Boron Isotopic Composition Research Articles

Boron isotope compositions establish the origin of marble from metamorphic complexes: Québec, New York, and Sri Lanka

Abstract The origin of an array of marble samples found in both the Grenville Province and southwestern Sri Lanka remains uncertain, whether magmatic, sedimentary, or mixed, due to their proximity to both carbonatite bodies and carbonate-rich metasedimentary rocks. This study reports boron and trace element abundances, in addition to carbon, oxygen, boron, and strontium isotopic compositions, to determine the petrogenesis of these carbonate-rich samples. Boron abundances for all of the samples are relatively high and variable (1.48–71.1 ppm) compared to those for carbonatites worldwide (≤1 ppm), and mostly overlap those documented for sedimentary sources (up to 54 ppm). The rare earth element (REE) abundances (0.5–1068 ppm) for the marbles studied are similar to those for local sedimentary units and thus contain, in general, lower REE contents than both the average worldwide calciocarbonatite and respective neighboring carbonatite bodies. The δ13CV-PDB and δ18OV-SMOW compositions for all of the samples range between –2.9 to +3.2 ± 0.1‰ and +14.3 to +25.8 ± 0.2‰, respectively, and are considerably heavier than those reported for magmatic or metamorphosed carbonatites. The 87Sr/86Sr ratios reported here range from 0.70417 to 0.70672, which are more radiogenic than the average 87Sr/86Sr (~0.70345) reported for carbonatites included for comparison in this study. Importantly, the boron isotopic compositions (δ11B‰) for samples from the Grenville Province range from +7.5 to +15.7 ± 0.5‰, which are consistent with those reported for biogenic carbonate (+4.9 to +35.1‰). In contrast, δ11B values for the samples of marble from Sri Lanka vary from –9.8 to –14.3 ± 0.5‰ overlapping with those estimated for average bulk continental crust (–9.1 ± 2.4‰). Together, the boron compositions, chemical data, stable (C, O), and radiogenic Sr isotopic data overwhelmingly point to a sedimentary origin for the marble samples examined here. Specifically, the samples from the Grenville Province represent marble formed during high-temperature regional metamorphism of limestone units. The Sri Lankan samples were formed from carbonate-rich and 11B-poor fluids derived from a crustal source. The boron isotopic compositions for the samples studied here are also compared to those reported for mantle-derived carbonate (i.e., carbonatites) worldwide, along with their associated δ13CV-PDB and 87Sr/86Sr values. This comparison results in defining three isotopically distinct fields; mantle-derived carbonates, sedimentary carbonates derived from heterogeneous limestone protoliths, and carbonates derived from meteoric water interacting with crustal material. This work establishes the effective use of boron isotopic compositions in determining the origin of carbonate-rich rocks of contentious petrogenesis.

Hydrogeochemistry and fractionation of boron isotopes in the inter-dune aquifer system of Badain Jaran Desert, China

Groundwater flow systems can be topography-controlled in arid, mountainous, and/or permeable areas. Previous theoretical studies indicated that local groundwater flow systems were usually less developed or even did not exist in recharge-controlled flow systems, and the whole flow system was dominated by regional groundwater flow system. However, the occurrence and role of local groundwater flow systems in real recharge-controlled aquifer systems are rarely addressed. The Badain Jaran Desert is featured by hyper dry climate, mega sand dune-lake landscapes, and highly permeable loose sediments, making it an ideal natural laboratory to address these issues. In this study, the local groundwater flow system and its interaction with saline lake water in the desert inter-dune aquifer are investigated via hydrogeochemical and isotopic approaches. The stable boron isotopic compositions reveal the existence of local groundwater flow systems in the hyper arid climate condition. The fresh local flow systems regulate not only the lake water volume and the lake water-groundwater mixing process, but also the ion contents and isotopic compositions of saline groundwater and lake water. Lake water-groundwater interaction manifests as fresh groundwater discharge and saline lake water intrusion. This study demonstrates that boron fractionation is applicable in deciphering groundwater flow patterns and lake water-groundwater interaction in the arid aquifer systems.

Accurate correction for the matrix interference on laser ablation MC-ICPMS boron isotope measurements in CaCO3 and silicate matrices

Refinement of the measurement of the boron isotopic composition of carbonate and silicate materials via LA-ICPMS pushes samples size requirements into the range of SIMS (figure redrawn after Foster et al., 2018).

Molybdenum and Boron Isotopic Compositions of Porphyry Cu Mineralization‐Related Adakitic Rocks in Central‐Eastern China: New Insights Into their Petrogenesis and Crust‐Mantle Interaction

AbstractThe voluminous Late Mesozoic porphyry ore‐related adakitic magmatism in central‐eastern China is crucial to understand the deep geodynamic process of the region and controls on porphyry deposit formation globally. Here, we present a novel study of whole‐rock Mo‐B isotopes on the mineralization‐related adakitic rocks from the Dexing (located in the interior of South China and associated with giant porphyry Cu deposits), and Middle‐Lower Yangtze River (MLYR) (host to one of the largest porphyry Cu‐Au deposit belts) areas in central‐eastern China. The Dexing adakitic rocks have δ98/95Mo ranging from −0.31‰ to −0.01‰ and δ11B from −17.65‰ to −11.49‰. The adakitic rocks from the MLYR area have δ98/95Mo values of −0.5‰ to 0.2‰ and δ11B values of −14.48‰ to −9.53‰. The δ98/95Mo values of both the Dexing and MLYR samples negatively correlate with the La/Yb and La/Sm ratios. Together with the high Mg#, Cr, and Ni contents, these results indicate a process of melt‐mantle interaction resulting in hybridization of initial adakitic melts with low δ98/95Mo and mantle‐derived melts/components with high δ98/95Mo. These adakitic rocks have distinct Mo isotope systematics compared with those of oceanic slab‐derived adakites and MORB‐type eclogites. Their relatively low δ11B values and the lack of correlation between δ11B and B contents support a mafic lower crust source for the initial adakitic melts. We thus suggest that these adakitic rocks were generated by partial melting of delaminated lower crust followed by interaction with the deep mantle, and that melt‐mantle interaction may facilitate Cu‐Mo‐Au mineralization.

Dynamic of boron in forest ecosystems traced by its isotopes: A modeling approach

Understanding the factors that control the cycling of nutrients in terrestrial ecosystems and in the Critical Zone is of fundamental importance given its role for example in nutrient availability to sustain forest productivity, and ultimately in soil carbon storage.In this paper, we developed a model to assess the dynamic of boron in forest ecosystems and to appraise how the impacts on boron cycling by internal or external factors should be reflected in the changes of its isotopic compositions across an ecosystem. Despite the scarcity of data, we tested this model on two case studies and were able to reproduce the distribution of boron isotopes between the different pools of these two contrasted ecosystems. The model shows a time dependency of the boron isotopic composition of the different biotic and abiotic compartments of the ecosystem. When the forest grows, a transient enrichment in the heavy isotope up to 20‰ relative to the values at steady-state is observed in the biomass and the soil solutions. The magnitude of this enrichment, and the return time to steady state, are sensitive to B supply and plant demand for boron. We also tested the responses of B dynamic to natural or anthropogenic disturbances and we show that they are well reflected in the variations of the B isotopic compositions of the different pools. As a whole, we show that B isotopes are a good potential tracer of nutrient cycling and by extension, a promising proxy for tracing the global functioning of terrestrial biosphere at present and in the past.

Boron isotopic signatures of melt inclusions from North Iceland reveal recycled material in the Icelandic mantle source

Trace element and volatile heterogeneity in the Earth’s mantle is influenced by the recycling of oceanic lithosphere through subduction. Oceanic island basalts commonly have high concentrations of volatiles compared to mid-ocean ridge basalts, but the extent to which this enrichment is linked to recycled mantle domains remains unclear. Boron is an ideal tracer of recycled subducted material, since only a small percentage of a recycled component is required to modify the bulk δ11B of the source mantle. Boron isotopic compositions of primary melts thus have potential to trace the fate of recycled subducted material in the deep mantle, and to constrain the lengthscales of lithologic and compositional heterogeneities in diverse tectonic settings.We present new measurements of volatiles, light elements and boron isotopic ratios in basaltic glasses and melt inclusions that sample the mantle at two endmember spatial scales. Submarine glasses from the Reykjanes Ridge sample long-wavelength mantle heterogeneity on the broad scale of the Iceland plume. Crystal-hosted melt inclusions from the Askja and Bárðarbunga volcanic systems in North Iceland sample short-wavelength mantle heterogeneity close to the plume centre. The Reykjanes Ridge glasses record only very weak along-ridge enrichment in B content approaching Iceland, and there is no systematic variability in δ11B along the entire ridge segment. These observations constrain ambient Reykjanes Ridge mantle to have a δ11B of −6.1‰ (2SD = 1.5‰, 2SE = 0.3‰). The North Iceland melt inclusions have widely variable δ11B between −20.7 and +0.6‰. We screen melt inclusions against influence from crustal contamination, identifying high [B] and low δ18O as fingerprints of assimilation processes. Only the most primitive melt inclusions with MgO ⩾ 8 wt.% reliably record mantle-derived δ11B. In North Iceland, incompatible trace element (ITE)-depleted primitive melt inclusions from Holuhraun record a δ11B of −10.6‰, a signal that has also been seen in melt inclusions from southwest Iceland (Gurenko and Chaussidon, 1997). In contrast, primitive ITE-enriched melt inclusions from nearby Askja volcano record a δ11B of −5.7‰, overlapping with our new constraint on the δ11B of Reykjanes Ridge mantle. Coupled [B], δ11B and δ18O signatures of more evolved melt inclusions from North Iceland are consistent with primary melts assimilating <5–20% of hydrothermally altered basaltic hyaloclastite as they ascend through the upper crust.Our data reveal the presence of a depleted, low-δ11B and an enriched, higher-δ11B mantle component, both intrinsic to the Icelandic mantle source and distinct from Reykjanes Ridge mantle. Non-modal melting calculations suggest that the enriched and depleted mantle components both contain ∼0.085 μg/g B, slightly lower than the 0.10–0.11 μg/g calculated for Reykjanes Ridge mantle. These data are consistent with the Icelandic mantle containing B-depleted dehydrated recycled oceanic lithosphere, in keeping with the low B/Pr of Icelandic melt inclusions in comparison to Reykjanes Ridge glasses or MORB. Our new data provide strong support for the role of recycled subducted lithosphere in melt generation at ocean islands, and highlight the need for careful screening of melt inclusion compositions in order to study global volatile recycling in ocean island basalts.

Sub‐Permil Interlaboratory Consistency for Solution‐Based Boron Isotope Analyses on Marine Carbonates

Boron isotopes in marine carbonates are increasingly used to reconstruct seawater pH and atmospheric pCO2 through Earth’s history. While isotope ratio measurements from individual laboratories are often of high quality, it is important that records generated in different laboratories can equally be compared. Within this Boron Isotope Intercomparison Project (BIIP), we characterised the boron isotopic composition (commonly expressed in δ11B) of two marine carbonates: Geological Survey of Japan carbonate reference materials JCp‐1 (coral Porites) and JCt‐1 (giant clam Tridacna gigas). Our study has three foci: (a) to assess the extent to which oxidative pre‐treatment, aimed at removing organic material from carbonate, can influence the resulting δ11B; (b) to determine to what degree the chosen analytical approach may affect the resultant δ11B; and (c) to provide well‐constrained consensus δ11B values for JCp‐1 and JCt‐1. The resultant robust mean and associated robust standard deviation (s*) for un‐oxidised JCp‐1 is 24.36 ± 0.45‰ (2s*), compared with 24.25 ± 0.22‰ (2s*) for the same oxidised material. For un‐oxidised JCt‐1, respective compositions are 16.39 ± 0.60‰ (2s*; un‐oxidised) and 16.24 ± 0.38‰ (2s*; oxidised). The consistency between laboratories is generally better if carbonate powders were oxidatively cleaned prior to purification and measurement.

Technical note: Single-shell &amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;&amp;lt;sup&amp;gt;11&amp;lt;/sup&amp;gt;B analysis of &amp;lt;i&amp;gt;Cibicidoides wuellerstorfi&amp;lt;/i&amp;gt; using femtosecond laser ablation MC-ICPMS and secondary ion mass spectrometry

Abstract. The boron isotopic composition (δ11B) of benthic foraminifera provides a valuable tool to reconstruct past deep-water pH. As the abundance of monospecific species might be limited in sediments, microanalytical techniques can help to overcome this problem, but such studies on benthic foraminiferal δ11B are sparse. In addition, microanalytics provide information on the distribution of δ11B at high spatial resolution to increase the knowledge of biomineralization processes, for example. For this study, we investigated the intra- and inter-shell δ11B variability of the epibenthic species Cibicidoides wuellerstorfi, which is widely used in paleoceanography, by secondary ion mass spectrometry (SIMS) and femtosecond laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS). While the average δ11B values obtained from these different techniques agree remarkably well with bulk solution values to within ±0.1 ‰, a relatively large intra-shell variability was observed. Based on multiple measurements within single shells, the SIMS and LA data suggest median variations of 4.8 ‰ and 1.3 ‰ (2σ), respectively, while the larger spread for SIMS is attributed to the smaller volume of calcite being analyzed in each run. When analytical uncertainties and volume-dependent differences in δ11B variations are taken into account for these methods, the intra-shell variability is estimated to be on the order of ∼3 ‰ and ∼0.4 ‰ (2σ) on a ∼20 and 100 µm scale, respectively. In comparison, the δ11B variability between shells exhibits a total range of ∼3 ‰ for both techniques, suggesting that several shells need to be analyzed for accurate mean δ11B values. Based on a simple resampling method, we conclude that ∼12 shells of C. wuellerstorfi must be analyzed using LA-MC-ICPMS to obtain an accurate average value within ±0.5 ‰ (2σ) to resolve pH variations of ∼0.1. Based on our findings, we suggest preferring the conventional bulk solution MC-ICPMS over the in situ methods for paleo-pH studies, for example. However, SIMS and LA provide powerful tools for high-resolution paleoreconstructions, or for investigating ontogenetic trends in δ11B.

Multiple generations of tourmaline from Yushishanxi leucogranite in South Qilian of western China record a complex formation history from B-rich melt to hydrothermal fluid

Abstract One tourmaline-bearing leucogranite dike occurs in the Yushishanxi Nb-Ta mining area in the Yushishan district of the South Qilian orogenic belt in western China. Abundant tourmalines have been identified in the leucogranite, including disseminated, crosscutting quartz-tourmaline veins and tourmaline veinlets. Detailed petrological, geochemical, and boron isotopic studies indicate that these tourmalines have distinctive core-rim zoning signatures and significant chemical variations, which can be divided into four paragenetic generations. Generation-I tourmalines in the leucogranite have a magmatic origin and were followed by hydrothermal Generation-II, Generation-III, and Generation-IV. Tourmalines from all four generations belong to the alkali group and the schorl-dravite solid-solution series. The most notable features are the variations in Mg, Fe, and Ca contents. The variable Mg/(Mg+Fe) ratios in generations I to IV may be attributed to the multiple influx and/or interaction of initially magma-derived hydrothermal fluid with surrounding rocks, magma-derived Fe-rich hydrothermal fluid, and external metamorphic fluid. In situ B-isotope analyses of tourmaline yield a total range of δ11B values from –11.7 to –6.0‰. The earliest Generation-I tourmalines have δ11B values of –11.1 to –9.6‰, whereas Generation-II and Generation-III tourmalines record a higher δ11B value of –9.5 to –6.0‰ and –9.3 to –6.9‰, respectively. Such an increase is mainly controlled by boron-isotope fractionation between melt-fluid and tourmaline-fluid, Rayleigh fractionation, and also, to some extent, by a hydrothermal recharge from the same magma source. Generation-IV tourmalines have the lightest B-isotope values (down to –11.7‰). This shift back to lighter B-isotopes in the Generation-IV cannot be explained by closed-system crystallization and isotope fractionation of known phases. Alternatively, a lighter source of boron via fluid circulation from the surrounding metamorphic rocks can explain this light boron isotope composition. Therefore, we propose that an isotopically light fluid may have derived from B release during mica breakdown within the surrounding metamorphic rocks.

Ocean acidification during the early Toarcian extinction event: Evidence from boron isotopes in brachiopods

Abstract The loss of carbonate production during the Toarcian Oceanic Anoxic Event (T-OAE, ca. 183 Ma) is hypothesized to have been at least partly triggered by ocean acidification linked to magmatism from the Karoo-Ferrar large igneous province (southern Africa and Antarctica). However, the dynamics of acidification have never been directly quantified across the T-OAE. Here, we present the first record of temporal evolution of seawater pH spanning the late Pliensbachian and early Toarcian from the Lusitanian Basin (Portugal) reconstructed on the basis of boron isotopic composition (δ11B) of brachiopod shells. δ11B declines by ∼1‰ across the Pliensbachian-Toarcian boundary (Pl-To) and attains the lowest values (∼12.5‰) just prior to and within the T-OAE, followed by fluctuations and a moderately increasing trend afterwards. The decline in δ11B coincides with decreasing bulk CaCO3 content, in parallel with the two-phase decline in carbonate production observed at global scales and with changes in pCO2 derived from stomatal indices. Seawater pH had declined significantly already prior to the T-OAE, probably due to the repeated emissions of volcanogenic CO2. During the earliest phase of the T-OAE, pH increased for a short period, likely due to intensified continental weathering and organic carbon burial, resulting in atmospheric CO2 drawdown. Subsequently, pH dropped again, reaching the minimum in the middle of the T-OAE. The early Toarcian marine extinction and carbonate collapse were thus driven, in part, by ocean acidification, similar to other Phanerozoic events caused by major CO2 emissions and warming.

Chemical and boron isotope composition of tourmaline from the Kiaka orogenic gold deposit (Burkina Faso, West African Craton) as a proxy for ore-forming processes

The Kiaka orogenic gold deposit (Burkina Faso), located in the Paleoproterozoic domain of the West African Craton, is characterized by a two-stage gold mineralization hosted in volcano-sedimentary metamorphic rocks that was formed during the Eoeburnean (2.20–2.13 Ga) and Eburnean (2.13–2.05 Ga) orogenic cycles. These two stages include an early disseminated low-grade gold mineralization and a late vein-hosted high-grade gold mineralization. Paragenetic studies indicate that the first gold stage was coeval with the deposition of hydrothermal tourmaline. The aim of this paper is twofold: (i) to determine the processes responsible for deposition of the economic disseminated gold mineralization and (ii) to identify the source of the mineralizing fluids. For this purpose, we performed an in situ study on tourmaline by combining electron probe microanalysis and secondary ion mass spectrometry measurements of boron isotopes. Hydrothermal tourmaline hosted in metamafic volcanic rocks and metagreywackes has a dravite composition but shows different δ11B values falling within the two intervals of − 25.1 to − 22.0‰ and − 19.6 to − 15.1‰, respectively. Our results suggest that tourmaline formed from a distal, high-temperature (ca. 400 °C), reduced, and low-salinity hydrothermal fluid that interacted with the local host rocks. Based on the modeling of tourmaline–fluid boron isotope fractionation, we propose a metamorphic fluid origin derived from devolatilization of deeply buried muscovite schists during the regional prograde to peak metamorphism prior ca. 2.13 Ga. This metamorphic fluid–rock interaction model may possibly extend to other orogenic gold deposits in the West African Craton.

Equilibria, kinetics, and boron isotope partitioning in the aqueous boric acid–hydrofluoric acid system

The aqueous boric, hydrofluoric, and fluoroboric acid systems are key to a variety of applications, including boron measurements in marine carbonates for CO2 system reconstructions, chemical analysis and synthesis, polymer science, sandstone acidizing, fluoroborate salt manufacturing, and more. Here we present a comprehensive study of chemical equilibria and boron isotope partitioning in the aqueous boric acid–hydrofluoric acid system. We work out the chemical speciation of the various dissolved compounds over a wide range of pH, total fluorine (FT), and total boron (BT) concentrations. We show that at low pH (0 ≤ pH ≤ 4) and FT ≫ BT, the dominant aqueous species is BF4−, a result relevant to recent advances in high precision measurements of boron concentration and isotopic composition. Using experimental data on kinetic rate constants, we provide estimates for the equilibration time of the slowest reaction in the system as a function of pH and [HF], assuming FT ≫ BT. Furthermore, we present the first quantum-chemical (QC) computations to determine boron isotope fractionation in the fluoroboric acid system. Our calculations suggest that the equilibrium boron isotope fractionation between BF3 and BF4− is slightly smaller than that calculated between B(OH)3 and B(OH)4−. Based on the QC methods X3LYP/6-311+G(d,p) (X3LYP+) and MP2/aug-cc-pVTZ (MP2TZ), α(BF3−BF4−) ≃ 1.030 and 1.025, respectively. However, BF4− is enriched in 11B relative to B(OH)4−, i.e., α(BF4−−B(OH)4−) ≃ 1.010 (X3LYP+) and 1.020 (MP2TZ), respectively. Selection of the QC method (level of theory and basis set) represents the largest uncertainty in the calculations. The effect of hydration on the calculated boron isotope fractionation turned out to be minor in most cases, except for BF4− and B(OH)3. Finally, we provide suggestions on best practice for boric acid–hydrofluoric acid applications in geochemical boron analyses.

Application of PIGE method for quantification of total boron in neutron absorbers and shielding materials and isotopic composition in in-house prepared enriched boron carbide samples

The in situ current normalized particle induced gamma-ray emission method using 4 MeV proton beam was utilized for total boron concentrations in boron based neutron absorbers and borated wood based neutron shielding materials. Boron carbide samples with varying masses were analyzed to examine the effect of matrix as well as the efficacy of the in situ current normalized method. Isotopic composition of boron (10B/11B atom ratio) has been determined in natural and 10B enriched boron carbide samples as well as in four synthetically prepared enriched boron carbide samples to serve as in-house reference standard for isotopic composition.

Exploration of driving mechanisms of equilibrium boron isotope fractionation in tourmaline group minerals and fluid: A density functional theory study

The equilibrium boron isotope fractionations (α3–4, Δ11B(Tur-fluid)) between tourmaline mineral groups and fluids at 0.5 GPa, 600–1000 K are investigated using density functional theory calculations. The first solvent shell controls boron isotope fractionation in solution, where the β values of both H3BO3 and B(OH)4− decrease with increasing numbers of hydrogen bonds. In supercritical fluids, the weakening of hydrogen bonds and the diversity in configurations of hydrated boron species both contribute to variations in the vibrational frequencies. The 1000lnαTur-fluid value increases with increasing Mg/(Fe+Mg) ratios in the dravite-schorl solid solution series. This effect, together with the crystallization sequence predicted from the Gibbs free energies of tourmaline formation, accounts well for the association of boron isotope and chemical zonation observed in zoned tourmaline grains. The dependence of boron isotope fractionation on the BO bond length in tourmaline group minerals reflects the changes in relevant molar volumes caused by differences in the mass and charge of the atoms occupying the X, Y, and Z sites of tourmalines. This study suggests that the chemical composition of the tourmaline plays an important role in controlling the boron isotope composition in tourmalines crystallized from hydrothermal systems.

Geochemistry of hydrothermal tourmaline from IOCG occurrences in the Great Bear magmatic zone: Implications for fluid source(s) and fluid composition evolution

Tourmaline crystallized during iron oxide and alkali-calcic alteration and IOCG mineralization in the Great Bear magmatic zone, Canada. Within the samples analyzed, tourmaline occurs as small crystals in the groundmass of hydrothermal breccias (Southern Breccia and Contact Lake), or as large prismatic crystals in quartz (±ore-mineralogy) veins (Contact Lake, DeVries Lake, and NICO). The tourmaline from these localities is predominantly shorlitic in composition, with minor dravitic, uvitic and feruvitic components. The boron isotopic composition ranges from −15‰ to −5‰, with the majority of values between −15‰ and −10‰. The lack of variation among samples supports a common fluid source across the belt for the metasomatic-hydrothermal systems. These results are isotopically similar to global magmatic and non-marine evaporitic reservoirs, and lighter than seawater and marine sedimentary rocks. The δ11B results support a single-fluid IOCG deposit model for the systems studied, wherein metasomatic-hydrothermal fluids derived by magmatism ascend and evolve over time through fluid-rock interactions.

Rapid determination of the original boron isotopic composition from altered basaltic glass by in situ secondary ion mass spectrometry

Rapid determination of the original boron composition from altered basaltic glass by SIMS boron isotopic profile analysis from the core to rim.

Origin of Underground Brine in Potassium-Bearing Strata in Khammouane, Central Laos

The Khorat Plateau is one of the largest potash deposits in the world. In recent years, exploitation of potash resources accompanied by leakage of underground brines (UB) in potassium-bearing strata has threatened mining safety. However, there is little research on origin of UB in Laos. In this study, twelve samples of UB, salt springs (SS) and river waters were collected in Khammouane mining area, central Laos and analyzed for ionic and boron isotopic compositions. The results show that both of UB, dominated by K+, Mg2+, Na+ and Cl– ions, and SS, enriched in Na+ and Cl–, are chloride-type brines, while river waters abundant in Ca2+ and HC$${\text{O}}_{3}^{ - }$$ ions are carbonate-type waters. K+ (35.1–53.7 g/L) and Rb+ (12.9–16.6 mg/L) concentrations of UB are higher than the peak (25.9 g/L, 9.4 mg/L) of seawater evaporation trajectories (SET). UB is also characterized by high Mg2+ (32.7–68.0 g/L) and Br– (1795.2–2801.3 mg/L) concentrations and its lg(Cl/Br) ratios (1.9–2.1) deviate from the halite dissolution line. These comparisons indicate that the water chemistries of UB are mainly influenced by dissolution of carnallite and sylvite. High Sr2+ concentrations (31.8–58.6 mg/L) in UB might be simultaneously associated with dissolution of potash salt and Ca-bearing minerals (carbonate and anhydrite). The negative saturation indices (SI) of different salt minerals in UB suggest that salt minerals are unsaturated in UB. Meanwhile, boron (B) concentrations (396.0–426.0 mg/L) and δ11B values (+19.8–+21.0‰) of UB fall in between low B–δ11B end member (0.02 mg/L, +2.31‰) of river water and high B-δ11B end member (126.7–890.8 μg/g, +25.94–+32.94‰) of evaporitic minerals. The comparison implies that UB was influenced and mixed by two end members. Therefore, combining with major and trace ionic concentrations, isotopic compositions and lithologies of salt-bearing strata revealed by drillings in mining area, we conclude that the major solute source in UB is derived from dissolution of carnallite and sylvite. These geochemical evidences provide insights into dealing with leakage of underground brines occurred in the underground mining of potash deposits in Laos.

Linking Internal Carbonate Chemistry Regulation and Calcification in Corals Growing at a Mediterranean CO2 Vent

Corals exert a strong biological control over their calcification processes, but there is a lack of knowledge on their capability of long-term acclimatization to ocean acidification. We used a dual geochemical proxy approach to estimate the calcifying fluid pH (pHcf) and carbonate chemistry of a Mediterranean coral (Balanophyllia europaea) naturally growing along a pH gradient (range: pHTS 8.07-7.74). The pHcf derived from skeletal boron isotopic composition (11B) was 0.3-0.6 units above seawater values and homogeneous along the gradient (mean  sem: Site 1 = 8.39  0.03, Site 2 = 8.34  0.03, Site 3 = 8.34  0.02). Also carbonate ion concentration derived from B/Ca was homogeneous (mean  sem (µmol kg-1): Site 1 = 579  34, Site 2 = 541  27, Site 3 = 568  30) regardless of seawater pH. Furthermore, gross calcification rate (GCR, mass of CaCO3 deposited on the skeletal unit area per unit of time), estimated by a “bio-inorganic model” (IpHRAC), was homogeneous with decreasing pH. The homogeneous GCR, internal pH and carbonate chemistry confirm that the features of the ‘building blocks’ – the fundamental structural components - produced by the biomineralization process were substantially unaffected by increased acidification. Furthermore, the pH up-regulation observed in this study could potentially explain the previous hypothesis that less ‘building blocks’ are produced with increasing acidification ultimately leading to increased skeletal porosity and to reduced net calcification rate obtained by including the total volume of the pore space. In fact, assuming that the available energy at the three sites is the same, this energy at the low pH sites could be partitioned among fewer calicoblastic cells that consume more energy given the larger difference between external and internal pH compared to the control, leading to the production of less building blocks (i.e., formation of pores inside the skeleton structure, determining increased porosity). However, we cannot exclude that also dissolution may play a role in increasing porosity. Thus, the ability of scleractinian corals to maintain elevated pHcf relative to ambient seawater might not always be sufficient to counteract declines in net calcification under ocean acidification scenarios.

Boron isotope composition of coexisting tourmaline and hambergite in alkaline and granitic pegmatites

The boron isotopic composition of tourmaline and hambergite (Be2BO3[OH,F]) from peraluminous (n = 12), peralkaline (n = 1), and peralkaline nepheline syenite (n = 16) pegmatites has been measured by secondary ion mass spectrometry, for which a new hambergite reference material was developed. The focus of this study is on nepheline syenite pegmatites from the Larvik Plutonic Complex (Norway) and one peralkaline pegmatite related to the nearby Eikeren-Skrim Complex (Norway), where we investigate the source of boron as being from magmatic vs. external fluids. Tourmaline-hambergite mineral pairs were also analysed from peraluminous pegmatite localities (Russia, Tajikistan, and Pakistan) to test for systematic B-isotope fractionation between these two minerals.Tourmaline and hambergite from peraluminous granitic pegmatites have light boron ratios (δ11B = −12.9to −1.0‰) associated with S-type granites, whereas peralkaline granitic and nepheline syenite pegmatites have boron ratios (δ11B = −1.7 to 11.8‰), which we interpret is a result of heavy‑boron enrichment from external fluids. Our data show that hambergite tracks isotope variations of its geochemical setting and could therefore be used as a proxy mineral in place of tourmaline when geochemical stability favours hambergite.The results suggest a slight but consistent partitioning of B-isotopes between tourmaline and hambergite, with Δ11B = δ11Btourmaline−δ11Bhambergite in the range of approximately −3‰ to −5‰. Boron is in trigonal coordination with oxygen in both of these mineral phases as verified by NMR. Single crystal XRD analyses of tourmaline and hambergite reveal consistent longer <B-O > distances of tourmaline relative to hambergite. We attribute the boron isotopic fractionation to the longer <B-O > bond-lengths in tourmaline compared with hambergite.

Network polymerization and cation coordination environments in boron-bearing rhyolitic melts: Insights from 17O, 11B, and 27Al solid-state NMR of sodium aluminoborosilicate glasses with varying boron content

Despite the geochemical implications for the dissolution behaviors of nuclear waste glasses and the properties of boron-bearing rhyolitic melts, a detailed atomic-level understanding of the overall effect of boron content on the structural characteristics of boron-bearing silicate glasses remains elusive. Herein, we explore the effects of B/Al and Si/B ratios on the local configurations around cations and the degree of network polymerization of Na2O-Al2O3-B2O3-SiO2 glasses in nepheline (NaAlSiO4) – malinkoite (NaBSiO4) and albite (NaAlSi3O8) – reedmergnerite (NaBSi3O8) joins using multi-nuclear magnetic resonance (NMR). The 11B NMR results confirmed an increase in the B coordination numbers from 3 ([3]B) to 4 ([4]B) as B/Al and Si/B ratios increase. The 27Al NMR spectra show that the Al is mainly four coordinated, except in NaAl0.25B0.75SiO4 glasses, with a minor amount of [5]Al species, revealing a more prominent control of Si over that of B on the Al coordination environments. The 17O NMR spectra resolve distinct bridging (BO, Si−O−Al, Al−O−Al, Si−O−Si, B−O−B, and Si−O−B) and non-bridging oxygens (NBO, Na−O−Si). In contrast, the absence of B−O−Al species confirmed the separation between B and Al and prevalent melt depolymerization exclusively in silicate-networks. The fractions of B−O−B and Na−NBO increase as B/Al ratio increases, accounting for a drastic enhancement in the dissolution rate of glasses. A dramatic increase also indicates an enhanced connectivity of relatively weaker bonds such as B−O−B and Na−O−Si as composition reached a certain boron content. This is linked to a decrease in the melt viscosity with increasing B2O3 component. Based on the measured fractions of [3,4]B in 11B NMR spectra with varying composition, the trend of the evolution of boron isotope composition in sodium aluminoborosilicate melts was estimated, confirming the preferential enrichment of 10B into rhyolitic melts with increasing boron content. The decreased fraction of [3]B species with increasing Si/B may partly explain the reduction in δ11B in boron-bearing volcanic rocks at deeper depths in the subduction zone. The results suggest that composition-induced changes in the atomic structures should be constrained to precisely interpret the melt viscosity, boron isotope composition of arc volcanic lavas, and the dissolution rates of nuclear waste glasses.