Boron Isotope Research Articles

Experimental Modeling of Boron-10 Isotope Enrichment during Chemical Exchange in a Liquid–Liquid System

Experimental Modeling of Boron-10 Isotope Enrichment during Chemical Exchange in a Liquid–Liquid System

Carbon Cycle and Circulation Change in the North Pacific Ocean at the Initiation of Northern Hemisphere Glaciation Constrained by Boron‐Based Proxies in Diatoms

AbstractThe intensification of Northern Hemisphere glaciation (iNHG) at 2.73 Ma is associated with a reorganization of the subarctic Pacific Ocean and abrupt drop in opal mass accumulation rates. Uncertainty, however, remains around the extent to which these changes altered carbon dynamics and contributed to a reduction in atmospheric pCO2 and global temperatures. These issues are addressed here using the boron isotope (δ11B) proxy in diatom frustules to reconstruct past changes in the pH and pCO2 of ambient seawater. Diatom δ11B and [B] indicate a subarctic Pacific surface water increase of 0.3–0.5 pH units over the iNHG. This confirms that delivery of carbon and nutrients into surface waters was reduced at this time, explaining the drop in opal productivity and limiting CO2 outgassing from the ocean interior. We consider two hypotheses to explain this based on potential changes in circulation from the late Pliocene to early Pleistocene: “ventilation to stratification” or “stratification to ventilation.” The ventilation to stratification hypothesis, which posits a switch from vigorous Pacific Meridional Overturning Circulation in the Pliocene to stratification over iNHG, has received more attention in the literature. The stratification to ventilation hypothesis, which posits a modest increase in ventilation, is more consistent with modern and late Pleistocene analogs, the majority of models and δ13C data. These late Pliocene changes in the subarctic Pacific, in conjunction with other external and internal processes including those in the Southern Ocean, would have contributed to a lowering of atmospheric pCO2 and the long‐term expansion of ice‐sheets across the Northern Hemisphere.

Imprints of Atlantic Multidecadal Oscillation on pantropical seawater pH inferred from coral δ11B records

Understanding natural variability in seawater pH (pHsw) is crucial for predicting future ocean acidification. Coral boron isotopes (δ11B) offer an alternative method to extend pHsw records back centuries, shedding light on how pHsw varies over time. This study compiles both new and existing coral δ11B records from pantropical oceans to investigate multidecadal pHsw variability and its connection to climate variability. A notable finding is the pronounced influence of the Atlantic Multidecadal Oscillation (AMO) on pantropical coral δ11B-pH variations. This is primarily interpreted as the effects of AMO-associated climate changes on reef pHsw, rather than influencing coral physiological processes related to calcifying fluid pH. Key factors like wind speeds and hydrological variability related to Intertropical Convergence Zone (ITCZ) shifts are identified as potential drivers of the AMO's impact on pHsw. This study thus provides valuable insights into the broader effects of the Atlantic climate on pantropical seawater CO2 system.

Luminescence Modulation in Boron-Cluster-Based Luminogens via Boron Isotope Effects.

Recent advances in luminescent materials highlight the significant impact of hydrogen isotope effects on improving optoelectronic properties. However, the research on the influence of the boron isotope effects on photophysical properties remains underdeveloped. This study focused on exploring the boron isotope effects in boron-cluster-based luminogens. In doing so, we designed and synthesized carborane-based luminogens containing 98% 10B and 95% 11B, respectively, and observed distinct photophysical behaviors. Compared to the 10B-enriched luminogens, the 11B-enriched counterparts can significantly enhance luminescence efficiency, prolong emission lifetime, and reduce full-width at half-maximum. Additionally, increased thermal stability, redshifted B-H vibrations, and a fourfold enhanced electrochemiluminescence intensity have also been observed. On the other hand, the biological assessments of a 10B-enriched luminogen reveals low cytotoxicity, high boron uptake, and excellent fluorescence imaging capability, indicating the potential application in boron neutron capture therapy (BNCT). This work presents the first comprehensive exploration on the boron isotope effects in boron clusters, and provides valuable insights into the rational design of organic luminogens for advanced optoelectronic and biomedical applications.

Luminescence Modulation in Boron‐Cluster‐Based Luminogens via Boron Isotope Effects

Recent advances in luminescent materials highlight the significant impact of hydrogen isotope effects on improving optoelectronic properties. However, the research on the influence of the boron isotope effects on photophysical properties remains underdeveloped. This study focused on exploring the boron isotope effects in boron‐cluster‐based luminogens. In doing so, we designed and synthesized carborane‐based luminogens containing 98% 10B and 95% 11B, respectively, and observed distinct photophysical behaviors. Compared to the 10B‐enriched luminogens, the 11B‐enriched counterparts can significantly enhance luminescence efficiency, prolong emission lifetime, and reduce full‐width at half‐maximum. Additionally, increased thermal stability, redshifted B–H vibrations, and a fourfold enhanced electrochemiluminescence intensity have also been observed. On the other hand, the biological assessments of a 10B‐enriched luminogen reveals low cytotoxicity, high boron uptake, and excellent fluorescence imaging capability, indicating the potential application in boron neutron capture therapy (BNCT). This work presents the first comprehensive exploration on the boron isotope effects in boron clusters, and provides valuable insights into the rational design of organic luminogens for advanced optoelectronic and biomedical applications.

A Sequential Leaching Protocol for δ11B and Trace Element Analyses of Multi‐Phase Carbonate Rocks

AbstractBoron geochemistry from biogenic carbonates offer valuable information about ocean pH and CO2 chemistry. However, application to geological carbonate deposits suffers from analytical difficulties in obtaining geochemical signals exclusively from the carbonate phase. Sequential leaching with reagents and acids has the potential to overcome such an issue. There is, however, little systematic investigation about the efficiency of sequential leaching in isolating carbonate‐associated boron from siliciclastic matrix. Here, we developed a sequential leaching protocol and applied it to methane‐derived‐authigenic‐carbonate samples. Using the leachate δ11B signatures, elemental composition, and mineral composition of residues, we show that the sequential leaching is able to improve the separation of boron from different phases. Buffered hydrogen peroxide removes organic matter and also some silicate phases resulting low δ11B values. Leaching with NH4Ac removes adsorbed boron though may also partially leach some carbonate phases. The first few leaching steps with diluted acetic acid dissolve carbonate phases. Depending on the sample type, these may also capture some remaining adsorbed boron from the preceding NH4Ac leaching. Once the adsorbed boron is completely removed, as indicated by the progressively higher δ11B values during the following acid leaching steps, representative carbonate composition can be derived. The accuracy of this protocol is demonstrated with leaching experiments using artificial deep sea coral carbonate and clay mixtures that give the representative carbonate‐associated δ11B within error of the pure coral value. Our results provide insights into characteristic signatures derived from silicates and organic matter that need to be considered in boron isotope analyses of impure marine carbonates.

Reconstructing CO2 uptake capacity and pH dynamics in the Middle Ordovician Taebaeksan Basin, Korea

This study presents the first continuous carbonate boron isotope (δ11Bcarb) record from Middle Ordovician carbonate rocks, focusing on the Taebaek section in Korea. The δ11Bcarb values analyzed from the Makgol, Jigunsan, and Duwibong formations range between 7.5 ‰ and 17.8 ‰, falling within the published Paleozoic range. Samples exhibit minimal diagenetic alteration, as evidenced by weak correlations between isotopic factors and elemental ratios (δ11Bcarb, δ13Ccarb, δ15Nbulk, B/Ca, Mn/Sr, and Na/Ca) as well as mineralogical observations used to select unaltered micrite portions. Using the δ11Bcarb data, we reconstruct pH levels (8.4 to 9.1) which suggest that this shallow shelf environment acted as a CO2 sink during the Middle Darriwilian, despite periods of acidification coinciding with sea level rise and expansion of anoxic zones. Weakening of the basin's CO2 uptake capacity via reduced CaCO3 precipitation and photosynthesis accompanied these oceanographic changes. Our findings provide new insights into the dynamics of CO2 accumulation and pH in Middle Ordovician shallow marine settings in relation to sea level fluctuations. This work lays the foundation for further research into ocean chemistry and carbon cycle perturbations during this important period in Earth's history.

Investigation and Development of the BODIPY-Embedded Isotopic Signature for Chemoproteomics Labeling and Targeted Profiling.

A common goal in mass spectrometry-based chemoproteomics is to directly measure the site of conjugation between the target protein and the small molecule ligand. However, these experiments are inherently challenging due to the low abundance of labeled proteins and the difficulty in identifying modification sites using standard proteomics software. Reporter tags that either generate signature fragment ions or isotopically encode target peptides can be used for the preemptive discovery of labeled peptides even in the absence of identification. We investigated the potential of BODIPY FL azide as a click chemistry enabled chemoproteomics reagent due to the presence of boron and the unique 1:4 natural abundance ratio of 10B:11B. The isotopes of boron encode BODIPY-labeled peptides with a predictable pattern between the monoisotopic (M) and M+1 peaks. BODIPY-labeled peptides were identified in MS1 spectra using an R script that filters for the signature 10B:11B intensity ratio and mass defect. Application of the boron detection script resulted in three times the labeled peptide coverage achieved for a BODIPY-conjugated BSA sample compared with untargeted data-dependent acquisition sequencing. Furthermore, we used the inherent HF neutral loss signature from BODIPY to assist with BODIPY-modified peptide identification. Finally, we demonstrate the application of this approach using the BODIPY-conjugated BSA sample spiked into a complex E. coli. digest. In summary, our results show that the commercially available BODIPY FL azide clicked to alkyne-labeled peptides provides a unique isotopic signature for pinpointing the site(s) of modification with the added potential for on- or off-line UV or fluorescence detection.

Historic ocean acidification of Loch Sween revealed by correlative geochemical imaging and high-resolution boron isotope analysis of Boreolithothamniom cf. soriferum

Ocean Acidification (OA) arises from the increase in atmospheric carbon dioxide concentration following the industrial revolution. The ecological and socio-economic consequences of OA were first identified around 10–15 years ago but remain poorly understood. This is particularly true in coastal regions where local processes can have dramatic consequences on pH trends through time, obscuring and compounding the long-term effects from rising atmospheric CO2. Here we explore the possibility of generating long records of coastal ocean pH using the skeletons of widely distributed coralline algae (CA). The skeletons of these slow growing (<1 mm/year) taxa often contain micron-scale heterogeneities, making sampling for high-resolution climate reconstructions using bulk sampling techniques difficult. Here we use laser ablation coupled to inductively coupled plasma mass spectrometers to generate high-resolution 2D images of the element/calcium ratios and boron isotope composition (δ11B) of a sample of Boreolithothamniom cf. soriferum from Loch Sween in Scotland, UK where we have been monitoring temperature since 2004 and pH during 2014. By carefully correlating the geochemical images with a scanning electron microscopy image we can segment them to remove the marginal portions of the skeleton, isolating the central growth axis to generate an age model and growth rate. The δ11B-pH is significantly elevated above the seawater pH in Loch Sween (8.4 to 8.9 vs. 7.9 to 8.1) consistent with other CA that show internal pH elevation. On a seasonal scale, internal pH is negatively correlated with temperature and also exhibits a long-term decline. By removing this temperature effect, internal pH can be correlated to seawater pH during the 2014 monitoring period allowing us to reconstruct a seawater acidification trend from 2004 to 2018 of -0.018 pH units per year, 10x higher than open ocean trends but consistent with contemporaneous monitoring efforts of UK coastal waters. Reconstructed aqueous CO2 suggests that prior to ∼2008 Loch Sween was a sink of CO2 but after this date, particularly during the early summer, it was a substantial CO2 source. Comparison of reconstructed aqueous CO2 with a record of calcification rate of our sample of Boreolithothamniom cf. soriferum suggests this acidification and associated rise in local seawater pCO2 may have freed this sample from carbon limitation leading to a recent increase in calcification.

Simultaneous Bulk‐Rock Elemental and Boron Isotope Ratio Measurement Using LA‐ICP‐MS on Silicate Micro‐Particulate Pellets

We have established a method for the simultaneous measurement of element abundances and boron isotopic compositions of bulk‐rock silicate samples by laser ablation coupled to two different ICP‐MS instruments. Fifteen silicate reference materials were prepared as micro‐particulate pressed powder pellets (MP) for analysis. Approximately 1/5th of the ablated material was transported to a single‐collector ICP‐MS for elemental measurement, while 4/5ths were sent to a multi‐collector ICP‐MS for boron isotope ratio measurement. Compositions of different milling materials (agate, zirconium oxide, tungsten carbide, sintered corundum and silicon nitride) were measured for the evaluation of potential contamination during sample preparation. Determined element mass fractions are in agreement with literature values within ±20%. For boron isotope ratios, the matrix‐dependent background elevation across the m/z from 9.9 to 11, and the effect of ablation mass was quantified. Measured δ11B values of most reference materials are consistent with literature values within ±1‰. The intermediate measurement precision is ±0.3‰ for JB‐2 with 30 μg g−1 B and ±0.4‰ for JA‐2 with 21 μg g−1 B. A publically available, web‐based streamlit app was developed for data processing (https://zenodo.org/doi/10.5281/zenodo.11150471). This paves the way for combined, accurate and precise elemental and boron isotope ratio measurements of geological materials.

Deep recycling of volatile elements in the mantle: Evidence from the heterogeneous B isotope in intra-plate basalts

Volatiles in the mantle are crucial for Earth’s geodynamic and geochemical evolution. Understanding the deep recycling of volatiles is key for grasping mantle chemical heterogeneity, plate tectonics, and long-term planetary evolution. While subduction transfers abundant volatile elements from the Earth’s surface into the mantle, the fate of hydrous portions within subducted slabs during intensive dehydration processes remains uncertain. Boron isotopes, only efficiently fractionating near the Earth’s surface, are valuable for tracing volatile recycling signals. In this study, we document a notably large variation in δ11B values (−14.3‰ to +8.2‰) in Cenozoic basalts from the South China Block. These basalts, associated with a high-velocity zone beneath East China, are suggested to originate from the mantle transition zone. While the majority exhibit δ11B values (−10‰ to −5‰) resembling the normal mantle, their enriched Sr-Nd-Pb isotope compositions and fluid-mobile elements imply hydrous components in their source, including altered oceanic crust and sediments. The normal δ11B values are attributed to the dehydration processes. Remarkably high δ11B values in the basalts indicate the presence of subducted serpentinites in their mantle source. A small subset of samples with low δ11B values and radiogenic isotope enrichments suggests a contribution from recycled detrital sediments, though retaining minimal volatile elements after extensive dehydration. These findings provide compelling evidence that serpentinites within subducted slabs predominantly maintain their hydrous nature during dehydration processes in subduction zones. They may transport a considerable amount of water into deep mantle reservoirs, such as the mantle transition zone.

Adsorption pathways of boron on clay and their implications for boron cycling on land and in the ocean

Reversible adsorption and isotope fractionation of boron on the surface of clay minerals is a key process that impacts boron isotope cycling in porewater, rivers and the ocean. However, the differences in boron isotope fractionation factors between various clay minerals and their dependence on fluid chemistry are not well known. We performed two sets of experiments, using solutions of pure water with added boron and seawater, to explore the isotope behavior during adsorption of boron onto kaolinite, smectite and illite. We found that the amount of sorbed boron increases with ionic strength of solutions and is proportional to the cation exchange capacity of a given clay mineral. Maximum adsorption is observed in alkaline seawater, which we attribute to the efficient fixation of magnesium-borate ion pairs onto negatively charged surface sites. Isotopic fractionation is modestly different between clays and demonstrates that clay surfaces preferentially sorb borate, even when the concentration of borate in solution is low. In both pure water and seawater, adsorbed complexes retain the isotopic composition of their dissolved precursors (borate or boric acid) with minimal isotopic fractionation. In other words, isotopic composition of adsorbed boron is set by the ability of clays to adsorb boron from an already fractionated boron pool rather than specific fractionation associated with the complexation reaction. Our experimental results allow us to provide revised constraints on the adsorbed boron being transported in terrestrial fluids and the ocean.

Boron Isotope Compositions and Environmental Significance of Sediments from the East Taijnar Salt Lake

Boron Isotope Compositions and Environmental Significance of Sediments from the East Taijnar Salt Lake

High-temperature boron partitioning and isotope fractionation between basaltic melt and fluid

In the last two decades, boron has gained significance as a geochemical tracer in mantle studies, particularly related to fluid-mediated processes. In our investigation, we explore how boron and its stable isotopes distribute between basaltic melt and hydrous fluid under conditions relevant to magmatic degassing in the shallow crust (1000–1250 °C, 150–250 MPa). We utilized a synthetic MORB-like composition with added boric-acid isotope standard (NIST-SRM951a) and additional trace elements, subjecting it to varying pressure, temperature, and melt-fluid ratios using an internally heated pressure vessel. The B isotope composition in the quenched glasses were determined through femtosecond laser ablation coupled to a multi-collector inductively-coupled-plasma mass spectrometer. Our experiments revealed that, even at the highest temperatures, boron strongly partitions into the fluid phase, accompanied by significant B isotope fractionation. This leads to an enrichment of the heavy B isotope in the fluid, with a constrained Δ11Bmelt-fluid range of -1.7 ± 0.9‰, consistent with ab-initio modeling results. These findings highlight the potential of B isotopes to trace geochemical processes at elevated temperatures with \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\Delta}^{11}{{B}}_{melt-fluid}=2.913-9.693\\frac{{10}^{6}}{{{T}}^{2}}$$\\end{document}. Our results have implications for predicting the δ11B of degassed, water-bearing basaltic magmas and estimating the B isotope composition of their mantle source.

Unveiling nitrate origins in semiarid aquifers: A comparative analysis of Bayesian isotope mixing models using nitrate and boron isotopes and a Positive Matrix Factorization model

Nitrate contamination of groundwater is a pressing global concern, affecting over 80 million people worldwide, with agricultural activities being the primary contributor to nitrogen inputs into aquifers. The primary objective of this study was to identify the predominant sources of nitrate pollution and biogeochemical transformations in the semiarid region of the Meoqui-Delicias aquifer, Mexico. In this region, the uncontrolled use of chemical fertilizers and manure leads to excessive nutrient input, which accelerates the deterioration of groundwater quality. This study introduces an innovative dual-model approach (δ15NNO3 vs. δ11B) to compare the Bayesian Isotope Mixing Model (BIMM) with Positive Matrix Factorization (PMF) for nuanced source apportionment. This approach enhanced the differentiation between manure and sewage as nitrate sources. Results from the δ15NNO3 vs. δ11B model revealed that manure (52.4%) was the most significant source of nitrate pollution in the aquifer, followed by soil (37.4%), chemical fertilizers (5.5%), and sewage (4.7%). The PMF model corroborated this finding and indicated that 60.2% of the NO3–-N pollution in the aquifer was attributed to manure and sewage, confirming the superior performance of the proposed BIMM in source differentiation. Our findings enhance the understanding of nitrate dynamics in semi-arid regions and can serve as a scientific evidence base for targeted interventions in nutrient management in the study area and elsewhere.

On the occurrence of kalsilite in melilite-bearing ultrapotassic lavas from the Roman Province (Vulsini Mts., central Italy)

The Montefiascone Volcanic Complex belongs to the Vulsini Mts. volcanic district, one of the several leucite-bearing ultrapotassic Pleistocene volcanoes of the Roman Province (Central Italy). It is characterized by abundant pyroclastic successions and minor lava flows emplaced during a caldera formation phase occurred ∼0.3–0.2 Ma. The volcanic products span from leucite-basanites and leucite-tephrites to rarer leucitites and melilitites, occasionally associated with very rare kalsilite melilitolite ejecta. Here we report for the first time the presence of primary kalsilite in two lava flows in Forcinella and Feltricci localities, identifying a kamafugitic component among the Montefiascone volcanics.Detailed petrographic, SEM and EPMA studies on ten samples from the two localities revealed, in addition to kalsilite, variable amounts of clinopyroxene, melilite, leucite and nepheline plus rarer olivine coupled with carbonate-bearing segregation pockets in the groundmass. Geochemical and isotopic data confirm the exotic nature of the analysed rocks, showing ultrabasic/basic (SiO2 = 42.15–45.94 wt%) and ultrapotassic (K2O = 6.17–8.48 wt%; K2O/Na2O = 5.3–8.7) compositions, as well as strongly radiogenic 87Sr/86Sr (0.71038–0.711068), poorly radiogenic 143Nd/144Nd (0.512057.512091) and uniform middle radiogenic 206Pb/204Pb (18.76–18.78). Boron isotopes (δ11B ranges from 7.36 to 8.82‰) almost overlaps the typical MORB-OIB range values.These geochemical features recall those of the nearby kamafugite rocks from the Intra-Apennine Province. Major oxide and trace element modelling support fractionation links among the Italian kamafugite types. The overall geochemical and isotopic signatures speak for a common strongly subduction-metasomatized and orthopyroxene-free lithospheric mantle source. As already proposed for some nearby kamafugite centres in central Italy, limestone assimilation during magma ascent is a viable option to explain the mineralogical and whole-rock composition of Montefiascone lavas (i.e., the presence of carbonate plagues in the lava groundmass). This is particularly relevant considering the thick sedimentary sequence pierced by the limited volume of the upwelling magma. On the other hand, many other key features (such as the general lack of crustal xenoliths, the overall incompatible element fractionation and the radiogenic Sr isotopic ratios) clearly require also derivation from subduction-modified mantle sources.

Ocean acidification in the tropical Indian Ocean over the past 37 years: Insights from [formula omitted]11B and B/Ca records in a Maldives coral

Boron isotopes (δ11B) in coral skeletons of Porites have been widely applied to reconstruct past seawater pH (pHSW) on decadal to centennial timescales. However, due to biological regulation within corals, an additional transfer function is required to estimate ambient seawater chemistry during the skeleton growth under the calcification site fluid pH. Temperature may also interfere with coral calcification fluid pH (pHCF) due to changes in kinetics of coral aragonite precipitation, or buffering capacity in coral calcification fluid. To decipher how coral Porites adjusts pHCF in response to pHSW from complex environmental controls, long-term records from sites with least fluctuations in environmental conditions other than pHSW are essential. Here we present a 37-year record of coral δ11B and B/Ca ratios derived from a coral core collected from southern Maldives, the tropical Indian Ocean. Our results show no clear seasonality in the coral δ11B and B/Ca ratios between monsoons, but a long-term decline in coral pHCF is evident across the entire record. When applying different existing transfer functions, we also observe discrepancies among the calculated pHCF values, model results and short-term instrumental data. Calculated calcification fluid dissolved inorganic carbon concentration ([DIC]CF) values are relatively low compared to literature, suggesting that coral calcification fluid carbonate chemistry may be under different levels of control, even within the same coral taxa. Thus, coral records from a wider geographic range are required to better quantify coral response to ocean acidification, and our results can serve as a baseline for future comparisons.

The Rare Earth Element Distribution in Marine Carbonates as a Potential Proxy for Seawater pH on Early Earth

Understanding the marine environment of early Earth is crucial for understanding the evolution of climate and early life. However, the master variable of Archean and Proterozoic seawater, the pH, is poorly constrained, and published ideas about the pH range encompass ~7 pH units from mildly acidic to hyperalkaline. To better infer ancient seawater pH, we examine the possibility of a seawater pH proxy using rare earth elements (REEs) in marine carbonates. The principle is based on increasing concentrations of heavy rare earth elements in solution relative to the light REEs with decreasing pH due to REE complexation and scavenging. We calibrated such an REE pH proxy using pH variability in modern seawater and tested the proxy with ~100 REE measurements from 13 separate carbonate formations. We compared our pH estimates derived from the REE proxy to published pH estimates of Cenozoic and Neoproterozoic seawater that use the established pH proxy of boron isotopes (δ11B). REE-pH estimates agree with the Cenozoic and the Ediacaran δ11B-pH proxy based on the type of carbonate and boron isotopic composition at corresponding times. The uncertainty in our REE-pH proxy can probably be explained by model assumptions, noise from freshwater influence, siliciclastic input, and diagenesis. This proof-of-concept study demonstrates that the REE-pH method provides pH estimates comparable to boron isotope pH estimates within uncertainties, which potentially could constrain changes in Precambrian seawater pH to better understand the coevolution of life and early Earth’s environment.

Cycling of fluid-mobile elements through the forearc: Insights from the Cl, B, and Li isotope composition of Costa Rican spring fluids

Loss of slab-derived volatile and fluid-mobile elements (FME) through the forearc may represent a large, unaccounted for flux out of convergent margins. To address volatile recycling through the forearc and potential changes in fluid source from the progressively dehydrating subducting slab, we collected water samples from 44 different cold and thermal (>40 °C) springs on a transect across the outer forearc (∼20–40 km to the subducting slab), forearc (∼40–80 km to the slab), and arc (∼80–120 km to the slab) in Costa Rica. We focused on the heavy halogens (Cl, Br, I), as well as B and Li, given their high fluid mobility and thus limited potential for modification by subsequent fluid-rock interaction. Chlorine, boron, and lithium stable isotope ratios show dramatic ranges from −1.7 to +1.0 ‰ (n = 43), −12.0 to +30.9 ‰ (n = 35), and −2.4 to + 27.5 ‰ (n = 38), respectively, with distinct changes across the transect consistent with a sedimentary pore fluid component in the outer forearc, fluids sourced from dehydration of the slab (sediment and altered oceanic crust) in the forearc, and minimal slab input near the arc. Variations in elemental ratios (Br/Cl, I/Cl, B/Cl, Li/B) across the transect are also largely consistent with evolving FME sources during subduction. Boron isotope ratios in some higher-temperature fluids are modified due to phase separation, whereas, lithium and chlorine isotope ratios are minimally modified. Springs located in the outer forearc along a major fault (Falla Morote) deviate from the consistent across-arc trend and record almost the full range of observed isotopic values. This extreme range in values along the fault likely represents a combination of adsorption/desorption effects, weathering, and mixing with crustal fluids. Mass balance calculations show that up to ∼ 70 % of Cl, ∼50 % of Br, ∼30 % of B, ∼15 % of I, and minimal Li may be recycled through the outer forearc and forearc springs of the Costa Rican margin, therefore fluid emission via springs in global forearc regions are a potentially significant output for FME at convergent margins.

Boron isotopes of Manganese ores from the northern part of the Kalahari Manganese Deposit, South Africa

Abstract The world-class deposits of the Kalahari Manganese Field (KMF) in South Africa constitute a key resource of manganese (Mn) ore for the global steel industry. Many aspects of the origin of high metal-grade ores in the northernmost KMF remain unresolved, especially with respect to the complex hydrothermal history of these ores and the source/s of fluids responsible for epigenetic metal enrichment. In this study, the geochemistry of boron (B) is employed as a potential proxy for such processes, and the results are contextualised against an assumed low metal-grade precursor. Samples collected from the northern part of the Kalahari Manganese Deposit allow for the distinction of three Mn ore types, of which the precursor ore is the closest candidate, petrographically and geochemically, to primary/diagenetic Mn deposition. This precursor ore is dominated by Mn-carbonate and braunite, and records B concentrations in the order of a few 100s to 1 000s ppm and positive δ11B values at an average of 11 ± 6‰. Ferruginous Mn ores appear to have formed from decarbonation of precursor ore and metasomatic ferric iron addition, particularly in the stratigraphically upper Mn ore bed, which is closest to the overlying Olifantshoek and Kalahari unconformities. Ferruginous Mn ores appear depleted in B (ca. 500 ppm on average) and have lower boron isotope values (δ11B: 0 ± 3‰). Hydrothermally Mn-enriched ores, in contrast, are dominated by hausmannite, braunite-II and bixbyite and record highest B concentrations (up to 10 000s ppm) and δ11B values as high as 41‰, pointing to metasomatic introduction of B by circulating evaporitic brines. While further research is needed to comprehensively unravel the geochemical intricacies of B within the complex interplay of fluids and rocks in the Hotazel region, the findings of this study still strongly suggest that B holds promise as a tracer for alteration processes in the KMF.