Average δ18O Values Research Articles

Diagenesis and the Conditions of Deposition of the Middle Jurassic Siderite Rocks from the Northern Margin of the Holy Cross Mountains (Poland)

The aim of the present study is to reconstruct sedimentary conditions of Middle Jurassic rocks that contain siderites to identify the mineral composition of the inserbeds and to recognize the origin of the siderite. Thin inserbeds of siderite rocks occur most frequently within Bajocian siliciclastic deposits and, more rarely, Aalenian and Bathonian. The research material comes from 11 boreholes located in the north and northeastern margins of the Holy Cross Mountains. The research methods included sedimentological analyses, and studies in polarizing and scanning electron microscopes, staining of carbonates, cathodoluminescence, X-ray structural analysis, and stable carbon and oxygen isotopic determinations were used. Middle Jurassic sideritic rocks are most often represented by clayey siderites, which also include muddy and sandy varieties and siderite sandstones. There are also local occurrences of coquinas, claystones, mudstones, and siderite conglomerates. The main component of sideritic rocks is sideroplesite. Berthierine, pistomesite, calcite, and ankerite are important components, too. The action of diagenetic processes of cementation, compaction, replacement, and alteration within the Middle Jurassic deposits was most intense during the eo- and mesodiagenesis. The sedimentological analysis showed that most of the studied siderites were formed in a low-oxygenated marine environment, mainly in the transition zone between the normal and storm wave bases and in the lower and middle shoreface zones. The results of the petrographic, mineralogical, and geochemical studies indicated the origin of the sideritic rocks mainly in the marine environment, with the participation of meteoric water. There were slight differences in the chemical composition of sideroplesite depending on the environment it crystallized in. There was no correlation between the values of the carbon isotope determinations in the sideroplesite and the environmental conditions of its crystallization. Slight differences were visible in the case of the average values of δ18O in the sideroplesite.

Spatial distributions of oxygen and hydrogen isotopes in multi-level groundwater across South Korea: A case study of mountainous regions

This study presents the spatial distributions of stable isotopes for groundwater according to well depth and spring water across South Korea, using an interpolation model to provide baseline information for hydrological studies. In total, 888 groundwater and 108 spring water samples were collected across South Korea; their oxygen and hydrogen isotopic compositions (δ18O and δ2H) were analyzed. δ18O and δ2H values biased toward the summer local meteoric water line and low d-excess values indicate that summer precipitation is important for groundwater recharge. The δ18O and δ2H values for groundwater and spring water decrease progressively from the southwest to the northeast on the Korean Peninsula. Based on eight hydrological regions, the average δ18O values of groundwater and spring water are negatively correlated with latitude, but they are positively correlated with temperature. This result indicates that the spatial distributions of groundwater isotopic values in South Korea are significantly influenced by latitude and altitude effects associated with the movement of the North Pacific air mass in summer. Spring waters showed a negative correlation between δ18O and d-excess, with more depleted 18O values than groundwater, indicating that local recharge and flow within mountainous areas is dominant. Considering that the correlation in multi-level groundwater located in northern regions is similar to that of spring water, the contribution of regional groundwater flow, which is recharged in mountainous areas, is considered to be higher in the northern regions. The spatial distribution of δ18O in groundwater gradually approached the spatial distribution of spring water with increasing well depth, indicating that the contribution of regional groundwater flow may be greater in deep groundwater. Our results provide estimates for data-poor regions, supporting the investigation of links between groundwater and other hydrological factors.

Zircon Hf[sbnd]O isotope and magma oxidation state evidence for the origin of Early Cretaceous granitoids and porphyry Mo mineralization in the Tongbai-Hong'an-Dabie orogens, Eastern China

The Tongbai-Hong'an-Dabie orogens constitute the east segment of the Qinling-Dabie orogenic belt, which host several large molybdenum deposits, with a proven reserve of over 8 million tons metal Mo. In this study, we present an integrated study of in situ zircon UPb dating and HfO isotopes, in combination with magma oxidation state estimation for the Early Cretaceous granitoids in the Tongbai-Hong'an-Dabie regions. These granitoids can be divided into three groups on the basis of zircon UPb ages, including early-stage ore-related granitoids from Mushan, Xiaofan and Wangwan deposits (143–136 Ma), ore-barren granites (136–130 Ma), and late-stage ore-related granitoids from Qian'echong, Bao'anzhai, Tangjiaping and Shapinggou deposits (130–110 Ma). Early-stage ore-barren granitoids have the lowest average εHf(t) and δ18O values of −27.1 ± 1.6 and 4.25 ± 0.63‰ as well as the oldest two-stage Hf model ages (TDM2) of ~2.8 Ga, which are roughly comparable with the inferred Dabie lower crust, indicating that they were mainly derived from the partial melting of the lower crust. Ore-forming granitoids in two stages show similar two-stage Hf model ages from 2.2 to 2.3 Ga which differ from any inferred layer of crusts, while they exhibit different zircon HfO isotope and oxygen fugacity evolutionary trends. Granitoids display decreasing average εHf(t) and δ18O values from −17.1 ± 1.9 to −27.1 ± 1.6, and 5.71 ± 0.50‰ to 4.25 ± 0.63‰, and increasing average logƒO2 values from −17.4 ± 4.5 to −6.7 ± 3.8 with a decrease in age from 143 to 130 Ma. In addition, relict zircon cores with Paleoproterozic and Archean ages were observed with εHf(t) and δ18O values analogous to those of the Kongling Complex. These characteristics suggest that the early-stage granitoids were possibly produced by the remelting of the subducted lower crust mixed with the Kongling Complex, with the involvement of reducing Mo-rich black shales in the Yangtze Block that seemingly account for the lower oxygen fugacity of ore-forming granitoids with younger two-stage Hf model ages and lead to the formation of those early-stage Mo deposits. As the increasing contributions of the lower crust in the magma source, the metallogenic gap developed from 136 to 130 Ma in the Tongbai-Hong'an-Dabie orogens. Zircon HfO isotopic compositions of ore-related granitoids increase simultaneously up to −14.2 ± 4.3 and 5.82 ± 0.33‰ respectively at a lower slope after 130 Ma. Part of zircon δ18O values is slightly higher than 6.5‰, implying the involvement of supercrustal materials. Meanwhile, magma oxygen fugacity exhibits an abrupt drop at ca. 130 Ma in the Qian'echong deposit with the lowest average logƒO2 value (−19.3 ± 4.5) and Ce4+/Ce3+ ratios (1–138), then rises quickly up to a maximum average logƒO2 value of −7.3 ± 2.3. As such, these granitoids were likely generated by mixing of a blended crustal magma, derived from the lower crust and supercrustal materials, and the mantle-derived component which also contributes to the late-stage Mo mineralization. We propose that not only the highly oxidized magma but also the continuous change of oxygen fugacity in the deep magma chamber may have positive effects on the formation of porphyry Mo deposits. Given the significant geochemical change at ca. 130 Ma instead of 125 Ma, we agree that the intensive magmatism and mineralization in the Tongbai-Hong'an-Dabie orogens are closely associated with the delamination of the thickened lower crust of the Yangtze Block, which caused the asthenosphere upwelling that provided the heat source and led to partial melting of overlaying lower crust.

The isotope record (δ13C, δ18O) of vertical mobility in incremental tissues (tooth enamel, hair) of modern livestock: A reference set from the Mongolian Altai

Identifying the isotopic signatures of vertical mobility and alpine meadows exploitation in the teeth of domesticated animals can be a key to understanding the subsistence strategies used by pastoral communities through history. Indeed, the oxygen (δ18O) and carbon (δ13C) isotopic composition of sequentially sampled tooth enamel, used alone or in combination have been commonly employed to investigate altitudinal mobility and alpine exploitation. However, conflicting interpretations of the outcomes of these analyses exist, due in part to the paucity of modern reference datasets. In this study, we propose a new reference set, composed of GPS monitored sheep, goats and horses from the Mongolian Altai, to investigate the influence of vertical mobility and mean grazing altitude, on the C and O isotopic compositions of sequentially sampled tooth enamel and on the C isotopic composition of horse tail hair. We found that δ13C values were negatively correlated with the mean grazing altitude and time of residency of alpine meadows. Although no correlation was found between the average δ18O values of tooth enamel and mean grazing altitude, vertically mobile livestock had a higher intra-tooth range in δ18O values than vertically immobile livestock, possibly reflecting the ingestion of isotopically more diverse sources of water. Moreover, the coefficient of the correlation between δ13C and δ18O values of tooth enamel was - although weakly - negatively correlated with the standard deviation of the animal mean grazing altitude (i.e. reflecting the frequency of altitudinal mobility). These results confirm that δ13C and δ18O analyses of tooth enamel and tail hair can be used to infer animal mobility and land use in modern and ancient times.

Hydrochemical and Isotopic Characteristics in the Shallow Groundwater of the Fenhe River Basin and Indicative Significance

The Fenhe River basin is the second largest tributary of the Yellow River. Piper diagrams, Gibbs, PCA, correlation analysis and forward derivation modeling were used to analyze the distribution characteristics and the controlling factors of the groundwater chemistry and stable isotopes in the Fenhe River basin, which revealed the water cycle and water quality evolution process. The results indicated that the groundwater is a weakly alkaline, micro-hard water, the dominant anions and cations are HCO3- and Ca2+, the major groundwater types are Mg-Ca-HCO3 and Mg-Ca-Cl-SO4, the groundwater quality is good, and more than 94% of the samples belong to classes Ⅰ-Ⅲ. The average values of δD and δ18O of the Fenhe River groundwater are -70.2‰ and -9.6‰, which are similar to the isotope values of the precipitation from July to September, indicating that the groundwater may have originated from this period and that the groundwater recharge mode (dominant flow and piston flow) has a spatial variation. Rock weathering is the dominant source of ions in the groundwater, with an average contribution of 87%, while the contributions of atmospheric input and human activity are 8% and 5%, respectively. For rock weathering, silicate, evaporate, and carbonate rock contribute equally to the groundwater solutes, accounting for 32%, 28%, and 26%, respectively. The results of this study provide the basis for promoting the sustainable development and utilization of groundwater resources in the Fenhe River basin.

Understanding Tropical Convection Through Triple Oxygen Isotopes of Precipitation From the Maritime Continent

AbstractMonthly precipitation samples from Singapore were collected between 2013 and 2019 for stable isotope analysis to further our understanding of the drivers of tropical precipitation isotopes, in particular, 17O‐excess. δ18O ranges from –11.34‰ to –2.34‰, with a low correlation to rainfall (r = –0.31, p = 0.014), suggesting a weak amount effect. d‐excess is relatively consistent and has an average value of 10.89‰ ± 3.45‰. Compared to high‐latitude regions, 17O‐excess in our samples generally falls in a narrower range from 2 to 47 per meg with an average of 21 ± 11 per meg. Moreover, 17O‐excess shows strong periodic variability; spectral analysis reveals 3‐month, 6‐month, and 2.7‐year periodicities, likely corresponding to intraseasonal oscillations, monsoons, and the El Niño–Southern Oscillation (ENSO), respectively. In contrast, d‐excess shows no clear periodicities. Although spectral analysis only identifies 6‐month periodicity in the δ18O time series, δ18O tracks the Nino3.4 sea surface temperature variability; the average δ18O value (–5.2‰) is higher during El Niño years than ENSO neutral years (–7.6‰). Therefore, regional convection associated with monsoons and ENSO has different impacts on δ18O, d‐excess, and 17O‐excess. 17O‐excess and d‐excess are anticorrelated and do not relate to the relative humidity in moisture source regions. Extremely low humidity and drought conditions in moisture source regions would be required to account for high 17O‐excess. Processes during transport and precipitation likely modify these two parameters, especially 17O‐excess, which no longer record humidity conditions of moisture source regions. Our findings will be useful for further modeling studies to identify physical processes during convection that alter d‐excess and 17O‐excess.

Oceanographic transport along frontal zones forms carbon, nitrogen, and oxygen isoscapes on the east coast of New Zealand: Implications for ecological studies

Characterizing the spatial distribution of δ18O in seawater and δ15N and δ13C in marine particulate organic matter pools can be useful in identifying isotopic discriminators of major water masses and serve as a baseline for food web and other ecological studies. Nevertheless, we do not know how isoscapes of δ18O, δ15N and δ13C are distributed and interact with mesoscale (10–100 km) physical oceanographic features for large parts of the global ocean. Here samples and measurements were collected during research cruises in the austral summer of 2017/18 and 2018/19 along the east coast of the South Island of New Zealand. The physical structure of the water column, δ18O of seawater, and δ15N and δ13C of suspended particulate organic matter were analyzed. Interannual differences in inferred circulation, oceanographic features, and the interaction of Subtropical and Subantarctic water masses in the study area were described. The relationships between nutrient concentrations, physical oceanographic variables and δ18O, δ15N and δ13C isoscapes were investigated by running linear models with all combination of variables, then averaging the most parsimonious ones that best fit the data. Results demonstrate that different environments along the east coast are connected via northward transport of Subtropical Water near the surface and Subantarctic Mode Water at depth (220–500 m). The northward transport of modified STW was associated with nutrient inputs and deepening of the maximum fluorescence depth. The spatial distribution of isotopic values had minimal temporal variability and was primarily linked to patterns of nutrient input (δ15N), coastal productivity (δ13C) and water mass type (δ18O). δ15N values demonstrated significant spatial variation, while δ18O displayed distinct values among depths. The present study was the first to resolve the distribution of δ15N and δ13C isotopes at a regional scale along the east coast of the South Island of New Zealand, δ18O in New Zealand waters, and to investigate the extent to which these distributions were influenced by patterns in physical oceanography. Average values of δ18O, δ15N and δ13C for different fishery management areas and depths are presented to support future ecological studies along Southeast New Zealand.

Significance of active speleothem δ18O at annual-decadal timescale——A case study from monitoring in Furong Cave

Stalagmites are the product of the combined effects of the surface environment and the cave environment, and their stable carbon and oxygen isotope ratios (δ18O and δ13C) have been widely used as proxies for palaeoclimatic and palaeoenvironmental reconstructions. Modern cave monitoring is essential for the accurate interpretation of multiple-proxies in stalagmites. However, the interpretation of the climatic and environmental signals indicated by the carbon and oxygen stable isotopes in stalagmites remains controversial. Based on the ongoing monitoring work of Furong Cave in Southwest China over 12 years (2008-2019 AD) involving a combination of field monitoring and experimental data, the following conclusions were drawn. (1) On the interannual scale, the isotopic fractionation of oxygen in active speleothems (AS) exhibits equilibrium fractionation. On the seasonal scale, the crystallization of a mixture of calcite and aragonite minerals is one of the reasons for the deviation of oxygen isotopic composition from the value expected under equilibrium fractionation. (2) There was a significantly positive correlation between δ18O and δ13C values in the AS, which may have been controlled by climate change rather than dynamic fractionation. (3) Mainly because of the mixing effect in the 300–500 m of bedrock overlying Furong Cave, the δ18O value of the drip water reflected the annual average δ18O value of precipitation. (4) The AS δ18O (δ18OAs) values over the 12-year monitoring records did not show significant seasonal variations but exhibited multiyear trend, which might reflect changes in the external atmospheric circulation and “amount effect” on decadal and longer timescales. Consequently, speleothem δ18O values in Furong Cave can be used for isotope-based palaeoclimate reconstruction.

Monitoring of stable isotopes (δ2H, δ18O) in precipitations of Moscow city (Russia): comparison for 2005–2014 and 1970–1979 periods

The isotopic composition of oxygen (δ18O) and hydrogen (δ2H) of atmospheric precipitation in Moscow in 2005–2014 was studied by sampling single precipitations (a total of 842 measurements after rejection of unreliable samples). A comparison is made with similar studies carried out by VSEGINGEO and IVP RAS for the IAEA-WMO GNIP network in 1969–1979, when monthly average samples were taken by the accumulation method (61 pair determinations of deuterium and oxygen-18, mainly in 1975–1979). The 2005–2015 series is reduced to a similar form for the 1969–1979 series, recalculated through the data on the volume of precipitation at the nearest meteorological station. It was found that in the last decade there has been a significant change in the equation of the local line of meteoric waters, which for the first period had the form δ2H = 6.09×δ18O – 23.0 ‰ (R2 = 0.87), and is currently described by the relation δ2H = 6.93×δ18O – 11.3 ‰ (R2 = 0.944). There is also a decrease in depletion of the average values of δ2H and δ18O, compared with the first observation period, which apparently reflects the course of climatic changes. At the same time, within each of the periods, a negative (albeit relatively small) slope of the line of approximation of chronological data is noted. Deuterium excess naturally changes seasonally, taking negative values in summer, primarily due to non-equilibrium fractionation during evaporation. In 2005–2014, the temperature dependence of the isotopic composition of precipitation changed significantly in comparison with 1969–1979, which makes it impossible to reconstruct the composition of precipitation in the past from meteorological observations.

Using Stable Isotopes to Determine the Water Balance of Utah Lake (Utah, USA)

To investigate the hydrology of Utah Lake, we analyzed the hydrogen (δ2H) and oxygen (δ18O) stable isotope composition of water samples collected from the various components of its system. The average δ2H and δ18O values of the inlets are similar to the average values of groundwater, which in turn has a composition that is similar to winter precipitation. This suggests that snowmelt-fed groundwater is the main source of Utah Valley river waters. In addition, samples from the inlets plot close to the local meteoric water line, suggesting that no significant evaporation is occurring in these rivers. In contrast, the lake and its outlet have higher average δ-values than the inlets and plot along evaporation lines, suggesting the occurrence of significant evaporation. Isotope data also indicate that the lake is poorly mixed horizontally, but well mixed vertically. Calculations based on mass balance equations provide estimates for the percentage of input water lost by evaporation (~47%), for the residence time of water in the lake (~0.5 years), and for the volume of groundwater inflow (~700 million m3) during the period April to November. The short water residence time and the high percentage of total inflow coming from groundwater might suggest that the lake is more susceptible to groundwater pollution than to surface water pollution.

The formation of TTGs by hydrous partial melting and anatexis from a gabbroic mantle source in Viti Levu, Fiji Islands

Tonalite, trondhjemite, granodiorite (TTG) rocks in Viti Levu, Fiji Islands formed through hydrous partial melting of gabbroic oceanic crust at low-pressure amphibolite-facies conditions caused by flat subduction of an oceanic plateau from Yavuna creek during late-Eocene time. This was followed by the formation of a quartz diorite unit by anatexis of the gabbro in the mid-Miocene. The TTGs occur as migmatitic discordant stocks and dykes within the older gabbroic unit of the Yavuna Pluton. Zircon ages show the parental gabbro to be ∼ 47.5 Ma in age, whereas the TTGs, consisting of a tonalite and a quartz diorite suite, are ∼ 37.1 Ma and ∼ 16.5 Ma old, respectively. The average δ18O value of 4.8‰ in zircons from the gabbro and the tonalite suggest a homogenous mantle source. However, about 50% of the analyzed zircons from the gabbroic and tonalitic rock samples show lower δ18O values than this average, and are interpreted as reflecting interaction of hydrothermally altered seafloor with the deep mantle source. The zircon εHf of 13 in the analyzed TTGs is interpreted as reflecting typical juvenile continental crust. PerpleX whole-rock calculations supports that the tonalite formed by hydrous partial melting of the gabbro through decompression under water-saturated amphibolite-facies conditions at a temperature of ∼ 770 °C at ∼ 3.8 kbar, whereas the quartz diorite formed by anatexis under granulite-facies conditions at ∼ 800 °C at ∼ 1.7 kbar at very shallow depth. Our investigation provides new evidence that the tonalite formed by hydrous partial melting and the quartz diorite by metasomatism and anatexis of an extant gabbroic rock/crust at < 0.1 Ga in the South Pacific region.

Stratospheric Incursion as a Source of Enhancement of the Isotopic Ratios of Atmospheric N2O at Western Pacific

AbstractWe present analyses of nitrogen and oxygen isotopic ratios and site‐preference (SP) values of atmospheric nitrous oxide samples obtained from Taiwan in west Pacific. Air samples were collected during September 2013 to January 2016 from Taipei and during November 2011 to December 2014 from Keelung, located in the northeastern coast of Taiwan. The average values of δ15N, δ18O, and SP are 6.4 ± 0.3 ‰, 44.0 ± 0.4 ‰, and 17.1 ± 1.0 ‰, respectively, at Taipei and 6.4 ± 0.2 ‰, 44.6 ± 0.4 ‰, and 17.9 ± 1.3 ‰, for δ15N, δ18O, and SP respectively, at Keelung, comparable to the average values (6.6 ± 0.2, 44.2 ± 0.2, and 18.1 ± 0.5 ‰, scaled to year 2014) from Hateruma, an open ocean station in western Pacific. Strong temporal variations in the delta values, however, are seen. During a subtropical jet‐strengthening period in October to mid‐December 2014, the δ15N, δ18O and SP values increased at the rate of 0.006 ± 0.002 ‰/day, 0.010 ± 0.002 ‰/day and 0.008 ± 0.002 ‰/day, respectively at Taipei and 0.013 ± 0.005 ‰/day, 0.011 ± 0.005 ‰/day and 0.011 ± 0.027 ‰/day at Keelung. This suggests intrusion of air to the lower troposphere from the stratosphere where δ15N, δ18O and SP values are higher. This is consistent with earlier observations of oxygen isotope anomaly in atmospheric CO2 and elevated 35S in sulfate aerosols, indicating frequent stratospheric intrusions in this region. The observed δ15N and δ18O values, along with the oxygen isotope anomaly in CO2 and multiple sulfur isotopes in sulfate, can be used to constrain local emissions from the surface, improving our knowledge of anthropogenic contribution to the global budget of N2O.

Warmer–wetter climate drives shift in δD–δ18O composition of precipitation across the Queen Elizabeth Islands, Arctic Canada

We examine how recent increases in air temperature and precipitation, together with reductions in sea ice extent, may have affected the regional δD–δ18O composition of precipitation. In spring 2014, 80 snow samples were collected from six glaciers and ice caps across the Queen Elizabeth Islands, and in 2009 and 2014, two shallow ice cores were collected from Agassiz Ice Cap and White Glacier, respectively. The snow samples showed average δ18O values from 2013 to 2014 to be approximately 2‰–3‰ higher than those recorded in 1973–1974 in nearby locations, with the ice cores showing similar trends in δ18O values. A zonal average water isotope model was used to help understand the causes of the increased δ18O values, using inputs calibrated for observed changes in temperature, vapour flux, and sea ice extent. Model results indicate that atmospheric temperature changes account for &lt;1‰ of the observed change in δ18O values, and that changes in local water input and precipitation driven by changes in sea ice extent only have an effect in coastal regions. Enhanced meridional vapour flux to the Queen Elizabeth Islands is, therefore, also required to explain the observed increases in δ18O values, with fluxes ∼7% higher today than in the 1970s, consistent with the change in precipitation.

Geochronology, Geochemistry, Whole Rock Sr‐Nd and Zircon Hf‐O Isotopes of the Early Neoproterozoic Volcanic Rocks in Jiangshan, Eastern Part of the Jiangnan Orogen: Constraints on Petrogenesis and Tectonic Implications

AbstractThis paper presents the results of combined studies of geochronology, geochemistry, whole rock Sr‐Nd and zircon Hf‐O isotopes carried out upon the rhyodacite and ignimbrite of Shangshu village, Shangyu town and Shanghupeng village of Jiangshan City in Zhejiang Province, along the northwestern side of the Jiangshan–Shaoxing suture. SHRIMP zircon U‐Pb dating of samples in the three areas yielded weighted mean 206Pb/238U ages of 842.8 ± 6.9 Ma and 850.0 ± 7.3 Ma, 839 ± 9 Ma and 832.2 ± 8.1 Ma, 828.3 ± 8.5 Ma and 836.9 ± 9.9 Ma, respectively. These ages are older than the volcanic rocks of the Shangshu Formation dated at around 780 Ma distributed in Fuyang City, Hangzhou City, Kaihua County, etc. The volcanic rocks generally have high SiO2 (54.08–76.80 wt%) and Al2O3 (12.40–21.31 wt%), low Fe2O3 (0.68–8.92 wt%), MgO (0.29–2.49 wt%), CaO (0.12–2.86 wt%), TiO2 (0.10–1.59 wt%) and P2O5 (0.01–0.39 wt%), with variable total alkalis (K2O + Na2O =5.42–8.29 wt%). There exists a clear negative correlation between SiO2 and P2O5. The volcanic rocks have A/CNK ratios of 1.03–2.77 and thus are peraluminous. They are characterized by enrichment in LREE, Rb, Ba, Zr, Hf, K, Th, La, U and depletion in Nb, Sr, P, Ti, with distinct LREE and HREE fractionation of (La/Yb)N values of 5.68–11.67, and with a moderate negative Eu anomaly (&amp;Eu=0.58–0.89). Whole‐rock geochemical data shows that the Jiangshan volcanic rocks are possibly I‐type granitic rocks, even though they have some characteristics of A&amp;S‐type granites due to the magma fractional crystallization and water‐rock interaction. Zircon δ18O values are 3.97‰–5.49‰ (average 4.50‰), 2.90‰–5.21‰ (average 4.32‰) for ignimbrite from Shangshu village section, and Shanghupeng village section, respectively. They are slightly lower than the average δ18O values of igneous zircons in equilibrium with mantle magmas (5.3 ± 0.6‰ (2σ)), the lower δ18O value also demonstrating the presence of high temperature water‐rock interactions. The ignimbrite rocks have positive ∊Nd(t) (4.02, 3.37, 3.91, 4.74, 2.85, 4.39, totals from the three areas) and ∊Nd(t) (in‐situ zircon) (4.3–14.6, a weighted mean of 8.4; 6.6–12.7, a weighted mean of 9.0; 8.1–12.0, a weighted mean of 9.5, respectively, from the three areas). In conjunction with the trace element studies, they indicate that the source region of the Jiangshan volcanic rocks was mainly composed of juvenile lower crustal material, mixed with some mantle‐sourced magma. Detailed elemental and isotopic data suggest that the Jiangshan volcanic rocks were formed in a continental arc setting. There is a series of ca. 860–830 Ma volcanic rocks formed in a back‐arc extensional setting in the southern margin of the eastern Jiangnan Orogen, along the northwest side of the Jiangshan–Shaoxing suture. The first stage rift‐related anorogenic magmatism may have occurred as early as ca. 860 Ma in the eastern Jiangnan Orogen.

The Origin of Plagiogranites: Coupled SIMS O Isotope Ratios, U–Pb Dating and Trace Element Composition of Zircon from the Troodos Ophiolite, Cyprus

AbstractU–Pb ages, trace element content and oxygen isotope ratios of single zircons from five plagiogranite intrusions of the Troodos ophiolite were measured to determine their crystallization age and assess the importance of fractional crystallization versus crustal anatexis in their petrogenesis. The results indicate that oceanic magmatism in Troodos took place at 94·3 ± 0·5 Ma, about 3 Myr earlier than previously recognized. Later hydrothermal alteration has affected most of the Troodos plagiogranitic rocks, resulting in growth of new zircon and/or partial alteration of zircon domains, causing slightly younger apparent crystallization ages. The new age inferred for seafloor spreading and ocean crust accretion in Troodos nearly overlaps that of the Semail ophiolite in Oman (95–96 Ma), strengthening previous indications for simultaneous evolution of both ophiolites in similar tectonic settings. Average δ18O(Zrn) values in the Troodos plagiogranites range between 4·2 and 4·8 ‰. The lower values in this range are lower than those expected in equilibrium with mantle-derived melt (5·3 ± 0·6 ‰), indicating variable contribution from hydrothermally altered, deep-seated oceanic crust in most of the Troodos plagiogranite intrusions. The inferred substantial involvement of crustal component is consistent with the existence of a shallow axial magma chamber, typical of fast-spreading mid-ocean ridge settings, within the Troodos slow-spreading ridge environment. This apparent contradiction may be reconciled by episodically intense magmatism within an otherwise slow, magmatically deprived spreading axis.

Kinetic isotope effects during CO2 hydration: Experimental results for carbon and oxygen fractionation

Kinetic isotope effects (KIEs) during the hydration of carbon dioxide (CO2) in aqueous solution are key to understanding various geochemical, physicochemical, and biological processes. Despite the growing interest in KIEs in natural systems (e.g., speleothem calcites, skeletal formation of corals, formation of cryogenic carbonates, and more), the currently available experimental data is limited to only a few studies. Here, we report results from laboratory experiments to constrain kinetic isotope fractionation (KIF) during the hydration of CO2. The experimental setup consisted of a stock solution vessel containing a NaHCO3 solution, a reaction chamber in which BaCO3 was rapidly precipitated from a Tris-buffered BaCl2 solution, and a pump to internally circulate gas throughout the system that was isolated from the ambient laboratory air. Resultant BaCO3 samples, as well as the NaHCO3 and deionized H2O used for our experiments were analyzed for stable carbon and oxygen isotopes by mass spectrometry. Our experimental results were grouped into four different batches, which were obtained during discrete time periods, while some adjustments to the experimental setup and protocols were implemented to improve the data quality. We discuss possible experimental errors among the four batches and conclude that Batch-4 provided the most reliable results. The average δ13C and δ18O values of the Batch-4 BaCO3 samples produced at 25 °C and pHNBS = 8.0 are −29.7 ± 0.7‰ (VPDB) and 18.8 ± 0.6‰ (VSMOW), respectively, which is strongly depleted relative to gaseous CO2 in the system (δ13CVPDB ≅ −10.7‰, δ18OVSMOW ≅ 37.9‰) . From the isotope data, we calculate our experimental 13KIF and 18KIF, which refer to the 13C/12C and 18O/16O fractionation between CO2(g) and BaCO3, where the δ13C and δ18O values of CO2(g) were calculated using known equilibrium fractionation factors. From the mean of Batch-4 results, we conclude 13KIF = 19.6 ± 0.8‰ and 18KIF = 18.8 ± 0.6‰ at 25 °C and pHNBS = 8.0. If full carbon KIF of ∼ 2.0‰ is assumed between CO2(g) and CO2(aq), then the mean 13KIF relative to CO2(aq) is 17.6 ± 0.8‰. Our KIFs are the largest values compared to previously reported experimental KIFs (except for one study), suggesting that our values are closest to the full isotope disequilibrium during CO2 hydration, as any partial re-equilibration tends to reduce observed KIFs.

Precipitation chemistry and stable isotopic characteristics at Wengguo in the northern slopes of the Himalayas

The major ions in precipitation can reflect the conditions of the atmosphere, while stable isotopic characteristics provide information on the moisture source. In order to understand the local hydro-chemical features and regional geochemical cycle, it is essential to assess the chemical composition of precipitation and the associated sources. Therefore, a total of 57 precipitation samples (2016 to 2017) for major ions and 178 samples (2013 to 2017) for stable isotopes were collected from the Wengguo station and analyzed to explore the major ionic deposition and stable isotopic characteristics in the northern slopes of the Himalayas. The average pH and electrical conductivity were 6.82 ± 0.45 and 15.36 ± 11.67 μS cm−1, respectively. Ca2+ followed by K+ and Mg2+ played a crucial role in neutralizing the precipitation acidity. The major ionic sources in the region were terrigenous (Ca2+, HCO3−, and Mg2+) and sea salt (Na+, Cl−, and Mg2+), as well as anthropogenic emissions (SO42− and NO3−) and biomass burning (K+). The total deposition flux of the major ions was higher in 2016 than in 2017 and was influenced by the higher precipitation. The average values of δ18O and δD in precipitation were − 15.22 ± 5.17 ‰ and − 116.01 ± 41.31 ‰, respectively. The precipitation stable isotopes were not significantly correlated to the local air temperature but the precipitation amount. Moreover, the variation in stable isotopes, local meteoric water line, and d-excess indicated the existence of continental and monsoon moisture transport systems. The transport of chemicals over the high elevation region from polluted cities in South Asia via moisture originating in the Bay of Bengal and the Arabian Sea was determined based on the source identification, clusters of air mass backward trajectory analysis, and the National Center for Environmental Prediction Final dataset. Thus, the ionic concentrations and stable isotopic characteristics of the precipitation from this study provided a valuable dataset to assess the atmospheric environment in the northern slopes of the Himalayas at Southern Tibetan Plateau.

Refining the timing and mechanism of the Triassic partial melting in the Sulu UHP orogen, China: Zircon and garnet evidence from a felsic vein and its host granitic gneiss

Partial melting of high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic rocks in subduction zones has great influence on both the chemical and physical properties of deeply subducted continental crust. Felsic veins in HP-UHP metamorphic rocks are products of partial melting and thus would be a key target to trace partial melting in HP-UHP terranes. This contribution is an integrated study of petrology, zircon and titanite U-Pb ages, as well as zircon Hf-O and major minerals O isotope compositions for a felsic vein and its host granitic gneiss in the Sulu UHP terrane to decipher the timing and mechanism of partial melting. Zircon U-Pb dating for the inherited magmatic zircon cores of the granitic gneiss constrains the protolith age at ca. 750 Ma. The inherited zircons had an average δ18O value of 0.03 ± 0.15‰ and yielded a weighted mean εHf(t) value of −1.16 ± 0.54 and two-stage Hf model ages of 1707 ± 65 Ma. These results suggest that the protolith was derived from reworking of Paleoproterozoic crust in a Neoproterozoic rift. In the two samples, broad garnet core/mantle domains are of metamorphic origin and narrow garnet rims of peritectic origin. There are large differences in δ18O values between the protolith magmatic zircon (δ18O = 0.03 ± 0.15‰) and metamorphic garnet (−4.14 ± 0.21‰ for the felsic vein and −4.13 ± 0.11‰ for the host granitic gneiss), suggesting that the negative δ18O fingerprints might result from hydrothermal alteration or multi-episodes of rift magmatism after the formation of the protolith. Zircon from the felsic vein yielded a U-Pb age of 212 ± 3 Ma. Titanites from the two studied samples show undistinguishable U-Pb ages of 227 ± 14 Ma and 212 ± 6 Ma within the analytical uncertainties. These ages register the timings of the partial melting. The approximately consistent δ18O values of the metamorphic zircon (−4.78 ± 0.15‰) and metamorphic garnet indicate that the melt was sourced from the surrounded gneiss in a closed system, which might be triggered by breakdown of phengite. According to a compilation of partial melting in the Sulu UHP terrane, there exists a clear spatio-temporal variation. The ca. 237-224 Ma partial melting in the northern UHP zone might be due to its relatively high temperature metamorphism; the most extensive partial melting occurred at ca. 220-215 Ma in both the southern and northern UHP zones, which might be triggered by the detachment of the preceding subducted slab; and partial melting of the northern UHP zone continued to ca. 210 Ma due to heat supply from the underneath mantle. Such a variation might be dictated by the tectonic evolution of the different UHP slices. Accordingly, the whole partial melting process of UHP rocks in the Sulu UHP terrane might have lasted for about 27 Myr with two major episodes at ca. 230–224 and 220-215 Ma.

Stable isotopes of atmospheric water vapour and precipitation in the northeast Qinghai‐Tibetan Plateau

AbstractStable water isotopes (δ18O and δ2H) are an important source signature for understanding the hydrological cycle and altered climate regimes. However, the mechanisms underlying atmospheric water vapour isotopes in the northeast Qinghai‐Tibetan Plateau of central Asia remain poorly understood. This study initially investigated water vapour isotopic composition and its controls during the premonsoon and monsoon seasons. Isotopic compositions of water vapour and precipitation exhibited high variability across seasons, with the most negative average δ18O values of precipitation and the most positive δ18O values of water vapour found during the premonsoon periods. Temperature effect was significant during the premonsoon period but not the monsoon period. Both a higher slope and intercept of the local meteoric water line were found during the monsoon period as compared with in the premonsoon period, suggesting that raindrops have been experienced a greater kinetic fractionation process such as reevaporation below the cloud during the premonsoon periods. The δ2H and δ18O signatures in atmospheric water vapour tended to be depleted with the occurrence of precipitation events especially during the monsoon period and probably as a result of rainout processes. The monthly average contribution of evaporation from the lake to local precipitation was 35.2%. High d‐excess values of water vapour were influenced by the high proportion of local moisture mixing, as indicated by the gradually increasing relative humidity along westerly and Asian monsoon trajectories. The daily observation (observed ε) showed deviations from the equilibrium fractionation factors (calculated ε), implying that raindrops experienced substantial evaporative enrichment during their descent. The average fraction of raindrops reevaporation was estimated to be 16.4± 12.9%. These findings provide useful insights for understanding the interaction between water vapour and precipitation, moisture sources, and help in reconstructing the paleoclimate in the alpine regions.

C and Sr Isotope Chemostratigraphy and Pb–Pb Age of Carbonate Deposits of the Vorogovka Group (Neoproterozoic), Northwest of the Yenisei Range

The C and Sr isotopic signatures and the Pb–Pb age of carbonate sedimentary rocks of the Vorogovka Group in the reference section in the Vorogovka River valley, northwestern Yenisei Range, were obtained. The deposits of the group ascribed to the Severnaya Rechka, Mutnina, and Sukhaya Rechka Formations fill a large trough, and lithological and textural features indicate a high sedimentation rate. Most of the carbonate rocks are represented by limestones with low Fe/Sr and Mn/Sr ratios and average δ18O value (–6.0 ± 1.6‰ V-PDB), indicating the well-preserved isotope systems of the rocks. The 87Sr/86Sr ratios and δ13С values vary within narrow limits of 0.70816–0.70826 and –1.4 to +0.8‰ for Upper Severnaya Rechka limestones and 0.70818–0.70829 and +1.0 to +2.0‰ for Sukhaya Rechka limestones, respectively. The Pb–Pb isochrone ages of carbonate rocks of the Severnaya Rechka and Sukhaya Rechka formations are 580 ± 40 Ma (MSWD = 1.4) and 565 ± 90 Ma (MSWD = 1.1), respectively. According to chemostratigraphic and isotope-geochronological data, the accumulation of limestones of the Vorogovka Group could have occurred at 580–550 Ma, that corresponds to the Late Vendian or Late Ediacaran. It is evident that the duration of accumulation of Vorogovka Group did not exceed 10–15 m.y. The Vorogovka Trough is different from in the sedimentation pattern from other Late Vendian marginal troughs at the southwestern part of the Siberian Platform and formed independently.