Distinct Isotopic Compositions Research Articles

Wintertime decoupling of urban valley and rural ridge hydrological processes revealed through stable water isotopes

Water from fossil fuel combustion can represent >10% of urban specific humidity, but this fraction is difficult to constrain from meteorological measurements alone. Stable water vapor isotopes can be used to estimate the fraction of combustion-derived vapor (CDV) and characterize the contrast between anthropogenically-altered and natural systems due to CDV's distinctive isotope composition. However, accurate estimates of the CDV fraction of urban humidity requires information on vapor isotope ratios in the absence of anthropogenic emissions. We present data from an urban site in Salt Lake City, UT and from a high-elevation site in the adjacent Wasatch Mountains. Urban vapor d-excess values (d-excess = δ2H–8δ18O) closely track CO2 concentrations on diurnal to weekly timescales, but high-elevation vapor values do not. Instead, the high-elevation site captures large-scale atmospheric variability, with d-excess changes largely following changes in δ18O and humidity. Isotope ratios at the two sites remain distinct throughout most of the winter, indicating that these sites are regularly decoupled from each other and that high-elevation winter measurements may rarely be representative of valley conditions in the absence of urban emissions. Furthermore, high-elevation d-excess may be higher than emissions-free values at the same elevation as the urban area, as vapor d-excess changes non-linearly during condensation at low specific humidity, and would result in an overestimate of urban humidity amounts. Therefore, the high-elevation site may not help place additional constraints on the amount of CDV in urban systems, yet the paired sites show changes in vapor isotope ratios, particularly in d-excess, that capture differences in urban and natural impacts on water vapor cycling.

An environmental baseline for food webs at deep-sea hydrothermal vents in Manus Basin (Papua New Guinea)

Food webs and trophic interactions provide a measure of ecosystem function and have been highlighted as an important element for environmental baselines and environmental impact assessments. Stable isotopes have long been used in the deep sea as a means to explore and summarize food webs, especially in chemosynthetic ecosystems, where there may be multiple sources of primary production with distinctive isotopic compositions. Hydrothermal vents in Manus Basin have been targeted for mining and, if mining proceeds, vent food webs will be altered. Here we describe a pre-mining baseline for the food-web state for 3 active vent fields. Manus vents support at least 9 feeding guilds, and both reverse TriCarboxylic Acid (rTCA) and Calvin-Benson-Bassham (CBB) carbon fixation pathways are inferred for symbionts of holobiont taxa. There are site differences in the C-N-S isotopic trait spaces (convex hulls) of Manus Basin vent food webs that likely reflect environmental variability. Isotopic diversity metrics (divergence, dispersion, evenness) are similar in Solwara 1, South Su, and PACMANUS vent food webs, but South Su has a higher degree of isolation in isotopic space (uniqueness). Because convex hull and isotopic diversity metrics are easy to calculate from multidimensional isotopic data, they may be useful food-web indicators for environmental management.

Effect of air pollution control devices on mercury isotopic fractionation in coal-fired power plants

Coal combustion is an important source of anthropogenic mercury (Hg) emissions. Air pollution control devices (APCDs) are widely used in coal-fired power plants (CFPPs) to reduce pollutant emissions including Hg. To gain insight into the Hg isotopic fractionation produced by APCDs, the Hg isotopic compositions of feed coal (FC) samples and combustion byproducts from 7 CFPP utility boilers in Hebei Province of China were studied. Significant mass-dependent fractionation (MDF) was observed in combustion byproducts when comparing with FC samples, while mass-independent fractionation (MIF) was not identified. Compared to FC samples, no significant difference in bottom ash (BA, δ202HgBA-FC = 0.08 ± 0.16‰, 1SD, n = 3) samples while light isotopes were enriched in fly ash (FA, δ202HgFA-FC = −0.38 ± 0.75‰, 1SD, n = 7) samples. Boilers lacking of a selective catalytic reducer (SCR) unit produced gypsum (GY) samples with negative MDF signatures compared to FC samples (δ202HgGY-FC, no SCR = −0.22 ± 0.10‰, 1SD, n = 3). In contrast, boilers equipped with a SCR unit produced GY samples with higher MDF signatures (δ202HgGY-FC, SCR = 0.54 ± 0.40‰, 1SD, n = 3), probably as a result of the oxidation of Hg0 in the SCR unit. Calculated isotopic signatures of gaseous Hg in the stack emission (SE, δ202HgSE-FC = 0.03 ± 0.20‰, 1SD, n = 7) had no significant difference from that of FC samples. The results suggested distinct isotopic compositions should be fully considered during tracing Hg pollution via Hg isotopic signatures, especially when solid produced by CFPPs were involved.

Geochemical Variability Along the Northern East Pacific Rise: Coincident Source Composition and Ridge Segmentation

AbstractNew trace element abundances and isotope compositions for more than 100 mid‐ocean ridge basalts from 5.5°N to 19°N on the East Pacific Rise show step function variations in isotopic composition along the ridge axis that coincide with ridge discontinuities. Transform faults, overlapping spreading centers, and devals (deviation from axial linearity) mark the separation of individual clusters of distinct isotopic composition and trace element ratios that indicate source variations. This correlated chemical clustering and morphological segmentation indicates that source composition and segmentation can be closely related even on a fine scale. Substantial chemical variations within a segment are related to source composition. This suggests that even within segments the magma transport is mainly vertical, and there is limited along‐ridge transport, and there is little evidence for magma chambers that are well mixed along strike. Trace element concentrations show good correlations with isotopic compositions on a segment scale but less so on a regional scale. The trace element and isotopic variability along the northern East Pacific Rise can be explained by three mantle components: a depleted peridotite endmember, an enriched peridotite endmember, and a recycled gabbro‐like component. The gabbroic component has an isotopic signature indicating an ancient origin. The high‐resolution sampling indicates that within a segment the chemical variability is largely binary but that the endmembers of the binary mixing change from segment to segment. The endmembers of the binary variation within a segment are a combination of three of the endmembers.

New insights into the heterogeneity of the Tagish Lake meteorite: Soluble organic compositions of variously altered specimens

AbstractThe Tagish Lake carbonaceous chondrite exhibits a unique compositional heterogeneity that may be attributed to varying degrees of aqueous alteration within the parent body asteroid. Previous analyses of soluble organic compounds from four Tagish Lake meteorite specimens (TL5b, TL11h, TL11i, TL11v) identified distinct distributions and isotopic compositions that appeared to be linked to their degree of parent body processing (Herd et al. 2011; Glavin et al. 2012; Hilts et al. 2014). In the present study, we build upon these initial observations and evaluate the molecular distribution of amino acids, aldehydes and ketones, monocarboxylic acids, and aliphatic and aromatic hydrocarbons, including compound‐specific δ13C compositions, for three additional Tagish Lake specimens: TL1, TL4, and TL10a. TL1 contains relatively high abundances of soluble organics and appears to be a moderately altered specimen, similar to the previously analyzed TL5b and TL11h lithologies. In contrast, specimens TL4 and TL10a both contain relatively low abundances of all of the soluble organic compound classes measured, similar to TL11i and TL11v. The organic‐depleted composition of TL4 appears to have resulted from a relatively low degree of parent body aqueous alteration. In the case of TL10a, some unusual properties (e.g., the lack of detection of intrinsic monocarboxylic acids and aliphatic and aromatic hydrocarbons) suggest that it has experienced extensive alteration and/or a distinct organic‐depleted alteration history. Collectively, these varying compositions provide valuable new insights into the relationships between asteroidal aqueous alteration and the synthesis and preservation of soluble organic compounds.

Origin of the non-carbonaceous–carbonaceous meteorite dichotomy

The isotopic composition of meteorites reveals a fundamental dichotomy between non-carbonaceous (NC) and carbonaceous (CC) meteorites. However, the origin of this dichotomy—whether it results from processes within the solar protoplanetary disk or is an inherited heterogeneity from the solar system's parental molecular cloud—is not known. To evaluate the origin of the NC–CC dichotomy, we report Ni isotopic data for a comprehensive set of iron meteorites, with a special focus on groups that have not been analyzed before and belong to the CC group. The new Ni isotopic data demonstrate that the NC–CC dichotomy extends to Ni isotopes, and that CC meteorites are characterized by a ubiquitous 58Ni excess over NC meteorites. These data combined with prior observations reveal that, in general, the CC reservoir is characterized by an excess in nuclides produced in neutron-rich stellar environments, such as 50Ti, 54Cr, 58Ni, and r-process Mo isotopes. Because the NC–CC dichotomy exists for refractory (Ti, Mo) and non-refractory (Ni, Cr) elements, and is only evident for nuclides produced in specific, neutron-rich stellar environments, it neither reflects thermal processing of presolar carriers in the disk, nor the heterogeneous distribution of isotopically anomalous Ca–Al-rich inclusions (CAI). Instead, the NC–CC dichotomy reflects the distinct isotopic composition of later infalling material from the solar system's parental molecular cloud, which affected the inner and outer regions of the disk differently. Simple models of the infall process by themselves can support either infall of increasingly NC-like material onto an initially CC-like disk, or infall of increasingly CC-like material in the absence of disk evolution by spreading. However, provided that CAIs formed close to the Sun, followed by rapid outward transport, their isotopic composition likely reflects that of the earliest infalling material, implying that the composition of the inner disk (i.e., the NC reservoir) is dominated by later infalling material, whereas the outer disk (i.e., the CC reservoir) preserved a compositional signature of the earliest disk. The isotopic difference between the inner and outer disk was likely maintained through the rapid formation of Jupiter, which prevented complete homogenization between material from inside (NC reservoir) and outside (CC reservoir) its orbit.

An isotopically distinct Zealandia–Antarctic mantle domain in the Southern Ocean

The mantle sources of mid-ocean ridge basalts beneath the Indian and Pacific oceans have distinct isotopic compositions with a long-accepted boundary at the Australian–Antarctic Discordance along the Southeast Indian Ridge. This boundary has been widely used to place constraints on large-scale patterns of mantle flow and composition in the Earth’s upper mantle. Sampling between the Indian and Pacific ridges, however, has been lacking, especially along the remote 2,000 km expanse of the Australian–Antarctic Ridge. Here we present Sr, Nd, Hf and Pb isotope data from this region that show the Australian–Antarctic Ridge has isotopic compositions distinct from both the Pacific and Indian mantle domains. These data define a separate Zealandia–Antarctic domain that appears to have formed in response to the deep mantle upwelling and ensuing volcanism that led to the break-up of Gondwana 90 million years ago, and currently persists at the margins of the Antarctic continent. The relatively shallow depths of the Australian–Antarctic Ridge may be the result of this deep mantle upwelling. Large offset transforms to the east may be the boundary with the Pacific domain. A separate mantle domain, distinct from both the Pacific and Indian domains, exists beneath the Southern Ocean, according to isotope compositions of samples from the Australian–Antarctic ridge.

Experimental evidence for mineral-controlled release of radiogenic Nd, Hf and Pb isotopes from granitic rocks during progressive chemical weathering

The role of accessory minerals in the incongruent release of Hf and Pb during continental weathering and its implications for the generation of distinct seawater isotope compositions is subject of debate. While it has been suggested that radiogenic Hf and Pb isotope signatures released during the dissolution of rocks are controlled by the relative abundances of minerals with distinct isotope compositions and differences in their resistance to dissolution there has not been a comprehensive experimental investigation of these processes to date. We carried out systematic sequential leaching experiments on fresh and partly weathered granitic rock samples as well as separated zircons from the Central Aar Granite in Switzerland. Combined with major and rare earth element concentrations our new quantitative experimental data reveal systematic preferential release of radiogenic Nd, Hf and Pb isotopes primarily controlled by dissolution characteristics of the host rock's easily dissolvable accessory and major minerals, in particular apatite and sphene, during weak chemical weathering. Moreover, Pb isotope signatures of incipient weathering conditions, contrary to expectations, indicate initial congruent release of Pb from freshly exposed mineral surfaces that becomes subsequently incongruent. During more advanced chemical weathering stages, as well as enhanced physical weathering conditions, the dissolution of major minerals (i.e. feldspars) becomes dominant for Nd and Pb isotope signatures, whereas Hf isotopes are still dominated by contributions from highly radiogenic accessories.Additional leaching experiments of zircon separates were performed to test the specific role of zircons for Hf isotope compositions of riverine runoff. It is demonstrated that zircon is more efficiently dissolved when physical weathering is enhanced. This increased Hf release originating from partial dissolution of zircons, however, is quantitatively not sufficient to explain less radiogenic Hf isotope signatures in seawater during episodes of enhanced mechanical erosion alone. Moreover, the observed addition of Hf from the more congruent dissolution of the zircon-free fractions of the parent rock due to enhanced physical weathering indicates that these minerals also play an important role in controlling Hf isotope signatures released under deglacial conditions.

Seasonality of nitrate sources and isotopic composition in the Upper Illinois River

To improve understanding of spatial, seasonal, and inter-annual variations in nitrate sources and in-stream processes in the Illinois River system, nitrate concentrations and isotopic compositions were measured in 445 water samples collected over a four-year period (2004–2008) from the Upper Illinois River Basin (UIRB). Samples included surface water in the river and major tributaries, effluent samples from Chicago’s largest wastewater treatment plant (WTP), and representative groundwater from shallow wells in agricultural land. Two principal nitrate endmember sources within the UIRB had distinctive isotopic compositions: WTP effluent with δ15N = 8.6 ± 1.7‰ and δ18O = 0.8 ± 1.4‰ and agricultural groundwater with δ15N-NO3 = 3.4 ± 0.6‰ and δ18O = 3.7 ± 0.5‰ (when minimally affected by nitrate reduction). Isotopic data indicated that the large pulse of nitrate exported from the river basin during the spring was mostly derived from agricultural land drainage, while nitrate from large WTP effluent point sources was predominant in the upper reaches of the river near Chicago. During low base-flow conditions in late-summer and fall, the agricultural nitrate source was greatly diminished and the headwater WTP source was predominant in the river basin export. Our results indicated biogeochemical nitrate reduction and isotopic fractionation occurred within the river network, affecting both agricultural and urban sources during surface-water transport. In addition, diminished agricultural nitrate export was attributable to preferential discharge of biogeochemically reduced groundwater during low base flow. Isotopic indicators of spatial and seasonal variations in the relative importance of different nitrate sources, and their relative susceptibility to natural attenuation, might be useful for guiding monitoring and management practices to reduce nitrate export from complex watersheds with mixed land uses.

Magnesium Isotope Composition of Subduction Zone Fluids as Constrained by Jadeitites From Myanmar

AbstractSubduction zone fluids are critical for transporting materials from subducted slabs to the mantle wedge. Jadeitites from Myanmar record fluid compositions and reactions in the forearc subduction channel. Here we present high‐precision Mg isotope data of the Myanmar jadeitites and associated rocks to understand the Mg isotope composition of subduction zone fluids at forearc depths. Two types of jadeitites (white and green) exhibit distinct Mg isotope compositions. The white jadeitites precipitated from Na‐Al‐Si‐rich fluids and have low δ26Mg values, varying from −1.55‰ to −0.92‰, whereas the green jadeitites have higher δ26Mg values (−0.91‰ to −0.74‰) due to metasomatic reactions between fluids and Cr spinel. The amphibole‐rich blackwall in the contact boundaries between jadeitites and serpentinites also exhibits low δ26Mg values (−1.17‰ to −0.72‰). Therefore, the jadeite‐forming fluids have not only high concentrations of Na‐Al‐Si but also low δ26Mg values. The low δ26Mg signature of the fluids is explained by the dissolution of Ca‐rich carbonate in subducted sediments or altered oceanic crust, which is supported by the negative correlation of δ26Mg with CaO/TiO2, CaO/Al2O3, and Sr in the white jadeitites. Given the common occurrence of Ca‐rich carbonates in the subduction channel, the Mg isotope composition of low‐Mg aqueous fluids would be significantly modified by dissolved carbonates. Metasomatism by such fluids along conduits has the potential to generate centimeter‐scale Mg isotope heterogeneity in the forearc mantle wedge. Therefore, Mg isotopes could be a powerful tracer for recycled carbonates not only in the deep mantle but also in the shallow regions of subduction zones.

Pb Isotopic Composition and Metal Sources of Au and Ag Deposits of the South Verkhoyansk Region (Yakutia, Russia) According to High-Precision MC-ICP-MS Data

The paper considers the results of high-precision Pb–Pb isotopic analysis of 120 galena samples from 27 Au and Ag deposits of the South Verkhoyansk Synclinorium (SVS) including large Nezhdaninsky deposit (628.8 t Au). The Pb isotopic composition is analyzed on a MC-ICP-MS NEPTUNE mass-spectrometer from solutions with an error of no more than ±0.02% (2σ). Four types of deposits are studied: (i) stratified vein gold–quartz deposits (type 1) hosted in metamorphosed Upper Carboniferous–Lower Permian terrigenous rocks and formed during accretion of the Okhotsk Block to the North Asian Craton synchronously with dislocation metamorphism and related granitic magmatism; (ii) vein gold–quartz (Nezhdaninsky type) deposits also hosted in Lower Permian metasedimentary rocks; (iii) Au–Bi deposits localized at the contact zones of the Late Cretaceous granitic plutons; and (iv) Sn–Ag polymetallic deposits related to granitic and subvolcanic rocks of the Okhotsk Zone of the SVS. The deposits of types 2, 3, and 4 are postaccretionary. The general range of 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios is 18.1516–18.5903 (2.4%), 15.5175–15.6155 (0.63%), and 38.3010–39.0481 (2.0%), respectively. In 206Pb/204Pb–207Pb/204Pb and 206Pb/204Pb–208Pb/204Pb diagrams, the data points of Pb isotopic compositions of all deposits occupy restricted, partly overlapping areas along a general elongated trend. The various SVS Au–Ag deposits can be classified according to the Pb isotopic composition in accordance with all three Pb ratios. Deposits of the same type show distinct Pb isotopic compositions that strongly exceed the scale of analytical error (±0.02%). The differences in Pb isotopic composition within specific deposits are low and subordinate and have little effect on variations in the Pb isotopic composition of the SVS deposits. The μ2 values (Stacey–Kramers model), which characterize the 238U/204Pb ratios of ore lead sources of the SVS deposits, widely vary from 9.7 to 9.38. The ω2 values (232Th/204Pb) are 39.82–36.61, whereas the Th/U ratios are 4.04–3.86. The content of all three radiogenic Pb isotopes and μ2 values of feldspars from SVS intrusive rocks are strongly distinct from those of galena of stratified gold–quartz and vein gold–quartz deposits and are identical to Pb of galena from Au–Bi and Sn–Ag polymetallic deposits, indicating a mostly magmatic origin for the Pb of these deposits. Detailed isotopic study of the Nezhdaninsky deposit shows different Pb isotopic composition of two consecutive mineral assemblages (gold–sulfide and Ag polymetallic): ~0.30, ~0.07, and ~0.22% for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios, respectively. These differences are interpreted as a result of involvement of at least two metal sources during the evolution of an ore-forming system: (i) host Lower Permian terrigenous rocks and (ii) a magmatic source similar in Pb isotopic composition to that of Sn–Ag polymetallic deposits. The Pb isotopic composition and μ2 and Th/U values show that lead of stratified gold–quartz deposits combines isotopic tracers of lower and upper crustal sources (Upper Carboniferous–Lower Permian terrigenous rocks), lead of which was mobilized by ore-bearing fluids. The high 208Pb/206Pb ratios and Th/U evolutionary parameter are common to all Pb isotopic composition of all studied Au–Ag deposits and SVS Cretaceous intrusive rocks and indicate that Pb sources were depleted in U relative to Th. Taking into account the structure of the region and conceptions on its evolution, we can suggest that the magma source was related to lower crustal subducted rocks of the Archean (~2.6 Ga) North Asian Craton and the Okhotsk terrane.

The Spatial Origin of Chondrules in Individual Chondrites: Constraints from Modeling Chondrule Mixing

Abstract Chondrules are a major component of chondritic meteorites and potentially populated the entire protoplanetary disk before planet formation. Chondrules provide insights into the physical and chemical evolution of the protoplanetary disk. An important constraint for the protoplanetary disk is whether chondrules in individual chondrite groups formed in spatially separate reservoirs and were then transported and mixed throughout the disk, finally accreting in chondrites, or did chondrules in individual chondrite groups form and then accrete in the same reservoir and locality, without large-scale transport and mixing involved. Both scenarios have been proposed. Here we use bulk chondrule compositional data from the recently published ChondriteDB database in combination with a mixing model we developed to test whether the compositional distributions of chondrule populations in individual chondrites (1) are the result of mixing chondrules from multiple parental reservoirs or (2) originated from single parental reservoirs. We thereby provide a fundamental framework that each mixing model needs to obey. Although one mixing model is principally possible, this particular model is unlikely, and it therefore appears more reasonable that chondrules in individual chondrites originated from single, although different, parental reservoirs. Significant disk-wide transport or mixing of chondrules seems unlikely, while chondrule-forming models that produce chondrules from single reservoirs seem more likely. Anomalous minor element and nucleosynthetic isotope chondrule compositions are possibly best explained by admixing tiny nuggets such as refractory or presolar grains with distinct elemental or isotopic compositions into chondrules.

Sr and Mg isotope geochemistry of the basal Ediacaran cap limestone sequence of Mongolia: Implications for carbonate diagenesis, mixing of glacial meltwaters, and seawater chemistry in the aftermath of Snowball Earth

The seawater chemistry and oceanographic information associated with Snowball Earth are commonly inferred from the geochemistry of cap carbonates deposited on continental margins during and after deglaciation. However, interpretation of such records can be complicated by carbonate diagenesis and contamination from siliciclastic components. In an attempt to disentangle these effects, we studied the geochemistry of the post-Marinoan cap carbonate sequence from Mongolia using a step-leaching procedure, which revealed that most samples are heterogeneous with respect to multiple geochemical signatures, including trace element concentrations, Sr, Mg, C and O isotopic signatures, raising questions to previous studies applying carbonate bulk-rock geochemistry for paleoenvironment reconstructions. Such sample heterogeneity can be explained by contamination from non-carbonate phases and carbonate alteration. After stepped leaching, the least-altered/contaminated geochemical signatures for each sample were identified and the influences of carbonate diagenesis were evaluated. Our data indicate that mixing of glacial meltwater persisted to the maximum flooding surface within the cap carbonate sequence, below which carbonates record significant Mg and Sr isotope fluxuations that are most readily interpreted in the context of the mixing of water masses having distinct isotopic compositions. Only limestones deposited above the maximum flooding surface formed in a well-mixed ocean and exhibit Mg and Sr isotope values that record the integrated effects of Snowball Earth on ocean chemistry. Our study cautions against interpreting the geochemistry of cap carbonates in terms of whole ocean geochemical cycles.

Tracking nitrate sources in the Chaohu Lake, China, using the nitrogen and oxygen isotopic approach.

The Chaohu Lake is highly polluted and suffers from severe eutrophication. Nitrate is a key form of nitrogen that can cause water quality degradation. In this study, hydrochemical and dual isotopic approaches were utilized to identify the seasonal variation of nitrate sources in the Chaohu Lake and its inflowing rivers. The average nitrate concentrations ([NO3-]) of the lake and its inflowing rivers were 89.9 and 140.8μmolL-1, respectively. The isotopic values of δ15N-NO3- and δ18O-NO3- in the lake ranged from - 1.01 to + 16.67‰ and from - 4.39 to + 22.20‰, respectively. The two major rivers had distinct isotopic compositions, with average δ15N-NO3- values of + 17.52 and + 3.51‰, and average δ18O-NO3- values of + 2.71 and + 7.47‰ for the Nanfei River and Hangbu River, respectively. The results show that soil organic ammonium and urban wastewater discharge were the main sources of nitrate in the Chaohu Lake, and nitrate assimilation was an important process affected [NO3-] and isotopic compositions, especially in the western Chaohu Lake. The elevated [NO3-] and δ15N-NO3- values in the western Chaohu Lake indicate the high influence of human activities. Urban wastewater discharge was the primary nitrate source in the Nanfei River and soil organic ammonium was the main source in the Hangbu River. Although nitrate from direct atmospheric deposition was low, its strong flushing effect can substantially improve riverine [NO3-] and nitrate loading from terrestrial ecosystem. The relatively high nitrate fluxes from the Hangbu River indicate that nitrogen loading from agricultural watershed is unneglectable in watershed nitrogen sources reduction strategies.

Trace elements and isotope data of the Um Garayat gold deposit, Wadi Allaqi district, Egypt

Trace element composition of sulfides and O, C, Sr and S isotopic data are assessed to constrain the evolution and potential fluid and metal sources of the Um Garayat gold deposit. Ore microscopy and BSE investigations of quartz veins show blocky arsenopyrite and pyrite replaced in part by pyrrhotite, chalcopyrite, sphalerite, galena, and gersdorffite. Free-milling gold occurs commonly in close association with the late sulfides, and along fractures in pyrite. On the other hand, recrystallized pyrite is disseminated in host metavolcaniclastic/metasedimentary rocks that commonly contain carbonaceous material. In situ LA-ICP-MS analysis of sulfides shows the recrystallized pyrite enriched in most trace elements, while blocky pyrite contains only some traces of arsenic. Detected concentrations of gold (up to 17 ppm) were only reported in arsenopyrite disseminated in quartz veins. The δ34S values of blocky pyrite and pyrrhotite in quartz veins define a narrow range (1.6 to 3.7‰), suggesting a homogenous sulfur source which is consistent with the dominantly mafic host rocks. The recrystallized pyrite has a distinctive sulfur isotope composition (δ34S − 9.3 to − 10.6‰), which is rather comparable to diagenetic sulfides. Hydrothermal carbonate in quartz veins and wallrock have nearly constant values of δ18O (10.5 to 11.9‰) and δ13C (− 4.2 to − 5.5‰). Based on constraints from mineral assemblages and chlorite thermometry, data of six samples indicate that carbonate precipitation occurred at ~ 280 °C from a homogenous hydrothermal fluid with δ18OH2O 4.4 ± 0.7‰ and δ13C = 3.7 ± 0.8‰. Strontium isotope values of two samples (87Sr/86Sr = 0.7024 and 0.7025) are similar to the initial 87Sr/86Sr ratios of island arc metabasalts (~ 710 Ma) in the South Eastern Desert. The generally homogenous sulfur, C, O, Sr isotope data are suggestive of metamorphogenic fluids, likely produced from dominantly mafic volcanic rocks at the greenschist–amphibolite facies transition.

Subduction contribution to the magma source of the Okinawa Trough—Evidence from boron isotopes

Boron (B) has been an excellent geochemical tracer for the investigation of magmatism in arcs and back‐arc basins, due to its high fluid mobility and distinct isotope compositions in the mantle wedge and subducting slabs. In this study, B isotope compositions of the volcanic rocks collected from the northern, middle, and southern Okinawa Trough were determined to discuss the sources of B in the magma and assess the contribution of subduction components to the magma source. The results show that B in the volcanic rocks from the Okinawa Trough are mainly from the underlying mantle wedge but influenced by the fluids derived from the subducted slab. Compared with volcanic rocks from the middle Okinawa Trough, samples from the southern Okinawa Trough are generated from the source with larger proportion of subduction components. For the volcanic rocks from the northern Okinawa Trough, the influence of the subduction components is the most significant and the proportion of slab‐derived B in the magma reaches up to 30%. It is estimated that the subduction components are composed of altered oceanic crust/serpentinite fluid and sediment fluid with the proportion of 94:6. The B‐rich fluid, loaded by cold and fast dipping slab, released into overlying mantle wedge resulting in partial melting of mantle is the genetic mechanism of B‐rich volcanic rocks in the Okinawa Trough.

The Origin and Evolution of Nucleosynthetic Sr Isotope Variability in Calcium and Aluminum-rich Refractory Inclusions

Abstract Nucleosynthetic isotope anomalies in meteorites are useful for investigating the origin of materials in the protoplanetary disk and dynamical processes of planetary formation. In particular, calcium and aluminum-rich inclusions (CAIs) found in chondrites are key minerals for decoding the initial conditions of the solar system before the accretion of small planetary bodies. In this study, we report isotopic analyses for three Allende CAIs, fluffy type A (FTA), type B, and fine-grained spinel rich (FS) inclusions, with a specific emphasis on the measurements of 84Sr/86Sr ratios. It was found that the average μ 84Sr values (106 relative deviations from a standard material) were 175, 129, and 56 ppm for the samples of FTA, type B, and FS inclusions, respectively. Additionally, the FTA samples exhibited heterogeneous μ 84Sr values, while those for the type B and FS inclusions were homogeneous within individual inclusions. The elevated μ 84Sr values were most likely explained by the relative enrichment of r-process nuclides in the CAI formation region. The variation of μ 84Sr values between the FTA and type B inclusions, as well as within the FTA inclusion, suggests the presence of multiple CAI source reservoirs with distinct isotopic compositions, which is either inherited from isotopic heterogeneity in the molecular cloud or caused by the selective destruction of r-process-enriched supernova grains via nebular thermal processing. On the other hand, the reaction between a refractory precursor of the FS inclusion and a gaseous reservoir enriched in Mg, Si, and 16O resulted in the lowest μ 84Sr values for the FS inclusion.

Noble gases in conventional and unconventional petroleum systems

Abstract Petroleum systems represent complex multiphase subsurface environments. The properties of the noble gases as conservative physical tracers allow them to be used to gain insight into the physical behaviour occurring within hydrocarbon systems. This can be used to better understand the mechanisms of hydrocarbon migration, residence time of fluids, and measurement of the scale of the subsurface fluid system involved in the transport and trapping of the hydrocarbon phase. The noble gases in the subsurface derive from different sources with distinct isotopic compositions, allowing them to be resolved in any crustal fluid. We discuss the processes within petroleum systems that incorporate the noble gases from each of these sources into hydrocarbon accumulations. The dominant mechanism controlling the introduction of air-derived noble gases into petroleum systems is via subsurface groundwater, and this records key information about the interaction of the petroleum system with the hydrogeological regime. Radiogenic noble gases accumulate over time, recording information about the age and relative timing of processes within the petroleum system. We review the conceptual framework and quantitative models describing these processes using examples from previous studies, and discuss both their current limitations and the potential for their application to unconventional hydrocarbon systems.

Using environmental isotopes along with major hydro-geochemical compositions to assess deep groundwater formation and evolution in eastern coastal China

Hydrochemical analysis and environmental isotopic tracing are successfully applied to study groundwater evolution processes. Located in eastern China, the Jiangsu Coastal Plain is characterized by an extensively exploited deep groundwater system, and groundwater salinization has become the primary water environmental problem. This paper provides a case study on the use of a hydrochemical and environmental isotopic approach to assess possible mixing and evolution processes at Yoco Port, Jiangsu Province, China. Hydrochemical and isotopic patterns of deep groundwater allow one to distinguish different origins in deep water systems. HCO3– is the dominant anion in the freshwater samples, whereas Na+ and Cl− are the dominant major ions in the saline samples. According to δ18O, δ2H and 14C dating, the fresh water is derived from precipitation under a colder climate during the Glacial Maximum (Dali Glacial), while the saline groundwater is influenced by glacial-interglacial cycles during the Holocene Hypsithermal. The δ18O, δ2H and 3H data confirm that deep groundwater in some boreholes is mixed with overlying saline water. The deep groundwater reservoir can be divided into a saline water sector and a fresh water sector, and each show distinct hydrochemical and isotopic compositions. The saline groundwater found in the deep aquifer cannot be associated with present seawater intrusion. Since the Last Glacial Maximum in the Late Pleistocene, the deep groundwater flow system has evolved to its current status with the decrease in ice cover and the rising of sea level. However, the hydraulic connection is strengthened by continuous overexploitation, and deep groundwater is mixed with shallow groundwater at some points.

Diverse lamprophyres origins corresponding to lithospheric thinning: a case study in the Jiurui district of Middle-Lower Yangtze River Belt, South China Craton

Late Mesozoic magmatic rocks that range in composition from mafic to felsic are widespread in the Jiurui district of the Middle-Lower Yangtze River belt in eastern Yangtze Craton of southeastern China. Among which the lamprophyre dikes represent mantle-derived magma and record deep mantle magmatic processes in the region. In this paper, we report a detailed study of geochronology, mineral chemistry, petrochemistry and Sr-Nd-Pb isotopes on the lamprophyres from the Nangang area in the Jiurui district, in an attempt to provide a comprehensive understanding in their petrogenesis, as well as the Mesozoic lithospheric mantle characteristics, and geodynamical control of the extensive Mesozoic magmatism in the Jiurui district. The Nangang lamprophyres belong to alkaline lamprophyre. They are characterized by low contents of SiO2 and TiO2, high Mg# values and high contents of compatible elements, and are enriched in LREE and LILE but depleted in HFSE (with Nb-Ta-Ti negative anomalies). They show moderate enriched Sr-Nd isotopic compositions with initial 87Sr/86Sr values of 0.70598 to 0.70715 and εNd(t) values of −2.2 to −1.6. They also show radiogenic Pb isotopic compositions with (206Pb/204Pb)i values of 17.678 to 18.558, (207Pb/204Pb)i values of 15.544 to 15.591 and (208Pb/204Pb)i values of 38.349 to 38.664. Combining new data from the Nangang lamprophyres with those of previously published data for lamprophyres in the Jiurui (i.e. Wushan I and II lamprophyres) and Edong districts (i.e. Ruanjiawan lamprophyres), we suggest that the Jiurui lamprophyres were likely derived from partial melting of the enriched lithospheric mantle characterized by phlogopite-bearing lherzolite in spinel-garnet transition zone at 75–85km depth. The feature of enriched-mantle source region was formed by metasomatism of subducted slab-derived fluids and melts, which is likely related to the Mesozoic subduction of the paleo-Pacific plate underneath the eastern China continent and the Neoproterozoic subduction of Cathaysia Block towards the Yangtze Craton. In contrast, the Ruanjiawan lamprophyres from the Edong district have the geochemical characteristics (such as the Nb-Ta positive anomalies) which are obviously different from the Jiurui lamprophyres, and we conclude that the Ruanjiawan lamprophyres, which are located more inland than the Jiurui district, were likely derived from the depleted asthenospheric mantle that show no significant influence by the subduction-related metasomatism. Furthermore, interpretation of the Sr-Nd-Pb isotope data for the Jiurui lamprophyres suggests slightly different petrogenesis for the various lamprophyre dikes, namely, the Nangang and Wushan–I lamprophyres were derived mainly from the enriched lithospheric mantle, but with addition of depleted asthenospheric mantle materials, whereas the Wushan–II lamprophyre dikes were product of the enriched lithospheric mantle without any signature for addition of depleted asthenospheric mantle materials. Given that these lamprophyres with distinct different isotopic compositions are sympatric and coeval, we consider that in the Late Jurassic to Early Cretaceous period (ca. 155–125Ma), the lithosphere was thinned to about 75–85km depth. The enriched lithospheric mantle was heated by the underlying convective asthenosphere and underwent partial melting to generate the Wushan–II lamprophyres. Subsequently, the enriched lithospheric mantle delaminated into depleted asthenospheric mantle, followed by partial melting coupled with an intensive interaction with the asthenospheric mantle to generate the Nangang and Wushan–I lamprophyres, respectively. Towards more inland, the Ruanjiawan lamprophyres were derived from the depleted asthenospheric mantle.