Low δ11B Values Research Articles

Boron Isotopes in the Mount Vulture Groundwaters (Southern Italy): Constraints for the Assessment of Natural and Anthropogenic Contaminant Sources

The Mt. Vulture volcanic aquifer is an important reservoir of effervescent mineral water in southern Italy. This area is included in the Vulture Regional Park and is located on a popular tourist route for its high scenic and ecological value. Unfortunately, signs of anthropogenic impact have recently increased. Among minor elements, boron and its isotopes are considered useful environmental tracers since different anthropogenic and natural sources can be distinguished. In this study, B content and (δ11B) isotope composition were determined in the Mt. Vulture groundwaters. The groundwater chemistry is strongly influenced by input of CO2-rich volcanic gases, and two different hydrofacies are observed. The first water type (BAW) has bicarbonate alkaline and alkaline-earth composition and relatively low salinity, whereas the second one (HSW) has bicarbonate-sulphate alkaline composition and higher salinity. The HSW are enriched in boron and have low δ11B values (from –9.6‰ to –4.3‰) similar to those measured in the local magmas suggesting that B contents in these waters are influenced by prolonged interaction between local meteoric water and volcanic rocks. As to BAW, a wide variability in B concentrations (from 14 μg/l to 769 μg/l) and B isotope composition (from -16.1‰ to +23‰) has been observed. A possible anthropogenic input is thought to be responsible for such variability. The water samples with high B concentrations and negative δ11B values (−12.2‰ to −16.1‰) are probably influenced by agricultural activities; for water samples characterized by positive δ11B values (from +5.8‰ to +7.9‰) and low B contents, a slight contamination by sewage effluents cannot be excluded. These conclusions are also supported by B/Cl molar ratios (7.8E−3 to 1.6E−2) different from those of the HSW group (1.1E−2 to 7.0E−2). This study highlights that also in the Mt. Vulture volcanic area B isotopes coupled to the B/Cl ratio can be a useful tool for the assessment of natural and anthropogenic contaminant sources.

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

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

Tourmaline as a petrogenetic indicator in the Pfitsch Formation, Western Tauern Window, Eastern Alps

Tourmaline is a common accessory mineral in the metasedimentary Pfitsch Formation located in the Pfitscher Joch (Passo de Vizze) area in the Tauern Window of the Eastern Alps. These post-Variscan metasedimentary units experienced peak metamorphic conditions of ~550°C, 1.0GPa during the Alpine orogeny. Tourmaline is most abundant in a ~25m thick unit of feldspathic gneiss (~20–200μg/g B), where it occurs as idiomorphic crystals typically 10mm in length. The abundance and size of the tourmaline crystals increase near coarse-grained quartzofeldspathic segregations (~1200μg/g B), reflecting the mobilization and concentration of B by metamorphic fluids. Near segregations, individual tourmaline crystals have up to three growth zones, recording pro- (~350–500°C, 0.7–1.0GPa) and retrograde (~400°C, 0.2GPa) growth as determined by combining textural information and sector-zoning thermometry. Retrograde tourmaline occurs as individual crystals as well as overgrowths on prograde tourmaline crystals, especially on the surface of extensional fractures formed by E–W extension during regional decompression.Tourmaline in the Pfitsch Formation is dravitic with a variable Fe content that correlates with the Fe content of its respective host unit. Charge balance calculations suggest that a significant proportion of Fe in tourmaline is ferric, supporting the interpretation of a subaerial continental sedimentary protolith. Tourmaline near segregations has the highest inferred ferric iron content, which decreases across growth zones, potentially reflecting a reduction of the fluid during metamorphism. The Mg/(Mg+Fe) ratio increases with prograde tourmaline growth and decreases in retrograde overgrowths. In contrast, the Ca/(Ca+Na) ratio increases gradually from 0.05 to 0.20 with prograde growth and continues to increase up to 0.25 in the retrograde overgrowths, recording the maximum Ca/(Ca+Na) ratio of the fluid during Alpine metamorphism.The metasediments of the Pfitsch Formation have very low and variable whole-rock δ11B values (−14.1 to −33.6‰), with the highest values (−17.7 to −14.1‰) found in B-rich samples (165–1200μg/g B) containing abundant tourmaline, and the lowest values (−24.2 to −33.6‰) in B-depleted samples (21–40μg/g), which lack tourmaline. This observation supports preferential loss of 11B from the rocks during prograde metamorphism. Zoned tourmaline crystals in the Pfitsch formation show successively decreasing B isotope ratios from −7.8 to −11.2‰ in their cores and −17.3 to −20.3‰ in their rims. As supported by a Rayleigh fractionation model, the B-isotope values of the host rocks and the tourmaline crystals are most easily explained by the internal redistribution of B from a B-rich precursor mineral (e.g. mica) to the tourmaline during Alpine metamorphism.

Boron isotope fractionation in magma via crustal carbonate dissolution.

Carbon dioxide released by arc volcanoes is widely considered to originate from the mantle and from subducted sediments. Fluids released from upper arc carbonates, however, have recently been proposed to help modulate arc CO2 fluxes. Here we use boron as a tracer, which substitutes for carbon in limestone, to further investigate crustal carbonate degassing in volcanic arcs. We performed laboratory experiments replicating limestone assimilation into magma at crustal pressure-temperature conditions and analysed boron isotope ratios in the resulting experimental glasses. Limestone dissolution and assimilation generates CaO-enriched glass near the reaction site and a CO2-dominated vapour phase. The CaO-rich glasses have extremely low δ11B values down to −41.5‰, reflecting preferential partitioning of 10B into the assimilating melt. Loss of 11B from the reaction site occurs via the CO2 vapour phase generated during carbonate dissolution, which transports 11B away from the reaction site as a boron-rich fluid phase. Our results demonstrate the efficacy of boron isotope fractionation during crustal carbonate assimilation and suggest that low δ11B melt values in arc magmas could flag shallow-level additions to the subduction cycle.

Boron isotope geochemistry of vent fluids from arc/back-arc seafloor hydrothermal systems in the western Pacific

Boron concentrations and isotopic compositions were determined for vent fluids from 11 sites in arc/back-arc seafloor hydrothermal systems. This new dataset allowed a systematic comparison of boron behavior during water–rock interaction in different geological settings. In sediment-starved hydrothermal systems (e.g., Manus Basin, Izu-Bonin Arc, Mariana Trough), end-member fluids showed large variations in boron concentration (0.53–1.52mmol/kg) and δ11B value (13.5–29.7‰), reflecting the interaction of seawater with varied types of rock that occur in arc/back-arc settings. Based on the model calculations, the boron concentration and δ11B value of the fluid are better explained by an equilibration between seawater and rock than a mixing model. The Chlorine-depleted fluids from phase-separated hydrothermal systems (e.g., North Fiji Basin) had low boron concentrations (0.44–0.55mmol/kg) and high δ11B values (34.5–36.1‰). The small fractionations of boron and boron isotopes during phase separation suggest that these characteristics were acquired during water–rock interaction rather than phase separation and segregation processes. The fluids from sediment-hosted hydrothermal systems (e.g., Okinawa Trough) are characterized by high boron concentrations (3.9–4.8mmol/kg) and low δ11B values (2.5–2.9‰). These fluids also showed high Cs/B ratios, indicating interactions with terrigenous sediments. Model calculations demonstrate that the combined evaluation of δ11B values and Cs/B ratios are useful for a quantitative estimate of sedimentary contributions in seafloor hydrothermal systems.

Boron isotopic constraints on the Nb and Ta mineralization of the syenitic dikes in the ~ 260 Ma Emeishan large igneous province (SW China)

Both Nb–Ta-mineralized and Nb–Ta-poor syenitic dikes in the Panxi region (SW China) are spatially and temporally associated with syenitic plutons, which are part of the ~260Ma Emeishan large igneous province. These syenitic dikes are NW-striking, and have width varying from 1 to 5m and length from 50 to 300m. The dikes are mainly composed of K-feldspar, albite, aegirine and arfvedsonite, however, mineral modes are different in the Nb–Ta-mineralized and Nb–Ta-poor syenitic dikes. The major Nb–Ta-bearing mineral in the dikes is pyrochlore, which is closely associated with albite and occurs in places with intensive albitization. Rocks of the Nb–Ta-mineralized syenitic dikes contain more albite and less K-feldspar than the Nb–Ta-poor dikes, and have compositions more evolved than the Nb–Ta-poor dikes, indicating that the Nb–Ta-mineralized syenitic dikes formed from a highly evolved magma. We analyzed the B concentrations and B isotopic compositions of the samples of both Nb–Ta-mineralized and Nb–Ta-poor syenitic dikes and associated syenitic pluton using a single column purification method and ICP-AES and MC-ICP-MS techniques. The samples of the Nb–Ta-mineralized syenitic dikes have whole-rock B concentrations ranging from 11.4 to 23.9ppm and δ11B values from −17.95 to −14.54‰, whereas the samples of the Nb–Ta-poor dikes and syenitic plutons have B concentrations varying from 3.32 to 16.5ppm and δ11B values from −13.45 to −10.02‰. The high B concentration of the Nb–Ta-mineralized dikes relative to the Nb–Ta-poor dikes is consistent with that B is incompatible and tends to be rich in more evolved magma. The relatively low δ11B values of the Nb–Ta-mineralized dikes indicate that the B isotopes may have fractionated between fluids and rocks in a transitional, magmatic–hydrothermal stage. We propose that the highly evolved magmas in a transitional, magmatic–hydrothermal stage may have Nb– and Ta–fluorine complexes dissolved in the hydrothermal fluids in the presence of Na+. Albite crystallization due to intensive albitization in this stage resulted in the decrease of Na+ in the fluids, decomposing the Nb– and Ta–fluorine complexes. The released Nb and Ta from the complexes were then dissolved in the fluids and finally entered the lattice of pyrochlore crystals in the stage of albitization.

Chemical and boron isotopic compositions of tourmaline from the Paleoproterozoic Houxianyu borate deposit, NE China: Implications for the origin of borate deposit

The Houxianyu borate deposit in northeastern China is one of the largest boron sources of China, hosted mainly in the Paleoproterozoic meta-volcanic and sedimentary rocks (known as the Liaohe Group) that are characterized by high boron concentrations. The borate ore-body has intimate spatial relationship with the Mg-rich carbonates/silicates of the Group, with fine-grained gneisses (meta-felsic volcanic rocks) as main country rocks. The presence of abundant tourmalinites and tourmaline-rich quartz veins in the borate orebody provides an opportunity to study the origin of boron, the nature of ore-forming fluids, and possible mineralization mechanism. We report the chemical and boron isotopic compositions of tourmalines from the tourmaline-rich rocks in the borate deposit and from the tourmaline-bearing fine-grained gneisses.Tourmalines from the fine-grained gneisses are chemically homogeneous, showing relatively high Fe and Na and low Mg, with δ11B values in a narrow range from +1.22‰ to +2.63‰. Tourmalines from the tourmaline-rich rocks, however, commonly show compositional zoning, with an irregular detrital core and a euhedral overgrowth, and have significantly higher Mg, REE (and more pronounced positive Eu anomalies), V (229–1852ppm) and Sr (208–1191ppm) than those from the fine-grained gneisses. They show varied B isotope values ranging from +4.51‰ to +12.43‰, which plot intermediate between those of the terrigenous sediments and arc rocks with low boron isotope values (as represented by the δ11B=+1.22‰ to +2.63‰ of the fine-grained gneisses of this study) and those of marine carbonates and evaporates with high boron isotope values. In addition, the rim of the zoned tourmaline shows notably higher Mg, Ti, V, Sn, and Pb, and REE (particularly LREEs), but lower Fe, Co, Cr, Ni, Zn, Mn, and lower δ11B values than the core. These data suggest that (1) the sources of boron of the borate ore-body are mainly the Paleoproterozoic meta-volcanic and sedimentary rocks, and (2) the ore-forming fluids should be the high temperature metamorphic fluids related to the amphibolite-facies metamorphism of the Paleoproterozoic foldbelt, which leach boron from the boron-rich meta-volcanic and sedimentary rocks of the Liaohe Group, and the boron-rich metamorphic fluids subsequently interacted with the marine Mg-rich carbonates and evaporates, forming borate deposit, the tourmaline overgrowth in the rim and the tourmaline-rich rocks.

Boron isotopic composition as a provenance indicator for the flux raw material in Roman natron glass

Boron is a trace element present in natron glass, which largely enters the glass via the flux. Therefore, the B isotope ratio is targeted as a means of provenancing this flux. In this work, 33 Greco-Roman natron glasses and 18 natron samples were analysed for their B isotopic composition (expressed as δ11B). All glasses, except one of mixed flux origin, show a δ11B value close to the average value of +29‰. 11 natron samples from the Wadi Natrun show an average δ11B of +29.8‰, while the sample from al-Barnuj shows a δ11B of +28.5‰, the average for 4 Fezzan samples from Libya is +25.3‰ and that for the 2 samples from Lake Pikrolimni in Greece +10.5‰. There is an influence of the sand on the δ11B of natron glass, complicating the picture, but in general, the silica source will slightly lower the δ11B of the glass compared to that of the natron source. The high melting temperature has no significant effect on the B isotope ratio. It can be concluded that Greco-Roman natron glasses show a rather homogenous δ11B. The Wadi Natrun salts analysed fit a model in which the δ11B of the glasses is somewhat lower than that of the natron source due to the input of low δ11B sand. The samples from Lake Pikrolimni in Greece analysed in this study show too low δ11B values to be used in glass making according to this model.

Boron sources and transport mechanisms in river waters collected from southwestern Taiwan: Isotopic evidence

Boron (B) isotopes are sensitive tracers for constraining water–rock interactions and pollution sources in river systems. In this study, we analyzed δ11B using multi-collector inductive coupled plasma mass spectrometry, in addition to major and trace elements, to delineate B sources and transport mechanisms in two river catchments, the largest Kao-ping River and the most polluted Erren River in southwestern Taiwan.Dissolved δ11B values vary more than 35‰ in Kao-ping River, but keep relatively constant in Erren River, 12.2–26.7‰. Four major B sources identified in these river waters are cyclic seasalt, water/rock interaction, geothermal water and anthropogenic inputs. All specimens collected near the estuaries were influenced strongly by the seawater, B=4.5mg/L and δ11B=39.5‰. The Erren River results provide unique information on B and δ11B compositions in anthropogenic sources, in particular for fertilizer and hog manure, characterized with high B (up to 3.43mg/L) and intermediate δ11B (average 13‰). The low δ11B values in 2006 Kao-ping River are attributed to contributions from local geothermal inputs, average 2.5ppm and 2‰, prior to massive landslides caused by 2009 Morakot typhoon. Mass balance calculations suggest that major contribution of B in river waters was derived from silicate weathering, characterized by low B and intermediate δ11B, contributed approximately 49–97% in 2010 Kao-ping River. The detected changes of δ11B (Δw-rock) reaches a maximum variation of 18‰ during chemical weathering in catchment and highlights the importance of water/rock interaction enhancement after major climatic events.

B, Sr, O and H Isotopic Compositions of Formation Waters from the Bachu Bulge in the Tarim Basin

Abstract In order to elucidate the origin and migration of basinal brines in the Bachu Bulge, Tarim Basin, we have carried out analyses on chemical composition, and boron, hydrogen and oxygen isotopes of formation waters together with the XRD of clay minerals from the Paleozoic strata. The waters show Ca, B, Li and Sr enrichment and SO4 depletion in the Carboniferous and Ordovician and K enrichment in part of the Ordovician relative to seawater. The relationship between δD and δ18O shows that all the data of the waters decline towards the Global Meteoric Water Line with the intersection of them close to the present-day local meteoric water, suggesting that modern meteoric water has mixed with evaporated seawater. The 87Sr/86Sr ratios range from 0.7090 to 0.7011, significantly higher than those of the contemporary seawater. The δ11B values range from +19.7 to +32.3‰, showing a decrease with the depth and B concentrations. The results suggest that isotopically distinct B and Sr were derived from external sources. However, since the percentages of illite are shown to increase with depth among clay minerals in the study area, i.e., illite is due to precipitation rather than leaching during deeper burial, it is unlikely for illite to have contributed a significant amount of B to the waters. Thus, B with low δ11B values is interpreted to have been added mainly from thermal degradation of kerogen or the basalts in the Cambrian and Lower Ordovician.

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.

Halite dissolution derived brines in the vicinity of a Permian salt dome (N German Basin). Evidence from boron, strontium, oxygen, and hydrogen isotopes

The paper presents the isotopic (B, Sr, O, H) and chemical composition of solid salts, brines, groundwater, and surface water in the vicinity of a Permian salt dome in Northern Germany (diapir of Gorleben-Rambow) being considered for disposal of high level radioactive waste. The site is situated in the NW European Basin that is dominated by intensive diapirism of Permian salt and overprinted by glacial erosion/sedimentation. The local hydrogeology comprises an upper aquifer system of heterogeneous Pleistocene sediments and a lower aquifer system of Tertiary clastics tectonically disturbed by diapirism. There is a clear chemical and isotopic evolution with TDS, that generally increases with depth up to 328 g · l−1. The chemistry and isotope ratios of the saline groundwater can be explained by a two end-member mixing system. The Na-Cl brine end-member is derived from evaporite dissolution as indicated by major ion chemistry, Cl/Br ratios, Sr and B isotopes. It is mixed with fresh-brackish groundwater dominated by water-silicate interaction. This low salinity end-member (TDS from 0.3–10 g · l−1) shows radiogenic 87Sr/86Sr ratios ranging from 0.70910 to 0.71030 and low δ11B values between 7.4 and 16.5‰ vs. NBS951. Two halite samples and a borate sample from the unaltered parts of the diapir show 87Sr/86Sr ratios from 0.70696 to 0.70868 and δ11B values of 20.8 and 27.7‰. Brine (TDS from 100–328 g/l) Sr isotope ratios, ranging from 0.70758 to 0.70788, lie in the uppermost range of late Permian ocean waters and are within the range of halite. The δ11B values of the brines vary between 25.1‰ to 33.5‰, which is close to the range of solid salt but well below those of evaporating present-day sea water. B isotope composition is thus compatible with salt dissolution rather than with residual evaporite brines. The chemistry and isotope characteristics of the brine end-member derived from halite dissolution are not significantly modified by subsequent interaction with the siliciclastic aquifer material. Most highly saline groundwaters show cold climate O and H isotopic signatures with δ18O values ranging from −10.5 to −9‰. A limited group of four rather deep-seated brines (>200 m) plots in the range of Holocene signatures with δ18O > −9‰ close to those of the shallow groundwater which is interpreted as an indication of rapid groundwater circulation from the surface and ongoing salt dissolution.

Geochemical and boron, strontium, and oxygen isotopic constraints on the origin of the salinity in groundwater from the Mediterranean Coast of Israel

In order to identify the origin of the salinity and formation of saline plumes in the central part of the Mediterranean coastal aquifer of Israel, we determined the elemental and boron, strontium, and oxygen isotopic compositions of fresh and brackish groundwater (Cl up to 1500 mg/L). We distinguish between two key anthropogenic sources: (1) sewage effluents used for irrigation with high Na/Cl, SO4/Cl, and B/C1 ratios and low Br/Cl ratios relative to seawater ratios, low δ11B values (0–10‰) and high δ18O values (>−4‰); and (2) imported water from the Sea of Galilee that is artificially recharged to the aquifer with high Br/Cl (3 × 10−3) and δ18O values (−1‰) and a low 87Sr/86Sr ratio of 0.70753. The brackish groundwater from the saline plumes have relatively low Na/Cl ratios (0.5–0.8) and high Ca/Mg, Mg/Cl, and Ca/(SO4 + HCO3) (>1) ratios relative to seawater ratios; marine SO4/Cl and Br/Cl ratios; δ11B values of 24.8–49.9‰; δ18O of −2.95‰ to −4.73‰ and 87Sr/86Sr ratios of 0.708275–0.708532. The composition of most of the investigated groundwater from the saline plumes differs from those of the anthropogenic sources, imported water, fresh uncontaminated groundwater (87Sr/86Sr of 0.70866, δ11B of 20–30‰), and saline water from the adjacent Eocene aquitard. Only in areas of artificial recharge does local groundwater have high Br/Cl and δ18O values that are typical to the Sea of Galilee. The linear relationships between chloride and most of the ions, including B and Sr, the relatively high δ11B (>30‰) and low δ18)O (<−4‰) values, and the chemical signature of the saline plumes (e.g., marine Br/Cl and SO4/Cl ratios), suggest that (1) mixing processes control the chemical composition of the brackish water within the aquifer, and (2) the saline postulated end‐member has a chemical composition that resembles modified seawater with a marine and higher δ11B values, and a 87Sr/86Sr ratio of <0.7083. We propose that most of the salinization phenomena and the formation of saline plumes in the inner parts of the coastal aquifer are derived from upconing of underlying natural saline water bodies and enhanced by overexploitation and draw‐down of the overlying fresh groundwater.

The B isotopic composition of arc lavas from Martinique, Lesser Antilles

Island-arc samples from Martinique, Lesser Antilles, are enriched in B relative to oceanic crust or the mantle, with B contents varying between 5 and 54 ppm, and have δ11B falling in a range between 1.8‰ and −5.3‰, less than the δ11B of altered oceanic crust. The combination of these results indicates that both sediments and altered oceanic crust must have been involved in the generation of those magmas, since the abundance of B in the mantle is too low to produce such low δ11B values with such high B concentrations. B content and δ11B are not correlated, showing that the amount of B added to the arc magma was decoupled from its isotopic composition by the mixing of at least three distinct components. Boron concentrations, considered in conjunction with 87Sr/86Sr, 143Nd/144Nd, δ18O and δ11B data, show that variable amounts of B were added by a B-enriched fluid to different batches of mantle-derived magma. The B, Sr and Nd isotopic compositions of these samples are consistent with a two-stage process in which enriched magma is produced in the mantle wedge by the addition of an aqueous fluid derived from dehydrating, subducting lithosphere and then modified during ascent by assimilation of variable amounts of sediment and altered oceanic crust. Most of the B contained in the fluid was probably derived from altered oceanic crust plus as little as 3% sediment, while the material involved in assimilation was apparently largely continentally derived sediment. A two-stage process like this also explains the distribution of δ18O, within the context of the B, Sr and Nd isotopic compositions of Martinique arc lavas.

95/00147 The Falkland islands and the early fragmentation of Gondwana: Implications for hydrocarbon exploration in the Faikland Plateau

Both Nb–Ta-mineralized and Nb–Ta-poor syenitic dikes in the Panxi region (SW China) are spatially and temporally associated with syenitic plutons, which are part of the ~ 260 Ma Emeishan large igneous province. These syenitic dikes are NW-striking, and have width varying from 1 to 5 m and length from 50 to 300 m. The dikes are mainly composed of K-feldspar, albite, aegirine and arfvedsonite, however, mineral modes are different in the Nb–Ta-mineralized and Nb–Ta-poor syenitic dikes. The major Nb–Ta-bearing mineral in the dikes is pyrochlore, which is closely associated with albite and occurs in places with intensive albitization. Rocks of the Nb–Ta-mineralized syenitic dikes contain more albite and less K-feldspar than the Nb–Ta-poor dikes, and have compositions more evolved than the Nb–Ta-poor dikes, indicating that the Nb–Ta-mineralized syenitic dikes formed from a highly evolved magma. We analyzed the B concentrations and B isotopic compositions of the samples of both Nb–Ta-mineralized and Nb–Ta-poor syenitic dikes and associated syenitic pluton using a single column purification method and ICP-AES and MC-ICP-MS techniques. The samples of the Nb–Ta-mineralized syenitic dikes have whole-rock B concentrations ranging from 11.4 to 23.9 ppm and δ11B values from − 17.95 to − 14.54‰, whereas the samples of the Nb–Ta-poor dikes and syenitic plutons have B concentrations varying from 3.32 to 16.5 ppm and δ11B values from − 13.45 to − 10.02‰. The high B concentration of the Nb–Ta-mineralized dikes relative to the Nb–Ta-poor dikes is consistent with that B is incompatible and tends to be rich in more evolved magma. The relatively low δ11B values of the Nb–Ta-mineralized dikes indicate that the B isotopes may have fractionated between fluids and rocks in a transitional, magmatic–hydrothermal stage. We propose that the highly evolved magmas in a transitional, magmatic–hydrothermal stage may have Nb– and Ta–fluorine complexes dissolved in the hydrothermal fluids in the presence of Na+. Albite crystallization due to intensive albitization in this stage resulted in the decrease of Na+ in the fluids, decomposing the Nb– and Ta–fluorine complexes. The released Nb and Ta from the complexes were then dissolved in the fluids and finally entered the lattice of pyrochlore crystals in the stage of albitization.

Clutch pedal in glass reinforced nylon

Tourmaline is a common accessory mineral in the metasedimentary Pfitsch Formation located in the Pfitscher Joch (Passo de Vizze) area in the Tauern Window of the Eastern Alps. These post-Variscan metasedimentary units experienced peak metamorphic conditions of ~ 550 °C, 1.0 GPa during the Alpine orogeny. Tourmaline is most abundant in a ~ 25 m thick unit of feldspathic gneiss (~ 20–200 μg/g B), where it occurs as idiomorphic crystals typically 10 mm in length. The abundance and size of the tourmaline crystals increase near coarse-grained quartzofeldspathic segregations (~ 1200 μg/g B), reflecting the mobilization and concentration of B by metamorphic fluids. Near segregations, individual tourmaline crystals have up to three growth zones, recording pro- (~ 350–500 °C, 0.7–1.0 GPa) and retrograde (~ 400 °C, 0.2 GPa) growth as determined by combining textural information and sector-zoning thermometry. Retrograde tourmaline occurs as individual crystals as well as overgrowths on prograde tourmaline crystals, especially on the surface of extensional fractures formed by E–W extension during regional decompression.Tourmaline in the Pfitsch Formation is dravitic with a variable Fe content that correlates with the Fe content of its respective host unit. Charge balance calculations suggest that a significant proportion of Fe in tourmaline is ferric, supporting the interpretation of a subaerial continental sedimentary protolith. Tourmaline near segregations has the highest inferred ferric iron content, which decreases across growth zones, potentially reflecting a reduction of the fluid during metamorphism. The Mg/(Mg + Fe) ratio increases with prograde tourmaline growth and decreases in retrograde overgrowths. In contrast, the Ca/(Ca + Na) ratio increases gradually from 0.05 to 0.20 with prograde growth and continues to increase up to 0.25 in the retrograde overgrowths, recording the maximum Ca/(Ca + Na) ratio of the fluid during Alpine metamorphism.The metasediments of the Pfitsch Formation have very low and variable whole-rock δ11B values (− 14.1 to − 33.6‰), with the highest values (− 17.7 to − 14.1‰) found in B-rich samples (165–1200 μg/g B) containing abundant tourmaline, and the lowest values (− 24.2 to − 33.6‰) in B-depleted samples (21–40 μg/g), which lack tourmaline. This observation supports preferential loss of 11B from the rocks during prograde metamorphism. Zoned tourmaline crystals in the Pfitsch formation show successively decreasing B isotope ratios from − 7.8 to − 11.2‰ in their cores and − 17.3 to − 20.3‰ in their rims. As supported by a Rayleigh fractionation model, the B-isotope values of the host rocks and the tourmaline crystals are most easily explained by the internal redistribution of B from a B-rich precursor mineral (e.g. mica) to the tourmaline during Alpine metamorphism.

Boron isotope geochemistry of Australian salt lakes

Boron isotope geochemistry has been investigated in brines, groundwaters, and sediments from the modern Australian salt lakes of Victoria, South Australia, and Western Australia by applying negative thermal-ionization mass spectrometry techniques. The geochemical history of the brines has been reconstructed by using δ11B, BCl, and NaCl ratios.The Victorian volcanic-crater lakes of southeastern Australia have water salinities of up to 60 g/L, molar NaCl ratios (0.87) similar to the marine ratio, molar BCl ratios of 2.9 × 10−4 to 4.9 × 10−4, and δ11B values of 54%. to 59%. (relative to NBS 951). The depletion of total B and the high positive δ11B values relative to seawater (BClratio = 7.9 × 10−4; δ11B = 39%.) are attributed to a marine (cyclic) salt origin together with adsorption processes in closed systems with low water/sediment (WR) ratios.In contrast, salt lakes from South Australia and Western Australia which are large shallow playas associated with halite, gypsum, and detrital clay minerals have interstitial and surface brines characterized by salinities of 80 to 280 g/L, molar NaCl ratios of 0.85 to 1, molar BCl ratios of 4 × 10−6 to 4 × 10−4, and δ11 values of 25%. to 48%.. The δ11 values of these brines are different from those of groundwaters from the Great Artesian Basin (δ11 = −15.9%. to 2.2%.; with high molar BCl ratios of 1 × 10−3 to 3.8 × 10−2), country rocks (δ11B = −8.7%. to 6.8%.), and modern detrital sediments present in these salt lakes (δ11B = −3.2%. to 12.3%.). The δ11B values of these salt lakes overlap with those of surface and brackish waters (δ11B = 28%. to 35%.) and with the B isotopic composition of seawater (δ11B = 39%.). Both low molar NaCl ratios (<1) and high δ11B values suggest that the source of the bulk of the dissolved solids in the Australian brines is dominated by cyclic salts, derived from seawater, rather than from local rock weathering. While the low BCl ratios and high δ11 values (>39%) of some brines indicate interaction of the brines with detrital sediments within the salt lake systems, δ11 values < 39% suggest mixing of brines of marine origin from which B was partly removed by adsorption, with waters of terrestrial origin with low δ11 values.NaCl ratios are used as indicators of the origin of the salts as well as of halite dissolution-precipitation. The δ11 values and BCl ratios are sensitive to a marine or non-marine origin, adsorption of boron onto clays, and the effective water/sediment ratio. At low WR ratios, the preferential removal of 10B from the solution affects the bulk solution, whereas at high WR ratios, the δ11 value of a solution is not affected by adsorption. Although the δ11 value of borate minerals may be a discriminant of marine or non-marine origin, boron isotopes are less distinctive in evaporative environments where boron is not an abundant component and where water/sediment interaction occurs.