Boron Isotopic Composition Research Articles

Boron content and isotopic composition of tektites and impact glasses: Constraints on source regions

Abundances of Li, Be, and B, as well as boron isotopic compositions, were determined in twenty-seven tektite and impact glass samples, using an ion microprobe. Samples included tektites from the Australasian, North American, and Ivory Coast strewn fields, and Aouelloul and Darwin impact glasses. Variations of B abundance and isotopic composition in a flanged australite were also studied. δ 11B variations of only a few permil were found within the australite flange. The isotopic composition shows no correlation with the B contents or with the distance from the rim of the flange. The mean δ 11B value for the flanged australite is very similar to that of Muong-Nong type tektites (−1.9 ± 1.9‰). Thus, vapor fractionation has been unimportant during tektite formation. This is supported by the observation that B contents and the δ 11B values of the different samples from the Australasian tektite strewn field are not correlated with each other. Most tektites show a rather limited range of δ 11B values (−9.3 ± 1.5 to +2.7 ± 1.5%o), which is small compared to the range observed for common terrestrial rocks (−30 to +40‰). The B abundance and isotopic data can be used to place constraints on the tektite source rocks. Australasian tektites have high B and Li abundances; only clay-rich sediments, such as pelagic and neritic sediments, as well as river and deltaic sediments have B contents (up to 100 ppm) and δ 11B values that are in agreement with the range shown by Australasian tektites (−4.9 to + 1.4‰). 10Be and RbSr data indicate continental crustal source rocks and exclude pelagic and neritic sediments. However, deltaic sediments, e.g., from the Mekong river, which are of continental crustal origin, agree with 10Be, RbSr, and B data, and support a possible source locality close to the coast of SE Indochina in the South China Sea. On the other hand, one bediasite sample has a very high δ 11B value of +15.1 ± 2.1‰, requiring the presence of marine carbonates or evaporites among the source rocks. This finding supports an offshore locality for the North American tektite source crater.

Chemical and boron isotope compositions of non-marine brines from the Qaidam Basin, Qinghai, China

The general chemistry and boron isotope composition were investigated in fresh waters and hypersaline brines from the Qaidam Basin, northern Qinghai-Xizang (Tibet) Plateau, China. This basin is a large, tectonically active system, isolated from the ocean and composed of thick clastic and evaporite sediments. The modern playas are subject to intense evaporation and are characterized by hypersaline brines as well as potash and borate evaporites. The chemical composition of the dissolved solutes in the modern brines and waters reveals three main sources: (1) inflow of hot springs enriched in sodium, sulphate and boron. Evaporation of these waters leads to a high Na/Cl ratio (>1), a NaClSO 4 brine and an evaporite mineral assemblage of halite-mirabilite-borate (Lakes Daqaidam and Xiaoqaidam); (2) inflow surface river waters which are modified by preferential dissolution of halite and potassium and magnesium salts characterized by a Na(Mg)Cl solute type with low Na/Cl (<1), Br Cl, Li Cl and B Cl ratios; and (3) Ca-chloridic subsurface brines which are controlled by both salt dissolution and dolomitization processes. Evaporation and salt crystallization of mixtures of the latter two types leads to a “marine-like” brine (e.g., NaMgCl type, Na Cl ratio ⪡ 1) and mineral assemblages similar to that predicted for progressive evaporation of seawater (e.g., Qarhan playa: halite-sylvite-carnallite-bischofite). The δ 11 B-values of the input waters to the Qaidam Basin (range of −0.7 to + 10.9%. vs. NBS- 951 and brines from salt lakes (+0.5 to + 15.0%.) are similar to those of associated granitic rocks ( δ 11B= −2.3 to +3.7%.; n=3) and hence indicate the non-marine origin of these fluids. The highest δ 11B-values are associated with fluids with low B Li ratios, indicating selective removal of elemental boron and 10B by adsorption onto clay minerals. The magnitude of 11B enrichment due to adsorption is ∼15–20%., and thus non-marine brines are well distinguished from marine-derived brines ( δ 11B of +39 to +59%.) preserving the large isotopic difference (∼40%.) of their source waters. It is proposed to use this distinctive isotopic signature for tracing the origin of ancient evaporite environments.

Measurement of boron isotope ratios in groundwater studies.

Boron is present at low levels in groundwater and rainfall in the UK, ranging between 2 and 200 ng ml-1. A sensitive technique has been developed using inductively coupled plasma mass spectrometry (ICP-MS) to measure boron isotope ratios at low concentrations with a precision (Sr) of between 0.1 and 0.2%. Samples were evaporated to increase elemental boron concentrations to 200 ng ml-1 and interfering matrix elements were removed by an adapted cation-exchange separation procedure. The validity of measuring boron isotopic ratios by ICP-MS at this concentration level is discussed in relation the theoretical instrument precision attainable based on counting statistics. Using the developed procedure it is possible to use ICP-MS to establish a reliable database documenting the natural variation in boron isotope compositions in aquifers and rain water in the UK. Preliminary results demonstrate a trend in boron isotopic compositions with increasing boron concentration. The method may be used to evaluate possible levels of boron pollution from anthropogenic inputs into natural aqueous systems.

Mineral-fluid partitioning and isotopic fractionation of boron in synthetic calcium carbonate

Aragonite, high-Mg calcite, and Mg-free calcite have been grown under monitored laboratory conditions to investigate mineral-fluid partitioning and isotopic fractionation of boron into carbonates. At pH ≈ 8, both polymorphs have boron isotopic compositions I6.5%0 lower than the boron isotopic composition of the experimental fluid ( δ 11 B = 0 ± 0.7ℵ. ). The isotopic composition of the synthetic minerals is identical to the calculated boron isotopic composition of the B(OH) 4 − species in the parent solutions, consistent with only B(OH) 4 − interacting with the crystal surface. The fractionation factor calculated from this isotopic offset is within analytical error of that measured between seawater and modern marine carbonates. The boron concentrations of the experimentally grown aragonite are 13.4, 85.1, and 379 ppm for total fluid boron concentrations of 0.59, 5.1, and 37.0 ppm, respectively. High-Mg calcite (HMC) coprecipitating with the aragonite has bulk boron concentrations of 5.6, 56.2, 260, and 1771 ppm for fluid boron concentrations of 0.59, 5.1, 37, and 272 ppm, respectively. Calcite boron concentrations are 1.78, 14.0, 137.7, and 1228 ppm for fluid boron concentrations of 0.45, 4.0, 44.l, and 499 ppm, respectively. Boron uptake by aragonite is greater than boron uptake by calcite grown under nearly identical conditions, while boron uptake by HMC is intermediate. Synthetic calcite crystals grown in solutions with greater than 4 ppm boron exhibit habit modification with the appearance of a subordinate form {11 2 0} in addition to the dominant {10 1 4}, the perfect cleavage rhombohedron, found in low-boron calcites. This is evidence of strong interaction of the borate with the crystal surface.

Boron isotope analysis. A review

Over the last ten years there has been a rapid increase in the number of studies of the boron isotope composition of natural samples. These studies have employed a variety of methods, with varying degrees of accuracy and precision. The relevant techniques are discussed in this paper and the problems associated with the measurement of boron isotope are divided into those dealing with (i) the extraction of boron from different types of samples (ii) purification of the extracted boron and (iii) mass spectrometric analysis of boron isotopes. The advantages and disadvantages of the different techniques are discussed. In addition, some refinements of these published methods and a new purification stage are presented. No one single method is appropriate for all studies; instead the choice of analytical method depends on the type of sample under consideration, the level of precision required to solve the problem, and the analytical equipment that is available. There is, however, a general requirement for more consideration to be given to assessing the reproducibility of some techniques and there is also a need for the development of working standards appropriate to the different types of natural samples, and the natural range of boron isotope ratios in these different types of samples.

Reply to the comment on “Boron content and isotopic composition of oceanic basalts: geochemical and cosmochemical implications”

Reply to the comment on “Boron content and isotopic composition of oceanic basalts: geochemical and cosmochemical implications”

Comment on “Boron content and isotopic composition of oceanic basalts: Geochemical and cosmochemical implications” by M. Chaussidon and A. Jambon

Comment on “Boron content and isotopic composition of oceanic basalts: Geochemical and cosmochemical implications” by M. Chaussidon and A. Jambon

Boron content and isotopic composition of oceanic basalts: Geochemical and cosmochemical implications

Ion microprobe determination of boron content and δ 11B values has been performed for a set of 40 oceanic basalt glasses (N-MORB, E-MORB, BABB and OIB) whose chemical characteristics (major and trace elements and isotopic ratios) are well documented. Boron contents, determined at ±10% relative, range from 0.34 to 0.74 ppm in N-MORB, whereas E-MORB, BABB and OIB extend to higher concentrations (0.5–2.4 ppm). After correction for crystal fractionation, this range is reduced to 0.5–1.3 ppm. N-MORB and E-MORB also exhibit different B/K ratios, 1.0 ± 0.3 × 10 −3 and 0.2 to 1.4 × 10 −3 respectively. This can be interpreted as resulting from the incorporation into the upper mantle of a K-rich and B-poor component (e.g., subducted oceanic crust having lost most of its initial boron). δ 11B values range between −7.40 ± 2 and +0.6 ± 2‰, with no significant difference between N-MORB, E-MORB, OIB or BABB. The Hawaiian samples define a strong linear correlation between boron contents, δ 11B values, MgO and water contents and δD values. This is interpreted as resulting from assimilation-fractionation processes which occurred within a water-rich oceanic crust, and which produced high δ 11B values associated with high δD values. The low level of 11B enrichment in the upper mantle constrains the amount of boron reinjected by subduction to a maximum of about 2% of the boron present in the subducted slab. This is turn corresponds to a maximum net Boron transfer of about 3 x 10 10 g/a towards the surface reservoirs. Finally, a boron content of 0.25 ± 0.1 ppm is estimated for the bulk silicate Earth (i.e., primitive mantle), corresponding to a depletion factor relative to C1 chondrites of about 0.15 and suggesting that B was moderately volatile upon terrestrial accretion.

The Boron Isotopic Composition of Tourmaline as a Guide to Fluid Processes and Boron Source in the South-West England Orefield: An Ion Microprobe Study

Recent work has shown the boron isotopic composition of tourmaline to be sensitive to a number of factors including boron source, waterrock interaction and temperature and pressure of formation (Palmer and Slack, 1989; Palmer et al, 1992). The boron isotopic composition of hydrothermal fluids has also been shown to be sensitive to boiling (Leeman et al, 1992). The boron isotopic composition of individual tourmalines from S.W. England has been determined in order to investigate both fluid source and fluid processes b y attempting to detect isotopic differences within crystals and between samples. Samples were analysed from three main rock types:granites, quartz-tourmaline (tourmalinite) and quartz-topaz-tourmaline (QTT) rocks and greisen(Sn,W)-related veins. Three main sample areas were used the St.Austell granite and its associated tourmalinites and QTT rocks at Roche and St. Mewan's Beacon, the roof zone of the Land's End granite exposed at Porth Ledden and the Cligga Head granite and sheeted vein system 9 Tourmalinites and QTT rocks are important as they are considered to represent the transition from magmatic to hydrothermal conditions (Manning, 1981;Bottrell and Yardley, 1988) 9

Boron isotope systematics of marine sediments

Boron contents and boron isotopic compositions were determined for modern and ancient (Permian to Miocene) marine sediments, including pelagic clay, calcareous ooze, siliceous ooze and neritic clay sediments. δ 11B values of modern marine sediments range from −6.6 to +4.8‰. Isotopic variation is controlled by the simple mixing of four major constituents, detritus of continental origin, marine smectite, biogenic carbonates and biogenic silica. Detritus of continental origin, with an average δ 11B value of −13 to −8‰, is the low-δ 11B end-member constituent of marine sediments and its boron is largely controlled by the concentration of illite which originates from wind or fluvial transport. Marine smectite, biogenic carbonate and biogenic silica, on the other hand, represent the high-δ 11B end members, with δ 11B values of +2.3 to +9.2‰, +8.0 to +26.2‰ and +2.1 to +4.5‰, respectively. These high δ 11B values are the result of the equilibrium uptake of boron from seawater. Spatial variations in boron isotopes in the Pacific sediments are essentially due to the distribution of the above four constituents. Although ancient argillaceous sediments (shale and slate) have boron contents that are identical with those of modern equivalents, boron contents of limestone and chert are distinctly lower than those of modern calcareous and siliceous oozes. Ancient marine sediments have systematically lower δ 11B values (−17.0 to −5.6‰) than those of the modern sediments. The lower δ 11B values can be caused by diagenesis, which induces (1) preferential removal of high-δ 11B boron in calcium carbonate and silica during recrystallization and (2) boron isotopic exchange in the course of the smectite/illite transition. The observed boron isotopic compositions of ancient argillaceous sediments are distinctly different from those of fresh and altered MORB. Therefore, boron isotope systematics will be useful in identifying components from the descending oceanic slab involved in the formation of island arc magma and in investigating mantle-crust recycling through subduction processes.

Strontium, boron, oxygen, and hydrogen isotope geochemistry of brines from basal strata of the Gulf Coast sedimentary basin, USA

Significant spatial heterogeneities exist in the stable isotopic composition of saline formation waters from reservoirs of the Smackover Formation (Upper Jurassic). We focused on the southwest Arkansas shelf, a structurally simple portion of one of the interior basins of the northern Gulf Coast sedimentary basin. Here, faulting and facies changes juxtapose dominantly oolitic carbonate strata against basal evaporites, red beds, and siliciclastics, as well as metamorphosed basement rocks. Brines from this area have exceptionally high Br and alkali element concentrations and have spatially heterogeneous hydrogen sulfide concentrations. Strontium, boron, oxygen, and hydrogen isotope compositions exhibit coherent relations with other aspects of brine geochemistry. Sr isotope compositions range from those expected for carbonates and evaporites deposited from Jurassic seawater (0.7071) to radiogenic ratios as high as 0.7107. Generally, most radiogenic Sr isotope values are associated with H 2S-rich waters which also have elevated alkali element (Li, B, K, Rb) concentrations. These alkali element-rich waters are associated with portions of the South Arkansas fault system which reach basement. Boron isotope compositions are similarly heterogeneous, ranging from values of +26 to +50%.. Brines with highest B contents are most depleted in 11B, consistent with boron input from brines generated from high-temperature siliciclastic diagenetic reactions. Normalizing B contents to Br in the brines reveals a reasonable mixing trend between a Dead Sea-type composition and Texas Gulf Coast-type shale/sand reservoir waters. Oxygen and hydrogen isotope data exhibit regional variations which are controlled by meteoric water invasion along the northern limb of the southwest Arkansas Fault, which has surface expression. Although oxygen isotope compositions are often near equilibrium with respect to reservoir carbonate, it is more difficult to ascribe trends in δD values to local water-rock interaction. The stable isotope trends can be explained either in terms of an evolved marine evaporite brine or a water evolved via water-rock interaction, and mixing with meteoric water in the vicinity of the southwest Arkansas Fault. Spatial heterogeneity in isotopic composition of formation waters from this relatively restricted study area suggests that assumptions of a homogeneous Sr reservoir in dating regional mineralization events require careful assessment in ancient systems.

Foraminiferal boron isotope ratios as a proxy for surface ocean pH over the past 21 Myr

THE pH of the surface ocean is a sensitive function of its alkalinity and total inorganic carbon concentration, properties which also control the partial pressure of atmospheric carbon dioxide17. Thus, an accurate proxy for past ocean pH could yield information about variations in atmospheric CO2. Recently, it has been suggested that the boron isotopic composition of foraminiferal tests depends on the pH of sea water as well as its isotopic composition1,2. Here we present boron isotope and elemental data for sedimentary pore fluids and isotope data for bulk foraminiferal samples from a deep-sea sediment core. The composition of the pore waters implies that sea water boron concentrations and isotopic composition have been constant during the past 21 Myr, allowing us to reconstruct past ocean pH directly from the foraminiferal isotope data. We find that 21 Myr ago, surface ocean pH was only 7.4 ±0.2, but it then increased to 8.2 ±0.2 (roughly the present value) about 7.5 Myr ago. This is consistent with suggestions3–5 that atmospheric CO2 concentrations may have been much higher 21 Myr ago than today.

Formation waters from Mississippian-Pennsylvanian reservoirs, Illinois basin, USA: Chemical and isotopic constraints on evolution and migration

We have analyzed a suite of seventy-four formation-water samples from Mississippian and Pennsylvanian carbonate and siliciclastic strata in the Illinois basin for major, minor, and trace element concentrations and for strontium isotopic composition. A subset of these samples was also analyzed for boron isotopic composition. Data are used to interpret origin of salinity and chemical and Sr isotopic evolution of the brines and in comparison with a similar data set from an earlier study of basin formation waters from Silurian-Devonian reservoirs. Systematics of Cl-Br-Na show that present Mississippian-Pennsylvanian brine salinity can be explained by a combination of subaerial seawater evaporation short of halite saturation and subsurface dissolution of halite from an evaporite zone in the middle Mississippian St. Louis Limestone, along with extensive dilution by mixing with meteoric waters. Additional diagenetic modifications in the subsurface interpreted from cation/Br ratios include K depletion through interaction with clay minerals, Ca enrichment, and Mg depletion by dolomitization, and Sr enrichment through CaCO 3 recrystallization and dolomitization. Ste. Genevieve Limestone (middle Mississippian) formation waters show 87Sr 86Sr ratios in the range 0.70782–0.70900, whereas waters from the siliciclastic reservoirs are in the range 0.70900–0.71052. Inverse correlations between 87Sr 86Sr and B, Li, and Mg concentrations suggest that the brines acquired radiogenic 87Sr through interaction with siliciclastic minerals. Completely unsystematic relations between 87Sr 86Sr and 1/Sr are observed; Sr concentrations in Ste. Genevieve and Aux Vases (middle Mississippian) waters appear to be buffered by equilibrium with respect to SrSO 4. Although there are many similarities in their origin and evolution, these formation waters are distinguished from Silurian-Devonian brines in the basin by elevated Cl Br and Na Br ratios and by unsystematic Sr isotope relationships. Thus waters from these two major segments of the Illinois basin stratigraphic column form distinct geochemical regimes which are separated by the New Albany Shale Group (Devonian-Mississippian) regional aquitard. Geochemical evolution appears to have been influenced significantly by Paleozoic and Mesozoic hydrologic flow systems in the basin.

Host Rock Control on the Boron Concentration and Isotopic Composition of Ground Water from the Edwards Aquifer of Central Texas

Host Rock Control on the Boron Concentration and Isotopic Composition of Ground Water from the Edwards Aquifer of Central Texas

Negative thermal ionisation mass spectrometry: a new approach to boron isotope geochemistry

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.BO2− ions are formed in a single-filament ion source. With the addition of La(NO3)3 as an activator, a 100- to 1000-fold increase in sensitivity as compared to the classical sodium or cesium metaborate method is achieved. With this high sensitivity, as little as 0.1 ng B (0.1 ppb B with 1 g sample size) is needed for analysis. The precision is typically∼ ± 0.2%.Boron concentrations are determined with a standard isotope dilution technique using a10B-enriched spike. Precisions are in the range of±0.01% (> 100ppm B)to± 4% (< 0.1ppm B).For aqueous solutions, water- and HCl-soluble salts and minerals (e.g., borates, brines, seawater, groundwater, carbonates) no chemical treatment is required. The untreated solutions are directly loaded onto the filament.The method has successfully been tested on a variety of hydrothermal waters and borate minerals from Larderello (Italy), Boron and the Death Valley (California, U.S.A.), and on seawater, tourmalines and carbonatites from different origins. Preliminary results on these investigations are presented.

Boron isotopic compositions of brine, sediments, and source water in Da Qaidam Lake, Qinghai, China

The isotopic compositions of boron in Da Qaidam Lake, Quinghai, have been determined by using the technique of thermal ionization mass spectrometry of cesium tetraborate. The brines are enriched in 11B, with an average δ 11B of +5.49 ± 2.34%., compared with those of lake sediments and boron minerals produced by the lake. The apparent isotopic fractionation factors between brine and lake sediments as well as boron minerals are 0.988 ± 0.001 and 0.986 ± 0.001, respectively. Source water δ 11B values range from −11.72 to + 16.69%., depending on their origin.

Boron isotope geochemistry of the oceanic crust from DSDP/ODP Hole 504B

Boron contents and boron isotopic compositions were determined for the uppermost 1.3 km section of typical 6.2 Ma oceanic crust from DSDP/ODP Hole 504B, Costa Rica Rift, Galapagos Spreading Center. Both the boron content and the δ 11 B value in the oceanic crust are controlled by two types of alteration: 1. (1) low-temperature alteration (0 to 60°C; Zones I and II) and 2. (2) high-temperature hydrothermal alteration (200 to 400°C; Zones III and IV). Basalts subjected to the low-temperature alteration are characterized by their relatively high boron contents (0.69 to 19.3 ppm) and high δ 11 B values (+2.2 to +10.6%.), indicating uptake of boron into secondary phases in equilibrium with seawater or evolved seawater. Hydrothermally altered basalts contain less abundant boron (0.17 to 0.52 ppm) and relatively constant δ 11 B values (−0.1 to +1.0%.). Although basalts from the upper part of these hydrothermal zones (< 1300 mbsf) show equilibrated boron content and δ 11 B value with aqueous fluid, effective leaching of boron from basalt is predominant in the lower part (>1300 mbsf). Original boron content and δ 11 B value of the Hole 504B MORB were 0.35 ppm and +0.2%., respectively. The present data provide fundamental information in understanding of the distribution of boron and boron isotopes in the oceanic crust.

Precise boron isotopic analysis of natural rock samples using a boron-mannitol complex

Newly developed techniques for boron chemical separation and isotopic analysis in natural silicate rocks and waters are described. Sample dissolution and the subsequent ion-exchange chromatography were conducted using hydrofluoric and hydrochloric acids in the presence of mannitol which suppresses boron volatilization and isotopic fractionation by the formation of a boron-mannitol complex. Thermal ionization mass spectrometry using the Cs 2BO 2 +-graphite method was employed for the determination of boron isotopic composition. No boron isotopic fractionation was observed in the course of chemical separation and mass spectrometry. In the whole analytical procedure, procedural blank and recovery yield of boron were 3–4 ng and 99±1%, respectively. The analytical precision and reproducibility of measured 11B/ 10B ratios were ±0.1−0.1% and ±0.2‰ for the measurements of basalt and seawater, respectively. The present method enables us to determine the isotopic composition of < 1 μg B in silicate samples and in natural fluids with the above-mentioned analytical errors. This method also provides a remarkable improvement in the measurement of boron concentration by isotope dilution mass spectrometry because of the achievement of complete mixing between sample and spike during sample decomposition.

Precise boron isotopic analysis of natural rock samples using a boron-mannitol complex

Newly developed techniques for boron chemical separation and isotopic analysis in natural silicate rocks and waters are described. Sample dissolution and the subsequent ion-exchange chromatography were conducted using hydrofluoric and hydrochloric acids in the presence of mannitol which suppresses boron volatilization and isotopic fractionation by the formation of a boron-mannitol complex. Thermal ionization mass spectrometry using the Cs 2BO 2 +-graphite method was employed for the determination of boron isotopic composition. No boron isotopic fractionation was observed in the course of chemical separation and mass spectrometry. In the whole analytical procedure, procedural blank and recovery yield of boron were 3–4 ng and 99 ± 1%, respectively. The analytical precision and reproducibility of measured 11B/ 10B ratios were ±0.1−0.1% and ±0.2‰ for the measurements of basalt and seawater, respectively. The present method enables us to determine the isotopic composition of < 1 μg B in silicate samples and in natural fluids with the above-mentioned analytical errors. This method also provides a remarkable improvement in the measurement of boron concentration by isotope dilution mass spectrometry because of the achievement of complete mixing between sample and spike during sample decomposition.

Boron isotopic composition and concentration in modern marine carbonates

The boron isotopic compositions and boron concentrations of selected modern marine carbonates were analysed by negative thermal ionization mass spectrometry with a 2σ reproducibility of standards and samples better than 0.7%. It was found that the boron isotopic compositions of modern marine carbonates fall within a relatively narrow range ( +22.1 ± 3%. relative to NBS SRM951 boric acid standard) for all samples (biogenic, nonbiogenic, warm water, cold water, aragonitic, or calcitic). This is about 17%. lighter than seawater. This indicates that mineralogy, temperature, and vital effects are not primary controls on the observed isotopic composition. The narrow range in isotopic composition is in contrast to the wide range in boron concentrations of these samples (10.9–71.4 ppm). The boron isotopic compositions and boron concentrations in modern marine carbonates are consistent with preferential adsorption of the isotopically lighter B(OH) 4 1− aqueous species over the B(OH) 3 species before incorporation into the carbonate site. Because there is a pH control on the availability of B(OH) 1− 4, the result is a pH control on the boron abundance in the carbonates. Possible second-order biomineralogic controls on pH may account for boron concentrations in calcareous organisms that produce low pH microenvironments at the site of CaCO 3 precipitation.