Re-evaluating boron speciation in biogenic calcite and aragonite using B MAS NMR

Understanding the partitioning of aqueous boron species into marine carbonates is critical for constraining the boron isotope system for use as a marine pH proxy. Previous studies have assumed that boron was incorporated into carbonate through the preferential uptake of tetrahedral borate B(OH)₄⁻. In this study we revisit this assumption through a detailed solid state ¹¹B magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopic study of boron speciation in biogenic and hydrothermal carbonates. Our new results contrast with those of the only previous NMR study of carbonates insofar as we observe both trigonal and tetrahedral coordinated boron in almost equal abundances in our biogenic calcite and aragonite samples. In addition, we observe no strict dependency of boron coordination on carbonate crystal structure. These NMR observations coupled with our earlier re-evaluation of the magnitude of boron isotope fractionation between aqueous species suggest that controls on boron isotope composition in marine carbonates, and hence the pH proxy, are more complex that previously suggested.     

Boron isotopic fractionation between minerals and fluids: New insights from in situ high pressure-high temperature vibrational spectroscopic data

Equilibrium boron isotopic fractionations between trigonal B(OH)₃ and tetragonal B(OH)₄⁻ aqueous species have been calculated at high P-T conditions using measured vibrational spectra (Raman and IR) and force-field modeling to compute reduced partition function ratios for B-isotopic exchange following Urey’s theory. The calculated isotopic fractionation factor at 300 K, α_3/4 = 1.0176(2), is slightly lower than the formerly calculated value of α_3/4 = 1.0193 (Kakihana and Kotaka, 1977), due to differences in the determined vibrational frequencies. The effect of pressure on α_3/4 up to 10 GPa and 723 K is shown to be negligible relative to temperature or speciation (pH) effects. Implications for the interpretation of boron fractionation in experimental and natural systems are discussed. We also show that the relationship between seawater-mineral B isotope fractionation and pH can be expressed using two variables, α_3/4 on one hand, and the pK_a of the boric acid-borate equilibrium on the other hand. This latter value is given by the equilibrium of boron species in water for the carbonate-water exchange, but could be governed by mineral surface properties in the case of clays. This may allow defining intrinsic paleo-pHmeters from B isotope fractionation between carbonate and authigenic minerals. Finally, it is shown that fractionation of boron isotopes can be rationalized in terms of the changes in 1) coordination of B from trigonal to tetrahedral in both fluids and minerals; and 2) the ligand nature around B from OH⁻ in the fluid and some hydrous minerals to non-hydrogenated O in many minerals. Relationships are established that allow predicting the isotopic fractionation factor of B between minerals and fluid.     

ATR-FTIR spectroscopic studies of boric acid adsorption on hydrous ferric oxide

Boron is an important micronutrient for plants, but high B levels in soils are often responsible for toxicity effects in plants. It is therefore important to understand reactions that may affect B availability in soils. In this study, Attenuated Total Reflectance Fourier transform Infrared (ATR-FTIR) spectroscopy was employed to investigate mechanisms of boric acid (B(OH)₃) and borate (B(OH)₄⁻) adsorption on hydrous ferric oxide (HFO). On the HFO surface, boric acid adsorbs via both physical adsorption (outer-sphere) and ligand exchange (inner-sphere) reactions. Both trigonal (boric acid) and tetrahedral (borate) boron are complexed on the HFO surface, and a mechanism where trigonal boric acid in solution reacts to form either trigonal or tetrahedral surface complexes is proposed based upon the spectroscopic results. The presence of outer-sphere boric acid complexes can be explained based on the Lewis acidity of the B metal center, and this complex has important implications for boron transport and availability. Outer-sphere boric acid is more likely to leach downward in soils in response to water flow. Outer-sphere boron would also be expected to be more available for plant uptake than more strongly bound boron complexes, and may more readily return to the soil solution when solution concentrations decrease.     

Boron isotopes as pH proxy: A new look at boron speciation in deep-sea corals using B MAS NMR and EELS

Dissolved boron in modern seawater occurs in the form of two species, trigonal boric acid B(OH)₃ and tetrahedral borate ion . One of the key assumption in the use of boron isotopic compositions of carbonates as pH proxy is that only borate ions, , are incorporated into the carbonate. Here, we investigate the speciation of boron in deep-sea coral microstructures (Lophelia pertusa specimen) by using high field magic angle spinning nuclear magnetic resonance (¹¹B MAS NMR) and electron energy-loss spectroscopy (EELS). We observe both boron coordination species, but in different proportions depending on the coral microstructure, i.e. centres of calcification versus fibres. These results suggest that careful sampling is necessary before performing boron isotopic measurements in deep-sea corals. By combining the proportions of B(OH)₃ and determined by NMR and our previous ion microprobe boron isotope measurements, we propose a new equation for the relation between seawater pH and boron isotopic composition in deep-sea corals.     

Ab initio molecular orbital calculations for boron isotope fractionations on boric acids and borates

Ab initio quantum chemistry calculations have been performed on the isotopic exchange reaction between B(OH)₃ and B(OH)₄⁻. Several calculation methods have been carefully compared and evaluated. The “water-droplet” method is chosen to investigate this isotope exchange reaction using cluster models with up to 34 water molecules surrounding the solute. HF/6-31G* level calculations coupled with a 0.920 scaling factor are used for the frequency calculations. A larger K value (1.027) is obtained from this study compared to the commonly used 1.0194 (Kakihana et al., 1977).The fractionations for several boric acid polymers and boron minerals are also studied. Our results suggest that assuming the BO₄ bonding in B(OH)₄⁻ is identical to that in borosilicates is wrong. Tetrahedral boron in silicates has a significantly smaller reduced isotopic partition function ratio (RPFR) and hence will be much isotopically lighter than in B(OH)₄⁻.The new theoretical curve of pH vs. δ¹¹B composition of B(OH)₄⁻ using our calculated 1.027 can be used to predict pH values for equilibrium cases such as incorporation into inorganic calcite. We also find that the shape of this curve is very sensitive to both K and pK_a value, giving the possibility of also predicting salinity from the different shapes of the curve.     

Boron isotopic fractionation related to boron sorption on humic acid and the structure of surface complexes formed

Boron isotopic fractionation during adsorption onto Ca-flocculated Aldrich humic acid (HA) has been investigated experimentally as a function of solution pH at 25°C and I = 0.15 M. Boron aqueous concentration and isotopic composition were determined by Cs₂BO₂⁺ Positive Thermal Ionization Mass Spectrometry analysis, while the structure of B surface complexes on HA was characterized using ¹¹B Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR). Significant B sorption on HA was observed at 6 < pH < 12 with a maximum value of Kd, the partition coefficient between adsorbed and aqueous boron, equal to 40 at pH = 9.5-10. Combined ¹¹B MAS NMR analysis and FITEQL modeling of B sorption on HA showed that this element forms tetrahedrally coordinated five- or six-membered ring chelates, most likely 1,2-diol and 1,3-diol complexes at alkaline pH (8 < pH < 11) and dicarboxylic complexes at near neutral conditions (6 < pH < 9). Results of this study demonstrate for the first time that boron sorption on HA induces a strong pH-dependent isotope fractionation—with ¹¹B depleted at the surface of HA—that reaches a maximum at 5 < pH < 9 (α = 0.975, Δ = −25‰) and decreases sharply at pH >9. The measured isotope fractionation cannot be modeled assuming that the isotopic composition of the sorbed borate species is identical to that of B(OH)₄^- species in the parent solution. It is shown that the extent of isotopic fractionation depends not only on B aqueous speciation but also on the distribution and structure of the borate surface complexes formed. In agreement with energetic constrains, calculation of the isotope fractionation factors between aqueous boric acid and boron surface complexes suggests that the formation of the strained six-membered ring 1,3-diol complex yields a much higher fractionation (α_BLP1−III = 0.954-0.960, Δ = −41/-47‰) than that of the very stable five-membered ring 1,2-diol (α_BLP2−III = 0.983, Δ = −18‰). The results of this study open new perspectives to understand and model boron biogeochemical cycle. It is predicted that boron sorption onto organic matter can have important consequences for the boron isotopic composition of surface water reservoirs (seawater, groundwater, soil waters) in which either abundant organic surfaces or significant boron concentrations are available. In addition, the large isotope fractionation between aqueous boric acid and surface boron-organic complexes found in the present work makes boron a promising tracer of biologic activity.     

Boron K-edge XANES of boron oxides: tetrahedral B–O distances and near-surface alteration

Synchrotron radiation boron K-edge XANES spectra collected in fluorescence yield mode are reported for monoclinic metaboric acid [HBO₂(II)], sinhalite (MgAlBO₄), and a selection of boron oxides in which B is exclusively in trigonal coordination (^[3]B). The anomalously high divergence of tetrahedral (^[4]B–O) bond lengths in HBO₂(II) and sinhalite is used to resolve fine structure at the ^[4]B K edge due to splitting of σ*(t₂) antibonding orbitals. For HBO₂(II), XANES peaks at 196.9 and 199.3?eV are assigned to ^[4]B–O distances of 1.564 and ～1.440 (×3) Å, respectively, and, for sinhalite, peaks at 196.8, 197.9, and 199.6?eV are assigned to distances of 1.586, 1.483 (×2), and 1.442?Å, respectively. A correlation between peak splitting at the ^[4]B K edge and divergence of tetrahedral bond length is established for borates and borosilicates using data for sinhalite, HBO₂(II), ferroaxinite, danburite, datolite, and BPO₄. B K-edge XANES spectra collected in total electron yield mode, which probes to <60?Å, show that almost all ^[4]B in HBO₂(II) and about one-third of the ^[4]B in sinhalite are converted to ^[3]B in the near-surface structure. Moreover, HBO₂(II), HBO₂(III), sassolite (boric acid; H₃BO₃), and v-B₂O₃, which have markedly different bulk structures, have a similar near-surface layer composed of a relaxed anhydrous network of BO₃ groups.     

Coordination of boron in nominally boron-free rock forming silicates: Evidence for incorporation of BO3 groups in clinopyroxene

To explore mechanisms of B-incorporation in common chain silicates we have investigated synthetic diopside samples produced under boron-saturated conditions by ¹¹B and ²⁹Si magic-angle spinning (MAS) NMR and single-crystal NRA, FTIR, EMP and XRD/SREF techniques. Our samples contain 0.14-0.65 wt.% B₂O₃. NMR reveals that B is predominantly present in trigonal coordination in the clinopyroxene structure. This observation is supported by vibrational bands characteristic for B-O stretching in BO₃ groups in the range 1250-1400 cm⁻¹ in polarised single crystal FTIR-spectra. Single crystal structure refinements suggest that boron replaces Si at the T site. Combined, these results suggest that boron replacement for Si at the T-site leads to disruption of one of the T-O bonds of the nominal clinopyroxene structure resulting in replacement of SiO₄ tetrahedra by BO₃ groups. Our results show that high concentrations of boron can be incorporated in the nominally boron-free diopside. Elevated B-concentrations in the present calcic clinopyroxenes are accompanied by modifications of the diopside crystal structure involving the breaking of one T-O bond and simultaneous formation of vacancies at the octahedral M2 site. These structural modifications destabilize the structure and constitute thereby limiting factors for incorporating higher boron concentrations in diopside.     

Boron speciation in aqueous fluids at 22 to 600°C and 0.1 MPa to 2 GPa

The speciation of boron in H₂O+H₃BO₃±NaCl and H₂O+Na₂B₄O₇ fluids was studied in situ at temperatures between 22 and 600°C and pressures from 0.1 MPa to ∼2 GPa using Raman spectroscopy and a hydrothermal diamond anvil cell. Additionally, we determined the frequency shifts of the 877 cm⁻¹ Raman line of [B(OH)₃]⁰ in aqueous fluids with temperature (∂ν₈₇₇/∂T)_{p = 0.1 MPa} = −0.02532 cm⁻¹K⁻¹ and pressure (∂ν₈₇₇/∂P)_{T = 22°C} = 4.06 cm⁻¹GPa⁻¹. The observed species in acidic fluids were [B(OH)₃]⁰ and smaller amounts of a four-coordinated boron species which may be attributed to dissolved metaboric acid HBO₂(aq). The ratio of this B^[4]-O species to [B(OH)₃]⁰ increases with temperature and decreases slightly with addition of NaCl. In alkaline solutions, polyboric ions depolymerize rapidly with temperature. Thus, [B(OH)₃]⁰ and [B(OH)₄]⁻ were the only remaining detectable species at 500 and 600°C. The Raman spectra showed an increase of [B(OH)₃]⁰ relative to [B(OH)₄]⁻ with temperature and an increase of [B(OH)₄]⁻ relative to [B(OH)₃]⁰ with pressure.The general trend in the boron speciation is a higher stability of simpler complexes with temperature. The experimental observations strongly indicate that planar three-coordinated [B(OH)₃]⁰ is the predominant boron species in the aqueous phase over a wide range of P-T-pH conditions. This supports the validity of previous assumptions on boron coordination in crustal and mantle wedge fluids.     

Stable boron isotope fractionation between dissolved B(OH)3 and B(OH)4

The stable boron isotope ratio (¹¹B/¹⁰B) in marine carbonates is used as a paleo-pH recorder and is one of the most promising paleo-carbonate chemistry proxies. Understanding the thermodynamic basis of the proxy is of fundamental importance, including knowledge on the equilibrium fractionation factor between dissolved boric acid, B(OH)₃, and borate ion, B(OH)₄⁻ (, hereafter α_(B3-B4)). However, this factor has hitherto not been determined experimentally and a theoretically calculated value (Kakihana and Kotaka, 1977, hereafter KK77) has therefore been widely used. I examine the calculations underlying this value. Using the same spectroscopic data and methods as KK77, I calculate the same α_(B3−B4) = 1.0193 at 300 K. Unfortunately, it turns out that in general the result is sensitive to the experimentally determined vibrational frequencies and the theoretical methods used to calculate the molecular forces. Using analytical techniques and ab initio molecular orbital theory, the outcome for α_(B3-B4) varies between ∼1.020 and ∼1.050 at 300 K. However, several arguments suggest that α_(B3-B4) ? 1.030. Measured isotopic shifts in various ¹⁰B-, ²D-, and ¹⁸O-labeled isotopomers do not provide a constraint on stable boron isotope fractionation. I conclude that in order to anchor the fundamentals of the boron pH proxy, experimental work is required. The critics of the boron pH proxy should note, however, that uncertainties in α_(B3-B4) do not bias pH reconstructions provided that organism-specific calibrations are used.     

Isotopic composition of dissolved boron and its geochemical behavior in a freshwater-seawater mixture at the estuary of the Changjiang （Yangtze） River

The isotopic composition of dissolved boron, in combination with the elemental concentrations of B, Cl and salinities in freshwater-seawater mixed samples taken from the estuary of the Changjiang River, the largest one in China, was investigated in detail in this study. Brackish water and seawater samples from the estuary of the Changjiang River were collected during low water season in November, 1998. Boron isotopic compositions were determined by the Cs2BO^＋2-graphite technique with a analytical uncertainty of 0.2‰ for NIST SRM 951 and an average analytical uncertainty of 0.8‰ for the samples. The isotopic compositions of boron, expressed in δ^11B, and boron concentrations in the Changjiang River at Nanjing and seawater from the open marine East Sea, China, are characterized by δ^11B values of -5.4‰ and 40.0‰, as well as 0.0272 and 4.43 mg B/L, respectively. Well-defined correlations between δ^11B values, B concentrations and Cl concentrations are interpreted in terms of binary mixing between fiver input water and East Sea seawater by a process of straightforward dilution. The offsets of δ^11B values are not related to the contents of clastic sediment and to the addition of boron. These relationships favor a conservative behavior of boron at the estuarine of the Changjiang River.     

Liquid-vapor fractionation of boron and boron isotopes: Experimental calibration at 400°C/23 MPa to 450°C/42 MPa

We experimentally determined the boron partitioning and boron isotope fractionation between coexisting liquid and vapor in the system H₂O−NaCl−B₂O₃. Experiments were performed along the 400 and 450°C isotherms. Pressure conditions ranged from 23 to 28 MPa at 400°C and from 38 to 42 MPa at 450°C. Boron partitions preferentially into the liquid. Its overall liquid-vapor fractionation is, however, weak: Calculated boron distribution coefficients D_B^liquid-vapor are < 2.5 at all run conditions. With decreasing pressure (i.e. increasing opening of the solvus) D_B^liquid-vapor increases along the individual isotherms. Extrapolation to salt saturated conditions yields maximum boron liquid-vapor fractionations of D_B^liquid-vapor = 1.8 at 450°C and D_B^liquid-vapor = 2.7 at 400°C. ¹¹B preferentially fractionates into the vapor. Calculated Δ¹¹B_vapor-liquid = {[(¹¹B/¹⁰B)_vapor - (¹¹B/¹⁰B)_liquid]/(¹¹B/¹⁰B)_{NBS 951}}*1000 are small and range from 0.2 (± 0.7) to 0.9 (± 0.5) ‰ at 450°C and from 0.1 (± 0.6) to 0.7 (± 0.6) ‰ at 400°C. The data indicate increasing isotopic fractionation with decreasing pressure (i.e. increasing opening of the solvus). Extrapolation to salt saturated conditions yields maximum boron isotope liquid-vapor fractionations of Δ¹¹B_vapor-liquid = 1.5 (± 0.7) ‰ at 450°C and Δ¹¹B_vapor-liquid = 1.3 (± 0.6) ‰ at 400°C. The weak boron isotope fractionation suggests similar trigonal speciation in liquid and vapor. Although the boron and boron isotope fractionation between liquid and vapor is only weak, mass balance calculations indicate that for high degrees of fractionation liquid-vapor phase separation in an open system can significantly alter the boron and boron isotope signature of low-salinity hydrous fluids in hydrothermal systems. Comparing the model calculations with natural oceanic hydrothermal fluids, however, indicate that other processes than fluid phase separation dominate the boron geochemistry in oceanic hydrothermal fluids.     

Boron isotopic composition of atmospheric precipitations and liquid-vapour fractionations

Boron isotope compositions (δ¹¹B) and B concentrations of rains and snows were studied in order to characterize the sources and fractionation processes during the boron atmospheric cycle. The ¹¹B/¹⁰B ratios of instantaneous and cumulative rains and snows from coastal and continental sites show a large range of variations, from −1.5 ± 0.4 to +26.0 ± 0.5‰ and from −10.2 ± 0.5 to +34.4 ± 0.2‰, respectively. Boron concentrations in rains and snows vary between 0.1 and 3.0 ppb. All these precipitation samples are enriched in ¹⁰B compared to the ocean value (δ¹¹B = +39.5‰). An empirical rain-vapour isotopic fractionation of +31‰ is estimated from three largely independent methods. The deduced seawater-vapour fractionation is +25.5‰, with the difference between the rain and seawater fractionations principally reflecting changes in the speciation of boron in the liquid with ∼100% B(OH)₃ present in precipitations. A boron meteoric water line, δD = 2.6δ¹¹B − 133, is proposed which describes the relationship between δD and δ¹¹B in many, but not all, precipitations. Boron isotopic compositions of precipitations can be related to that of the seawater reservoir by the seawater-vapour fractionation and one or more of (1) the rain-vapour isotopic fractionation, (2) evolution of the δ¹¹B value of the atmospheric vapour reservoir via condensation-precipitation processes (Rayleigh distillation process), (3) any contribution of vapour from the evaporation of seawater aerosols, and (4) any contribution from particulate matter, principally sea salt, continental dust and, perhaps more regionally, anthropogenic sources (burning of biomass and fossil fuels). From the δ¹¹B values of continental precipitations, a sea salt contribution cannot be more than a percent or so of the total B in precipitation over these areas.     

A critical evaluation of the boron isotope-pH proxy: The accuracy of ancient ocean pH estimates

The boron isotope-pH technique is founded on a theoretical model of carbonate δ¹¹B variation with pH that assumes that the boron isotopic composition of carbonates mirrors the boron isotopic composition of borate in solution (δ¹¹B_carb = δ¹¹B_borate). Knowledge of the fractionation factor for isotope exchange between boric acid and borate in solution (α_4-3), the equilibrium constant for the dissociation of boric acid (pK_B*), as well as the isotopic composition of boron in seawater (δ¹¹B_sw) are required parameters of the model.The available data suggests that both the value of α_4-3 and the history of δ¹¹B_sw are poorly constrained. However, if one assumes that δ¹¹B_carb = δ¹¹B_borate, an empirical value for α_4-3 can be estimated from the results of inorganic carbonate precipitation experiments. This exercise yields an α_4-3 value of ∼0.974 in accordance with recent theoretical estimates, but substantially deviates from the theoretical value of 0.981 often used to estimate paleo-ocean pH. Re-evaluation of ocean pH using an α_4-3 value of 0.974 and published foraminiferal δ¹¹B values for the Cenozoic yield pH estimates that are relatively invariant, but unrealistically high (∼8.4-8.6). Uncertainty increases as foraminiferal ‘vital effects’ are considered and different models for secular changes in seawater δ¹¹B are applied.The inability to capture realistic ocean pH possibly reflects on our understanding of the isotopic relationship between carbonate and borate, as well as the mechanism of boron incorporation in carbonates. Given the current understanding of boron systematics, pH values estimated using this technique have considerable uncertainty, particularly when reconstructions exceed the residence time of boron in the ocean.     

The structure of the Monte Amiata volcano-geothermal area (Northern Apennines, Italy): Neogene-Quaternary compression versus extension

The tectonic evolution of the Mt Amiata volcano-geothermal area is under discussion. Some authors state that this region, as well as the hinterland of the Northern Apennines, were affected by compression from the Cretaceous to the Quaternary. In contrast, most authors believe that extension drove the tectonic evolution of the Northern Apennines from the Early Miocene to the Quaternary. Field data, seismic analyses and borehole logs have been integrated in order to better define the structural features of the continental crust in the Mt Amiata geothermal area. In this paper I propose the hypothesis that the structure of the crust in the Mt Amiata volcano-geothermal area derives from two main geological processes: (1) contractional tectonics related to the stacking of the Northern Apennines (Cretaceous–Early Miocene), (2) subsequent extensional collapse of the hinterland of the mountain chain, and related opening of the Northern Tyrrhenian Sea (Early Miocene–Quaternary). Compressional and extensional structures characterise the Mt Amiata region, although extensional structures dominate its geological framework. In particular the extension produced: (a) Middle-Late Miocene boudinage of the previously stacked tectonic units; (b) Pliocene–Quaternary normal faulting which favoured the emplacement of a magmatic body in the middle-upper crust; and (c) the eruption of the Mt Amiata volcano, which gave rise to an acid and intermediate volcanic complex (0.3–0.19 Ma). The extension produced the space necessary to accommodate the Middle-Late Miocene marine and continental sediments. Pliocene and Quaternary normal and transtensional faults dissected the previous structures and influenced the Early Middle Pliocene marine sedimentation within the structural depressions neighbouring the Mt Amiata volcano. The magmatic body was emplaced at depth (about 6–7 km) during the Pliocene extension, and produced the eruption of the Mt Amiata volcano during the Late Pleistocene. This gave rise to local uplift, presently reaching about 3,000 m, as well as a negative Bouguer anomaly (−16 mgal), both centred on the Mt Amiata area. The crustal dome shows a good correspondence with the convex shape of the regional seismic marker known as the K-horizon, which corresponds to the 450°C isotherm, and the areas with greatest heat flow. This is probably a consequence of the above-cited magmatic body presently in the process of solidification. A Late Pleistocene eruption occurred along a crustal fissure striking N50° (Mt Amiata Fault), which crosscuts the crustal dome. Hydrothermal circulation, proven by the occurrence of thermal springs and gas vents (mainly CO₂ and H₂S), mainly occurs along the Mt Amiata Fault both in the northeastern ans southwestern sides of the volcano.     

Multi-proxy approach (H/H, O/O, C/C and Sr/Sr) for the evolution of carbonate-rich groundwater in basalt dominated aquifer of Axum area, northern Ethiopia

Isotopic and chemical composition of groundwater from wells and springs, and surface water from the basalt-dominated Axum area (northern Ethiopia) provides evidence for the origin of water and dissolved species. Shallow (depth < 40 m) and deep groundwater are distinguished by both chemical and isotopic composition. Deep groundwater is significantly enriched in dissolved inorganic carbon up to 40 mmol l⁻¹ and in concentrations of Ca²⁺, Mg²⁺, Na⁺ and Si(OH)₄ compared to the shallow type.The δ²H and δ¹⁸O values of all solutions clearly indicate meteoric origin. Shifts from the local meteoric water line are attributed to evaporation of surface and spring water, and to strong water–rock interaction. The δ¹³C_DIC values of shallow groundwater between −12 and −7‰ (VPDB) display the uptake of CO₂ from local soil horizons, whereas δ¹³C_DIC of deep groundwater ranges from −5 to +1‰. Considering open system conditions with respect to gaseous CO₂, δ¹³C_DIC = +1‰ of the deep groundwater with highest PCO₂ = 10^−0.9 atm yields δ¹³C_CO2(gas) ≈ −5‰, which is close to the stable carbon isotopic composition of magmatic CO₂. Accordingly, stable carbon isotope ratios within the above range are referred to individual proportions of CO₂ from soil and magmatic origin. The uptake of magmatic CO₂ results in elevated cations and Si(OH)₄ concentrations. Weathering of local basalts is documented by ⁸⁷Sr/⁸⁶Sr ratios of the groundwater from 0.7038 to 0.7059. Highest values indicate Sr release from the basement rocks. Besides weathering of silicates, neoformation of solids has to be considered, which results in the formation of, e.g., kaolinite and montmorillonite. In several solutions supersaturation with respect to calcite is reached by outgassing of CO₂ from the solution leading to secondary calcite formation.     

Experimental study of the stability of aluminate-borate complexes in hydrothermal solutions

Formation of aqueous aluminate-borate complexes was characterized at 25°C using ²⁷Al NMR spectroscopy, and at 50-200°C via measurements of gibbsite and boehmite solubility in the presence of boric acid. ²⁷Al spectra performed at pH = 9 in Al-B solution with m(B) = 0.02 show the presence of two peaks at 80.5 and 74.5 ppm which correspond to Al(OH)₄⁻ and a single Al-substituted Q¹_Al dimer, Al(OH)₃OB(OH)₂⁻, respectively. In 0.08 m and 0.2 m borate solution, a third peak appears at 68.5 ppm which can be assigned to the Q²_Al trimer Al(OH)₂O₂(B(OH)₂)₂⁻. These chemical shifts are close to those measured for Al(OH)₃OSi(OH)₃⁻ and Al(OH)₂O₂(Si(OH)₃)₂⁻ (74 and 69.5 ppm, respectively; Pokrovski et al., Min. Mag.62a (1998), 1194) which demonstrates the similar structure of Al-B and Al-Si complexes formed in alkaline solutions. Gibbsite and boehmite solubility were measured in weakly basic solutions as a function of boric acid concentration at 50°C and 78 to 200°C, respectively. Equilibrium was reached within several days at m(B) = 0.01-0.1, but more slowly at higher boron concentrations, and at 50°C and m(B) = 0.2, Al concentration increased continuously during at least 3 months as a result of the sluggish formation of Al-polyborates. The equilibrium constant of the reaction Al(OH)₄⁻ + B(OH)₃⁰_(aq) = Al(OH)₃OB(OH)₂⁻ + H₂O decreases very slowly with increasing temperature to 200°C. The log K values are 1.58 ± 0.10, 1.46 ± 0.10, 1.52 ± 0.15, and 1.25 ± 0.15 at 50, 78, 150 and 200°C, respectively, which result in the following values of the standard thermodynamic properties for this reaction: Δ_rG⁰ = −9.22 ± 3.25 kJ/mol, Δ_rH⁰ = −4.6 ± 2.5 kJ/mol, Δ_rS⁰ = 15.5 ± 6.9 J/mol K. The thermodynamic data generated in this study indicate that Al-B complexes can dominate aqueous aluminum speciation in solutions containing ≥0.7 g/L of boron at temperature to at least 400°C.     

Boron isotopic constraints on the source of Hawaiian shield lavas

Boron isotopic compositions of lavas from three representative Hawaiian shield volcanoes (Kilauea, Mauna Loa, and Koolau) were analyzed by thermal ionization mass spectrometry. The boron isotopic composition of each sample was analyzed twice, once with and once without acid leaching to evaluate the effect of posteruptive boron contamination. Our acid-leaching procedure dissolved glass, olivine, secondary zeolite, and adsorbed boron; this dissolved boron was completely removed from the residue, which was comprised of plagioclase, pyroxenes, and newly formed amorphous silica. We confirmed that an appropriate acid-leaching process can eliminate adsorbed and incorporated boron contamination from all submarine samples without modifying the original ¹¹B/¹⁰B ratio. On the other hand, when the sample was weathered, i.e., the olivine had an iddingsite rim, ¹¹B/¹⁰B of the acid-resistant minerals are also modified, thus it is impossible to get the preeruptive ¹¹B/¹⁰B value from the weathered samples. Through this elimination and evaluation procedure of posteruptive contamination, preeruptive δ¹¹B values for the shield lavas are −4.5 to −5.4‰ for Koolau (N = 8), −3.6 to −4.6‰ for Kilauea (N = 11), and −3.0 to −3.8‰ for Mauna Loa (N = 6).Historical Kilauea lavas show a systematic temporal trend for B content and Nb/B coupled with other radiogenic isotopic ratios and trace element ratios, at constant δ¹¹B, indicating little or no assimilation of crustal materials in these lavas. Uncorrelated B content and δ¹¹B in Koolau and Mauna Loa lavas may also indicate little or no effect of crustal assimilation in these lavas. The source of KEA-component (identical to the so-called Kea end member in Hawaiian lavas) of the Hawaiian source mantle, represented by Kilauea, should be derived from lower part of subducted oceanic crust or refractory peridotite in the recycled subducted slab. The systematic trend from Kilauea to Koolau—decreasing δ¹¹B coupled with decreasing εNd as well as increasing ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb—is consistent with involvement of subducted sediment components in the EMK(enriched Makapuu)-component, represented by Makapuu-stage of Koolau lavas.     

Controlling factors of the δ 11B-pH proxy and its research direction

Significant boron isotope fractionation occurs in nature (?70 ‰ to +75 ‰) due to the high geochemical reactivity of boron and the large relative mass difference between ¹⁰B and ¹¹B. Since the 1990s, reconstruction of ancient seawater pH using the isotopic composition of boron in bio-carbonates (δ ¹¹B_carb), and then calculation of the past pCO₂ have become important issues for the international isotope geochemistry community, and are called the δ ¹¹B-pH proxy. Although many achievements have been made by this proxy, various aspects of boron systematics require rigorous evaluation. Based on the previous researches, mechanism of boron isotope fractionation, variation of boron isotope (δ ¹¹B) in nature (especially in bio-carbonates) and controlling factors of the δ ¹¹B-pH proxy, such as the dissociation constant of B(OH)₃ in seawater (pK_a), the δ ¹¹B of seawater (δ ¹¹B_SW), the boron isotopic fractionation factor between B(OH) ₄ ^? and B(OH)₃ (α _4–3), and the incorporated species of boron into bio-carbonates, are reviewed in detail and the research directions of this proxy are proposed. Generally, the controversy about pK_a, δ ¹¹B_sw, and α _4–3 is relatively less, but whether boron incorporated into bio-carbonates only in the form of B(OH) ₄ ^? remains doubtful. In the future, it is required that the physicochemical processes that control boron incorporation into carbonates be rigorously characterized and that the related chemical and isotopic fractionation be quantified. It is also necessary and important to establish a “best-fit empirically equation” between δ ¹¹B_carb and pH of seawater based on the precipitation experiments of inorganic or culture experiments of corals or foraminifera. In addition, extended application of the δ ¹¹B-pH proxy to the earlier part of the Phanerozoic relying on the Brachiopods is worthy of studying. Like other geochemical indicators, there are limiting factors of δ ¹¹B; however, it remains a very powerful tool in the reconstruction of past seawater pH at present.