Average δ18O Research Articles

Isotopic composition (δ15N, δ18O) of nitrate in high-frequency precipitation events differentiate atmospheric processes and anthropogenic NOx emissions

Reactive nitrogen (Nr) species from the atmosphere contribute to adverse impacts like acid deposition and eutrophication; thus, information about local or distal emission N sources towards wet deposition of nitrogen oxides (NOx) is important information. Given N emissions source regions and atmospheric chemistry processes can change on an hourly basis, a thorough assessment of Nr removal from the atmosphere requires high resolution precipitation sampling. We measured nitrate isotopes (δ15Ν, δ18O of NO3−) and δ18Ο-H2O of all precipitation events at high-frequency (5–30 min) for Vienna, Austria, in 2019. Average monthly δ15N of NO3− ranged from −7.0‰ (AIR) to −0.1‰ with lowest and higher values in summer and winter, respectively. Similarly, the average δ18O of NO3− was highest in winter (+75.0‰, VSMOW) and lowest in summer (+61.2‰). Nitrate in nighttime precipitation and intense rainouts had higher δ15N-NO3− values (−4.4‰ and − 4.6‰, respectively) compared to daytime and washout samples (−5.7‰ and − 5.5‰). Atmospheric oxidation and cloud scavenging on a diel basis were correlated to air temperature, with an opposite non-linear effect between the δ18O of NO3− and H2O. The δ15N of NO3− showed a negative correlation with rain intensity, but a positive relationship to the number of days without rain prior to a precipitation event. Overall, nitrate source partitioning with and without photochemical equilibrium isotope fractionation suggested strong year-round influences from vehicle emissions and biomass burning by day and night. Biogenic, fertilized soil, and coal combustion N sources (power plants) showed a lower contribution to precipitation NO3− and was linked to both harvest periods and long-range transport of air masses from North-Eastern and southern Europe. N emissions from local municipal waste incineration were significant in the autumn. Our study reveals that nitrate isotopes in wet deposition combined with δ18O-H2O contribute new information to inform NOx source management strategies aimed at improving environmental air and water quality.

Inter- and intra-crystal quartz δ18O homogeneity at Okataina volcano, Aotearoa New Zealand: Implications for rhyolite genesis

The sources and processes involved in the genesis of the voluminous rhyolitic magmas of cataclysmic caldera-forming eruptions, and the intervening lower-volume intra-caldera extrusions, have been subject to much debate. To better understand generation of high-volume and low-volume silicic eruptions within a single volcanic centre, and how they may differ, we examined ten volumetrically varied high-SiO2 rhyolite eruptions from the Okataina Volcanic Centre (OVC) in Aotearoa New Zealand. The OVC is one of the world's most recurrently active silicic volcanoes. In the last ~600 ky, the OVC was the focus of three known caldera-forming events and numerous intermittent dome-building and fissure eruption episodes, with rhyolitic eruption activity as recent as 1314 CE. To elucidate how mass contributions from the mantle and crust may have fluctuated over the lifespan of the OVC magmatic system, oxygen isotopic ratios (δ18O) of quartz in rhyolites were investigated for the first time at inter-crystal and intra-crystal scales. Quartz crystals from four eruption episodes (two caldera-forming events, Utu, ~557 ka, Rotoiti, ~45 ka, and two intra-caldera dome-building events, Rotoma, ~9.5 ka, and Kaharoa, ~0.7 ka) yielded intra-crystal δ18O isotopic homogeneity (±0.23‰, 2sd) based on secondary ion mass spectrometry (SIMS). These samples also display inter-crystal and inter-unit homogeneity within slightly lower precision (7.6 ± 0.5‰, 2sd). Whole-crystal quartz from the same four units, as well as six other units (two intra-caldera dome-building episodes, Okareka, ~21.8 ka, Whakatane, ~5.5 ka, three pre-Rotoiti extra-caldera domes, Round Hill, Haparangi, Kakapiko, and one immediately post-Rotoiti eruption, Earthquake Flat), were then examined using high-precision laser fluorination. Single crystals also yielded mostly homogenous ratios with average δ18O = 7.6 ± 0.5‰ (2sd), which is consistent with intra-crystal SIMS analyses, albeit for a larger set of samples. Stable and radiogenic isotope mixing models using the newly obtained δ18O ratios demonstrate that OVC rhyolites can be produced by ≥25% assimilation of a regional (Torlesse-like) metasedimentary endmember by a depleted mantle source with slightly variable amounts of subduction flux, and that any incorporation of hydrothermally altered material to the system is limited to <5% in caldera and intra-caldera eruptions. The δ18O records of the OVC are among the most homogenous currently known and indicate stable and consistent mantle and crustal contributions across the lifespan of the magmatic system, with assimilation largely occurring prior to segregation of rhyolitic melts within the silicic reservoir. This isotopic homogeneity may be due to a relatively high-volume and constant magma flux at the OVC, which contrasts to other rhyolitic caldera volcanoes with greater isotopic variability.

Oxygen isotopic alteration rate of continental crust recorded by detrital zircon and its implication for deep-time weathering

Weathering plays a significant role in the Earth system through the exchange of material among the lithosphere, atmosphere, hydrosphere, and biosphere. Variation of continental weathering in deep-time, however, remains elusive. This work investigates continental weathering recorded by detrital zircon. Zircon can record the oxygen isotopic composition (δ18O) of its parent crust at the time of crystallization, the value of which principally reflects the time-integrated effect of crustal alteration. The Hf isotopes and U-Pb isotopes of zircon also help to constrain the alteration history between crust generation and zircon crystallization. A new algorithm is introduced to reconstruct the average δ18O alteration rate of continental crust (Rδ18O-CC) through time by solving a set of linear equations based on a large population of detrital zircons with varying temporal coverage across the history of crustal alteration. A nearly three-billion-year history of Rδ18O-CC from 3.2 Ga to 0.3 Ga can be reconstructed using more than 5,000 globally distributed detrital zircons with coupled U-Pb-Hf-O isotopic records. The reconstructed Rδ18O-CC shows an overall bell-shape long-term evolution centered at ∼2 Ga superposed with variations that are coupled with supercontinental assembly cycles. The long-term evolution of the reconstructed Rδ18O-CC seems to be correlated with solid-earth CO2 degassing expected from the age distribution of deleted mantle and the supercontinental cycles. Thus, the Rδ18O-CC is interpreted to reflect weathering considering the control of solid-earth CO2 degassing on the total weathering flux of continental crust. However, independent evidence on the solid-earth CO2 degassing is unavailable, interpreting Rδ18O-CC as a weathering record requires further testing. Nevertheless, this work provides an example of how the time-integrated signal, with large noise-to-signal ratio, preserved in geological archives can be deconvolved using a large dataset. The result also demonstrates the great potential that weathering history may have in reconstructing the operation of the Earth system across deep-time.

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.

The Mina Justa Iron Oxide Copper-Gold (IOCG) Deposit, Peru: Constraints on Metal and Ore Fluid Sources

Abstract Iron oxide copper-gold (IOCG) deposits are major sources of Cu, contain abundant Fe oxides, and may contain Au, Ag, Co, rare earth elements (REEs), U, and other metals as economically important byproducts in some deposits. They form by hydrothermal processes, but the source of the metals and ore fluid(s) is still debated. We investigated the geochemistry of magnetite from the hydrothermal unit and manto orebodies at the Mina Justa IOCG deposit in Peru to assess the source of the iron oxides and their relationship with the economic Cu mineralization. We identified three types of magnetite: magnetite with inclusions (type I) is only found in the manto, is the richest in trace elements, and crystallized between 459° and 707°C; type Dark (D) has no visible inclusions and formed at around 543°C; and type Bright (B) has no inclusions, has the highest Fe content, and formed at around 443°C. Temperatures were estimated using the Mg content in magnetite. Magnetite samples from Mina Justa yielded an average δ56Fe ± 2σ value of 0.28 ± 0.05‰ (n = 9), an average δ18O ± 2σ value of 2.19 ± 0.45‰ (n = 9), and Δ’17O values that range between –0.075 and –0.047‰. Sulfide separates yielded δ65Cu values that range from –0.32 to –0.09‰. The trace element compositions and textures of magnetite, along with temperature estimations for magnetite crystallization, are consistent with the manto magnetite belonging to an iron oxide-apatite (IOA) style mineralization that was overprinted by a younger, structurally controlled IOCG event that formed the hydrothermal unit orebody. Altogether, the stable isotopic data fingerprint a magmatic-hydrothermal source for the ore fluids carrying the Fe and Cu at Mina Justa and preclude significant input from meteoric water and basinal brines.

Potential for Volcanogenic Massive Sulfide Mineralization at the A6 Anomaly, North-West British Columbia, Canada: Stratigraphy, Lithogeochemistry, and Alteration Mineralogy and Chemistry

The Middle Jurassic A6 Anomaly is located 30 km southeast of Eskay Creek, north-central British Columbia and consists of thick, altered felsic igneous rocks overlain by a mafic volcano-sedimentary package. Lithogeochemistry on igneous rocks, X-ray diffraction on altered felsic units, and electron probe microanalysis and secondary ion mass spectrometry on illite and quartz were applied to explore the volcanogenic massive sulfide (VMS) potential, characterize alteration, and determine fluid conditions at the A6 Anomaly. Lithogeochemistry revealed calc-alkaline rhyodacite to trachyte of predominantly FII type, tholeiitic basalts with Nb/Yb &lt; 1.6 (i.e., Group A), and transitional to calc-alkaline basalts and andesites with Nb/Yb &gt; 2.2 (i.e., Group B). The felsic units showed weakly to moderately phyllic alteration (quartz–illite with minor orthoclase and trace chlorite–pyrite–calcite–barite–rutile). Illite ranged in composition from illite/smectite (K = 0.5–0.69 apfu) to almost endmember illite (K = 0.69–0.8 apfu), and formed from feldspar destruction by mildly acidic, relatively low temperature, oxidized hydrothermal fluids. The average δ18O composition was 10.7 ± 3.0‰ and 13.4 ± 1.3‰ relative to Vienna Standard Mean Ocean Water for illite and quartz, respectively. Geothermometry involving illite composition and oxygen isotope composition on illite and quartz yielded average fluid temperatures of predominantly 200–250 °C. Lithogeochemical results showed that the A6 Anomaly occurred in a late-Early to Middle Jurassic evolving back-arc basin, further east then previously recognized and in which transitional to calc-alkaline units formed by crustal assimilation to enriched Mid-Ocean Ridge Basalt (EMORB) (i.e., felsic units, Group B), followed by thinning of the crust resulting in tholeiitic normalized MORB basalts (i.e., Group A) with a minor crustal component. The alteration assemblage is representative of distal footwall alteration, and metal transport in this zone was limited despite favorable temperature, pH, and redox state, indicating a metal depleted source (i.e., felsic units).

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.

Regional meteoric water line of the Yucatan Peninsula, Mexico

AbstractThe data set represents the Regional Meteoric Water Line (RMWL) for the Yucatan Peninsula (Mexico), a region with karstic aquifers but little knowledge of the isotopic composition of meteoric water. This data paper comprises the isotopic composition of meteoric water collected from November 2018 to November 2019 as monthly composite samples expressed as weighted monthly mean precipitation from 16 locations across the Yucatan Peninsula. The RMWL was δ2H = 7.803 δ18O + 12.075, with a slope and intercept suggesting precipitation from air that has undergone condensation, air masses with variable moisture or recycled moisture. The average δ18O is −2.57‰ (−12.23 to 1.2‰) and δ2H is −7.94‰ (−83.39 to 18.32‰). This data set was collected by the implementation of an isotopic monitoring network, gathering information useful as a tool for better understanding of the hydrology and hydrogeochemistry of karstic aquifers, and to develop proxies for paleohydrology, ecohydrology, climate change and paleoclimate studies in the Gulf of Mexico and the Great Caribbean Area.

Two-stage crustal growth in the Arabian-Nubian shield: Initial arc accretion followed by plume-induced crustal reworking

Island-arc accretion during the assembly of Gondwana has been widely regarded as the main mechanism for Neoproterozoic crustal growth in the Arabian-Nubian Shield (ANS). However, processes involved to transform the newly accreted juvenile terranes into a typical continental crust remain unclear. Here, we present geochemical, isotopic, and U-Pb geochronological data from the El-Shadli volcanic province (80 km × 35 km and > 10 km thick) in the Eastern Desert of Egypt, which lies in the north-western part of the ANS, and overlies strongly deformed, previously accreted arc terranes. The El-Shadli volcanic province consists mainly of a mafic-felsic bimodal suite and subordinate intermediate rocks that intrude the mafic rocks of the suite. The bimodal suite rocks are tholeiitic, whereas the intermediate rocks have a calc-alkaline affinity. The bimodal suite and the intermediate rocks both yield U-Pb zircon ages of ~ 700 Ma implying they are coeval. High zircon εHf(t) values for the bimodal suite (average εHf(t) = +11.46) as well as the intermediate rocks (average εHf(t) = +9.76) indicate they were either magmatic extractions derived directly from a depleted mantle source, or the products of remelting of juvenile crust. Oxygen isotope data for zircon yield similar δ18O values for both the bimodal suite (average δ18O = 4.94‰) and intermediate rocks (average δ18O = 4.79‰). These are lower than typical mantle values indicating the parental magma in both cases interacted with hydrothermal fluids. Based on the petrological, geochemical, and isotopic data, we suggest that the El-Shadli bimodal suite and the intermediate rocks were produced by reworking of MORB-like and arc-like oceanic lithosphere, respectively, most likely driven by a mantle plume during the break-up of Rodinia. The recognition of the El-Shadli volcanic province as a likely mantle plume-induced post-kinematic magmatism provides a mechanism for the transformation of newly accreted juvenile crustal terranes into a chemically stratified normal continental crust. In addition, such plume events may result in new mantle extractions that are converted into new continental 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.

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.

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.

Distance-dependence from volcano for Asian dust inclusions in Andosols: A key to control soil ability to retain radiocesium

Deposition of Asian dust, i.e. mica-bearing aeolian particles, throughout geological times has contributed greatly to the increased ability of soil to specifically retain radiocesium (137Cs), particularly in soils derived from mica-deficient parent materials. Allophanic Andosols, volcanic mica-deficient soils, in Japan were hypothesized to contain fewer micaceous minerals proximal to volcanoes due to the dilution of Asian dust by thicker volcanic ash depositions. To test this hypothesis, we collected soils from stratified horizons mainly composed of mafic volcanic ash at four sites near the volcanic crater of Mt. Aso, Japan. The profiles were in volcanic ash soil dominated by poorly ordered clay minerals, and classified as Allophanic Andosols. Radiocesium interception potential (RIP) and mineral content were determined on soil particles fractionated to < 20 µm effective spherical diameter. The RIP for these particles ranged widely from < 0.1 to 4.5 mol kg soil−1, showed a strong-positive correlation with mica and quartz content, and increased with distance away from the crater. Isolated quartz grains, ranging from 2 to 20 µm, were analyzed for oxygen isotopic ratio (δ18O) as an indicator of Asian dust contribution. The average δ18O was 16.0 ± 0.4‰, very close to those of fine quartz in the Gobi Desert, while clearly differentiated from high temperature formation environments of SiO2 grains from volcanic origins, δ18O < 10‰. Results showed that RIP of Allophanic Andosols is primarily controlled by the proportion of Asian dust incorporated into volcanic ashes, which increased with distance from the main volcanoes. These findings improve predictions of the spatial distribution patterns soil radio cesium (RC) and the soil-to-plant transfer of RCs in Andosols in Japan.

Using Soil Water Stable Isotopes to Investigate Soil Water Movement in a Water Conservation Forest in Hani Terrace

Water conservation forests significantly contribute to the stability of mountain agricultural ecosystems in Hani Terrace. In this study, we analyzed the relationship between the stable isotopic composition of soil water and precipitation to determine the mechanisms of soil water movement in the small watershed of Quanfuzhuang. We observed significant seasonal variations in soil water sources: antecedent precipitation was the dominant supply during the dry season, and current precipitation dominated during the rainy season. The recharge ratio of precipitation to soil water in the grassland was significantly higher than that in the arbor land and shrubland. The influence of water infiltration, old and new soil water mixing, and soil evaporation on the soil water stable isotopes gradually decreased from the surface (0–20 cm) to the deep (60–80 cm) soil. We observed significant seasonal variability in average soil water δ18O in the upper 0–60 cm and lower variability at 60–100 cm. The average soil water δ18O was generally higher in the dry season than in the rainy season. The mixing of old and new water is a continuous and cumulative process that is impacted by soil structure, soil texture, and precipitation events. We therefore identified a significant time delay in soil water supply with increasing soil depth. Moreover, the piston flow of soil water co-occurred with preferential flow, and the latter was the dominant supply during the rainy season.

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.

The Oxygen Isotope Compositions of Large Numbers of Small Cosmic Spherules: Implications for Their Sources and the Isotopic Composition of the Upper Atmosphere

AbstractCosmic spherules are micrometeorites that melt at high altitude as they enter Earth's atmosphere, and their oxygen isotope compositions are partially or completely inherited from the upper atmosphere, depending on the amount of heating experienced and the nature of their precursor materials. In this study, the three oxygen isotope compositions of 137 cosmic spherules are determined using 277 in situ analyses by ion probe. Our results indicate a possible correlation between increasing average δ18O compositions of silicate‐dominated (S‐type) spherules along the series scoriaceous &lt; porphyritic &lt; barred &lt; cryptocrystalline &lt; glass &lt; CAT (calcium‐aluminum‐titanium) spherules (~12‰, 20‰, 22‰, 25‰, 26‰, and 50‰). This is consistent with the evolution of oxygen isotopes by mass fractionation owing to increased average entry heating and thus suggests mass fractionation dominates changes in isotopic composition, with atmospheric exchange being less significant. The Δ17O values of spherules, therefore, are mostly preserved and suggest that ~80% of particles are samples of C‐type asteroids. The genetic relationships between different S‐types can also be determined with scoriaceous, barred, and cryptocrystalline spherules mostly having low Δ17O values (≤0‰) mainly derived from carbonaceous chondrite (CC)‐like sources, while porphyritic spherules mostly have positive Δ17O (&gt;0‰) and are largely derived from ordinary chondrite (OC)‐like sources related to S (IV)‐type asteroids. Glass and CAT spherules have variable Δ17O values indicating they formed by intense entry heating of both CC and OC‐like materials. I‐type cosmic spherules have a narrow range of δ17O (~20–25‰) and δ18O (~38–48‰) values, with Δ17O (~0‰) suggesting their oxygen is obtained entirely from the Earth's atmosphere, albeit with significant mass fractionation owing to evaporative heating. Finally, G‐type cosmic spherules have unexpected isotopic compositions and demonstrate little mass fractionation from a CC‐like source. The results of this study provide a vital assessment of the wider population of extraterrestrial dust arriving on Earth.

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.

Hydrothermal fluid evolution at the Tiegelongnan porphyry-epithermal Cu(Au) deposit, Tibet, China: Constraints from H and O stable isotope and in-situ S isotope

The Tiegelongnan porphyry-epithermal Cu (Au) deposit is located in the Duolong porphyry district, north of the Bangong-Nujiang suture zone, Tibet, China. Mineralization is hosted by Jurassic sedimentary sandstone, and several phases of diorite and granodiorite porphyry dikes intruded between 123 and 116 Ma. The hydrothermal alteration is characterized by alunite-kaolinite-dickite overprinting quartz-muscovite-pyrite and biotite alteration zones. Porphyry chalcopyrite-pyrite ± molybdenite (Stage 1) mineralization is associated with biotite alteration. Porphyry chalcopyrite-bornite (Stage 2), and covellite (Stage 3) mineralization is associated with quartz-muscovite-pyrite alteration formed at ~121 Ma. Epithermal mineralization, consisting of pyrite-alunite (Stage 4), chalcopyrite-bornite-digenite (Stage 5), and tennantite-enargite (Stage 6), is hosted by two pulses of alunite-kaolinite breccia and veins at ~116 Ma and ~112 Ma. The fluid composition related to muscovite, with average δ18O of 8.9‰ and δD of −56‰, indicates a magmatic water origin. Fluid δ18O composition in equilibrium with quartz veins decreases from 6.7 to 2.3‰, which are likely the results of the water-rock isotopic exchange. Quartz fluid inclusions δD values between −50 to −84‰ are partly lower than that obtained from muscovite alteration fluids, which may result from H fractionation during fluid inclusions decrepitation. Epithermal stage fluid composition equilibrium with alunite yield δ18O from −1.2 to 2.7‰ and δD from −71 to −51‰, n = 11, which is comparable to the fluid composition equilibrium with Type Ⅰ kaolinite (hosting ores) with δ18O between −2.5 and 2.9‰, and δD between −72 and −51‰. It suggests that alunite and Type Ⅰ kaolinite formed with mixing between magmatic and high altitude Cretaceous meteoric water. Late Types Ⅱ and III kaolinite (filling alunite and quartz veins) fluid δ18O and δD values plot along a mixing line between magmatic and low altitude Cretaceous meteoric water, probably following the erosion and plateau subsidence. Porphyry mineralization sulfide stage 1 chalcopyrite and pyrite yield δ34S values between −5.8 and 0.9‰, with an average fluid δ34SH2S = −2.5‰ (n = 10), whereas stage 2 chalcopyrite returns δ34S values from −8.7 to −3‰ with an average δ34SH2S = −5.6‰ (n = 5). The lower fluid δ34SH2S values during sulfides stage 2 compared to that of stage 1, suggest that the chalcopyrite-bornite mineralization formed under higher oxidation conditions than that of the chalcopyrite-pyrite mineralization. Alunite yields δ34S values from 11 to 18.3‰ (n = 8), and the associated sulfide stage 4 pyrite have varying δ34S values from −32.2 to 5.4‰. Disequilibrium S isotope in alunite-pyrite pairs was likely because of rapid cooling and retrograde S isotope exchange during later sulfides emplacement. Epithermal mineralization sulfide Stage 4 S-equilibrated pyrite (−14.9 to −9.5‰), Stage 5 chalcopyrite (−11.6 to −8.2‰), and Stage 6 enargite (−5.4 to −2.6‰) display increasing δ34S values suggesting epithermal fluid compositions evolve towards more reducing conditions.