Boron Isotope Geochemistry Research Articles

Boron isotope fractionation between B(OH)3 and B(OH)4− in aqueous solution: A theoretical investigation beyond the harmonic and Born−Oppenheimer approximations

Boron isotope fractionation between boric acid and borate ion in aqueous solution (hereafter, α3–4) is a key coefficient for reconstructing paleo-pH and paleo-pCO2 records, and a valuable parameter for understanding boron isotope geochemistry related to mineral-water interactions. Although boron isotopes have wide geologic application, the effect of temperature and ion pairing on boron isotope fractionation is poorly described. Moreover, conventional density functional theory (DFT) calculations do not provide an accurate estimate of the boron isotope fractionation factor. Here, we provide a new strategy to accurately calculate α3–4 values in aqueous solution by evaluating harmonic frequencies, higher-order energy terms, and solvation effects. Using benchmark coupled cluster (CCSD(T)) calculations, our 25 °C results for α3–4 ranges from 1.0259 to 1.0275, which is in good agreement with recent experimental data. Solvation effects reduced the α3–4 value by ∼6‰, while high-order corrections raised the α3–4 value by ∼2‰. Our calculations evaluating the effect of aqueous ion pairs suggest that the α3–4 values in seawater and pure water should be almost identical. Our results provide a reasonable estimation of the partition function ratios of dissolved boron species, which will benefit studies of boron isotope fractionation in aqueous environments.

Application of AI Identification Method and Technology to Boron Isotope Geochemical Process and Provenance Tracing of Water Pollution in River Basins

River water is the most important water source that people can use. Since the 20th century, human influence on river courses has become increasingly serious. The quantitative analysis of water quality is even more difficult. According to the characteristics of Fenhe water chemistry, pollution time and pollution control factors, the contribution rate of people in the polluted water body is not clear. Therefore, this paper aims to use AI identification methods and technologies to study water pollution and provenance tracing. The combination of major elements, trace elements and stable isotopes was used to study the chemical characteristics, water quality status, and sources of pollution of the Fenhe water in the Fenhe area. Because the water contains a large number of pollution sources, it is difficult to find the source using traditional methods. Using correlation analysis, principal component analysis, multi-factor regression analysis, trend analysis and other methods, the macroelements and trace elements in the water body of the Fenhe River were analyzed. The boron sources in the Fenhe river were qualitatively and quantitatively analyzed using mass spectrometry equilibrium equation. Using the boron isotope value of the river, it showed a spatial variation of upstream (+5.1‰) < middlestream (+8.6‰) < downstream (+9.5‰) in dry season, and showed a spatial variation of upstream (+6.1‰) < downstream (+7.2‰) < middlestream (+9.0‰) in the wet season. The contribution of silicate to B is calculated by subtracting the contribution of other resources from the comprehensive contribution rate. It is found that the contribution of silicate is about 38.8%, 22% in dry season and 49.2%, 17% in wet season. The research results have provided a reliable scientific basis for the protection of water resources and pollution control in the Fenhe River Basin. Therefore, the above research confirms the role of AI identification method in the process of boron isotope geochemistry and provenance tracing of water pollution in river basins.

Boron isotope geochemistry of Bangor Co Salt Lake (central Tibet): implications for boron origin and uneven mixing of lake water

Boron isotope geochemistry of Bangor Co Salt Lake (central Tibet): implications for boron origin and uneven mixing of lake water

Boron Isotope Geochemistry of the Lakkor Co Salt Lake (Tibet) and Its Geological Significance

The origin of boron in modern salt lakes has long been a controversial subject. This study focuses on the boron geochemistry of Lakkor Co salt lake, located on the Shiquanhe-Lakkor Co-Yongzhu-Jiali Ophiolite Mélange Zone (SYMZ). Lakkor Co is one of the main boron-rich salt lakes in the Tibetan plateau where the boron concentration ranges from 401 to 849 mg L-1. The boron isotope value ( δ 11 B ) of the carbonate deposit within the Lakkor Co region is -28.08‰. The boron isotope value of the Somme river decreases from -9.97‰ to -11.15‰ after it flows through the carbonate deposit region, indicating that the carbonate deposit in this region releases boron, which has a more negative isotope value. However, the boron isotope value of Bai Kok river increases from -15.63‰ to -13.76‰, when it flows through the Quaternary clay region in Ganga Co, indicating that the carbonate deposit undergoes a process of boron adsorption. It can be concluded that the Quaternary carbonate deposits may be a source of boron and also serve as a temporary boron reservoir, thus playing an important role in the formation of modern salt lake boron deposits in Tibet.

Paleoproterozoic Rajapalot Au-Co system associated with evaporites: Chemical composition and boron isotope geochemistry of tourmaline, and sulfur isotopes of sulfates, Peräpohja belt, northern Finland

The Raja Au-Co mineralization in the Paleoproterozoic Peräpohja belt in northern Finland is part of the wider Rompas-Rajapalot mineralized area with several known Au-Co occurrences. The area is characterized by distinct tourmaline occurrences, spatially associated with Au-Co mineralization. Four texturally distinct tourmaline types were identified although all analysed tourmalines belong to the alkali-group and are classified as dravite. δ 11B values and fractionation modelling indicates that at least two distinct fluids were responsible for the tourmaline formation with initial end member δ11B values of the fluids at −8‰ and −1‰. Possible sources for boron rich fluids are Svecofennian orogeny related (ca. < 1.9 Ga) metamorphic fluids and magmatic-hydrothermal fluids related to the late-orogenic ca. 1.78 Ga granitoids. Structural data from the quartz-tourmaline veins outlines a strong linear trend towards the Raja high-grade mineralization trend. This is interpreted as evidence for structural control of the Raja mineralization, which possibly is contained in shear-zones or within the hinge regions of local high degree folding.A distinctive rock unit with bright purple anhydrite layers and white gypsum veins has been intersected by drilling. While the sulfate-rich unit is unmineralized, the mineralogy and regional geological setting suggests an evaporitic origin. Anhydrite has δ34S values in a narrow range from 8.1 to 9.8 ‰. Gypsum has slightly heavier δ34S from 10.6 to 12.2 ‰. Together with isotope fractionation constraints, textural evidence suggests the authigenic formation of gypsum with SO42- sourced from anhydrite. Significant similarities in textures and sulfur isotope values to well-known evaporite successions of the Onega basin, western Russia, further supports the presence of evaporitic strata within the Peräpohja belt.

Variations of the East Asian monsoon over the past 800 kyr constrained by the boron isotope composition of paleo-rainwater inferred from loess-paleosol deposits in NE China

Based on a study of the boron (B) isotope geochemistry of the loess-paleosol sequence of the Niuyangzigou section (NYZG) in Northeast (NE) China, the boron isotopic composition of paleo-rainwater (δ11Bpr) is used for the first time as a potential proxy for tracing changes in the East Asian summer monsoon (EASM) since the mid-Pleistocene. The acid-soluble phase of loess in NE China, which is characteristically a sandy loess, has lower boron concentrations (0.26 to 0.89 μg/g) and more positive δ11B values, from −12.1‰ to +9.4‰, compared to the loess of the Chinese Loess Plateau (CLP). According to the mass balance of boron accumulation from three distinct sources, a mathematical inversion calculation is used to quantify the proportion of boron from each source and to capture the original δ11Bpr signature. The resulting δ11Bpr values range from +6.8 ± 0.95‰ in loess layers to +17.3 ± 3.32‰ in paleosol layers, and the boron accumulated in loess-paleosol layers is shown to be mainly from atmospheric precipitation and silicate weathering. Synchronous variations of reconstructed δ11Bpr and other loess-based proxies, both in NE China and in the CLP, are observed over a large spatial range and on several temporal scales. The linkage of δ11Bpr to the intensity of the EASM is attributed to the kinetic boron isotope fractionation associated with vapor-precipitate phase separation along the transport path of moisture-bearing air masses. A Rayleigh fractionation model is proposed to interpret δ11Bpr in terms of the proportion of water vapor loss after multi-step fractionation processes, which is independent of temperature. The monsoon record based on δ11Bpr shows a significant shift at ∼430 ka, implying an abrupt weakening of the EASM after the Mid-Brunhes Event (MBE), which provides evidence for regional disparities in monsoon evolution in response to the MBE. The ice volume in Northern Hemisphere is a possible factor controlling changes in the EASM in NE China over the past 800 kyr.

Stable Isotope (S, Mg, B) Constraints on the Origin of the Early Precambrian Zhaoanzhuang Serpentine-Magnetite Deposit, Southern North China Craton

The origin of the Zhaoanzhuang serpentine-magnetite deposit in the southern North China Craton (NCC) is highly disputed, with some investigators having proposed an ultramafic origin, whereas others favor a chemical sedimentary origin. These discrepancies are largely due to the difficulty in determining the protolithic characteristics of the highly metamorphosed rocks. Sulfur, magnesium, and boron isotope geochemistry combined with detailed petrography was carried out in this study to constrain the original composition of the Zhaoanzhuang iron orebodies. Anhydrite is present as coarse crystals intergrown with magnetite, indicating that the anhydrite formed simultaneously with the magnetite during metamorphism rather than as a product of later hydrothermal alteration. The anhydrite has a narrow range of positive δ34S values from +19.8 to +22.5‰ with a mean value of +21.1‰. These values are significantly higher than that of typical magmatic sulfur (δ34S = 0 ± 5‰) and deviate away from primary igneous anhydrite towards mantle-sulfur isotopic values, but they are similar to those of marine evaporitic anhydrite and gypsum (~+21‰). The sulfur isotopic compositions of several samples show obvious signs of mass-independent sulfur fractionation (Δ33S = −0.47‰ to +0.90‰), suggesting that they were influenced by an external sulfur source through a photochemical reaction at low oxygen concentrations, which is consistent with the Neoarchean-Paleoproterozoic atmosphere. Coarse-grained tourmaline from the tourmaline-rich interlayers of the orebodies occurs closely with Mg-rich minerals such as phlogopite, talc, and diopside, indicating that it has a metamorphic origin. The δ11B values of the tourmaline range from −0.2‰ to +3.6‰ with a mean value of +2.0‰, which is much positive relative to that of magmatic tourmaline but is consistent with that of carbonate-derived tourmaline. The magnesium isotopic analyses of the serpentine–magnetite ores and the magnesium-rich wall rocks revealed a wide range of very negative δ26Mg values from −1.20‰ to −0.34‰ with an average value of −0.80‰. The value is higher than that of ultramafic rocks (δ26Mg = −0.25‰) and exhibits minor Mg isotopic fractionation. However, these values are consistent with those of marine carbonate rocks, which have lower δ26Mg values and larger Mg isotopic variations (δ26Mg = −0.45‰ to −4.5‰). Collectively, the S–Mg–B isotopic characteristics of the Zhaoanzhuang iron orebodies clearly indicate a chemical sedimentary origin. The protoliths of these orebodies most likely reflect a series of Fe–Si–Mg-rich marine carbonate rocks with a considerable evaporite component, indicating a carbonate-rich superior-type banded iron formation precipitated in an evaporitic shallow marine sedimentary environment.

Boron isotope geochemistry of a brine-carbonate system in the Qaidam Basin, western China

The boron (B) isotope-pH proxy for marine carbonates has been widely used to reconstruct past ocean pH values. Unlike seawater, the apparent dissociation constant of boric acid (KB) and the δ11B values of brines from the terrestrial salt lakes in western China vary. However, relevant studies concerning the B isotope geochemistry of brine-carbonate systems in terrestrial salt lakes are limited. In this study, thirty-nine clastic samples from a sediment core (ISL1A) from the Qarhan Salt Lake in the Qaidam Basin were collected. These carbonates were analyzed for δ11B, B, Mg, Sr and Ca. Using our new data as well as published pH values, total dissolved solids (TDS), element concentrations, and δ11B values of brines in western China, we reached the following conclusions. (1) During evaporation process, salinity increases, the δ11B values of brines in salt lakes increase, and the pKB (−logKB) and pH values simultaneously decrease. (2) The large heterogeneity in the δ11B values of the carbonates (−2.74‰ to +7.64‰) from core ISL1A is mainly due to the δ11B values of the brines, rather than pKB and pH. (3) B isotopic fractionation with a small αcarbonate-brine of 0.997 in a brine-carbonate system conforms to the Rayleigh fractionation model. (4) Our comparison of the δ11B values of these carbonates with other records from core ISL1A suggests that the δ11B values of the carbonates in the arid Qaidam Basin can be used as a new proxy for the salinity of paleo-lake water.

The boron isotope geochemistry of smectites from sodium, magnesium and calcium bentonite deposits

The mineralogical, chemical and isotopic analyses of smectites, with variable interlayer cation occupancies, from bentonite deposits in various depositional environments, reveal new insights into the boron sources and the fluids involved in bentonitization in marine and non-marine environments.Smectites from bentonites have non-exchangeable, structural boron concentrations of 0.2 to 196 μg/g B. Smectites from sodium bentonites have higher boron concentrations (>30 μg/g) than those from magnesium or calcium bentonites. Most smectites have a small, interstratified illitic component that has a major influence on boron concentrations, requiring the use of modified fluid-mineral boron partitioning coefficients, which indicate that bentonites formed from fluids of highly variable boron concentrations of <0.1 mg/L B to >100 mg/L B, and a chlorine content of 76.4 mg/L to 59,076 mg/L. The sodium bentonites formed from boron-rich saline fluids or brines whereas the fluids involved in calcium and magnesium bentonite formation have a more variable boron composition and salinity.The δ11B values of the structural boron in tetrahedral sites of smectites range from −30.1‰ to +12.2‰. The smectites from terrestrial depositional settings have δ11B values of −30.1‰ to about 0‰, whereas smectites from marine depositional settings have negative as well as positive δ11B values. The boron isotope values indicate that all examined bentonites from terrestrial depositional settings as well as many bentonites from marine depositional settings formed from basinal fluids or brines.The boron isotope geochemistry of smectites is demonstrated to be a tool for elucidating the fluids involved in the formation of clay mineral deposits. It also has great potential for tracing fluids in other settings involving authigenic clay minerals such as sedimentary basins and surficial crystalline rocks, as well as man-made applications such as in disposal sites for highly active nuclear waste.

Boron, lithium and nitrogen isotope geochemistry of NH4-illite clays in the fossil hydrothermal system of Harghita Bãi, East Carpathians, Romania

Coarse (2.0–0.2μm) and fine (<0.2μm) clay fractions of NH4-illite-smectite (I-S) mixed-layered and K-illite/(NH4, K)-illite (I) mixed phases that vary in age and trace element composition were collected from the fossil hydrothermal system of Harghita Bãi, East Carpathians. Boron and Li isotope ratios were measured by secondary ion mass spectrometry (SIMS), and N by isotope ratio mass spectrometry (IRMS) with the aim to characterize the isotope geochemistry and source of light elements fixed in authigenic NH4-illitic clays.Boron in NH4-I-S clays ranges from 513 to 1457ppm and reached ~1000ppm in the K-I/(NH4, K)-I. The δ11B (‰) measured in NH4-I-S ranges from −12.6 to −22.4 (±0.3‰) and in K-I/(NH4, K)-I is consistently −5.5 to −5.1 (±0.3‰). Boron isotopes systematically become lighter in the NH4-I-S series as temperature increased from 90 to 270°C. Low Li content (1 to 8ppm) was found in illitic clay fractions. The δ7Li (‰) shows negative values ranging from −8.6 to −12.3 (±0.8‰) for the coarser (2.0–0.2μm) NH4-I-S clays and from +4.3 to +14.1 (±1‰) for the finer (<0.2μm) fraction of NH4-I-S and K-I/(NH4, K)-I clays. The N (%) measured in the NH4-I-S clays ranges from 0.70 to 1.50 (±0.2%), whereas in the K-I/(NH4, K)-I is about 0.70 (±0.2%). The δ15N (‰) ranges from +4.8 to +7.4 (±0.6) for most NH4-I-S and NH4, K-I clays, with one outlier for NH4-I-S of +14.6 (±0.6).The δ11B of K-I/(NH4, K)-I clays reflect a magmatic source, whereas the NH4-I-S series is consistent with the influx of isotopically light-B waters derived from hydrothermal leaching of continental evaporites and/or organic-rich sediments. The δ7Li signature measured on K-I/(NH4, K)-I clays also support a magmatic fluid, enriched in heavy Li, followed by precipitation of coarser NH4-I-S from more recent sedimentary contributions of isotopically light Li. This interpretation is also supported by the δ15N, which reflect an influx of waters from an organic sediment origin. The δ15N of +14.6‰ (±0.6) measured on NH4-I-S could be attributed to the presence of meteoric waters mixed with hydrothermal fluids. The isotopic data obtained trace the mobility of magmatic and organic – sedimentary components in the upper continental crust.

Fluid flow and water–rock interaction across the active Nankai Trough subduction zone forearc revealed by boron isotope geochemistry

Compositional changes, dehydration reactions and fluid flow in subducted sediments influence seismogenesis and arc magmatism in subduction zones. To identify fluid flow and water–rock interaction processes in the western Nankai Trough subduction zone (SW Japan) we analyzed boron concentration and boron isotope composition (δ11B) of pore fluids sampled across the subduction zone forearc from depths of up to ∼922m below seafloor during four Integrated Ocean Drilling Program (IODP) Expeditions. The major structural regimes that were sampled by coring include: (1) sedimentary inputs, (2) the frontal thrust zone, (3) the megasplay fault zone, and (4) the forearc basin. From mass balance consideration we find that consumption of boron (B) by ash alteration and desorption of B from the solid phase, mediated by organic matter degradation, produces a net decrease in B concentrations with depth down to ∼120μM and variable δ11B values in the range of ∼+20‰ and +49‰. Interstitial water in sediments on the incoming oceanic plate are influenced by more efficient mobilization of exchangeable B from the solid phase due to higher temperatures and alteration of the oceanic crust that acts as a sink for 10B. At the tip of the megasplay fault zone, elevated B concentration and B isotopic composition suggest that underthrust coarse-grained slope sediments provide a pathway for fluids out of the upper (<2km) accretionary prism. Silt and sand layers in the underthrust section of the downgoing plate favor fluid escape in seaward direction from depths equivalent to the temperature range of 60–150°C. At both locations the δ11B signature evolves during updip migration through re-adsorption. Mass balance considerations suggest a shallower fluid source depth compared to pore fluids sampled previously near the décollement zone along the central portion of the Nankai margin.

Boron occurrence in halite and boron isotope geochemistry of halite in the Qarhan Salt Lake, western China

Previous studies have investigated boron (B) isotope composition of salt lake brines in the Qaidam Basin, western China. However, the research on B isotope geochemistry of halite from a sediment core in a typical sedimentary basin has been very limited. In this study, a 102-m-long drill core (ISL1A) was recovered from Qarhan Salt Lake in eastern Qaidam Basin. Forty-three halite samples from upper 44.0m in ISL1A were collected and analyzed for chemical compositions (K+, Na+, Ca2+, Mg2+, Cl−, SO42−, Sr2+ and B3+) and B isotopes in order to investigate their relationships, and B isotope fractionation between halite and brines in the Qarhan Salt Lake. The results show that: (1) more B3+ and Mg2+ values in halite are low concentrations and more concentrated, and low B3+ concentrations have a strong correlation with low Mg2+ in halite, which imply that they might be the same source; (2) low Mg2+ values in halite from ISL1A have a similar trend with mMg2+ (molarity) of fluid inclusion brines from two sediment cores (east to ISL1A about 50km) in the study area, suggesting that low B3+ and Mg2+ concentrations should derive from fluid inclusions in halite based on X-ray diffraction results in ISL1A that no borate and small amount of magnesium salts were deposited; (3) δ11B values of halite in ISL1A range from −0.35 to +5.84‰, which are in the range of those of river water and brines (−1.0 to +9.6‰), and almost overlapped with those of brines (+2.9 to +7.5‰) in the Qarhan Salt Lake. These results suggest that no or minor B isotope fractionation between halite and brine occurred.

Boron isotope systematics of a fossil hydrothermal system from the Troodos ophiolite, Cyprus: Water–rock interactions in the oceanic crust and subseafloor ore deposits

Abstract We determined concentrations and isotopic compositions of boron in a complete section of the hydrothermally altered Cretaceous oceanic crust of the Troodos ophiolite. The boron content and δ 11 B value for each lithological section are: pillow lava (3.8–206.8 μg/g, 63 μg/g average; δ 11 B = + 0.17‰ to + 15.6‰, + 8.1‰ average), sheeted dike complex (0.6–18.0 μg/g, 4.0 μg/g average; δ 11 B = + 3.3‰ to + 10.6‰, + 6.0‰ average), and plutonic complex (0.3–8.4 μg/g, 1.7 μg/g average; δ 11 B = − 1.7‰ to + 18.5‰, + 4.5‰ average). These boron contents are higher than the estimated original igneous values throughout the oceanic crust, indicating uptake of boron from seawater and hydrothermal fluid at temperatures ranging from ≤ 50 °C to > 300 °C. Although our boron data for the Troodos ophiolite are generally consistent with those for the Oman ophiolite of similar age, the distinctly low δ 11 B values of the lower gabbro section in the Troodos ophiolite ( 11 B-depleted fluid at a very small water/rock ratio. The boron content of the bulk oceanic crust (12.3 μg/g) estimated for the Troodos ophiolite is relatively high as a result of strong boron enrichment in the pillow lava section, which underwent prolonged seafloor weathering. Despite these differences, the weighted average δ 11 B value of the bulk oceanic crust (+ 7.6‰) is similar to that of the Oman ophiolite (+ 7.9‰). We also analyzed the boron isotope geochemistry of a subseafloor hydrothermal stockwork sulfide deposit in the Troodos ophiolite to investigate its formation processes. In contrast to the normal upper oceanic crust, the δ 11 B values of the rocks below the ore body decrease with increasing depth and have large negative values (− 6‰) in the highly altered uppermost dike section. These low δ 11 B values are coupled with high boron contents (2.5–17 μg/g) and high and uniform 87 Sr/ 86 Sr ratios (0.7064 average), and are unlikely to have resulted from interactions with fluids at a small water/rock ratio. These characteristics are better explained by interaction of ore-forming hydrothermal fluids with oceanic crust that had previously been enriched in boron through hydrothermal alteration at low temperatures. These observations demonstrate that boron and boron isotopes are useful for quantitative evaluation of fluid-related processes with multiple stages, including petrogenesis of hydrothermal ore deposits.

Boron isotope geochemistry ofNa‐bicarbonate,Na‐chloride, andCa‐chloride waters from theNorthernApennineForedeep basin: other pieces of the sedimentary basin puzzle

AbstractThe boron stable isotope ratio δ11Bof 12 water samples representative of three chemical facies (freshNa‐bicarbonate, brackishNa‐chloride, saline, and brineCa‐chloride) has been analyzed. Interpretation of the δ11Bdata, along with the chemical compositions, reveals thatNa‐carbonate waters from theNorthernApennine are of meteoric origin, with boron contributions from clay desorption and mixing with seawater‐derived fluids ofNa‐chloride orCa‐chloride compositions. The comparison of our new results with the literature data on other sedimentary basins ofMediterranean, and worldwide, confirms the contribution ofNa‐bicarbonate waters to the genesis of mud volcano fluids. TheNa‐chloride sample ofSalvarola (SAL), which may represent the end‐member of the mud volcanoes, and theCa‐chloride brine water fromSalsomaggiore (SM) indicate boron release from clays compatible with the diagenetic process. The empirical equation:urn:x-wiley:14688115:media:gfl12124:gfl12124-math-0001relating boron concentration and the stable isotope composition of the fluids observed in this study and the literature is proposed to trace the effect of diagenesis in sedimentary basins. A geothermometer associated to the diagenetic equation is also proposed:urn:x-wiley:14688115:media:gfl12124:gfl12124-math-0002The application of this equation to obtain reservoir temperatures from δ11B compositions of waters should be carefully evaluated against the results obtained from other chemical and isotopic geothermometers from other basins around the world.

Implication of boron isotope geochemistry for the pedogenic environments in loess and paleosol sequences of central China

We investigated the boron isotopic composition in loess–paleosol sequences in five different profiles in the Chinese Loess Plateau. Three possible boron sources are identified: atmospheric input, carbonates, and weathered silicate rocks. Variations of [Sr], [B], δ11B and the magnetic susceptibility correlate well with the pedogenetic intensity in three out of the five studied profiles, where pedogenesis under a cold–dry climate indicates lower δ11B, lower [B], lower magnetic susceptibility and higher [Sr] values. Exceptions to the variations between the δ11B and other known proxies were observed in arenaceous soils and the Red Clay sequence: the former suggested that vertical redistribution probably occurred with the boron migration, and the latter indicated an unknown mechanism of susceptibility enhancement. A better correlation between the δ11B and magnetic susceptibility and the quantitative estimation of boron budget from each source confirms the influence of paleoenvironmental changes on boron geochemical cycle. Significant positive correlations in Sr/Ca vs. B/Ca and Mg/Ca vs. B/Ca reflect consistent enrichment behavior of those mobile elements into calcium carbonate. The preliminary results imply that boron isotopic compositions in soils can be a potential geochemical proxy to reconstruct the paleoenvironmental changes in loess–paleosol sequences.

Boron isotope geochemistry of vent fluids from arc/back-arc seafloor hydrothermal systems in the western Pacific

Boron concentrations and isotopic compositions were determined for vent fluids from 11 sites in arc/back-arc seafloor hydrothermal systems. This new dataset allowed a systematic comparison of boron behavior during water–rock interaction in different geological settings. In sediment-starved hydrothermal systems (e.g., Manus Basin, Izu-Bonin Arc, Mariana Trough), end-member fluids showed large variations in boron concentration (0.53–1.52mmol/kg) and δ11B value (13.5–29.7‰), reflecting the interaction of seawater with varied types of rock that occur in arc/back-arc settings. Based on the model calculations, the boron concentration and δ11B value of the fluid are better explained by an equilibration between seawater and rock than a mixing model. The Chlorine-depleted fluids from phase-separated hydrothermal systems (e.g., North Fiji Basin) had low boron concentrations (0.44–0.55mmol/kg) and high δ11B values (34.5–36.1‰). The small fractionations of boron and boron isotopes during phase separation suggest that these characteristics were acquired during water–rock interaction rather than phase separation and segregation processes. The fluids from sediment-hosted hydrothermal systems (e.g., Okinawa Trough) are characterized by high boron concentrations (3.9–4.8mmol/kg) and low δ11B values (2.5–2.9‰). These fluids also showed high Cs/B ratios, indicating interactions with terrigenous sediments. Model calculations demonstrate that the combined evaluation of δ11B values and Cs/B ratios are useful for a quantitative estimate of sedimentary contributions in seafloor hydrothermal systems.

Rapid, high-precision measurements of boron isotopic compositions in marine carbonates.

The isotopic composition and elemental abundance of boron (B) in marine carbonates provide a powerful tool for tracking changes in seawater pH and carbonate chemistry. Progress in this field has, however, been hampered by the volatile nature of B, its persistent memory, and other uncertainties associated with conventional chemical extraction and mass spectrometric measurements. Here we show that for marine carbonates, these limitations can be overcome by using a simplified, low-blank, chemical extraction technique combined with robust multi-collector inductively couple plasma mass spectrometry (MC-ICPMS) methods. Samples are dissolved in dilute HNO3 and loaded first onto on a cation-exchange column with the major cations (Ca, Mg, Sr, Na) being quantitatively retained while the B fraction is carried in the eluent. The eluent is then passed directly through an anion column ensuring that any residual anions, such as SO4(2-), are removed. Isotopic measurements of (11)B/(10)B ratios are undertaken by matching both the B concentration and the isotopic compositions of the samples with the bracketing standard, thereby minimising corrections for cross-contamination. The veracity of the MC-ICPMS procedure is demonstrated using a gravimetrically prepared laboratory standard, UWA24.7, relative to the international reference standard NIST SRM 951 (δ(11)B = 0‰). This gives values consistent with gravimetry (δ(11)B = 24.7 ± 0.3‰ 2sd) for solutions ranging in concentration from 50 to 500 ppb, equivalent to ~2-10 mg size coral samples. The overall integrity of the method for carbonate analysis is demonstrated by measurements of the international carbonate standard JCp-1 (δ(11)B = 24.3 ± 0.34‰ 2sd). A streamlined, integrated approach is described here that enables rapid, accurate, high-precision measurements of boron isotopic compositions and elemental abundances in commonly analysed biogenic carbonates, such as corals, bivalves, and large benthic forams. The overall simplicity of this robust approach should greatly facilitate the wider application of boron isotope geochemistry, especially to marine carbonates.

New tracers identify hydraulic fracturing fluids and accidental releases from oil and gas operations.

Identifying the geochemical fingerprints of fluids that return to the surface after high volume hydraulic fracturing of unconventional oil and gas reservoirs has important applications for assessing hydrocarbon resource recovery, environmental impacts, and wastewater treatment and disposal. Here, we report for the first time, novel diagnostic elemental and isotopic signatures (B/Cl, Li/Cl, δ11B, and δ7Li) useful for characterizing hydraulic fracturing flowback fluids (HFFF) and distinguishing sources of HFFF in the environment. Data from 39 HFFFs and produced water samples show that B/Cl (>0.001), Li/Cl (>0.002), δ11B (25-31‰) and δ7Li (6-10‰) compositions of HFFF from the Marcellus and Fayetteville black shale formations were distinct in most cases from produced waters sampled from conventional oil and gas wells. We posit that boron isotope geochemistry can be used to quantify small fractions (∼0.1%) of HFFF in contaminated fresh water and likely be applied universally to trace HFFF in other basins. The novel environmental application of this diagnostic isotopic tool is validated by examining the composition of effluent discharge from an oil and gas brine treatment facility in Pennsylvania and an accidental spill site in West Virginia. We hypothesize that the boron and lithium are mobilized from exchangeable sites on clay minerals in the shale formations during the hydraulic fracturing process, resulting in the relative enrichment of boron and lithium in HFFF.

Boron isotope geochemistry of salt sediments from the Dongtai salt lake in Qaidam Basin: Boron budget and sources

Dongtai salt lake in Qaidam Basin contains abundant boron and lithium resources, and has the highest reserve of lithium found in the world, but the origin of these resources is highly controversial. In this study, we carried out a detailed study on chemical compositions and boron isotope characteristics of salt sediments collected from a drill core in the Dongtai salt lake. The evaporite, carbonate and silicate phases of the sediments show δ11B values of −6.4‰ to +2.2‰, −17.3‰ to +0.5‰ and −14.6‰ to −2.8‰, respectively, which suggest a non-marine origin for the salt lake. The variation of δ11B vs. K% identified the evolution process of the salt lake. The geochemical/hydrochemical signatures and the reserve estimation of boron resources approved the dominant sources of saline resources containing abundant B, K and Li from the inflowing Nalenggele River. The boron isotopic fractionation factors between evaporite and brine (i.e. αevaporite–brine) vary from 0.9869 to 0.9955, and the linear variation of αevaporite–brine vs. Li/Mg molar ratios reflects the boron isotopic fractionation during crystallization of the salts. The positive correlation between δ11B and [B] in the carbonate phase reflects the pH control on the incorporation of boron into the carbonates, and the equilibrium isotope fractionation follows the Rayleigh isotope fractionation rule in a closed system. The variation of δ11B values in the silicate phase behaved as an oscillating pattern, and the more negative shift of δ11B values corresponds to the mudstone strata deposited under warm–humid climate conditions, indicating the influence of intensive weathering.

The boron isotope geochemistry of tourmaline-rich alteration in the IOCG systems of northern Chile: implications for a magmatic-hydrothermal origin

Hydrothermal tourmaline is common in the iron oxide-copper-gold (IOCG) deposits of the Coastal Cordillera of Chile where it occurs as large crystals in the groundmass of magmatic-hydrothermal breccias, such as in the Silvita or Tropezon ore bodies, or as small grains in replacive bodies or breccia cement in the ore-bearing andesite, as seen at the Candelaria or Carola deposits. Tourmaline shows strong chemical zoning and has a composition of schorl–dravite with significant povondraite and uvite components. The observed boron isotope composition is fairly variable, between −10.4‰ and +6.0‰ with no major differences among the different deposits, suggesting a common genetic mechanism. The δ11B values are significantly lower than those of seawater or marine evaporites and very similar to those of younger porphyry copper deposits and volcanic rocks in the region, indicating that the boron has a common, likely magmatic, origin. The predominant boron source was ultimately dewatering of the subducting slab with a significant contribution derived from the overlying continental basement. The range of δ11B values is between those of the porphyry copper deposits and the porphyry tin deposits of the Andes, suggesting that the IOCG mineralization might be genetically related to fluids having more crustal contamination than the porphyry copper deposits; such an interpretation is at odds with current models that propose that the Andean IOCG deposits are related to juvenile melts or to the circulation of basinal brines. Furthermore, the obtained δ11B data are markedly different from those of the tourmaline in the Carajas IOCG district (Brazil), suggesting that IOCGs do not form by a unique mechanism involving only one type of fluids.