Boron Isotopic Composition Research Articles

Assessing scleractinian corals as recorders for paleo-pH: Empirical calibration and vital effects

Laboratory experiments on the branching, symbiont-bearing coral genus Porites and Acropora have been carried out to determine the dependence of the skeletal boron isotopic composition (δ 11B) on the pH of seawater. The results show a clear relationship similar to previously established empirical calibrations for planktonic foraminifera and inorganic calcite. A −0.6‰ offset exists between P. cylindrica and A. nobilis which is systematic over the pH range of 7.7–8.2. To test whether the δ 11B of coral skeletons changes with physiological processes such as photosynthesis and respiration, corals were grown along a depth transect in their natural environment and under controlled conditions in the laboratory at varying light intensities and food supply. Although we also observe an isotopic offset between P. compressa and Montipora verrucosa, neither experimental treatment systematically changed the δ 11B of the two species. These findings are encouraging for using the boron isotope paleo-pH proxy in corals, because it appears that seawater pH is the dominant control on the boron isotopic composition in corals.

Boron isotope composition of geothermal fluids and borate minerals from salar deposits (central Andes/NW Argentina)

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 δ 11B 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 δ 11B=−8.9±2.2‰ (1 SD); another important boron contribution comes from Neogene-Pleistocene ignimbrites ( δ 11B=−3.8±2.8‰, 1 SD). Cenozoic andesites and Mesozoic limestones ( δ 11B≤+8‰) provide a potential third boron source.

Boron isotope composition of coals: a potential tracer of organic contaminated fluids

The B-content of coals from sedimentary basins is a function of the primary depositional environment and secondary enrichments. Boron has been used to infer paleo-environments of coal deposits, and environmental impact of coal burning power plants. To improve our understanding of B in organic matter, this study used secondary ion mass spectrometry to measure the B-isotopic composition of 25 coals and three kerogens (Type I, II and III). Results show that coal is 10B-enriched compared to most terrestrial waters. The δ 11B values measured show large variability within a single `homogenized' coal sample indicating that B is heterogeneously bound in various organic macerals. Of greater importance is the observation that all coal δ 11B values are negative. The lowest δ 11B measured was −70‰ representing the lightest B-isotopic ratios ever reported for terrestrial materials. Bulk analyses of coals may not reveal isotope ratios this low because of the averaging of values from different organic macerals having different δ 11B values. This discovery may be important for tracing organic contaminants in fluids. When organic matter is heated during burial, the isotopically light B partitions into fluid phases. The very low δ 11B of organic products compared to most groundwaters may be useful in tracing fluids derived from organic sources.

The origin and mechanisms of salinization of the lower Jordan river

The chemical and isotopic ( 87Sr/ 86Sr, δ 11B, δ 34S sulfate, δ 18O water, δ 15N nitrate) compositions of water from the Lower Jordan River and its major tributaries between the Sea of Galilee and the Dead Sea were determined in order to reveal the origin of the salinity of the Jordan River. We identified three separate hydrological zones along the flow of the river: (1) A northern section (20 km downstream of its source) where the base flow composed of diverted saline and wastewaters is modified due to discharge of shallow sulfate-rich groundwater, characterized by low 87Sr/ 86Sr (0.7072), δ 34S sulfate (−2‰), high δ 11B (∼36‰), δ 15N nitrate (∼15‰) and high δ 18O water (−2 to–3‰) values. The shallow groundwater is derived from agricultural drainage water mixed with natural saline groundwater and discharges to both the Jordan and Yarmouk rivers. The contribution of the groundwater component in the Jordan River flow, deduced from mixing relationships of solutes and strontium isotopes, varies from 20 to 50% of the total flow. (2) A central zone (20–50 km downstream from its source) where salt variations are minimal and the rise of 87Sr/ 86Sr and SO 4/Cl ratios reflects predominance of eastern surface water flows. (3) A southern section (50–100 km downstream of its source) where the total dissolved solids of the Jordan River increase, particularly during the spring (70–80 km) and summer (80–100 km) to values as high as 11.1 g/L. Variations in the chemical and isotopic compositions of river water along the southern section suggest that the Zarqa River ( 87Sr/ 86Sr∼0.70865; δ 11B∼25‰) has a negligible affect on the Jordan River. Instead, the river quality is influenced primarily by groundwater discharge composed of sulfate-rich saline groundwater (Cl -=31–180 mM; SO 4/Cl∼0.2–0.5; Br/Cl∼2–3×10 -3; 87Sr/ 86Sr∼0.70805; δ 11B∼30‰; δ 15N nitrate ∼17‰, δ 34S sulfate=4–10‰), and Ca-chloride Rift valley brines (Cl -=846–1500 mM; Br/Cl∼6–8×10 -3; 87Sr/ 86Sr∼0.7080; δ 11B>40‰; δ 34S sulfate=4–10‰). Mixing calculations indicate that the groundwater discharged to the river is composed of varying proportions of brines and sulfate-rich saline groundwater. Solute mass balance calculations point to a ∼10% contribution of saline groundwater (Cl −=282 to 564 mM) to the river. A high nitrate level (up to 2.5 mM) in the groundwater suggests that drainage of wastewater derived irrigation water is an important source for the groundwater. This irrigation water appears to leach Pleistocene sediments of the Jordan Valley resulting in elevated sulfate contents and altered strontium and boron isotopic compositions of the groundwater that in turn impacts the water quality of the lower Jordan River.

B/Nb and δ 11B systematics in the Phlegrean Volcanic District, Italy

Boron concentration and isotopic compositions were determined in representative products from the Phlegrean Volcanic District (PVD), which includes Campi Flegrei (CF), and the islands of Procida and Ischia. The most primitive products (from Procida and Ischia islands) are moderately enriched in B (4.6–12 ppm), whereas more evolved products of CF and Ischia Island are more enriched (21–118 ppm). The content of B is positively correlated with Nb, Zr, Th, La and, generally, with all incompatible elements. δ 11B values are generally lower and more uniform in samples from CF (−6.8 to − 10.6‰) compared with those from Procida (−3.6 to −8.5‰) and Ischia (−2.8 to −8.4‰) islands. Overall, B-enrichments relative to fluid-immobile elements of PVD are only slightly higher than those observed in mid-ocean ridge basalts and there is no significant correlation between δ 11B and B/immobile element ratios. For CF samples including the Campanian Ignimbrite (CI) and most post-CI products, δ 11B is negatively correlated with 87Sr/ 86Sr, with extreme compositions represented by evolved products of the CI (higher δ 11B, lower 87Sr/ 86Sr) and less evolved shoshonite (lower δ 11B, higher 87Sr/ 86Sr). In contrast, samples from Procida, Ischia, and pre-CI products and one young shoshonite from CF define a distinct correlation with primitive trachybasalt as the high δ 11B, low 87Sr/ 86Sr end-member, and young shoshonites as the low δ 11B, high 87Sr/ 86Sr end-member. The overall interpretation of the geochemical and isotopic data suggests three conclusions: (1) Geochemical and isotopic variations of post-CI products from CF can be explained by mixing/mingling between at least two distinct magmas, i.e. the CI and the young shoshonite (Minopoli 1). (2) The isotopic trend described by Procida trachybasalts, Ischia samples, pre-CI samples from CF and Pigna St. Nicola shoshonite is a mantle trend suggesting the involvement in their genesis of a distinct component depleted in B, with low δ 11B and high 87Sr/ 86Sr. A similar component has been invoked to explain the differences between Stromboli potassic lavas and more typical calc-alkaline rocks of the Aeolian Islands. This component is consistent with ‘sediments or melts thereof’ introduced into the mantle by subduction, but depleted in B due to metamorphic reactions accompanying subduction. (3) The displacement from the mantle trend of CI and Minopoli 1 shoshonite magmas is probably due to crustal contamination processes.

In situ, multiple-multiplier, laser ablation ICP-MS measurement of boron isotopic composition ( δ11B) at the nanogram level

We present a new in situ, multiple electron-multiplier LA-ICP-MS technique for the analysis of boron isotopes ( δ 11B) at <1‰ 2 σ precision and at the nanogram level. Sample materials analyzed were all natural and synthetic glasses, spanning a range of boron concentrations from 0.39 to 30.2 ppm and 11B/ 10B values from 4.0254 to 4.0799 ( δ 11B −6.08‰ to +7.35‰). The in situ analyses were duplicated at higher concentrations by solution analyses of chemically separated boron using conventional nebulization on the same ICP-MS system. Sample-standard switching between glass chips on an approximate 5-min cycle is used to monitor and correct for instrumental fractionation and drift. Instrumental fractionation of more than 100‰ in measured 11B/ 10B ratios is successfully corrected and the final 11B/ 10B values are up to 100-fold more accurate than the measured ratios. Reproducibility of <1‰ at 2 σ are routinely produced at natural sample boron concentrations as low as 0.39 ppm. The amount of boron measured to produce these data is dependent on the boron concentration of the sample, and ranges from 0.014 μg at 30.2 ppm boron to 0.0008 μg at 0.39 ppm boron. These nanogram amounts of boron measured are approximately two orders of magnitude lower than conventional P-TIMS and solution multiple faraday ICP-MS methods which typically require 0.3–3.3 μg of boron. The advantages of this technique are the elimination of procedures for the chemical separation for boron, low instrument memory and background (<300 cps compared to >100 000 cps for solutions), high sensitivity, small sample size, and spatial control on material analyzed. These characteristics will permit boron isotopic analysis on virtually any glass collection. The boron isotope compositions of four fresh MORB glass samples from the northern EPR (15–18°N) were determined using the laser ablation technique presented here. The results indicate a narrow range of boron compositions for these samples ( δ 11B −6.92‰ to −7.68‰), which overlap at the 2 σ level. The light δ 11B composition of these samples precludes substantial seawater ( δ 11B +39.5‰) contamination of their pre-eruptive MORB magmas.

Possible roles of pH, temperature, and partial dissolution in determining boron concentration and isotopic composition in planktonic foraminifera

We present the first continuous records from 0 to 5 Ma (in 0.333 m.y. integrated time steps) of paired boron/calcium (B/Ca) ratios and boron isotopes (δ11B) in the planktonic foraminifera Globogerinoides sacculifer (without sacc) from a site in the western equatorial Pacific Ocean (Ocean Drilling Program Site 806). These measurements, the first made in conjunction with calcification temperature (magnesium/calcium ratios) and average shell mass measurements, indicate that pH is not the sole environmental variable controlling B in planktonic foraminiferal calcite. Our data are consistent with calcification temperature exerting a primary control on B concentration and isotopic composition in planktonic foraminifera. If so, calcification temperature must be taken into account if pH for past oceans and atmospheric pCO2 are to be estimated from B isotope measurements in foraminiferal calcite. Doing so will substantially increase the uncertainty of pH estimates. Although this work was designed as a temporal study, its results define new aspects of calibrating the δ11B paleo‐pH tracer.

Tourmalinites from the Eastern Sierras Pampeanas, Argentina

In the eastern Sierras Pampeanas, Central Argentina, tourmalinites and coticules are found in close association with stratabound scheelite deposits in metamorphic terranes. In Sierra Grande (Agua de Ramón and Ambul districts) and Sierra de Altautina, tourmalinites are associated with stratabound scheelite deposits related to orthoamphibolites. In the Pampa del Tamboreo area, tourmalinites are located in biotite schists stratigraphically related to acid to intermediate metavolcanic rocks and scheelite-bearing quartzites. The mineral chemistry and boron isotopic compositions of tourmalinite-hosted and vein-hosted tourmalines are investigated. Overall, the tourmalines belong to the dravite-schorl series and are generally aluminous; Fe/(Fe+Mg) ranges from 0.33 to 0.85, Al/(Al+Fe+Mg) from 0.66 to 0.76 and the amount of X-site vacancy (0.12–0.48) indicates significant foitite components. Their boron isotopic compositions ( δ 11B) are from −24.0‰ to−15.0‰. Similar mineral chemistries and boron isotopic values for tourmaline in tourmalinites related to stratabound scheelite mineralisation and in tungsten-bearing quartz veins suggest a common source for the boron and probably the tungsten. The field, chemical and isotopic relationships are consistent with tungsten and boron in quartz-vein deposits being remobilised from stratabound scheelite and tourmalinite, dominantly by liquid-state transfer associated with regional shear zones. Tungsten and boron in the original sedimentary sequence (now meta-exhalites) are ascribed to volcanogenic exhalations.

PH control on oxygen isotopic composition of symbiotic corals

Boron, carbon and oxygen isotopic compositions were determined at the micrometre scale by high-resolution ion microprobe in a sample of modern coral (massive hermatypic coral, Porites lutea). The ion probe data show for B and O much larger isotopic variations at the micrometre scale than those measured at the millimetre scale by conventional techniques: δ18OPDB values range from −10.6±0.9‰ to −0.2±0.5‰ and δ11B values range from +18.6±1.5‰ to +30.6±1.6‰. By contrast, δ13C values show the same range of variations, from −4.6±0.65‰ to −2.2±0.67‰ at the micrometre and millimetre scales. The range of δ11B values indicates that significant pH variations, from ≈7.1 to ≈9.0, are present at the sites of calcification. The largest δ18O variations correspond to the highest δ11B values, i.e. to the highest pHs. This measurement of pH allows modelling the oxygen isotopic fractionation occurring during aragonite precipitation. Taking into account the rate of O isotopic equilibrium between dissolved carbonate species (H2CO3, HCO3− and CO32−) and water via the two reactions of hydration and hydroxylation, the full range of δ18O values measured at the micrometre scale can be modelled for residence times of dissolved carbonates in the calcifying fluid ranging between ≈1 h and at maximum ≈12 h. The pH controls the δ18O of the growing carbonate through the relative fractions of dissolved carbonate species and through the kinetics of their isotopic equilibration with water via hydration and hydroxylation. The so-called ‘vital effect’ systematically observed for δ18O in corals can thus be understood as representing an average of rapid pH variations due to coral biology during coral growth. Selectively measuring δ18O values in the zones of coral skeletons that have low δ11B values (i.e. formed at low pH) should significantly improve the quality of palaeoclimatic reconstructions based on δ18O values.

Volatile transport during the crystallization of anatectic melts: oxygen, boron and hydrogen stable isotope study on the metamorphic complex of Naxos, Greece

Crystallization of anatectic melts in high-temperature metamorphic terrains releases volatile-rich magmas that can be transported into adjacent lithologies. This study addresses the variations in the oxygen, boron and hydrogen isotopic composition of aplite-pegmatite dikes that formed during the crystallization of anatectic melts in regional high-temperature metamorphism on the island of Naxos, Greece, and propagated upward into the overlying sequences of metamorphic schist. The transport distance of these dikes was increased through a significant horizontal component of travel that was imposed by contemporaneous low-angle extensional shearing. Laser fluorination oxygen isotope analyses of quartz, tourmaline, garnet, and biotite mineral separates from the aplite-pegmatite dikes show a progressive rise in δ 18O values with increasing distance from the core. Oxygen isotope fractionations among quartz, tourmaline, and garnet show temperature variations from > 700°C down to ∼400°C. This range is considered to reflect isotopic fractionation beginning with crystallization at high temperatures in water-undersaturated conditions and then evolving through lower temperature crystallization and retrograde sub-solidus exchange. Two processes are examined for the cause of the progressive increase in δ 18O values: (1) heterogeneous δ 18O sources and (2) fluid-rock exchange between the aplite/pegmatite magmas and their host rock. Although the former process cannot be ruled out, there is as yet no evidence in the exposed sequences on Naxos for the presence of a suitable high δ 18O magma source. In contrast, a tendency for the δ 18O of quartz in the aplite/pegmatite dikes to approach that of the quartz in the metamorphic rock suggests that fluid-rock exchange with the host rock may potentially be an important process. Advection of fluid into the magma is examined based on Darcian fluid flow into an initially water-undersaturated buoyantly propagating aplitic dike magma. It is shown that such advective flow could only account for part of the 18O-enrichment, unless it were amplified by repeated injection of magma pulses, fluid recycling, and deformation-assisted post-crystallization exchange. The mechanism is, however, adequate to account for hydrogen isotope equilibration between dike and host rock. In contrast, variations in the δ 11B values of tourmalines suggest that 11B/ 10B fractionation during crystallization and/or magma degassing was the major control of boron geochemistry rather than fluid-rock interaction and that the boron isotopic system was decoupled from that of oxygen.

Volatile element (B, Cl, F) behaviour in the roof of an axial magma chamber from the East Pacific Rise

Understanding the behaviour of volatile elements at mid-ocean ridges is important for reasons ranging from their influence on mantle viscosity through to their role as a food source for the deep biosphere. With the aim of constraining what processes control the distribution of volatiles in the ocean crust at fast-spreading ridges, we present a detailed study of the compositional variability in magmatic amphibole formed in the upper part of the plutonic sequence at the East Pacific Rise (EPR). These amphiboles are massively enriched in chlorine (by more than an order of magnitude), and moderately enriched in boron, with respect to magmatic amphiboles in cumulates from the Mid-Atlantic Ridge (MAR). Similar enrichments have been reported for basaltic glasses from the EPR and are interpreted as indicative of assimilation. The greater enrichments observed in the plutonic section suggest both that assimilation occurs at the roof of the axial magma chamber (AMC) and that lava compositions may record minimum amounts of exogenic contamination. Amphiboles with compositions indicative of crystallisation from a contaminated magma occur to depths of 800 m beneath the sheeted dyke complex. This is interpreted to indicate that at least this upper portion of the plutonic section forms via crystallisation within the AMC followed by subsidence of a crystal mush. Amphibole boron isotope compositions show that assimilation of altered sheeted dykes plus hydrothermal fluids drives AMC magmas to heavier δ11B values (up to +5.8‰). Subsequent degassing within a solidifying crystal mush leads to a negative trend in δ11B–B with the most degassed magma having δ11B as low as −21.2‰. This degassing was associated with hydrofracturing of the partially molten crystal mush and could have facilitated a temporal link with the overlying hydrothermal system.

Fossil leaves: Effective bioindicators of ancient CO2 levels?

[1] Past variations in the concentration of the greenhouse gas CO2 are thought to have played a major role in controlling Earth’s climate on preQuaternary and Quaternary timescales. To identify the contribution of CO2 to past climatic change requires accurate quantification of its content in the ancient atmosphere, and a number of proxies have been developed for this purpose (for a review see Royer et al. [2001a]). For the Late Quaternary, there is the unique opportunity to measure directly the composition of fossil air samples trapped in bubbles preserved in the polar ice sheets. Results from Antarctic ice cores reveal that the glacialinterglacial changes characterizing Quaternary climate were accompanied by variations in the atmospheric concentration of CO2 [Petit et al., 1999]. Although detection of phase relations between isotope-derived temperature estimates and trace gas concentration values is hampered by the difference in age of ice and air from the same ice sample, it is believed that CO2 lags glacial-interglacial temperature change and has acted as an amplifier of orbitally forced changes in temperature [Petit et al., 1999; Shackleton, 2000; Mudelsee, 2001].

Vital effects in foraminifera do not compromise the use of δ11B as a paleo‐pH indicator: Evidence from modeling

The stable boron isotope composition (δ11B) of fossil foraminiferal shells is used as a paleo‐pH recorder and is therefore one of the most promising paleocarbonate chemistry proxies (“paleoacidimetry”). One crucial question regarding this proxy is whether foraminifera record the pH of the bulk seawater or the pH of the microenvironment (diffusive boundary layer, ∼500 μm), which is strongly influenced by life processes. Here we present a novel theoretical approach to address this question by using a diffusion‐reaction model. Model results indicate that the δ11B in planktonic foraminifera is primarily controlled by the pH of the microenvironment. We therefore predict that the δ11B of different species (e.g., symbiont‐bearing versus symbiont‐barren) or of foraminifera grown in the dark and in the light should be offset from the δ11B of inorganic calcite. This theoretical prediction was experimentally confirmed while this paper was written [Hönisch et al., 2003]. Most importantly, the model predicts that this offset is constant over a wide pH range. Thus the use of δ11B as a paleo‐pH indicator is not compromised through vital effects as modeled here.

The influence of symbiont photosynthesis on the boron isotopic composition of foraminifera shells

Culture experiments were carried out with the planktonic foraminifer Orbulina universa under high and low light levels in order to determine the influence of symbiont photosynthetic activity on the boron isotopic composition of shell calcite. Under low light (reduced photosynthetic rates) the boron isotopic composition of the tests is 1.5‰ lower compared to shells grown under high light (elevated photosynthetic rates). In terms of inferred pH, the lower boron isotope values correspond to a reduction in pH of approximately 0.2 units. The boron isotopic composition of Orbulina universa from plankton tows is similar to that of shells grown under low light conditions in the laboratory. These data are consistent with reduced symbiont concentrations in recently secreted shells. In addition to laboratory and field grown O. universa, we present the first data for a symbiont-barren foraminifer, Globigerina bulloides. Data obtained for G. bulloides fall ∼1.4‰ below those of the field grown O. universa. Although the plankton tow results are preliminary, they support the hypothesis that respiration and photosynthesis are the key physiological parameters responsible for species-specific vital effects.

Geochemical study of boron isotopes in the process of loess weathering

In this paper the boron contents and boron isotopic composition of acid-soluble phases in loess and paleosol samples are determined for the first time. The boron contents of acid-soluble phases in the Luochuan loess section (S0–S2) vary within the range of (0.8–2.7)×10−6 and their δ11 B values vary from −1.8‱ to +18.6‱, mostly within the range of 0−+10‱. The boron contents and δ11 B values of paleosol layers are higher than those of loess layers, especially in the loess layer S1. Varying chemical weathering intensity and loess adsorption capability are the main factors leading to the variations of boron contents and δ11B values of acid-soluble phases in the loess section. The variation of chemical weathering intensity in response to the variation of climatic conditions seems to be the main factor leading to the variations of boron contents and δ11B values of acid-soluble phases in the loess section.

Record in metamorphic tourmalines of subduction-zone devolatilization and boron cycling

Boron concentrations and isotope compositions of fluids and lavas from subduction-zone settings show great potential for elucidating mass flux at Earth's modern convergent margins. However, the fluid-mineral-melt behaviorof B and its two stable isotopes remains relatively poorly understood. Boron isotope analyses of tourmaline in metasedimentary rocks subducted to 15-90 km depths (1) demonstrate the ability of this mineral to retain information regarding prograde devolatilization history in even highly retrograded rocks and (2) indicate the importance of tourmaline in affecting whole-rock B loss and B isotope evolution during subduction-zone metamorphism. The B lost from micas during metamorphism of subducting sedimentary rocks and altered oceanic crust is isotopically more enriched in 1 1 B than the B retained in the micas. Beneath forearcs and volcanic arcs, the B from micas is either removed from the subduction-zone rocks via metamorphic fluids or sequestered by growing tourmaline, in which the B can be entrained to even greater depths. Here we demonstrate that these metamorphic fluids could contribute to the relatively high 811B signatures observed in most arc lavas and the across-arc trends of decreasing δ 1 1 B observed in several arcs.

Chemical and boron isotopic compositions of tourmaline from the Lavicky leucogranite, Czech Republic

Chemical and boron isotopic compositions of tourmaline from the Lavicky leucogranite, Czech Republic

Exploring intra-crystalline B-isotope variations in mixed-layer illite-smectite

The isotopic composition of boron in illite-smectite (I-S) can be important for monitoring fluid/rock interactions in sedimentary basins. Boron substitutes for Si during reaction of smectite to illite and can preserve information about paleofluid B-isotopic composition. Boron is enriched in oilfield brines, therefore the isotopic composition of those brines may be recorded during illitization and represent a monitor of hydrocarbon maturity and migration. We re-examined previously published experimental results on B-isotope fractionation between I-S and water. By separating B from two crystallographic sites of I-S (tetrahedral and interlayer), we found differences in the δ 11 B that might be used as a single-mineral geothermometer. Boron incorporation in I-S follows a non-linear kinetic pathway. Maximum interlayer-B incorporation occurs during R1-ordering. R3-ordering approaches equilibrium with expulsion of interlayer-B leaving only tetrahedral layer-B. The important discovery is that tetrahedral layer δ 11 B does not change between R1 and R3 ordering. Boron substitutes in an equilibrium ratio early in the crystallographic reordering of I-S. Natural I-S samples were tested from Gulf Coast mudstones, increasingly illitized with burial depth. Diagenetic reaction kinetics differ from hydrothermal experiments, but still reveal large δ 11 B differences (up to 40‰) between the interlayer and tetrahedral layer. Interlayer δ 11 B decreases with increasing temperature and illitization. We propose that interlayer δ 11 B values represent metastable equilibrium, whereas tetrahedral layer B represents a temperature-dependent equilibrium. If this is true, then the B-isotope geochemistry of I-S can be used to determine paleotemperatures and monitor the influence of hydrocarbons on pore fluids associated with diagenetic I-S.

Chemical and boron isotopic compositions of tourmaline from the Archean Big Bell and Mount Gibson gold deposits, Murchison Province, Yilgarn Craton, Western Australia

Tourmaline is a commonly found gangue mineral in Archean greenstone belt lode-gold deposits worldwide. In this paper, we report major, trace, and rare-earth element (REE) and boron isotopic compositions of tourmaline from two major amphibolite-grade Archean gold deposits (Big Bell and Mount Gibson; Murchison Province, Yilgarn Craton, Western Australia), in an attempt to better understand the hydrothermal gold formation processes that were also responsible for tourmalinization. Tourmalines in these two major deposits all belong to the dravite–schorl series, and tend to be Mg-rich where closely associated with gold and sulfide ores. The ore-related tourmalines also contain high concentrations of ore metals such as Au, Ag, Pb, and Cu. In the Big Bell deposit, tourmalines occur along the foliation in lode schists, and show low Fe/(Fe+Mg) ratios of 0.11–0.30, TiO 2 contents of 0.15–0.73 wt.%, and negligible MnO (<0.05 wt.%). This type of tourmaline has δ 11B values of −17.7‰ to −15.0‰. In the Mount Gibson deposit, the tourmalines closely associated with the sulfide ores are the most Mg-rich, with low Fe/(Fe+Mg) ratios of 0.19–0.22, while tourmalines from garnet-bearing biotite–muscovite schists that overly the lode-gold orebody are more Fe-rich (Fe/(Fe+Mg)=0.30–0.63). Tourmalines from an amphibolite schist are Mg-rich, with Fe/(Fe+Mg) ratios (0.27–0.34) that lie between the above two groups of tourmalines. The compositional trend from Fe-rich tourmalines in the sulfide-poor schists to Mg-rich in the sulfide ores at Mount Gibson is similar to that observed in massive sulfide ore deposits worldwide. Alternatively, the Mg-rich nature of the sulfide-related tourmalines may be due to metamorphic reactions that are accompanied by Fe incorporation into sulfides, leaving coexisting silicates more Mg-rich. The REE geochemistry of the tourmalines from the Mount Gibson ore zone schists displays strong HREE enrichments and positive Eu anomalies with Eu/Eu* ratios up to 2.4, which are similar to tourmalines associated with massive sulfide deposits. The ore-related tourmalines disseminated in the host schists show δ 11B values of −18.6‰ to −17.0‰, whereas one coarse-grained tourmaline vein has a lower δ 11B value of −21.9‰ to −21.4‰. These data support hypotheses which suggest that the schists that host the gold mineralization are the metamorphosed equivalents of a base metal-rich, hydrothermally altered, seafloor horizon that is typical of VMS-style mineralization, with the boron possibly derived from leaching of footwall sedimentary and volcanic rocks during hydrothermal circulation.

Boron isotope systematics in large rivers: implications for the marine boron budget and paleo-pH reconstruction over the Cenozoic

The chemical composition of the oceans and long-term climate changes are believed to be linked. Reconstruction of seawater pH evolution is desirable as pH may be related to atmospheric pCO 2, and hence to climate evolution. Boron isotopes in oceanic carbonates have been suggested to be a proxy for oceanic paleo-pH reconstruction. Nevertheless, the calculation of paleo-pH values over geological periods requires a precise knowledge of the boron isotopic composition of the oceans when calcite precipitated. We present the systematics of boron isotopic composition of the world's main rivers. We deduce a continental boron flux to the oceans of 38×10 10 gB/year with a mean isotopic composition of +10‰. These results lead to a balanced boron budget in the oceans and allow the development of a model for the marine boron secular evolution over the past 100 Myr. It is shown that the oceanic boron cycle is mainly controlled by the boron continental discharge and the boron uptake from the oceans during low temperature alteration of oceanic crust. However, the recent important increase of the clastic sediment supply, linked to the Himalayan erosion, impacts the oceanic boron budget by enhancing significantly the boron uptake by adsorption on sediments. We predict a boron isotopic composition in the oceans lower during the Cenozoic and slightly higher during the Cretaceous than today. The modelled values for the marine boron isotopes follow the variations of boron isotopes in carbonates over the Cenozoic era provided by previous studies, suggesting that the variations of the seawater pH may not have been important on this time scale. If this is the case, it involves that buffering mechanisms occur in the oceans to maintain seawater pH at a roughly constant value against past atmospheric pCO 2 variations.