Boron isotopes

Boron, a group 13 metalloid, has two natural occurring stable isotopes, 11 B (11.00930536[45]) and 10 B (10.01293695[41]), with relative abundances of 0.199(7) and 0.801 , respectively, and hence occurs approximately in a 4:1 ratio (source of data: National Institute of Standards and Technology, http://www.nist.gov/pml/data/comp.cfm). Numerous radioisotopes of boron also occur with masses from 7 B to 17 B, but their half-lives are all <1 s. As with other stable isotopic systems, natural isotope variation is described using delta notation, i.e., the per mil variation from the 11 B/ 10

The 2016 Mw ≥ 6.0 Amatrice‐Norcia earthquakes (central Apennines, Italy) and the related seismic sequence were associated with increases in arsenic and vanadium concentrations recorded in groundwater springs a few months before the earthquakes occurred. To evaluate these signals as reliable seismic precursors and effective predictive tools, we studied the geochemical processes that caused these anomalies. Using chemical and isotope models we show that increased concentrations of arsenic and vanadium, a slight increase in boron concentrations, and a concomitant lowering of the boron isotope ratio may be due to mineral desorption (e.g., from iron oxides and/or clays). We argue that a displacing effect on the trace elements sorbed on minerals was triggered by an excess of deep CO2 in groundwater, which occurred prior to the main seismic event as a result of pre‐seismic crustal dilation. Our observations confirm the pivotal role of CO2 in the release of trace elements by alteration of solid phases, and provides a new understanding of earthquake‐related water chemical anomalies.

Water from ten thermal springs, five drinking-water boreholes and one hand-dug well from Hanlé-Gaggadé grabens has been analyzed for major-minor hydrogeochemical parameters and multi-isotope composition: δ2H(H2O), δ18O(H2O), δ18O(SO42−), δ34S(SO4
2−), δ13C(DIC), 14C(DIC), and δ11B. The interaction of the thermal
water with both silicic (basaltic) and sedimentary rocks and its low enthalpy (T ≤150 °C) regime make this area an optimal site for testing chemical and isotope geothermometers and optimizing them for similar difficult cases.
In this study, the use of radiogenic carbon (14C) reveals that the age of the deep aquifer is between 12,500 and 8700 years ago (late Pleistocene/early-to-mid Holocene), which places it within the East African Humid Period.
The oxygen and hydrogen stable isotope ratios of the water molecules, δ2H(H2O), δ18O(H2O), show common characteristics between the groundwater from this area and that of groundwater from nearby local lakes (Asal-Abhé) and palaeolakes (Sakalol-Hanlé-Gaggadé-Dobi), which were interconnected during that period to form the terminal Awash River Basin. The comparison of these aquifers within the transboundary aquifer located between northeast Ethiopia and Djibouti helps to reconstruct both the palaeoenvironmental history and the hydrogeochemical characteristics of this regional aquifer. At present, these data are poorly defined, but it is of fundamental importance for the constructive use of water resources in arid areas. Finally, a conceptual model was proposed for the Hanlé-Gaggadé low-enthalpy geothermal system on the basis of the geochemical and isotope data on the thermal and non-thermal groundwaters combined with geological and tectonic information and the results of recent geophysical studies.

The boron isotope-pH proxy, applied to mixed-layer planktic foraminifera, has great potential for estimating past CO2 levels, which in turn is crucial to advance our understanding of how this greenhouse gas influences Earth's climate. Previous culture experiments have shown that, although the boron isotopic compositions of various planktic foraminifera are pH dependent, they do not agree with the aqueous geochemical basis of the proxy. Here we outline the results of culture experiments on Globigerinoides ruber (white) across a range of pH (∼7.5–8.2) and analysed via multicollector inductively-coupled plasma mass spectrometry (MC-ICPMS), and compare these data to core-top and sediment-trap samples to derive a robust new species-specific boron isotope-pH calibration. Consistent with earlier culture studies, we show a reduced pH dependency of the boron isotopic composition of symbiont-bearing planktonic foraminifera compared to borate ion in seawater. We also present evidence for a size fraction effect in the δ11B of G. ruber. Finally, we reconstruct atmospheric CO2 concentrations over the last deglacial using our new calibration at two equatorial sites, ODP Site 999A and Site GeoB1523-1. These data provide further grounding for the application of the boron isotope-pH proxy in reconstructions of past atmospheric CO2 levels.

A global selection of 56 mid-ocean ridge basalt (MORB) glasses were analysed for Li and B abundances and isotopic compositions. Analytical accuracy and precision of analyses constitute an improvement over previously published MORB data and allow a more detailed discussion of the Li and B systematics of the crust-mantle system. Refined estimates for primitive mantle abundances ([Li] = 1.39 ±0.10 µg/g and [B] = 0.19 ±0.02 µg/g) and depleted mantle abundances ([Li] = 1.20 ±0.10 µg/g and [B] = 0.077 ±0.010 µg/g) are presented based on mass balance and on partial melting models that utilise observed element ratios in MORB. Assimilation of seawater (or brine) or seawater-altered material beneath the ridge, identified by high Cl/K, causes significant elevation of MORB d11B and variable elevation in d7Li. The B isotope ratio is, hence, identified as a reliable indicator of assimilation in MORB and values higher than -6‰ are strongly indicative of shallow contamination of the magma. The global set of samples investigated here were produced at various degrees of partial melting and include depleted and enriched MORB from slow and fast-spreading ridge segments with a range of radiogenic isotope signatures and trace element compositions. Uncontaminated (low-Cl/K) MORB show no significant boron isotope variation at the current level of analytical precision, and hence a homogenous B isotopic composition of d11B = -7.1 ±0.9‰ (mean of six ridge segments; 2SD). Boron isotope fractionation during mantle melting and basalt fractionation likely is small, and this d11B value reflects the B isotopic composition of the depleted mantle and the bulk silicate Earth, probably within ±0.4‰. Our sample set shows a mean d7Li = +3.5 ±1.0‰ (mean of five ridge segments; 2SD), excluding high-Cl/K samples. A significant variation of 1.0–1.5‰ exists among various ridge segments and among samples within individual ridge segments, but this variation is unrelated to differentiation, assimilation or mantle source indicators, such as radiogenic isotopes or trace elements. It, therefore, seems likely that kinetic fractionation of Li isotopes during magma extraction, transport and storage may generate d7Li excursions in MORB. No mantle heterogeneities, such as those generated by deeply recycled subducted materials, are invoked in the interpretation of the Li and B isotope data presented here, in contrast to previous work on smaller data sets. Lithium and boron budgets for the silicate Earth are presented that are based on isotope and element mass balance. A refined estimate for the B isotopic composition of the bulk continental crust is given as d11B = -9.1 ±2.4 ‰. Mass balance allows the existence of recycled B reservoirs in the deep mantle, but these are not required. However, mass balance among the crust, sediments and seawater shows enrichment of 6Li in the surface reservoirs, which requires the existence of 7Li-enriched material in the mantle. This may have formed by subduction of altered oceanic crust since the Archaean.

The boron stable isotope ratio δ11B of twelve water samples representative of three chemical facies (fresh Na-bicarbonate, brackish Na-chloride, saline and brine Ca-chloride) have been analyzed. Interpretation of the δ11B data, along with the chemical compositions, reveals that Na-carbonate waters from the Northern Apennine are of meteoric origin, with boron contributions from clay-desorption and mixing with seawater-derived fluids of Na-chloride or Ca-chloride compositions. The comparison of our new results with the literature data on other sedimentary basins of Mediterranean, and worldwide, confirms the contribution of Na-bicarbonate waters to the genesis of mud volcano fluids. The Na-chloride sample of Salvarola (SAL), which may represent the end-member of the mud volcanoes, and the Ca-chloride brine water from Salsomaggiore (SM) indicate boron release from clays compatible with the diagenetic process. The empirical equation: δ11B = [5.1364 x ln(1/B)mg/l] + 44.601 relating boron concentration and the stable isotope composition of the fluids observed in this study and the literature is proposed to trace the effect of diagenesis in sedimentary basins. A geothermometer associated to the diagenetic equation is also proposed: T°C = [δ11B - 38.873 (± 1.180)] / [-0.164 (± 0.012)] The application of this equation to obtain reservoir temperatures from δ11B compositions of waters should be carefully evaluated against the results obtained from other chemical and isotopic geothermometers from other basins around the world.

The boron isotopic composition (δ 11 B) of coral skeleton is a proxy for seawater pH. However, δ 11 B-based pH estimates must account for the pH difference between seawater and the coral calcifying fluid, ΔpH. We report that skeletal δ 11 B and ΔpH are related to the skeletal carbon isotopic composition (δ 13 C) in four genera of deep-sea corals collected across a natural pH range of 7.89–8.09, with ΔpH related to δ 13 C by ΔpH = 0.029 × δ 13 C + 0.929, r 2 = 0.717. Seawater pH can be reconstructed by determining ΔpH from δ 13 C and subtracting it from the δ 11 B-derived calcifying fluid pH. The uncertainty for reconstructions is ±0.12 pH units (2 standard deviations) if estimated from regression prediction intervals or between ±0.04 and ±0.06 pH units if estimated from confidence intervals. Our new approach quantifies and corrects for vital effects, offering improved accuracy relative to an existing δ 11 B versus seawater pH calibration with deep-sea scleractinian corals.

The Foça-Karaburun and Ayvalık-Lesvos grabens (western coast of Anatolia, Turkey) are two important
NW−SE-trending extensional areas generated in response to the Early Miocene−Holocene extension of the Western
Anatolian region, related to the opening of the ‘unconventional’ back-arc basin of the Aegean Sea. The abundance of
geo-structural evidence and the occurrence of volcanic rocks representing all the stages of the Aegean-Western Anatolia
volcanism render the Foça-Karaburun and Ayvalık-Lesvos Grabens key localities to exemplify the petrogenetic and
geodynamic evolution of the area. In this context, the Foça-Karaburun and Ayvalık-Lesvos grabens, possibly formerly
a single graben, formed along an original NE−SW-trending extension, later dissected by E−W-trending transtensional
faults, are investigated to constrain the petrogenetic and geodynamic evolution of the whole Aegean region. Calcalkaline
and shoshonitic volcanic rocks with scattered ultrapotassic-shoshonitic or lamproitic lavas and dykes represent
the orogenic phase of the magmatic activity, while the younger K- and Na-rich alkaline basaltic rocks are the result of
later magmatism characterized by an intraplate geochemical signature reflecting progressively decreasing subduction
rates.
While the tectonic lineaments and the structures of the study area allow the reconstruction of the tectonic evolution of
Western Anatolia and Aegean Sea, the volcanic rocks from the different stages of Neogene volcanism within the two
studied grabens and surrounding areas permit a precise record of the geochemical evolution of the magma sources

Optical isotopic measurement Laser ablation plasma Molecular emission spectra LIBS LAMIS of boron isotopes Boron isotope Laser Ablation Molecular Isotopic Spectrometry (LAMIS) was recently reported for optical isotopic analysis of condensed samples in ambient air and at ambient pressure. LAMIS utilizes molecular emissions which exhibit larger isotopic spectral shits than in atomic transitions. For boron monoxide 10 BO and 11 BO, the isotopic shifts extend from 114 cm −1 (0.74 nm) to 145-238 cm −1 (5-8 nm) at the transitions, respectively. These molecular isotopic shifts are over two orders of magnitude larger than the maximum isotopic shift of approximately 0.6 cm −1 in atomic boron. This paper describes how boron isotope abundance can be quantitatively determined using LAMIS and how atomic, ionic, and molecular optical emission develops in a plasma emanating from laser ablation of solid samples with various boron isotopic composition. We demonstrate that requirements for spectral resolution of the measurement system can be significantly relaxed when the isotopic abundance ratio is determined using chemometric analysis of spectra. Sensitivity can be improved by using a second slightly delayed laser pulse arriving into an expanding plume created by the first ablation pulse.

Cu-bearing elbaite from Paraíba (Brazil) is a highly-prized gem tourmaline. Specimens of similar quality from localities in Mozambique and Nigeria are being sold, and reliable provenance tools are required to distinguish specimens from the original locality from ‘Paraı´ba-type’ tourmaline from Africa. Here we present Li and B isotope analyses of Cu-bearing elbaite from all three localities and demonstrate the suitability of these isotope systems as a provenance tool. Isotopic profiles across chemically zoned grains revealed homogenous B and Li isotopic compositions, demonstrating a strong advantage of their application as a provenance tool as opposed to major, minor or trace element signatures.
Li and B isotopes of all investigated samples of Cu-bearing elbaites from the three localities are within the range of previously published granitic and pegmatitic tourmaline. Anomalous isotope compositions published previously for these samples are corrected by our results.

In the course of investigations of thermal neutron detection based on mixtures of 10BF3 with other gases, knowledge was required of the photoabsorption cross sections of 10BF3 for wavelengths between 135 and 205 nm. Large discrepancies in the values reported in existing literature led to the absolute measurements reported in this communication. The measurements were made at the SURF III Synchrotron Ultraviolet Radiation Facility at the National Institute of Standards and Technology. The measured absorption cross sections vary from 10−20 cm2 at 135 nm to less than 10−21 cm2 in the region from 165 to 205 nm. Three previously unreported absorption features with resolvable structure were found in the regions 135–145 nm, 150–165 nm, and 190–205 nm. Quantum mechanical calculations, using the TD-B3LYP/aug-cc-pVDZ variant of time-dependent density functional theory implemented in Gaussian 09, suggest that the observed absorption features arise from symmetry-changing adiabatic transitions.