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Equilibrium constants for boron isotope-exchange reactions
... These progresses not only refined the implications of these geochemical proxies recorded in carbonates, but also raised major concerns related to the proxy, such as the relatively large discrepancies in the empirical isotopic fractionation factor (α 4-3 ) obtained from theoretical and experimental approaches (Kakihana and Kotaka, 1977;Oi, 2000aOi, , 2000bOi and Yanase, 2001;Sanchez-Valle et al., 2005;Zeebe, 2005;Klochko et al., 2006Klochko et al., , 2009Rustad and Bylaska, 2007;Liu and Tossell, 2005;Rustad et al., 2010a;Palmer et al., 1987;Oi et al., 1991;Sanyal et al., 1995Sanyal et al., , 2000Sonoda et al., 2000;Lécuyer et al., 2002;Trotter et al., 2011), and the fundamental mechanisms of the exact incorporation of the aqueous boron species into the calcification process of CaCO 3 crystals. So far, the mechanism of lattice substitution has been widely accepted in the research community, which hypothesized that HBO 3 2− could substitute at a carbonate position to maintain the charge balance (Eq. ...
... For comparison, the results of boron isotope fractionation between biogenic calcite and their co-exiting natural seawater from previous studies are discussed here. As shown in the plot of δ 11 B vs. pH SWS (Fig. 4), the δ 11 B of inorganic calcite and cultured foraminifers are scattering over between two lines of δ 11 B in B(OH) 4 with the 11−10 K B of 1.0194 (Kakihana and Kotaka, 1977) and 1.0272 (Klochko et al., 2009). The δ 11 B values of carbonates were systematically depleted in 11 B relative to the expected value for aqueous B(OH) 4 -, believed to be primarily boron species incorporated into the carbonate mineral lattice, as suggested by Klochko et al. (2009). ...
... This observation also suggests that the boron isotope fractionation be Wang et al. Applied Geochemistry xxx (2018) xxx-xxx (Kakihana and Kotaka, 1977) and pK = 8.597 (Dickson, 1990) and empirical 11 −10 K B = 1.0272 ± 0.0006 (2σ) (Klochko et al., 2006), pK = 8.597 (shallow water station) or 8.697 (deep water station) (Dickson, 1990). tween carbonate and seawater is consistent with the pattern proposed by Hemming and Hanson (1992) and Vengosh et al. (1991), and justifies the applicability of the value of 11 −10 K B = 1.0272 ± 0.0006 by Klochko et al. (2006) for paleo-climate reconstruction using this proxy. ...
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.
... The variation of ␣ is inversely dependent on the temperature (Kakihana et al., 1977), and ␣ is generally expected to be small at magmatic temperatures. However, most studies on boron fractionation have concentrated on low-temperature environments (cf. ...
... If two boron minerals are in equilibrium or are formed from common solutions, the one with the larger 11 B-to- 10 B RPFR value would be most enriched in 11 B. The RPFRs of monomeric B(OH) 3 and B(OH) 4 Ϫ units were calculated by Kakihana and Kotaka (1977). A simplified formula that takes into account the effects of polynuclear species on RPFRs was proposed by Kakihana et al. (1977) ...
... The inferred crystallization temperatures for biotites are probably slightly different, ranging from 1000 to 1150 K. However, because T effects on RPFRs for B(OH) 3 and B(OH) 4 Ϫ (Kakihana and Kotaka, 1977) are smaller than analytical uncertainty for 1000ln␣, the exponential curves are virtually indistinguishable (Fig. 5). On the basis of these considerations, we suggest that (a) the absolute magnitude of boron fractionation could be large, even at magmatic temperature; (b) the B coordination in the melt dominates the boron isotopic fractionation between melt and crystallizing mineral phases such as biotite; and (c) the RPFR of glasses/melts is probably not properly expressed by a simple weighted sum of the RPFR of the monomeric B(OH) 3 and B(OH) 4 Ϫ species. ...
This paper is focused on the role of boron coordination in determining the 11B/10B isotopic fractionation between melt/glass and biotite at magmatic temperatures. For this purpose, three evolved volcanic rocks from Roccastrada, Mt. Amiata, and Mt. Cimini belonging to the Neogene-Quaternary magmatism of central Italy were studied. In these samples, the measured boron biotite-glass partition coefficient ranges between 0.004 and 0.011, indicating that boron behaves as an incompatible element during biotite crystallization. The 11B magic-angle spinning nuclear magnetic resonance (NMR) spectra reveal the presence of trigonal BO3/2 units, tetrahedral BO4/2− sites, and three-coordinated BO2/2O− species containing one nonbridging oxygen. The relative contributions of these different boron sites were estimated by spectral deconvolution, and it was observed that the fraction of trigonally coordinated boron decreases with increasing K2O concentration in the glass. The 11B/10B isotopic fractionation between biotite and melt/glass was observed to be large even at magmatic temperatures and was found to be 1.0066 (Roccastrada sample), 1.00535 (Mt. Amiata sample), and 1.00279 (Mt. Cimini sample). Fractionation is mostly related to the relative amount of trigonal and tetrahedral boron sites in the glass network rather than to other processes, including the speciation of hydrous species in the glass structure. The measured α values are significantly higher than the calculated ones obtained using the reduced partition function ratios (RPFRs) for B(OH)3 and B(OH)4− as reported by Kakihana et al. (1977) and the abundance of trigonal and tetrahedral boron obtained by 11B NMR spectra. Furthermore, a nonlinear relationship is observed between the percentage of BO4 in the glass structure and the measured 1000lnα, suggesting that the approximation of monomeric B(OH)3 and B(OH)4− species contributions through ideal mixing in calculating the RPFRs in polyanions (Oi et al., 1989) probably does not apply to silicate glasses.The large B isotopic fractionation measured between glass and biotite and its dependence on the boron coordination in the glass are a limitation to the use of δ11B in the mineral to characterize magmas. Nonetheless, the high incompatible behavior of boron in the most common magmatic minerals rules out that fractional crystallization significantly modified the B isotopic composition of the melt.
... In 1977, Kakihana et al. [19,20] provided the first theoretical estimate of the magnitude of isotope exchange between borate and boric acid ( 11-10 K B = 1.0194 at 25°C), based on reduced partition function ratio calculations from spectroscopic data on molecular vibrations (Table 1). In spite of the fact that in a later publication with new collaborators [21] it was suggested that the boron isotope equilibrium constant could be much larger than that predicted in 1977, it is the earlier value [19] that has been consistently used in paleo-pH reconstructions. ...
... These were specifically considered in order to evaluate the effect of different temperatures, medium composition and total boron on the isotopic equilibrium constant. Thousands of at-sea measurements of seawater pH [32][33][34] demonstrate Table 1 Published estimates of 11-10 K B in seawater Estimates based on spectroscopic data on molecular vibrations Treatment (T°C) K B Empirical spectra and Force Field modeling (26.8°C) [19,20] 1.0194 Empirical spectra and Force Field modeling (26.8°C) [22] 1.0176(2) Ab-initio molecular orbital theory (25°C) [23,24] 1.0260 Ab-initio molecular orbital theory (25°C) [25] 1.0267 Evaluation of all of the above treatments [26] ≥1.0300 ...
... Of greatest interest to oceanic pH reconstructions, the 11-10 K B result for artificial seawater [ 11-10 K B = 1.0272 ± 0.0006 (2σ m ′) at 25°C (B T = 0.01 mol kg − 1 H 2 O)] is significantly larger than the 1977 estimate of 1.0194 [19,20] and the spectral and the force field modeling estimate of 1.0176 [22]. On the other hand, our empirical result is in agreement with the recent ab-initio calculations [23][24][25], which indicate 11-10 K B values between 1.0260 to 1.0267. ...
The boron isotopic composition of marine carbonates is considered to be a tracer of seawater pH. Use of this proxy benefits from an intimate understanding of chemical kinetics and thermodynamic isotope exchange reactions between the two dominant boron-bearing species in seawater: boric acid B(OH)3 and borate ion B(OH)4−. However, because of our inability to quantitatively separate these species in solution, the degree of boron isotope exchange has only been known through theoretical estimates. In this study, we present results of a spectrophotometric procedure wherein the boron isotope equilibrium constant (11–10KB) is determined empirically from the difference in the dissociation constants of 11B(OH)3 and 10B(OH)3 in pure water, 0.6 mol kg− 1 H2O KCl and artificial seawater. Within experimental uncertainty, our results show no dependence of 11–10KB on temperature, but 11–10KB at 25 °C in pure water was statistically different than results obtained in solutions at high ionic strength. 11–10KB of the seawater (S = 35, BT = 0.01 mol kg− 1 H2O) at 25 °C is 1.0272 ± 0.0006. This result is significantly larger than the theoretical value used in numerous paleo-pH studies (11–10KB = 1.0194).
... Large differences in δ 11 B values of −30‰ (Williams and Hervig, 2004) to +75‰ (Hogan and Blum, 2003) exist between fluids and rocks. The large mass difference between 10 B and 11 B and the coordination change from dominantly trigonal boron in solution (at low pH) to tetrahedral boron in silicates is the reason for the large boron isotope fractionation during the incorporation of 10 B into clay minerals (Kakihana and Kotaka, 1977;Palmer et al., 1987). Because boron is not redox sensitive Tomascak, 2004) its fluid-mineral isotope fractionation make boron isotopes a good tracer of boron sources and fluid origin (Spivack, 1986;Vengosh et al., 1991;Williams et al., 2001a, b;Williams and Hervig, 2002;Hervig et al., 2002;Deyhle and Kopf, 2005;Pennisi et al., 2009). ...
... However, the pH susceptible fluid-mineral boron isotope fractionation com Köster et al. Chemical Geology xxx (2019) xxx-xxx plicates data interpretation (Kakihana and Kotaka, 1977;Palmer et al., 1987). The fluid-mineral boron isotope fractionation was experimentally evaluated (Williams et al., 2001a, b) for illite-smectite in pH~6 fluids and anchored at low temperature using the fluid-adsorbed boron isotope fractionation for which fractionation during boron adsorption is only a few per mil (Palmer et al., 1992). ...
The mineralogical, chemical and isotopic analyses of smectites, with variable interlayer cation occupancies, from bentonite deposits in various depositional environments, reveal new insights into the boron sources and the fluids involved in bentonitization in marine and non-marine environments.
Smectites from bentonites have non-exchangeable, structural boron concentrations of 0.2 to 196 μg/g B. Smectites from sodium bentonites have higher boron concentrations (>30μg/g) than those from magnesium or calcium bentonites. Most smectites have a small, interstratified illitic component that has a major influence on boron concentrations, requiring the use of modified fluid-mineral boron partitioning coefficients, which indicate that bentonites formed from fluids of highly variable boron concentrations of <0.1mg/L B to >100 mg/L B, and a chlorine content of 76.4 mg/L to 59,076 mg/L. The sodium bentonites formed from boron-rich saline fluids or brines whereas the fluids involved in calcium and magnesium bentonite formation have a more variable boron composition and salinity.
The δ11B values of the structural boron in tetrahedral sites of smectites range from −30.1‰ to +12.2‰. The smectites from terrestrial depositional settings have δ11B values of −30.1‰ to about 0‰, whereas smectites from marine depositional settings have negative as well as positive δ11B values. The boron isotope values indicate that all examined bentonites from terrestrial depositional settings as well as many bentonites from marine depositional settings formed from basinal fluids or brines.
The boron isotope geochemistry of smectites is demonstrated to be a tool for elucidating the fluids involved in the formation of clay mineral deposits. It also has great potential for tracing fluids in other settings involving authigenic clay minerals such as sedimentary basins and surficial crystalline rocks, as well as man-made applications such as in disposal sites for highly active nuclear waste.
... The isotopic fractionation factor a = ( 11 B(OH) 3 / 10 B(OH) 3 )/([ 11 B(OH) 4 ] À /[ 10 B(OH) 4 ] À ) between boric acid and borate ions was determined by Kakihana and Kotaka (1977) to 1.007 at 700 K and 1.004 at 1000 K. According to the equation Table 1. for mica-fluid partitioning experiments performed at near neutral and at basic conditions we calculated the isotopic fractionation a to 1.011 (F 0.001) for 500 8C and 1.0065 (F 0.0004) for 700 8C, and to 1.007 (F 0.001) for 400 8C and 1.005 (F 0.001) for 500 8C, respectively. ...
... However, it is unknown to which extent second-order effects might influence the fractionation. Second-order effects might include (i) differences in geometries and energetics between [BO 4 ]-tetrahedra in micas and B(OH) 4 À tetrahedra in fluids and (ii) the possible presence of various, chemically different species of boron associated with K, Al, and Si complexes within fluids at high pressures and temperatures, which do not appear in the simple system investigated by Kakihana and Kotaka (1977). High solubilities of alkalis, Si, Al, and particularly light elements at high pressure and temperature (Manning, 2004) indicate that such complexation of fluid-compatible elements within subduction zone fluids is likely. ...
The fractionation of boron isotopes between synthetic boromuscovite and fluid was experimentally determined at 3.0 GPa/500 °C and 3.0 GPa/700 °C. For near-neutral fluids Δ11B(mica-fluid) = δ11B(mica) − δ11B(fluid) is − 10.9 ± 1.3‰ at 500 °C, and − 6.5 ± 0.4‰ at 700 °C. This supports earlier assumptions that the main fractionation effect is due to the change from trigonal coordination of boron in neutral fluids to tetrahedrally coordinated boron in micas, clays and melts. The T-dependence of this effect is approximated by the equation Δ11B(mica,clay,melt–neutral fluid) = − 10.69 · (1000/T [K]) + 3.88; R2 = 0.992, valid from 25 °C for fluid–clay up to about 1000 °C for fluid–silicate melt. Experiments at 0.4 GPa that used strongly basic fluids produced significantly lower fractionations with Δ11B(mica–fluid) of − 7.4 ± 1.0‰ at 400 °C, and − 4.8 ± 1.0‰ at 500 °C, showing the reduced fractionation effect when large amounts of boron in basic fluids are tetrahedrally coordinated. Field studies have shown that boron concentrations and 11B/10B-ratios in volcanic arcs systematically decrease across the arc with increasing distance from the trench, thus reflecting the thermal structure of the subducting slab. Our experiments show that the boron isotopic signature in volcanic arcs probably results from continuous dehydration of micas along a distinct P–T range. Continuous slab dehydration and boron transport via fluid into the mantle wedge is responsible for the boron isotopic signature in volcanic arcs.
... Although the behavior of dissolved B includes other processes as trigonal anions B (OH) 3 and tetrahedral borate B(OH) 4 anions are attached to reactive sites on the hydroxyl surface, neutral [Min (OH)] or protonated [Min (OH 2 ) + ], or on clay minerals by adsorption reactions. Adsorption is pH-dependent as tetrahedral B(OH) 4 predominant at high pH is more readily adsorbed than its equilibrium competitor B(OH) 3 , trigonal and planar, which prevails at low pH (Kakihana and Kotaka, 1977;Zeebe, 2005;Liu and Tossell, 2005). Maximum adsorption of B has been reported at pH 9 (Goldberg, 1999). ...
The intensive use of the Aquifer of the Metropolitan Zone of Mexico City (AMZMC) has increased the concentrations of some trace elements due to natural or anthropogenic sources, as the release of elements from the heterogeneous mixture of the volcanic rocks and/or alluvial clay deposits, which play a very important hydrogeological role in the behavior of the aquifer. This study characterizes, examines, and compares the concentrations of Ba, B, Sr, and Rb in some wells of the AMZMC. Characterization of major elements and Ba, B, Sr concentration and their behavior in the aquifer, and some ionic ratios have been used to identify the dominant hydrogeochemical processes and the different sources of Rb, Ba, B, Sr in the water, in which mineral alteration, ion exchange, redox reactions are the dominant processes, and may intensify as the salinity, temperature and ionic strength in the medium increases. This information may be indirectly relevant as a contribution to the development of appropriate water management strategies in Mexico City. In addition to providing new data on the distribution and behavior of Rb, Ba, B, Sr in groundwater of the Mexico City Aquifer, which is used for drinking water supply.
... 8). Early studies frequently used a value of 1.0194, based on calculations by Kakihana and Kotaka (1977) using vibrational frequency data from Kotaka and Kakihana (1977). However it was later shown that a major vibrational frequency mode had been improperly assigned in these calculations (Rustad and Bylaska 2007), and recalculation using various methods yields higher values, though with considerable range (*1.020-1.050; ...
The boron isotope composition of foraminifera provides a powerful tracer for CO2 change over geological time. This proxy is based on the equilibrium of boron and its isotopes in seawater, which is a function of pH. However while the chemical principles underlying this proxy are well understood, its reliability has previously been questioned, due to the difficulty of boron isotope (δ¹¹B) analysis on foraminferal samples and questions regarding calibrations between δ¹¹B and pH. This chapter reviews the current state of the δ¹¹B-pH proxy in foraminfera, including the pioneering studies that established this proxy’s potential, and the recent work that has improved understanding of boron isotope systematics in foraminifera and applied this tracer to the geological record. The theoretical background of the δ¹¹B-pH proxy is introduced, including an accurate formulation of the boron isotope mass balance equations. Sample preparation and analysis procedures are then reviewed, with discussion of sample cleaning, the potential influence of diagenesis, and the strengths and weaknesses of boron purification by column chromatography versus microsublimation, and analysis by NTIMS versus MC-ICPMS. The systematics of boron isotopes in foraminifera are discussed in detail, including results from benthic and planktic taxa, and models of boron incorporation, fractionation, and biomineralisation. Benthic taxa from the deep ocean have δ¹¹B within error of borate ion at seawater pH. This is most easily explained by simple incorporation of borate ion at the pH of seawater. Planktic foraminifera have δ¹¹B close to borate ion, but with minor offsets. These may be driven by physiological influences on the foraminiferal microenvironment; a novel explanation is also suggested for the reduced δ¹¹B-pH sensitivities observed in culture, based on variable calcification rates. Biomineralisation influences on boron isotopes are then explored, addressing the apparently contradictory observations that foraminifera manipulate pH during chamber formation yet their δ¹¹B appears to record the pH of ambient seawater. Potential solutions include the influences of magnesium-removal and carbon concentration, and the possibility that pH elevation is most pronounced during initial chamber formation under favourable environmental conditions. The steps required to reconstruct pH and pCO2 from δ¹¹B are then reviewed, including the influence of seawater chemistry on boron equilibrium, the evolution of seawater δ¹¹B, and the influence of second carbonate system parameters on δ¹¹B-based reconstructions of pCO2. Applications of foraminiferal δ¹¹B to the geological record are highlighted, including studies that trace CO2 storage and release during recent ice ages, and reconstructions of pCO2 over the Cenozoic. Relevant computer codes and data associated with this article are made available online.
... Under the strong weathering conditions, fine clay minerals were delivered by the river and gradually deposited at the bottom of the lake. Previous studies have confirmed that the light isotope of 10 B tends to adsorb preferentially onto clay minerals during adsorption processes (Kakihana and Kotaka, 1977;Palmer et al., 1987;Vengosh et al., 1991;Rose et al., 2000), therefore, the depletion of 11 B causes negative shift in the δ 11 B in silicate phase of the mudstone strata. ...
... Given that the maximum theoretical B isotope fractionation between species B(OH) 3 and B(OH) 4 − is between +17.6‰ (Sanchez-Valle et al., 2005) and +19.3‰ (Kakihana and Kotaka, 1977), rain water, as the alteration fluid, is insufficient in raising the values of the δ 11 B values of the pristine crust, that would only reach a maximum of δ 11 B = −2‰. An alteration agent solely of meteoric origin is therefore considered unlikely, and a seawater δ 11 B (+40‰) influenced fluid/rain is more suitable in creating the high δ 11 B (+16.9‰) of the altered crust endmember. ...
We report new boron concentrations and isotope compositions measured by ion microprobe on Holocene tephra samples from 6 Icelandic volcanoes. The B concentrations range over almost a factor of 10, from 1.50 ±0.10 to 13.10 ± 0.10 ppm in basalt and obsidian, respectively. The highest δ11B ( + 16.9 ± 2.2 and + 6.1 ± 1.6‰) measured in rhyolites from the rift-related Askja and Krafla volcanoes. In contrast, the lowest δ11B values (− 5.3 and − 5.2‰) observed in samples with the some of the highest δ18O (+ 4.92 and + 5.18‰, respectively) and (230Th/232Th) (1.067 and 1.030, respectively), correspond to a “normal-mantle” signature. The variations of boron isotope compositions in the Icelandic tephra and their negative correlations with both δ18O and (230Th/232Th) strongly support the mixing between mantle derived basalts and crustal rhyolites produced from anatexis of hydrothermally altered basaltic crust. The low δ18O in combination with high δ11B values in rhyolite is best explained by the presence of seawater derived B in a hydrothermal fluid that is mostly of meteoric origin.
... During peridotite-seawater interaction, the observed B and Li isotope fractionation is controlled by temperature, by pH (for B) and by water/ rock ratio. Kakihana and Kotaka (1977) calculated the fractionation factor between BOH 3 -BOH 4 in a fluid (α 3/4 =1.0193 at 300 K). New ab initio molecular orbital calculations for boron isotope fractionation on boric acids and borates of Liu and Tossell (2005) showed that the B (OH) 3 -B(OH) 4 isotope fractionation is higher (α 3/4 = 1.0245 at 300 K), Salters and Stracke, 2004). ...
... Water-rock interaction clearly concentrates 11 B in the fluid phase relative to rocks, as evidenced by empirical studies of formation of pore fluids and associated sediments Brumsack and Zuleger, 1992;You et al. 1995a;Vengosh et al., 1998;Pennisi et al., 2000), geothermal fluids and rocks (Kakihana et al., 1987;Palmer and Sturchio, 1990;Musashi et al., 1988Musashi et al., , 1991, metamorphic fluids and minerals (Peacock and Hervig, 1999), and fumarolic condensates and associated rocks or sublimates (Nomura et al., 1982;Kanzaki et al., 1987;Oi et al., 1989;this study). As predicted by theoretical calculations (Kakihana and Kotaka, 1977) and indicated by limited experimental data, at temperatures above 300°C the magnitude of isotopic fractionation is on the order of a few permil between fluids and sediments (You et al., 1995b) or minerals like tourmaline (Palmer et al., 1992). In modeling, we assumed ⌬ water-rock values for 11 B/ 10 B between 1‰ (150 -200°C) and 0‰ (Ͼ500°C), which are consistent with the small differences in ␦ 11 B for La Fossa condensates and local volcanic rocks. ...
Temporal variation in the isotopic composition of boron has been monitored in fumarolic condensates collected over an extended time period (1970–1996) from La Fossa crater, Volcano Island. We also report comparative boron isotopic data for representative Vulcano lavas and for shallow hydrologic samples (seawater, wells, thermal springs) from the north flank of La Fossa. Combined with concurrent chemical and isotopic (δ18O, δD) data for the fumaroles, these results indicate that the fumarolic fluids record mixing relations between three distinct fluid end members: (1) a dominantly magmatic fluid (EM1); (2) a mixture of modified seawater with magmatic fluid (EM2); and (3) an aqueous fluid produced from seawater by extensive wall-rock reaction, evaporation, and boiling (AF). Differences between the latter two end members are most clearly accentuated on the basis of the boron isotopic data. Long-term compositional variations for crater fumaroles were dominated by EM1-AF mixing between 1979–88, with progressive decrease in EM1 contribution with time, and by EM2-AF mixing between 1988–96. The exact spatial distribution of these fluid reservoirs remains unclear, but all must have been present throughout the monitoring period to account for the observed variations. Moreover, the combined B-O-H data seem to preclude important contributions from shallow meteoric reservoirs. Marked short-term variations in δ11B closely coincide with episodes of local seismicity, which presumably triggered reorganization of hydrothermal circulation patterns; gradual variations over periods up to 3–4 years are associated with relatively low seismicity during which fluid circulation was likely influenced by effects of mineral precipitation on permeability of the hydrologic system.
... Reference to the fact that 611B values of sassolites near fumaroles are slightly lower than those of gases from the fumaroles ( Kanzaki et al., 1979;Nomura, 1990) indicates that K1 may be slightly larger than unity. Kakihana and Kotaka (1977) Table 2 that boron in hot spring water is slightly lighter than boron in fumarolic gas in an area (of course, the 811B values of hot spring water and fumarolic gas are the same, if all the boron in the gas is transferred to the water). ...
Compositions of major dissolved components and of boron isotopes were investigated for hot spring waters in Ibusuki and adjacent areas (the Ibusuki region), Kyushu, Japan. The chemical compositions of the hot spring waters examined were consistent with the previous contention that major dissolved compo nents of hot spring waters in the region are the resultant of the interaction of seawater with heated rocks underground. The 811B values (relative to the isotopic ratio of NBS SRM 951) of the hot spring waters ranged from +2.1 %o to +39.4%o. A close inspection of the boron isotopic data identified two boron sources; one is seawater with a 611B value of +39%o and the other is volcanic gases with a 611B value of ca. +6%o, which supply hot spring waters with boron with 811B = ca. +2%1o.
... This is mostly because of the geochemical characteristics of B, i.e., its high solubility in aqueous fluids, its generally high volatility, and its large isotopic variation in natural environments. The almost 100&-span of B fractionation in nature results mostly from the coordination change in B between the trigonal B(OH) 3 of dissolved to tetrahedral BðOHÞ À 4 , which substitutes for Al and Si in silicate minerals, whereas 10 B has a preference for tetrahedral bonds (Kakihana and Kotaka, 1977;Palmer and Swihart, 1996;Palmer et al., 1987). In the field of interactive processes between siliceous sediment/rock and aqueous fluid in diagenetic regimes, B is one of the key tracers in subduction zone processes Bebout et al., 1993;You et al., 1993;Kopf et al., 2000;Deyhle andKopf, 2001, 2002;, alteration of igneous rock (Smith et al., 1995;Benton et al., 2001), arc volcanism (e.g., Rosner et al., 2003;Ishikawa and Nakamura, 1474- 1994; Ishikawa and Tera, 1997;Smith et al., 1995;Ishikawa and Tera, 1999;Ishikawa et al., 2001), or diagenesis (Williams et al., 2001c;Kopf et al., 2003). ...
Boron and its two stable isotopes 10 B and 11 B are considered powerful tracers in low temperature regimes like ocean waters and subduction zones, while very little is known about the isotopic fractionation in high pressure–temperature (PT) regimes. However, recent studies indicate that boron fractionation in silicate–water systems may follow a systematic relationship with temperature, regardless of the geological regime. In this paper we review previous B isotope studies and test the empirical relationship proposed by Williams et al. [Boron isotope geochemistry during diagenesis. Part I. Experimental determination of fractionation during illiti-zation of smectite, Geochim. Cosmochim. Acta 65 (2001a) 1769–1782; Boron isotope geochemistry during diagenesis. Part II. Appli-cations to organic sediments, Geochim. Cosmochim. Acta 65 (2001b) 1783–1794; Application of boron isotopes to the understanding of fluid–rock interactions in a hydrothermally stimulated oil reservoir in the Alberta Basin, Canada, Geofluids 1 (2001c) 229–240] by comparing it with the wealth of earlier data from studies of different geological scenarios. Our main conclusion is that, given the large variations of B isotope fractionation patterns in natural silicate–water systems, the majority of these systems are not represented by the proposed relationship. Factors more complex than temperature alone appear to control B isotope geo-chemistry, which include species of the silicate mineral, starting fluid, pH, geological setting, fluid/rock ratio (i.e. porosity), or time for equilibration between the solid and fluid phase.
... During peridotite-seawater interaction, the observed B and Li isotope fractionation is controlled by temperature, by pH (for B) and by water/ rock ratio. Kakihana and Kotaka (1977) calculated the fractionation factor between BOH 3 -BOH 4 in a fluid (α 3/4 =1.0193 at 300 K). New ab initio molecular orbital calculations for boron isotope fractionation on boric acids and borates of Liu and Tossell (2005) showed that the B (OH) 3 -B(OH) 4 isotope fractionation is higher (α 3/4 = 1.0245 at 300 K), Salters and Stracke, 2004). ...
Spinel harzburgites from ODP Leg 209 (Sites 1272A, 1274A) drilled at the Mid-Atlantic ridge between 14°N and 16°N are highly serpentinized (50–100%), but still preserve relics of primary phases (olivine ≥ orthopyroxene >> clinopyroxene). We determined whole-rock B and Li isotope compositions in order to constrain the effect of serpentinization on δ11B and δ7Li. Our data indicate that during serpentinization Li is leached from the rock, while B is added. The samples from ODP Leg 209 show the heaviest δ11B (+ 29.6 to + 40.52‰) and lightest δ7Li (− 28.46 to + 7.17‰) found so far in oceanic mantle. High 87Sr/86Sr ratios (0.708536 to 0.709130) indicate moderate water/rock ratios (3 to 273, on the average 39), in line with the high degree of serpentinization observed.
... High-resolution boron isotope time series will demand the measurement of large sample sets. Ion probe techniques (Chaussidon and Albarède, 1992) provide fast measurements but precision is too low (2r = 3x ) with respect to the high sensitivity of boron isotope fractionation relatively to pH (Kakihana and Kotaka, 1977;Palmer et al., 1987;Sanyal et al., 1996). TIMS techniques are the most commonly used to perform boron isotope analyses of terrestrial materials. ...
The use of MC-ICP-MS for B isotopic measurements offers some advantages compared with other techniques such as the absence of interferences with atomic masses 10 and 11 and an enhanced speed of data acquisition. We have developed a fast and simple protocol for the separation by ion-exchange resins and concentration of B from natural waters, carbonates, phosphates, and silicates. Normalization of sample isotopic ratios to the NIST reference NBS 951 leads to an external reproducibility of ±0.3δ unit at 95% confidence level. The ionization yield is about 2 ions per 106 atoms of B in the analyzed solution. At least 2 μg of boron is required to obtain the above good-quality external reproducibility.Replicated analyses of Mediterranean seawater provided a δ11B of 40.26±0.29 (n=11). Aragonite-secreting fauna (corals and bivalves) living in tropical seawater have δ11B close to 25‰ that are much higher than calcite-secreting invertebrates such as brachiopods from tropical to high latitudes with δ11B ranging from 16.8‰ to 19.6‰. The first application of this method was to reevaluate the boron isotopic fractionation between calcite and water as a function of pH. We analyzed the calcitic shell of present-day brachiopods living in marine waters (surface down to 450 m) with pH varying from 7.6 to 8.4. By combining our data with those obtained by Sanyal et al. [Paleoceanography 11 (1996) 513] on cultured foraminifera, we derived an isotopic fractionation curve (αcalcite–seawater=0.023X[B(OH)4−]+0.976), resulting in the following relationship between the pH of seawater and the δ11B values of biogenic carbonatesThis equation has a slope equal to that of the experimental clay–water fractionation curve [Palmer et al., 1987, Geochim. Cosmochim. Acta 51 (1987) 2319], which precludes the use of the boron isotope fractionation among calcite–clay pairs as a “paleo-pH meter”.
This thesis has the following objectives: 1) To better understand how boron isotopes in modern fluvial sediments record the weathering regime at the catchment scale. 2) To better understand how the weathering “signal” carried by river sediments is transferred from source areas to the depositional environment. 3) To determine if boron isotopes in sediment deposits (paleochannels) can be used to reconstruct paleo-weathering and paleo-environmental conditions and reveal how continental weathering at large (production and sediment transport) responds to climatic variability over the last glacial-interglacial cycle (last 100 ka). These objectives were addressed by studying fluvial material from the Gandak (Himalayas) and Murrumbidgee (NSW, Australia) Rivers and fluvial sediment deposits from the Riverine Plain (Murrumbidgee catchment, Australia). Knowledge of the parameters that control boron isotope fractionation of river sediment during formation and transport was first gained in the modern systems and then applied to ancient paleochannel deposits.
This chapter explains the translation of δ¹¹B and B/Ca to carbonate system parameters. In addition to explaining the equations behind this translation process, we also provide a series of sensitivity experiments to demonstrate the pH and pCO2 uncertainties introduced by individual uncertainties of δ¹¹Bforam, temperature, salinity, alkalinity, δ¹¹Bsw, and varying seawater elemental composition over long geological time scales, including recommendations for propagating errors. The similarities between boron proxy calibrations published to date are further examined to explore potential systematics for proxy sensitivities in marine calcifiers that have not yet or can no longer be calibrated over a wide range of known environmental conditions. Finally, guidelines are provided for selecting sediment core sites and sample material for successful paleoreconstructions.
This chapter presents the theoretical background of the boron isotope and B/Ca proxies, starting with the dissociation of dissolved boron in aqueous solution, and constraints on the boron isotope fractionation between the dominant dissolved boron species in seawater. Using laboratory culture experiments with foraminifers, corals and inorganically precipitated calcium carbonate, as well as observations of naturally grown samples collected from the ocean, a strong empirical framework has been established for the boron isotope proxy. However, it is also clear that an organism's calcifying fluid is often chemically distinct from ambient seawater, and partial dissolution of skeletal remains at the seafloor can further modify original proxy records. These aspects need to be taken into account when selecting sample material and interpreting proxy records. Deep‐time paleoreconstructions further need to consider secular variations in the seawater boron concentration and isotopic composition; currently available estimates are summarized.
In comparison to the boron isotope proxy, the relationship between B/Ca and marine carbonate chemistry has only been studied over the past decade, and it is clear that B incorporation in different calcifiers responds to different environmental controls. While B/Ca in corals may be more sensitive to temperature than carbonate chemistry, planktic foraminiferal B/Ca increases with experimental seawater‐pH in laboratory culture, but sediment observations suggest that additional environmental parameters complicate the proxy. In contrast, strong and reproducible relationships have been established for B/Ca in benthic foraminifers and ocean bottom water carbonate saturation, albeit with significant species effects and without a mechanistic explanation as to why carbonate saturation should be the controlling parameter.
Isotope fractionation factors play a key role in modern geochemistry and are used to interpret a broad range of natural phenomena over a wide range of temporal scales. Experimental advances in this area have been driven by significant improvements in mass spectrometry techniques coupled to advances in computational chemistry methods due to advances in software and substantial improvements in hardware. The prediction of isotope fractionation factors requires the ability to predict harmonic frequencies to high accuracy due to the fact that the changes are often in parts per 1000 (per mil). This requires the choice of a good model system that captures the critical geochemical features, the appropriate choice of the computational electronic structure method (correlated molecular orbital theory at least at the second-order Møller–Plesset theory level vs. density functional theory with an appropriate exchange-correlation functional), the choice of the basis set, and the potential use of implicit models for solvation and/or the solid state environment. This chapter describes the computational approaches needed for the prediction of isotope fractionation factors and provides example of the application of these methods to geochemical systems containing atoms from across the periodic table.
This chapter concerns calculation of the energetics of isotope exchange reactions from electronic structure calculations. It focuses on the thermodynamics of equilibrium isotope exchange as derived from harmonic partition functions. In the harmonic approximation, solving the eigenvalue problem turns the complicated motions of the system of atoms into 3&;#x02009;N independent harmonic oscillator problems. The chapter also discusses some generalizations of the rules of thumb often used to qualitatively understand isotope fractionation that have been more completely understood through electronic structure calculations. It shows the relative error in the computed equilibrium constants for isotope exchange reactions involving small molecules, originally studied over a wide range of density functional theory (DFT) exchange&;#x02010;correlation functionals and basis sets. First&;#x02010;principles calculations have helped advance the science of isotope geochemistry, having played an important role in helping formulate the field of clumped isotopes.
Hydrogen isotope separation effect by electrolysis of water was theoretically investigated and was compared with experimental results. The separation mechanism was analyzed as the hydrogen isotope exchange reaction between water and diatomic hydride that consists of hydrogen and cathode material. The equilibrium constants of the isotope exchange reaction were calculated from reduced partition function ratio. Using the constants, the separation factor (SF) of the isotopes was calculated according to the two-phase distribution theory for isotopes. Experimentally, light or heavy water spiked with tritiated water was electrolyzed by a device with a solid polymer electrolyte, which equipped with SUS, Ni, or carbon cathode. Thus, the SFs were experimentally obtained. Calculated SFs were well agreed with the experimentally values for SUS and Ni cathodes, and that for carbon cathode was somewhat small then the experimental value.
The negative thermal ionisation mass spectrometry of boron, a powerful method for the determination of boron isotopic composition (11B/10B) and concentration in geological materials, is described.
The boron isotope paleo-pH proxy has been extensively studied due to its potential for understanding past climate change, and further calibrations were considered for accurate applications of the proxy because of significant variability related to biocarbonate microstructure. In this work, we studied the boron isotopic fractionation between modern marine corals and their coexisting seawater collected along shallow area in Sanya Bay, South China Sea. The apparent partition coefficient of boron (KD) ranged from 0.83×10−3 to 1.69×10−3, which are in good agreement with previous studies. As the analyzed coral skeleton (∼5 g) spanned the growth time period of 1–2 years, we discussed the boron isotopic fractionation between pristine corals and modern seawater using the annual mean seawater pH of 8.12 in this sea area. Without taking the vital effect into account, (11B/10B)coral values of all living corals spread over the curves of (11B/10B)borate vs. (11B/10B)sw with the α
4−3 values ranging from 0.974 to 0.982. After calibrating the biological effect on the calcifying fluid pH, the field-based calcification on calcifying fluid pH (i.e., Δ(pHbiol-pHsw)) for coral species of Acropora, Pavona, Pocillopora, Faviidae, and others including Proites are 0.42, 0.33, 0.36, 0.19, respectively, and it is necessary to be validated by coral culturing experiment in the future. Correlations in B/Ca vs. Sr/Ca and B/Ca vs. pHbiol approve temperature and calcifying fluid pH influence on skeletal B/Ca. Fundamental understanding of the thermodynamic basis of the boron isotopes in marine carbonates and seawater will strengthen the confidence in the use of paleo-pH proxy as a powerful tool to monitor atmospheric CO2 variations in the past.
The ocean's ‘biological pump’ refers to the coupled biological, chemical, and physical processes that work to concentrate carbon and other biologically active elements in the voluminous ocean interior, sequestering them from the surface ocean and the atmosphere. Current research seeks to understand the relationship of the ocean's biological pump to the Earth's environmental, chemical, and climatic history. Changes in the efficiency of the biological pump are central to most current hypotheses for the cause of the coherent variations of atmospheric CO2 over the ice age climate cycles (i.e., glacial vs. interglacial stages). Here, we review the concepts, tools, and observations relating to this topic. While the biological pump is driven by biological activity in the sunlit surface ocean, its global efficiency is shown to be affected by the ocean's physical circulation, and its net effect on atmospheric CO2 is shown to work through the ocean's acid–base chemistry. We integrate these findings into a proposed recipe for the major dynamics driving CO2 change over the past 800 000 years.
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, 3/4 1.0176(2), is slightly lower than the formerly calculated value of 3/4 1.0193 (Kakihana and Kotaka, 1977), due to differences in the determined vibrational frequencies. The effect of pressure on 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, 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. Copyright © 2005 Elsevier Ltd
Accurate records of the state of the ocean carbonate system are critical for understanding past changes in pCO2, ocean acidification and climate. The chemical principles underlying the proxy of oceanic pH provided by the boron isotope ratio of foraminiferal carbonate are relatively well understood, but the proxy's reliability has been questioned. We present 76 new Multi-Collector Inductively-Coupled Plasma Mass Spectrometry (MC-ICPMS) delta11B measurements on a range of benthic foraminifera from 23 late-Holocene samples from the Atlantic that reaffirm the utility of the delta11B-pH proxy. Our boron isotope measurements on ~ 10 benthic foraminifera tests typically yield a precision of ~ ± 0.250/00 at 2 s.d. (equivalent to ~ ± 0.03 pH units). delta11B values of epifaunal species are within analytical uncertainty of those predicted from a simple model assuming sole incorporation of B(OH)4- from seawater and no vital effects, using the independently determined fractionation factor of 1.0272 between 11B/10B of aqueous boron species. Infaunal foraminifera are consistent with this model, but record the combined effects of lower pore-water delta11B and pH. No influence of partial dissolution or shell size on delta11B is observed. We have also measured the B/Ca ratios of the same samples. For individual Cibicidoides species, B/Ca shows a good correlation with Delta[CO32-], but the B/Ca of different co-occurring species morphotypes varies considerably. These effects are not seen in delta11B, which may therefore provide a more robust proxy of the ocean carbonate system. Whilst in theory delta11B and B/Ca can be combined to provide a quantitative reconstruction of alkalinity and dissolved inorganic carbonate (DIC), in practice this is precluded by propagated uncertainties. delta11B data give significant constraints on foraminifera calcification mechanisms, and seem most simply explained by incorporation of B(OH)4- into a HCO3- pool, which is then completely incorporated in foraminiferal CaCO3. Our demonstration of the predictable variation of delta11B with pH, across a wide range of species and locations, provides confidence in the application of MC-ICPMS measurements of foraminiferal delta11B to reconstruct past changes in the ocean carbonate system.
Boron paleo-acidimetry is based on the stable boron isotope composition of foraminiferal shells which has been shown to be a function of seawater pH. It is cur- rently one of the most promising paleo-carbonate chemistry proxies. One important parameter of the proxy is the equilibrium fractionation between the dissolved boron species B(OH)3 and B(OH)- which was calculated to be 19 per mil at 25C by Kak- 4 ihana and Kotaka (1977), based on Urey's theory. The calculated equilibrium frac- tionation, however, depends on the vibrational frequencies of the molecules for which different values have been reported in the literature. We have recalculated the equilib- rium fractionation and find that it may be distinctly different from 19 per mil (this is the bad news). The good news is that - theoretically - the use of 11B as a paleo-pH indicator is not compromised through vital effects in planktonic foraminifera. We de- rive this conclusion by the use of a diffusion-reaction model that calculates pH profiles and 11B values in the vicinity of a foraminifer.
11B/10B ratios of the high temperature fumarolic gases (>465°C) of this island were found to be constant within the limits of experimental error (11B/10B = 4.066). This value may represent the 11B/10B ratio of boron in the andesite magma. 11B/10B ratios of the low temperature fumarolic gases (<235°C) were found to vary from 4.053 to 4.077. 11B/10B ratios of some sassolites were approximately equal to that of the fumarolic condensates and the other ones were slightly enriched in 10B compared to the fumarolic condensates.
This paper mainly reviews our recent work on the biology and geochemistry of foraminifera with respect to their use as palaeoceanographic proxies. Our approach to proxy validation and development is described, primarily from a modeler's point of view. The approach is based on complementary steps in understanding the inorganic chemistry, inorganic isotope fractionation, and biological controls that determine palaeo-tracer signals in organisms used in climate reconstructions. Integration of laboratory experiments, field and culture studies, theoretical considerations and numerical modelling holds the key to the method's success. We describe effects of life-processes in foraminifera on stable carbon, oxygen, and boron isotopes as well as Mg incorporation into foraminiferal calcite shells. Stable boron isotopes will be used to illustrate our approach. We show that a mechanism-based understanding is often required before primary climate signals can be extracted from the geologic record because the signals can be heavily overprinted by secondary, non-climate related phenomena. Moreover, for some of the proxies, fundamental knowledge on the thermodynamic, inorganic basis is still lacking. One example is stable boron isotopes, a palaeo-pH proxy, for which the boron isotope fractionation between the dissolved boron compounds in seawater was not precisely known until recently. Attempts to overcome such hurdles are described and implications of our work for palaeoceanographic reconstructions are discussed.
We have measured the boron isotope composition of seventeen samples of borate minerals (colemanite, ulexite, and borax) and the 87Sr/86Sr ratio in thirteen borate samples from the Kirka borate deposit in western Anatolia, Turkey. These Neogene deposits were formed by evaporation of playa lakes fed by geothermal springs. The δ11B values range from −14.9% o in colemanite to −1.6% o in borax. To a first approximation the relative differences in the δ11B values of the borate minerals are consistent with their basic boron atomic configuration, but the magnitude of the boron isotope fractionation between the three minerals precludes their simultaneous precipitation from a brine with the same boron isotope composition and pH. Rather the data are consistent with precipitation of colemanite from a brine with lower pH than that required for ulexite precipitation, which in turn requires a lower pH than is needed for borax precipitation. The boron isotope data also suggest that the borate minerals did not maintain boron isotopic equilibrium with the brine after they precipitated. Rayleigh fractionation models indicate that during borax precipitation the δ11B value of the brine was slightly heavier than during precipitation of ulexite and colemanite.
Boron minerals that have different structural formulae but are supposed to have the same geologic origin have been collected and analyzed for the "B/i% isotopic ratio. It has been reconfirmed that minerals of marine origin have higher "B/'"B ratios than those of nonmarine origin. It has been found that the sequence of decreasing "B/"B values among the minerals with the same geologic origin is; borax, tincal, kernite (Na borates) > ulexite (Na/Ca borate) > colemanite, iyoite, meyerhofferite (Ca borates). This sequence is explainable on the basis of the difference in crystal structure among the minerals. That is, minerals with higher B03/B04 ratios, (the ratio of the number of the BOX triangle units to the number of the B04 tetrahedron units in the structural formula of a mineral) have higher "B/'OB ratios.
A series of chromatographic experiments were carried out to determine the values of the separation factors for the "B llB isotopic pair in the boron isotope separation systems of an anion-exchange resin in halide forms at 25°C. In every experiment, the lighter isotope. '"B. was enriched in the rear part of the boron zone formed in the chromatographic column. The separation factors obtained were 1.0099 for the fluoride-form resin. 1 0018 for the chloride-form resin and 1.0014 for the bromide-form resin. independent of the boron concentration. A theoretical conside-ration of them is given INTRODUCTION
Isotopic fractionation between two phases can be calculated if the vibrational properties of isotopic end-members are fully characterized. We assessed a theoretical approach based on first-principles density-functional density (DFT) prediction of vibrational frequencies by comparing with spectroscopic data on isotopically substituted brucite, a model mineral for which detailed experimental data on isotopic effects and brucite-water D/H partitioning exist. The deviation from experimental values averages to less than 1% for lattice modes, and 4% for CH stretching modes. The reduced partition function ratio (RPFR) for D/H substitution in brucite was combined with experimental RPFR for water to calculate D/H fractionation factor between water and brucite. Results within the harmonic approximation systematically deviate from experimental data by + 20-25 parts per thousand. RPFR is very sensitive to the frequency and isotopic shift of high frequency CH stretching modes, for which anharmonic effects are important and usually not explicitly taken introduced in calculations for minerals. The asymmetry of the O-H potential was calculated by DFT and accounts for spectroscopic measurements of the anharmonic shifts of OH stretching mode overtones and isotopic frequency ratio. Calculations of D/H fractionation introducing anharmonic corrections to the RPFR of brucite are fitted with the expression 1000ln alpha(D/H)(brucite/H2O) = -23.3 10(3)/T + 2.55 10(6)/T-2 - 1.51 10(9)/T-3, that yields an improved agreement with experiments at high temperature, but the deviation at 25 degrees C is -30 parts per thousand. Uncertainties in calculated fractionation factors can arise from dispersion effects or from DFT errors. For instance, a 1% change of the stretching mode frequencies or a 3% change of lattice mode frequencies could account for the discrepancy between model and experimental fractionation factors. The pressure dependence of brucite RPRF was calculated, and is given by 1000ln Gamma(Pbrucite) = P (-1.005 10(3)/T + 2.18 10(6)/T-2 - 0.213 10(9)/T-3), with P in GPa. The large calculated temperature dependence of the fractionation factor suggests that a single experimental fractionation curve can be fitted to a partial consistent set of experimental data as 1000ln alpha(D/H)(brucite/H2O) = -27.9(30) 10(3)/T + 8.8(29) 10(6)/T-2 - 2.24(63) 10(9)/T-3. These equations allow calculating the temperature and pressure dependence of the D/H fractionation factor between brucite and water in the 300-900 K and 0.1-100 MPa range when combined with the pressure correction for water RPFR [Polyakov, V.B., Horita, J. and Cole, D.R., 2006. Pressure effects on the reduced partition function ratio for hydrogen isotopes in water. Geochimica Cosmochimica Acta, 70: 1904-1913.] for geochemical applications. The DFT model used here for estimating RPFR of the model mineral brucite can be extended to other hydrous minerals using vibrational spectroscopy as a test for the accuracy of the DFT prediction, and the brucite-water equations proposed here as a reference for determining mineral-water fractionation factors and geochemical applications. (C) 2009 Elsevier B.V. All rights reserved.
Boron isotopic fractionation between vapor and brine phases separated under supercritical conditions has been experimentally examined using a Na-Ca-K-Cl fluid. Experiments were conducted at 425, 440, and 450°C. Phase separation was controlled by adjusting pressure. Fractionation between the coexisting phases was less than 0.5%.. In contrast, fractionation between trigonal and tetrahedral B, at these temperatures, has been predicted to be approximately 8%.. The lack of fractionation suggests that the trigonal/tetrahedral speciation of B is similar in the two phases. Although this result may not be general for other fluid compositions, it is relevant to ridge crest hydrothermal solutions and indicates that the boron isotopic compositions of these solutions reflect the proportions of B from seawater and crustal sources and not phase separation.
In 1999 the Istituto di Geoscienze e Georisorse (IGG), with the support of the International Atomic Energy Agency (IAEA), undertook the collection, preparation and distribution of eight geological materials intended for a blind interlaboratory comparison of measurements of boron isotopic composition and concentration. The materials came from Italian sources and consist of three natural waters (Mediterranean seawater and two groundwaters) and five rocks and minerals (tourmaline, basalt, obsidian, limestone and clay). The solid materials were crushed, milled and mixed, in preparation for distribution. Extensive assays performed at the IGG on these materials demonstrated that their boron isotopic and chemical compositions are homogeneous.
Additional homogeneity tests were carried out on solid material fragments at the GeoForschungsZentrum Potsdam, with the specific objective of investigating the suitability of some of them for the calibration in situ of micro‐analytical techniques. Two materials, B4 (tourmaline) and B6 (obsidian), proved to be isotopically homogeneous and may become excellent references for in situ microanalyses of boron isotopes.
The materials described here were used as the basis of a major laboratory intercomparison study and are now available for further distribution from either the IAEA (solid materials) or the IGG (waters).
A series of experiments was conducted in which boron minerals were precipitated by water evaporation from solutions containing boron and potassium, sodium or lithium at 25°C, and boron isotope fractionation accompanying such mineral precipitation was investigated. In the boron-potassium ion system, K2[B4O5 (OH)4]·2H2O, santite (K[B5O6(OH)4] ·2H2O), KBO2·1.33H2O, KBO2· 1.25H2O and sassolite (B(OH)3) were found deposited as boron minerals. Borax (Na2[B4O5 (OH)4·8H2O) was found deposited in the boron-sodium ion system, and Li2B2 O4·16H2O, Li2B4 O7·5H2O, Li2B10 O16·10H2O, LiB2O3 (OH)·H2O and sassolite in the boron-lithium ion system. The boron isotopic analysis was conducted for santite, K2[B4O5 (OH)4]·2H2O, borax and Li2B2O4· 16H2O. The separation factor, S, defined as the 11B/10B isotopic ratio of the precipitate divided by that of the solution, ranged from 0.991 to 1.012. Computer simulations for modeling boron mineral formations, in which polyborates were decomposed into three coordinated BO3 unit and four coordinated BO4 unit for the purpose of calculation of their boron isotopic reduced partition function ratios, were attempted to estimate the equilibrium constant, KB, of the boron isotope exchange between the boric acid molecule (B(OH)3) and the monoborate anion (B(OH)4-). As a result, the KB value of 1.015 to 1.029 was obtained. The simulations indicated that the KB value might be dependent on the kind of boron minerals, which qualitatively agreed with molecular orbital calculations independently carried out.
The Castellón Plain alluvial aquifer, Spain, is intensively exploited to meet the demand for agricultural irrigation and industrial water supply. The geochemistry of its groundwater shows complex salinization in the northern and southern parts of the aquifer, with significant pollution from human origin in the central portion. Boron content and B isotope geochemistry are useful for distinguishing between various sources of pollution and their relative importance in different parts of this aquifer. Boron concentrations in the groundwater vary between 0.01 and 0.85 mg/L. In the more saline groundwaters, found at the northern and southern ends of the study area, the presence of B is linked to inputs from seawater and water with a calcium-magnesium sulphate facies, which feed the aquifer and clearly influence the chemistry of its waters. Evidence of B adsorption processes in some samples is shown by the low B/Cl ratios and the high values of δ11B. In the central portion of the aquifer, the high B/ Cl ratios and the strongly negative δ11B are related to pollution of human origin.
We have measured the boron concentration and isotope composition of regionally expansive borate deposits and geothermal fluids from the Cenozoic geothermal system of the Argentine Puna Plateau in the central Andes. The borate minerals borax, colemanite, hydroboracite, inderite, inyoite, kernite, teruggite, tincalconite, and ulexite span a wide range of δ11B values from −29.5 to −0.3‰, whereas fluids cover a range from −18.3 to 0.7‰. The data from recent coexisting borate minerals and fluids allow for the calculation of the isotope composition of the ancient mineralizing fluids and thus for the constraint of the isotope composition of the source rocks sampled by the fluids. The boron isotope composition of ancient mineralizing fluids appears uniform throughout the section of precipitates at a given locality and similar to values obtained from recent thermal fluids. These findings support models that suggest uniform and stable climatic, magmatic, and tectonic conditions during the past 8 million years in this part of the central Andes. Boron in fluids is derived from different sources, depending on the drainage system and local country rocks. One significant boron source is the Paleozoic basement, which has a whole-rock isotopic composition of δ11B=−8.9±2.2‰ (1 SD); another important boron contribution comes from Neogene-Pleistocene ignimbrites (δ11B=−3.8±2.8‰, 1 SD). Cenozoic andesites and Mesozoic limestones (δ11B≤+8‰) provide a potential third boron source.
Reliable reconstructions of deep ocean carbonate ion concentration, [CO32−], and pH are crucial to understand mechanisms responsible for the past atmospheric CO2 variations observed in ice cores. However, it is challenging to reconstruct past deep water [CO32−] and pH and literature results from different proxies conflict, warranting careful investigations on possible reasons for the existing inconsistencies. Here, we present the first down core B/Ca and δ11B records measured in an epifaunal benthic foraminifer Cibicidoides wuellerstorfi from the Caribbean Sea during the last 160 kyr. The two proxies yield quantitatively comparable deep water [CO32−] and pH results, showing high values during glacials relative to inter-glacials (differences in [CO32−] and pH are ∼ 35 μmol/kg and ∼ 0.15, respectively), consistent with past ocean circulation changes in the Caribbean Sea. Our data provide convincing evidence that both proxies serve as faithful proxies to estimate deep ocean [CO32−] and pH, despite our incomplete understanding of boron incorporation into foraminiferal carbonates.
The Cornia Plain alluvial aquifer, in Tuscany, is exploited intensely to meet the demand for domestic, irrigation and industrial water supplies. The B concentration of groundwater, however, is often above the European limit of 1 mg L−1, with the result that exploitation of these water resources requires careful management. Boron and Sr isotopes have been used as part of a study on the origin and distribution of B dissolved in groundwater, and indirectly as a contribution to the development of appropriate water management strategies.The geochemistry of the Cornia Plain groundwater changes from a HCO3 facies in the inland areas to a Cl facies along the coastal belt, where seawater intrusion takes place. The B concentration of groundwater increases towards the coastal areas, while the 11B/10B ratio decreases. This indicates that there is an increasing interaction between dissolved B and the sediments forming the aquifer matrix, whose B content is in the order of 100 mg kg−1. Adsorption–desorption exchanges take place between water and the sediment fine fraction rich in clay minerals, with a net release of B from the matrix into the groundwater, and a consequent δ11B shift from positive to negative values. The aquifer matrix sediments therefore seem to be the major source of B dissolved in the groundwater.The groundwater–matrix interactions triggered by the ionic strength increase caused by seawater intrusion can also be detected in the Ca–Na ion exchanges. Dissolved Sr follows a trend similar to that of Ca, while the 87Sr/86Sr ratio is equal to that of the exchangeable Sr of the aquifer matrix and therefore does not change significantly.These results have helped to define a new strategy for groundwater exploitation, with the final objective of reducing B concentration in the water extracted from the aquifer.
Boron/calcium ratios were measured in four benthic foraminiferal species (three calcitic: Cibicidoides wuellerstorfi, Cibicidoides mundulus, and Uvigerina spp., and one aragonitic: Hoeglundina elegans) from 108 core-top samples located globally. Comparison of coexisting species shows: B/Ca of C. wuellerstorfi > C. mundulus > H. elegans > Uvigerina spp., suggestive of strong “vital effects” on benthic foraminiferal B/Ca. A dissolution effect on benthic B/Ca is not observed. Core-top data show large intra-species variations (50–130 μmol/mol) in B/Ca. Within a single species, benthic foraminiferal B/Ca show a simple linear correlation with deep water Δ[CO32−], providing a proxy for past deep water [CO32−] reconstructions. Empirical sensitivities of Δ[CO32−] on B/Ca have been established to be 1.14 ± 0.048 and 0.69 ± 0.072 μmol/mol per μmol/kg for C. wuellerstorfi and C. mundulus, respectively. The uncertainties associated with reconstructing bottom water Δ[CO32−] using B/Ca in C. wuellerstorfi and C. mundulus are about ± 10 μmol/kg. A preliminary application shows that the Last Glacial Maximum (LGM) B/Ca ratios were increased by 12% at 1–2 km and decreased by 12% at 3.5–4.0 km relative to Holocene values in the North Atlantic Ocean. This implies that the LGM [CO32−] was higher by ∼ 25–30 μmol/kg at intermediate depths and lower by ∼ 20 μmol/kg in deeper waters, consistent with glacial water mass reorganization in the North Atlantic Ocean inferred from other paleochemical proxies.
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.
Density functional and correlated molecular orbital calculations (MP2) are carried out on B(OH) 3 · n H 2 O clusters ( n = 0, 6, 32), and inlMMLBox · n H 2 O ( n = 0, 8, 11, 32) to estimate the equilibrium distribution of 10 B and 11 B isotopes between boric acid and borate in aqueous solution. For the large 32-water clusters, multiple conformations are generated from ab initio molecular dynamics simulations to account for the effect of solvent fluctuations on the isotopic fractionation. We provide an extrapolated value of the equilibrium constant α 34 for the isotope exchange reaction 10 B(OH) 3 (aq) + inlMMLBox (aq) = 11 B(OH) 3 (aq) + inlMMLBox (aq) of 1.026–1.028 near the MP2 complete basis set limit with 32 explicit waters of solvation. With some exchange-correlation functionals we find potentially important contributions from a tetrahedral neutral B(OH) 3 ·H 2 O Lewis acid–base complex. The extrapolations presented here suggest that DFT calculations give a value for 10 3 ln α 34 about 15% higher than the MP2 calculations.
The (11)B/(10)B ratio exhibits wide variations in nature; thus, boron isotopes have found numerous applications in geochemistry, hydrology, and environmental studies. The main analytical techniques used are as follows: positive thermal ionisation mass spectrometry is the most precise (about 0.2 per thousand of the boron isotope ratio), but requires complex and laborious sample preparation; negative thermal ionisation mass spectrometry is less precise (about 0.5 per thousand), but rapid and suitable for water samples, whereas total evaporation-NTIMS allows for identification of the precise boron isotope composition of marine carbonates. It is expected that multi-collection system inductively coupled plasma mass spectrometry (MC-ICPMS) will eventually combine high precision with simple analytical procedures. Secondary ion mass spectrometry and laser ablation (LA)-MC-ICPMS allow in situ determinations on solid samples, but require the availability of calibration materials which are chemically and mineralogically similar to samples. These features of boron isotope measurement techniques were confirmed by the results of the first inter-laboratory comparison of measurements, organised by the Istituto di Geoscienze e Georisorse in Pisa. Finally, two examples of boron isotope applications in groundwater investigations are reported.
Large aqueous oxide ions as minerals! Minerals dissolve by repeated ligand exchange reactions and geochemists use polyoxometalate ions to establish structure–reactivity relations for environmentally important functional groups. Here, for example, are plotted the dissolution rates of two classes of minerals against rates of solvent exchanges around the corresponding aquo ions.
Geochemists and environmental chemists make predictions about the fate of chemicals in the shallow earth over enormously long times. Key to these predictions is an understanding of the hydrolytic and complexation reactions at oxide mineral surfaces that are difficult to probe spectroscopically. These minerals are usually oxides with repeated structural motifs, like silicate or aluminosilicate polymers, and they expose a relatively simple set of functional groups to solution. The geochemical community is at the forefront of efforts to describe the surface reactivities of these interfacial functional groups and some insights are being acquired by using small oligomeric oxide molecules as experimental models. These small nanometer-size clusters are not minerals, but their solution structures and properties are better resolved than for minerals and calculations are relatively well constrained. The primary experimental data are simple rates of steady oxygen-isotope exchanges into the structures as a function of solution composition that can be related to theoretical results. There are only a few classes of large oxide ions for which data have been acquired and here we review examples and illustrate the general approach, which also derives directly from the use of model clusters to understand for the active core of metalloenzymes in biochemistry.
Ab initio molecular dynamics calculations are used here to calculate vibration frequencies for B(OH)(3)(aq) and B(OH)(4)(-)(aq). We show that previous calculations have either underestimated or omitted altogether a major fractionating vibrational mode. The new results indicate that the B-11 partitions into B(OH)(4)(-) in water, in contrast to recent experimental measurement of the fractionation factor. The discrepancy appears to result from using finite-temperature vibrational frequencies in the standard harmonic expression for the fractionation factor. While our results connect the measured spectrum to previous harmonic electronic structure calculations, they indicate that harmonic frequencies must be extracted from experimental vibrational spectra before they can be used in the standard expressions.
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