Boron Isotope Fractionation Research Articles

Boron and molybdenum isotopic fractionation during crustal anatexis: Constraints from the Conadong leucogranites in the Himalayan Block, South Tibet

Partial melts derived from crustal anatexis exhibit large variations in field characteristics, composition, and isotope geochemistry. The relative influence of melting process and magmatic differentiation on such heterogeneity is a subject of ongoing debate. Boron and molybdeum isotopes have the potential to shed light on the genesis of crustal melts, but the behaviors of B and Mo isotopes during crustal anatexis and differentiation are unclear. Here, we present geochemical and isotopic data, including whole-rock Sr–Nd–B–Mo and mineral B–Mo data, for the Conadong leucogranites in the Himalayan Block, to provide new insights into these issues. The studied samples include two-mica granites (biotite + muscovite), muscovite leucogranites (muscovite ± garnet), and biotite-rich granites. The two-mica granites were derived from muscovite dehydration melting of metasedimentary rocks. Their initial Sr and Nd isotopic compositions show positive correlation, implying non-modal partial melting associated with dissolution of accessory phases. The B contents (3.44–21.2 ppm) of the two-mica granites decrease but their δ11B (−15.80‰ to −13.25‰) and δ98Mo (−0.15‰ to 0.34‰) values increase with increasing Sr and Ba contents. This may have been controlled by different phases involved in non-modal melting (i.e., muscovite and feldspar). The muscovite leucogranites display features of residual liquids. Their δ11B (−15.38‰ to −11.90‰) and δ98Mo (−0.15‰ to 0.17‰) values negatively correlate with Sr and Ba contents, which can be ascribed to shallow-crustal crystal–liquid separation and melt–fluid interaction. The biotite-rich granites are least evolved, with large variations in their initial Sr–Nd isotopic compositions. They show relatively heavy whole-rock B isotopic compositions of −11.97‰ to −9.00‰. These may indicate that biotite-rich granites may be contaminated by entrained restitic minerals (e.g., biotite, feldspar and accessory minerals). Overall, this study demonstrates that compositional diversity of anatectic melts is controlled primarily by melting processes in the source and secondarily by emplacement-level crystal–liquid separation. Both non-modal partial melting and crystal–liquid separation could result in B–Mo isotopic fractionation in anatectic melts.

Boron coordination and B/Si ordering controls over equilibrium boron isotope fractionation among minerals, melts, and fluids

The high mobility of boron during fluid-rock interaction makes it an effective tracer for the sources of magmatic and metamorphic fluids, as recorded in minerals such as tourmaline and muscovite. Although advances have been made in quantifying the fractionation of boron isotopes among different phases, boron isotope fractionation in complex silicate melts remains poorly understood. Here, we propose appropriate models for the BO3 and BO4 units in silicate melts covering a wide range of chemical compositions and boron coordination structures in silicate magmas, and report the results of a theoretical investigation of boron isotope fractionation among silicate melt, minerals and fluids using a first principles theoretical approach. A comparison of measured and calculated α factors in mineral-melt and fluid-melt systems shows good agreement, suggesting the applicability of a simplified treatment of boron coordination structures in silicate melt. The results of this study show that the proportion of trigonal/tetrahedral coordinated boron and the B/Si ordering in silicate tetrahedral layers control the boron isotope fractionation among different phases, and that the effect of chemical composition is minor (less than 2‰ at 600 K). The temperature-dependent boron isotope fractionations are described as 1000lnαmica-basic fluid = 0.8–2.4 × (1000/T) - 0.8 × (1000/T)2, 1000lnαmica-acidic fluid = 7.0–14.0 × (1000/T) - 1.2 × (1000/T)2 and 1000lnαmica-tur = 1.9–5.4 × (1000/T) -3.4 × (1000/T)2 (T is temperature in Kelvins). At magmatic temperatures, ∆11B values between mineral/fluid and melt also vary with the proportion of the BO4 unit in the melt. This study underpins the applicability of the white mica-tourmaline geothermometers and boron isotopes for fluid source identification, and also offers an explanation of boron isotope fractionation in systems that contain complex silicate melts.

Boron Isotope Fractionation between Liquid and Gas at Saturated Water Vapor Line

The first experimental data were obtained on the distribution of boron isotopes between gas and liquid at 100–350°С and the pressure of saturated water vapor, and the effect of pH on this distribution at 100°С was studied. The results showed that the value of δ11Bgas–δ11Bliquid was 5.9, 3.7, 1.1, 0.8, and –1.3 ‰ at 100, 200, 250, 300, and 350°С. In an alkaline solution, the boron distribution coefficient between gas and liquid decreases, and the isotopic shift increases; at 100°C and pH 9.1 it is 17.0. The isotopic composition of the boron of thermal waters of the Mutnovsky geothermal system (Kamchatka Peninsula, Russia) was investigated. In thermal springs, a large spread of δ11B values from 2.8 to 18‰ was determined. A wide range of isotopic compositions can be explained by the fractionation of boron between gas and liquid.

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.

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.

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.

Boron Isotope Fractionation Accompanying Formation of Calcium Borate Minerals at 25 ℃

Boron Isotope Fractionation Accompanying Formation of Calcium Borate Minerals at 25 ℃

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.

Boron Isotopes in the Puga Geothermal System, India, and Their Implications for the Habitat of Early Life.

Boron is associated with several Archean stromatolite deposits, including the tourmaline-rich Barberton stromatolites in South Africa and tourmaline-bearing pyritic laminae associated with stromatolites of the 3.48 Ga Dresser Formation in the Pilbara Craton, Australia. Boron is also a critical element in prebiotic organic chemistry, including in the formation of ribose, a crucial component in RNA. As geological evidence and advances in prebiotic chemistry are now suggesting that hot spring activity may be associated with the origins of life, an understanding of boron and its mobility and isotopic fractionation in geothermal settings may provide important insights into the setting for the origin of life. Here, we report on the boron isotopic compositions and elemental concentrations in a range of fluid, sediment, and mineral samples from the active, boron-rich Puga geothermal system in the Himalayas, India. This includes one of the lowest boron isotope values ever recorded in modern settings: diatom-rich sediments (δ11B = -41.0‰) in a multiphase fractionation system where evaporation is not the dominant form of isotope fractionation. Instead, the extreme boron isotopic fractionation is ascribed to the incorporation of tetrahedral 10B borate anions in precipitating amorphous silica. These findings expand the known limits and drivers of boron isotope fractionation, as well as provide insight into the concentration and fractionation of boron in Archean hot spring environments.

Boron geochemistry of the geothermal waters from two typical hydrothermal systems in Southern Tibet (China): Daggyai and Quzhuomu

The Tibetan plateau is characterized by a widely and intensively distributed geothermal activity and an extreme enrichment of boron in some geothermal waters. Twenty-five hot spring samples were collected from the Daggyai and Quzhuomu geothermal areas in Tibet to determine their boron concentrations and isotope composition. The boron concentrations and δ11B values for the neutral geothermal waters in Daggyai range from 72.5 to 106.9 mg/L and −16.4‰ to −12.3‰ while the Daggyai acid waters have much lower boron concentrations (1.1–1.2 mg/L) and slightly higher δ11B values (−12.9‰). The hot spring samples from the Quzhuomu geothermal area have boron concentrations varying from 21.9 to 44.6 mg/L and δ11B values from −11.3‰ to −7.1‰. The hydrochemical and B isotopic evidence imply that the boron in Daggyai geothermal waters is contributed by both magmatic fluid input and host rock leaching. In contrast, marine carbonates and granites, especially the former, are confirmed as the major boron sources for the Quzhuomu geothermal waters. In addition, there also exist some secondary processes affecting the boron isotope fractionation in geothermal waters, e.g. water-vapor separation and absorption or incorporation of B into hydrothermally altered minerals. Finally, based on the analysis of B geochemistry in these two typical geothermal areas in Southern Tibet with different heat sources and reservoir lithology, the mechanisms for the accumulation of boron in the Daggyai and Quzhuomu geothermal waters are suggested. This study is promising to provide insight into the sources of boron in the geothermal waters in other Tibetan geothermal areas.

Correlation between Boron Isotope of Coral Acropora and pH Value of Seawater Surface

Abstract A water circulation system with the almost same element composition and socket type was adopted in coral Acropora culture under different seawater pH value conditions and the data of the relationship between boron isotopic compositions of coral and seawater pH value by thermoelectric ionization mass spectrometer were obtained. According to the correlations between αcarb-3 of coral and the pH value of cultured seawater, αcarb-3 was not a constant but related to pH value, indicating that B(OH)3 also incorporated carbonate. Therefore, the theoretical formula could not be used to calculate the seawater pH value from the δ11Bcarb value of the measured marine biological carbonate. The empirical equations obtained experimentally would be an alternative method to calculate the seawater pH value. In addition, the mixed precipitation of CaCO3 and Mg(OH)2 was found in aquaculture tanks with high pH value, and the δ11B of the solid was significantly higher than that of cultured seawater. The result indicated that the presence of Mg(OH)2 had a significant effect on the boron isotope fractionation, which deserved our attention.

Boron circulation and recharge pattern in Zabuye drainage basin, Tibetan plateau

Zabuye salt lake is famous for abundance of B and Li resource, especially for natural product of lithium carbonate mineral. Based on systematic measurement of chemical compositions and boron isotope, the fractionation mechanisms of boron were discussed to constrain possible boron cycle among different reservoirs. The excellent consistency of borax isotopic composition on the basis of fractionation between hydroboracite and borax and basicboron structure indicates that boron isotopic fractionation occurred during lake water evolution, such as crystallization, adsorption, and alteration processes. The increase of δ11B values in brine with corresponding pH varying from 9.86 to 9.01 suggests that pH has predominent control over the separation because 11B will preferentially concentrated in trigonal (B(OH)3) species. Very low δ11B values of brine in Zabuye salt lake imply a negative origin as almost all natural geochemical processes will result in positive values without any other extra boron input. Similar fractionation factor between carbonate-rich clay and brine to that between sediment and brine in other salt lakes in Tibetan Plateau also strongly support the accumulation of boron from the input of carbonate-rich clay. Unlike the consecutive evaporation in Bange salt lake with relatively positive δ11B values, Zabuye salt lake has experienced crystallization from initial brine and subsequent redissolution of carbonate-rich clay processes and the extracted boron is carried into the lake by river water and rising springs.

Mechanism of boron incorporation into calcites and associated isotope fractionation in a steady-state carbonate-seawater system

An investigation on the interface reactions and boron isotope fractionation in a steady-state carbonate-seawater system was performed to better understand boron incorporation into CaCO3 in a natural environment. In seawater, inorganic carbonate crystals did not precipitate even when the solution reached saturation and the presence of carbonic anhydrase enzyme promotes the process, indicating the precipitation kinetics of inorganic carbonates from natural seawater is fairly slow. Both theoretical and experimental evidence proved that boron incorporation into calcite might be defined as an adsorption-precipitation process rather than a substitution of CO32− by HBO32− in the crystal lattice, where the charged B(OH)4- ions adsorb preferentially over H3BO3 onto positively charged calcite crystal surfaces and finally co-precipitate into occlusions or inclusions after new layers of calcites formed. The Langmuir isotherm well describes the adsorption process, yielding a maximum adsorption capacity [B]solid-max of 0.33 ± 0.04 mg⋅g−1 and an equilibrium constant K of 0.0053 ± 0.0003. Langmuir kinetics also describes previous studies with [B]solid-max of 71.4–418 μg⋅g−1 for calcite and aragonite, covering the variation of boron contents in a variety of biogenic carbonates. The boron isotope fractionations in ulexite-seawater and calcite-seawater systems are distinct. In the former one the fractionation factor (αulexite-seawater = 0.995 ± 0.001) suggests that both trigonal and tetrahedral species of boron enter into ulexite crystals via rapid precipitation, and in the latter one (α4-3 = 0.9736 ± 0.004) confirms that only the charged B(OH)4- species interacts with the crystal surface in calcite.

Boron isotopic fractionation during adsorption by calcite – Implication for the seawater pH proxy

Although adsorption at the solid surface is the first step controlling boron incorporation in the crystal lattice during the standard growth mechanism of calcite and aragonite, little is known about the identity, structure and isotopic composition of the boron complexes formed at the CaCO3-solution interface. To generate this important information, we investigated experimentally the boron chemical and isotopic fractionation during adsorption at the calcite-water interface as a function of pH (6.5–11.7) at 4 and 20 °C in 0.01 and 0.1 M NaCl aqueous solutions. The surface complexation modeling of B adsorption and isotopic composition data showed that boron is sorbed at the calcite surface as a tetrahedral complex (>CaB(OH)40) formed by reaction of borate ions with Ca-protonated surface sites (log Kint0 = 1.54 ± 0.37 at 20 °C) and excluded the formation of trigonal B surface complexes (>CaB(OH)3+). The B isotopic composition of >CaB(OH)40 is ∼5‰ and 2‰ heavier than that of aqueous B(OH)4− in 0.01 and 0.1 M NaCl solution, respectively. Consistently, these values suggest that adsorbed borate ions have isotopic compositions intermediate between those of aqueous borate and structural tetrahedral species in calcite, which have been recently predicted to be ∼12‰ heavier than aqueous borate using quantum mechanical calculations (Balan et al., 2018). The good agreement between the isotopic composition of adsorbed boron measured in this study and boron experimentally co-precipitated with calcite in the 8–9 pH range at close to equilibrium conditions (i.e. via ion-by-ion attachment at advancing steps) (Noireaux et al., 2015) indicate that the isotopic composition of structural boron can be inherited from the boron surface complexes formed at the calcite/water interface. The results of this study contradict the assumption of no isotopic fractionation between tetrahedral boron in calcite and the borate ion that sustains the boron paleo-pH proxy, but confirm that trigonal B cannot be directly incorporated in the crystal structure during near equilibrium growth of calcite.

Tourmaline geochemistry and boron isotopic variations as a guide to fluid evolution in the Qiman Tagh W–Sn belt, East Kunlun, China

The Qiman Tagh W–Sn belt lies in the westernmost section of the East Kunlun Orogen, NW China, and is associated with early Paleozoic monzogranites, tourmaline is present throughout this belt. In this paper we report chemical and boron isotopic compositions of tourmaline from wall rocks, monzogranites, and quartz veins within the belt, for studying the evolution of ore-forming fluids. Tourmaline crystals hosted in the monzogranite and wall rocks belong to the alkali group, while those hosted in quartz veins belong to both the alkali and X-site vacancy groups. Tourmaline in the walk rocks lies within the schorl–dravite series and becomes increasingly schorlitic in the monzogranite and quartz veins. Detrital tourmaline in the wall rocks is commonly both optically and chemically zoned, with cores being enriched in Mg compared with the rims. In the Al–Fe–Mg and Ca–Fe–Mg diagrams, tourmaline from the wall rocks plots in the fields of Al-saturated and Ca-poor metapelite, and extends into the field of Li-poor granites, while those from the monzogranite and quartz veins lie within the field of Li-poor granites. Compositional substitution is best represented by the MgFe−1, Al(NaR)−1, and AlO(Fe(OH))−1 exchange vectors. A wider range of δ11B values from −11.1‰ to −7.1‰ is observed in the wall-rock tourmaline crystals, the B isotopic values combining with elemental diagrams indicate a source of metasediments without marine evaporates for the wall rocks in the Qiman Tagh belt. The δ11B values of monzogranite-hosted tourmaline range from −10.7‰ and −9.2‰, corresponding to the continental crust sediments, and indicate a possible connection between the wall rocks and the monzogranite. The overlap in δ11B values between wall rocks and monzogranite implies that a transfer of δ11B values by anataxis with little isotopic fractionation between tourmaline and melts. Tourmaline crystals from quartz veins have δ11B values between −11.0‰ and −9.6‰, combining with the elemental diagrams and geological features, thus indicating a common granite-derived source for the quartz veins and little B isotopic fractionation occurred. Tourmalinite in the wall rocks was formed by metasomatism by a granite-derived hydrothermal fluid, as confirmed by the compositional and geological features. Therefore, we propose a single B-rich sedimentary source in the Qiman Tagh belt, and little boron isotopic fractionation occurred during systematic fluid evolution from the wall rocks, through monzogranite, to quartz veins and tourmalinite.

Differentiation analysis of boron isotopic fractionation in different forms within plant organ samples

As a critical micronutrient, boron (B) plays an important role in plant growth and embryonic development. To further understand the effects of B uptake, transportation and isotopic fractionation, the contents and isotopic compositions of hydro-soluble B in the sap and structural B fixed in the cell within individual plant tissues were investigated. The B isotope ratio was determined by multi-collector inductively coupled plasma mass spectrometry. The δ11B values in hydro-soluble and structural B in the investigated plant samples ranged from −1.57‰ to +11.30‰ and from +6.57‰ to +16.64‰, respectively. Different fractionation factors of the B isotopes, in the range of 0.9954–1.0150, were observed in these samples, indicating that in most plant tissues, the heavy isotope (11B) was preferentially enriched in structural B, which was fixed into the cell. However, there was a reversal in the fractionation of B isotopic compositions in the fruit samples compared with the other plant tissue samples. It is more powerful to examine the molecular mechanisms of B transport, uptake and utilization than the use of limited plant organ samples containing a mixture of hydro-soluble and structural B within different intra-plant compartments and in inter-plant interactions. These isotopic shifts, which may be used as important isotopic indicators, contribute to the surface processes interactions in the plant–soil system and the knowledge of the molecular mechanisms of B in the uptake and absorption by different plant species in nature.

An experimental approach to quantify the effect of tetrahedral boron in tourmaline on the boron isotope fractionation between tourmaline and fluid

An experimental approach to quantify the effect of tetrahedral boron in tourmaline on the boron isotope fractionation between tourmaline and fluid

Boron isotope geochemistry of Zigetang Co saline lake sediments, Tibetan Plateau

The origin of boron in boron-rich salt lakes in the Tibetan Plateau is highly controversial. In this study, we carried out a detailed study on boron geochemistry and isotope composition of lake sediments collected in Zigetang Co, central Tibet. Evaporites had high boron concentrations of 172.3–418.6 μg/g and δ11B values of −8.2‰ to −3.3‰, suggesting a non-marine origin for the saline lake. The boron isotopic fractionation factor, α, between evaporite and brackish water (αevaporite–brackish) decreased systematically with depth, from 0.9942 at the top of the drill core to 0.9893 at the bottom; the linear variation between αevaporite–brackish and depth reflects boron isotopic fractionation associated with progressive crystallization. The positive correlation between δ11B versus [B] and δ11B versus depth in the evaporite phase reflects pH and boron speciation in the solution control on the adsorption of boron, and B(OH)3 species incorporated preferentially into Mg(OH)2 precipitation at high pH.

The boron isotopic evolution of the Little Three pegmatites, Ramona, CA

Tourmaline, mica, hambergite, danburite, and axinite from three pegmatite dikes (named Main, Swamp, and Spessartine) at the Little Three mine near Ramona, California, have been analyzed by secondary ion mass spectrometry (SIMS) for their boron isotopic compositions (δ11B). Tourmaline is the sole host of B in the massive footwall and hanging wall sections of the dikes, and the principal sink for B in the miarolitic cavities. In the thinner Spessartine and Swamp dikes, the pocket mineralogy includes foitite, spessartine, muscovite, axinite, danburite, and rare beryl. The thicker Main dike culminates in cavities containing topaz, elbaite, lepidolite, boromuscovite, hambergite, and rare stibiotantalite.Values of δ11BTur from the Little Three pegmatites are strikingly heavy (−0.1‰ to +13.8‰) compared to other pegmatitic tourmaline (~−27 to +9‰), which is best explained by inheritance from an 11B-enriched source material such as altered oceanic crust. Flat patterns of δ11BTur from margin-to-core of each pegmatite attest to a negligible fractionation of boron isotopes between tourmaline-granitic melt and granitic melt-aqueous fluid. Tourmaline samples from intermediate zones and miarolitic cavities have identical boron isotopic compositions, except for miarolitic cavities that contain axinite and danburite, whose B is in 4-fold structural coordination. In these cavities, the δ11B of tourmaline increases relative to tourmaline from the intermediate zone.Interpretations of δ11BTur from margin-to-core in these and other pegmatites are hampered by uncertainties in the boron isotopic fractionation factors for melt-aqueous fluid and for tourmaline-granitic melt. Based on modeled cooling and crystallization of these dikes, the growth of tourmaline should have been fast relative to the diffusivity of B through granitic melt. Hence, an equilibrium distribution of 11B and 10B between tourmaline, bulk melt, and an aqueous solution (i.e. Rayleigh fractionation process) is unlikely. The negligible isotopic fractionation of boron in tourmaline from margins to center, therefore, probably records only the bulk isotopic composition of the pegmatite-forming melts.