolic inhibitor decreasing the active Cd uptake further supports the preference of heavier Cd isotopes during high-affinity transport. The Δ66Znplant-initialsolution or Δ66Znplant-finalsolution values were similar at different Cd concentrations, indicating negligible interaction of Cd in the Zn uptake process. However, decreasing Zn supply levels significantly increased the enrichment of light Cd isotopes in plants (Δ114/110Cd = -0.08‰) in low-Cd treatments but reduced the enrichment of light Cd isotopes in plants (Δ114/110Cd = 0.08‰) under high Cd conditions. A systematic enrichment of heavy Cd and light Zn isotopes was found in root-to-shoot translocation of the metals. The Cd concentrations of the growth solutions thereby had no significant impact on Zn isotope fractionation during root-to-shoot translocation. However, the Δ114/110Cdtranslocation values hint at possible competition between Cd and Zn for transporters during root-to-shoot transfer and this may impact the transport pathway of Cd. The stable isotope data demonstrate that the interactions between the two metals influenced the uptake and transport mechanisms of Cd in S. plumbizincicola but had little effect on those of Zn.

Sulfuric acid as a corrosive cryofluid and oxygen isotope reservoir in planetesimals

The Earth-Moon and other planetary objects are enriched in the heavier isotopes, 17O and 18O, relative to 16O by several %, compared to the Sun. A stunning exception is the magnetite/sulfide symplectite found in Acfer 094 meteorite, which shows 24% enrichment in 17,18O. Here we show that the SO and SO2 molecules in the molecular cloud, ∼106% enriched in 17,18O relative to the Sun, evolved through the protoplanetary disk and planetesimal stages to become a sulfuric acid, 24% enriched in 17,18O. The sulfuric acid provided a cryofluid environment in the planetesimal and by itself reacted with ferric iron to form an amorphous ferric-hydroxysulfate-hydrate, which eventually decomposed into the symplectite by shock. We indicate that the Acfer 094 symplectite and its progenitor, sulfuric acid, is strongly coupled with the material evolution in the solar system since the days of our molecular cloud.We interrogate a petrographic relationship of the symplectite to the Acfer 094 meteorite matrix for an additional clue to its origin. In light of the comparative study of Acfer 094 with other “primordial” chondrites that contain amorphous silicate matrices and with GEMS, we conclude that the cryofluid condition was critical to the formation of the Acfer 094 symplectite.

Subducted carbonates not required: Deep mantle melting explains stable Ca isotopes in kimberlite magmas

Ca isotope geochemistry has great potential for improving our understanding of magmatic systems and for tracing the deep Earth carbon cycle. There are still many open questions, however, regarding the proper application of this relatively novel proxy to the study of mantle-derived magmas, including (i) the possible effects of pressure on mineral-melt Ca isotope fractionation factors, and (ii) the potential for Ca isotopes to be used as tracers of recycled marine carbonates in mantle-derived magmas. Kimberlites are mantle-derived melts that are highly enriched in CO2 and are the deepest-sourced magmas (>200 km depth) known to erupt at Earth’s surface, providing an excellent opportunity to explore these questions. We present Ca isotope data combined with detailed petrographic observations, bulk-carbonate C-O isotope data, and bulk-rock major element analyses, for a suite of 23 well-characterized kimberlite samples from their type-locality (Kimberley, South Africa). These kimberlites have abundant previous evidence for recycled surface materials in their mantle source, including low S isotope and moderately radiogenic Sr isotope compositions, yet display only limited variations in their Ca isotope compositions (δ44CaBSE of −0.08‰ to −0.27‰), with an average of −0.17 ± 0.02‰ (2SE, n = 21). This composition is indistinguishable from average carbonatites [−0.19 ± 0.03‰ (2SE, n = 106)] and OIB from recent studies [−0.16 ± 0.01‰ (2SE, n = 41)], and slightly lower than average MORB [−0.11 ± 0.02‰ (2SE, n = 31)]. Although our samples display a wide range of emplacement styles, alteration conditions, extents of magmatic differentiation, and degrees of mantle-cargo entrainment (i.e., xenocryst accumulation), we find no correlations between Ca isotopes and any of these factors. Instead, we find that low-degree partial melting of the likely kimberlite source lithology (i.e., carbon-bearing garnet lherzolite) yields modelled melt δ44CaBSE values ranging between −0.12‰ and −0.16‰ (at 1400–1500 °C), in agreement with the measured Ca isotope compositions of the Kimberley kimberlites. This observation, and the lack of heavy carbon isotope signatures in the examined samples, indicates that kimberlites do not require subducted carbonates in their mantle sources, despite their very high CO2 contents. Although several recent studies have suggested that equilibrium mineral-melt Ca isotope fractionation factors (e.g., 1000lnαgrt-melt) could be significantly different at higher pressures (i.e., due to pressure-induced changes in CaO bond lengths and coordinations), our models successfully reproduce the kimberlite data using pressure-independent predictions for mineral-melt fractionations. It remains possible, however, that differences in isotopic fractionation due to the peculiar composition of kimberlite melts (e.g., high CO2, low SiO2) are effectively cancelled out by competing pressure effects, and future work independently targeting these factors will be especially important for our understanding of Ca isotope fractionation in mantle-derived melts and the Ca isotope systematics of Earth’s mantle.

Mobilization of Cu in the continental lower crust: A perspective from Cu isotopes

Recent studies have shown that Cu-rich sulfide accumulates in the lower continental crust and serves as a critical reservoir to balance Cu depletion in the upper crust. Recycling of Cu in the lower crust is also assumed to be a major metal source for non-arc setting porphyry Cu deposits. To test this hypothesis and further explore the behavior of Cu in the lower crust, we analyzed the elemental and Cu isotopic compositions of lower crustal rocks from different geological domains. The collected samples include hornblendites from the Kohistan arc, granulite xenoliths and hornblendites from the Gangdese arc, hornblendites and gabbros from the Laiyuan complex in the North China Craton, and hornblendite xenoliths from the western margin of the Yangtze Craton. These lower crustal rocks have experienced varying degrees of primary or secondary sulfide accumulation, with significantly varied Cu contents (11.2 to 145 ppm) and δ65Cu (−1.05‰ to 1.40‰). Petrography and geochemistry reveal varying degrees of metasomatism and fluid interaction in these rocks, and based on this, they can be further divided into three groups: Group I includes the Gangdese granulites and Yunnan hornblendites, which perhaps experienced significant metasomatism. This suite of rocks shows enrichment of 65Cu (δ65Cu = 0.01‰ to 1.40‰), positively correlated with metasomatism (δ65Cu vs. Ce/Pb). We suggest the secondary sulfides which transformed from sulfates during the interaction between lower crust and arc magma are dominant in these rocks, so the feature of heavy isotope enrichment is inherited. Group II includes Laiyuan hornblendites and gabbros, derived from the same parental magma and emplaced at different depths (hornblendites, 23.3 – 28.1 km; gabbros 8.4 – 11.1 km). The Cu isotopic compositions are strongly fractionated between these two kinds of rocks, with low δ65Cu in the hornblendites (0.00‰ to 0.28‰) and highly polarized δ65Cu in the gabbros (−1.05‰ to 0.81‰). Geochemical indicators and mineral assemblages reveal that fluid interaction is most likely responsible for this feature. Primary sulfides were decomposed by fluids and reprecipitated at shallower depths. Since this process involves multiple redox reactions, the Cu isotopic composition in the shallowed emplaced gabbros was large fractionated. Group III includes the Gangdese hornblendites and Kohistan hornblendites which show negligible impacts of subduction-like metasomatism and fluid interaction. The Gangdese hornblendites show a homogeneous and unfractionated Cu isotopic composition (−0.09‰ to 0.18‰) and Cu content (83.4 to 128 ppm), suggesting insignificant Cu migration and isotope fractionation. In contrast, the Cu isotopic composition of the Kohistan hornblendites is strongly fractionated (−0.36‰ to 1.27‰). Geochemistry and modeling results suggest partial melting plays a role in the Cu isotope fractionation. The light Cu isotopes are preferentially distributed into sulfide melts and removed from the source region during partial melting of the lower crust, resulting in a decrease in Cu content and enrichment of heavy Cu isotopes in residues. Results suggest that partial melting and fluid interaction are two efficient mechanisms that encourage Cu migration in the lower crust.

Evidence for late winter biogeochemical connectivity in permafrost soils

The permafrost active layer is a key supplier of soil organic carbon and mineral nutrients to Arctic rivers. In the active layer, sites of soil-water exchange are locations for organic carbon and nutrient mobilization. Previously these sites were considered as connected during summer months and isolated during winter months. Whether soil pore waters in active layer soils are connected during shoulder seasons is poorly understood. In this study, exceptionally heavy silicon isotope compositions in soil pore waters show that during late winter, there is no connection between isolated pockets of soil pore water in soils with a shallow active layer. However, lighter silicon isotope compositions in soil pore waters reveal that soils are biogeochemically connected for longer than previously considered in soils with a deeper active layer. We show that an additional 21% of the 0–1 m soil organic carbon stock is exposed to soil - water exchange. This marks a hot moment during a dormant season, and an engine for organic carbon transport from active layer soils. Our findings mark the starting point to locate earlier pathways for biogeochemical connectivity, which need to be urgently monitored to quantify the seasonal flux of organic carbon released from permafrost soils.

The Geochemical Characteristics of Source Rock and Oil in the Fukang Sag, Junggar Basin, NW China

The Fukang Sag in the Junggar Basin is the main exploration block. However, the origin and source of crude oil are still controversial, which seriously affects the well locating and exploration in this area. In the present work, 30 source rocks and 21 crude oils were collected for geochemical analysis to clarify the source of the organic matter, the sedimentary environment, and the evolution degree. Among them, the source rocks of the Pingdiquan Formation are type II1 organic matter with good quality, the source rocks of the Badaowan Formation are type II2-III organic matter with fair–good quality, and the source rocks of the Xishanyao Formation are type II2 organic matter with fair quality. All source rocks are in the mature stage. The results of the biomarker compounds show that the lacustrine mudstone of the Xishanyao Formation and the coal-measure mudstone of the Badaowan Formation were deposited in reducing environments. The former was mainly from lower aquatic organisms, and the latter was from terrestrial higher plants. The mudstone of the Pingdiquan Formation was formed in a weakly oxidizing–weakly reducing depositional environment, and its parent material was of mixed origin. Based on the results of the biomarker compounds and carbon isotopes, the crude oils were divided into three categories. The Family I crude oil has the characteristics of low maturity, low salinity, and more input of low-level aquatic organisms, and the carbon isotope has a good affinity with the lacustrine mudstone of the Xishanyao Formation. The Family II crude oil shows medium maturity, low salinity, mainly higher plant input, and heavy carbon isotope, mainly derived from the Badaowan Formation coal-measure mudstone. The Family III crude oil is characterized by high maturity, high salinity, mixed parent materials, and light carbon isotope and originates from the mudstone of the Pingdiquan Formation. The results provide a reference for oil and gas exploration and development in the eastern area of the Junggar Basin; the future research will focus on well areas with high maturity near the Fukang fault zone.

Isotopic Constraints on the Nature of Primary Precipitates in Archean–Early Paleoproterozoic Iron Formations from Determinations of the Iron Phonon Density of States of Greenalite and 2L- and 6L-Ferrihydrite

Iron formations (IFs) are chemical sedimentary rocks that were widely deposited before the Great Oxidation Event (GOE) around 2.4–2.2 Ga. It is generally thought that IFs precipitated as hydrated Fe3+ oxides (HFOs) such as ferrihydrite following surface oxidation of Fe2+-rich, anoxic deep waters. This model often implicates biological oxidation and underpins reconstructions of marine nutrient concentrations. However, nanoscale petrography indicates that an Fe2+ silicate, greenalite, is a common primary mineral in well-preserved IFs, motivating an alternative depositional model of anoxic ferrous silicate precipitation. It is unclear, however, if Fe2+-rich silicates can produce the Fe isotopic variations in IFs that are well explained by Fe2+ oxidation. To address this question, we constrain the equilibrium Fe isotopic (56Fe/54Fe) fractionation of greenalite and ferrihydrite by determining the iron phonon densities of states for those minerals. We use ab initio density functional theory (DFT + U) calculations and nuclear resonant inelastic X-ray scattering spectroscopy to show that ferrous greenalite should be isotopically lighter than ferrihydrite by ∼1–1.2‰ at equilibrium, and fractionation should scale linearly with increasing Fe3+ content in greenalite. By anchoring ferrihydrite–greenalite mineral pair fractionations to published experimental Fe isotopic fractionations between HFOs and aqueous Fe2+, we show that ferrous greenalite may produce all but the heaviest pre-GOE Fe isotopic compositions and mixed valence greenalites can produce the entire record. Our results suggest that heavy Fe isotope enrichments alone are not diagnostic of primary IF mineralogies, and ferrihydrite and partially oxidized or even purely ferrous greenalite are all viable primary IF mineralogies.

Thrive or survive: prokaryotic life in hypersaline soils

BackgroundSoil services are central to life on the planet, with microorganisms as their main drivers. Thus, the evaluation of soil quality requires an understanding of the principles and factors governing microbial dynamics within it. High salt content is a constraint for life affecting more than 900 million hectares of land, a number predicted to rise at an alarming rate due to changing climate. Nevertheless, little is known about how microbial life unfolds in these habitats. In this study, DNA stable-isotope probing (DNA-SIP) with 18O-water was used to determine for the first time the taxa able to grow in hypersaline soil samples (ECe = 97.02 dS/m). We further evaluated the role of light on prokaryotes growth in this habitat.ResultsWe detected growth of both archaea and bacteria, with taxon-specific growth patterns providing insights into the drivers of success in saline soils. Phylotypes related to extreme halophiles, including haloarchaea and Salinibacter, which share an energetically efficient mechanism for salt adaptation (salt-in strategy), dominated the active community. Bacteria related to moderately halophilic and halotolerant taxa, such as Staphylococcus, Aliifodinibius, Bradymonadales or Chitinophagales also grew during the incubations, but they incorporated less heavy isotope. Light did not stimulate prokaryotic photosynthesis but instead restricted the growth of most bacteria and reduced the diversity of archaea that grew.ConclusionsThe results of this study suggest that life in saline soils is energetically expensive and that soil heterogeneity and traits such as exopolysaccharide production or predation may support growth in hypersaline soils. The contribution of phototrophy to supporting the heterotrophic community in saline soils remains unclear. This study paves the way toward a more comprehensive understanding of the functioning of these environments, which is fundamental to their management. Furthermore, it illustrates the potential of further research in saline soils to deepen our understanding of the effect of salinity on microbial communities.

Granite petrogenesis and the δ44Ca of continental crust

Stable Ca isotopes are an increasingly useful tool for understanding the sources and processes leading to the formation of magmatic rocks, yet Ca isotope fractionation during genesis of silicic continental crust is still poorly understood. Here, we present Ca, Sr, and Nd isotope, as well as major- and trace-element whole-rock geochemical data for A-, I-, and S-type granites (n = 30) from Australia/Tasmania, Canada, and France (δ44CaBSE of -0.6‰ to +0.2‰) and compare them to phase-equilibrium models for partial-melting (pelite, greywacke, MORB, enriched Archean tholeiite) and crystallization (hydrous arc basalt, A-type granite) that incorporate novel ab-initio predictions for Ca isotope fractionation in epidote and K-feldspar. The ab-initio calculations predict that epidote has similar δ44Ca to anorthite and that K-feldspar is the isotopically lightest known silicate mineral at equilibrium (Δ44Cakspar-melt of -0.4‰ at 1000 K). Our phase-equilibrium model results suggest that δ44Ca variations in all three granite types can be fully explained through magmatic processes, without necessarily requiring addition of isotopically exotic components (e.g., carbonate sediments). Heavy Ca isotope enrichments in A-type granites from the Lachlan Fold Belt, however, require isotopic disequilibrium between plagioclase and melt, which we use to constrain average plagioclase growth rates in these systems. This also serves to illustrate that whole-rock Ca isotope measurements can be used to estimate crystal growth rates, even in the absence of analyzable phenocrysts. In general, low Ca diffusivities and strong isotopic diffusivity ratios (D44/D40) in low-H2O granitic magmas should lead to resolvable isotopic disequilibrium effects in plagioclase, even at relatively slow growth rates (e.g., > 0.03 cm/yr). Combining our data with those from previous studies, we demonstrate that average granitoids and upper continental crust (with newly estimated δ44CaBSE of -0.25 ± 0.02‰, 2SE) have resolvable low δ44Ca compared to basalts and oceanic crust. Given that pressure has a major influence on Ca isotope fractionation across all of our models, this implies that melts feeding upper crustal granitoids dominantly evolve in the lower crust (10-14 kbar, through either partial-melting or fractional crystallization). This observation also suggests that heavier Ca isotopes are preferentially recycled back into the mantle through subduction and/or lower-crustal delamination events, but this is unlikely to have had a significant influence on the δ44Ca evolution of the upper mantle through geologic time.