Ba Isotopic Compositions Research Articles

Constraints of barium isotopes on recycling of ancient oceanic crust in the mantle of the South China Sea

As surface reservoirs (e.g. sediments and altered oceanic crust) have extremely high barium (Ba) concentrations and distinctive Ba isotope compositions compared to the mantle, Ba isotopes have the potential to trace crustal recycling in the mantle. The sediment components and fluids derived from altered oceanic crust in arc lavas and continental basalts have been identified using Ba isotopes, while the geochemical behavior of Ba in subducted oceanic crust remains enigmatic. A series of volcanic breccia that temporally evolved from carbonated silicate clasts to alkali basalts in the South China Sea (SCS) drilled by the Integrated Ocean Discovery Program (IODP) Site U1431 was suggested to originate from the mantle source contaminated by recycled oceanic crust. To understand the potential of using Ba stable isotopes to trace recycled oceanic crust in the mantle, we report high-precision Ba isotope compositions of these samples, together with alkali basalts from adjacent Hainan Island to constrain the Ba isotope characteristics of the Hainan mantle plume. The carbonated silicate clasts and alkali basalts from the SCS display extremely high Ba concentrations and are thus insensitive to magma evolution during ascent through the thin lithospheric mantle and proper to trace crustal materials in their mantle source using Ba isotopes. These samples have variable Ba isotope compositions ranging from δ138/134Ba = −0.10 to +0.05‰, implying local mantle isotopic heterogeneity underlying the SCS. The Ba isotope characteristics of some samples, which are much lighter than those of the mantle-derived carbonatites and global mid-ocean ridge basalts (MORBs), in conjunction with trace element ratios and radiogenic isotope compositions, reflect significant incorporation of recycled carbonated oceanic crust after subduction-related dehydration. The rising of the Hainan mantle plume offers the most reasonable interpretation of the entrainment of the recycled oceanic crust in the carbonated silicate magmatism beneath the SCS. Alkali basalts from Hainan Island display Ba isotope characteristics slightly heavier than the SCS carbonated silicate clasts and alkali basalts. It implies that the Hainan mantle plume underlying Hainan Island contains less recycled-oceanic-crust components than that underneath the SCS. This study highlights Ba isotopes as effective tracers of recycled oceanic crust in the mantle.

Barium stable isotopic composition of chondrites and its implication for the Earth

Barium is a refractory and fluid mobile element that is highly incompatible in basaltic systems. The elemental and isotopic compositions of Ba are used to trace fluid processes or sediment recycling during subduction. However, the Ba isotopic composition of primitive meteorites (chondrites), which represents the possible building blocks of terrestrial planets, is presently poorly known. Here, we report high precision Ba isotopic compositions of major types of chondrites using a double-spike method. Chondritic meteorites without clear terrestrial weathering show relatively homogeneous Ba isotopic distributions and have an average δ137/134Ba (the permil deviation of the 137Ba/134Ba ratio from the SRM3104a standard) value of 0.09 ± 0.09‰ (2SD, N = 14). Given the refractory and lithophile behavior of Ba and the limited variations of Ba isotope composition between various chondrite groups, this value can represent the Ba isotopic composition of the bulk Earth and the Bulk Silicate Earth (BSE). It is actually consistent with the present estimate of BSE (0.05 ± 0.06‰, Li et al., 2020; Nan et al., 2022) based on the measurements of mid-ocean ridge basalts and carbonatites. Small Ba isotope variations are observed between carbonaceous chondrite groups where CO and CV show slightly heavier (by 0.09‰) Ba isotopic compositions than CI and CM. Significant light stable Ba isotopes enrichment has previously been documented in calcium, aluminum-rich inclusions (CAIs) in the Allende CV chondrite, and the Ba isotopic differences between CO and CV might result from the various proportions of CAIs based on a mass balance calculation. However, the parent body of CI and CM might have experienced aqueous alteration and secondary carbonates are most likely responsible for their light Ba isotopic signatures.

The influence of black shale weathering on riverine barium isotopes

Barium and its isotopes are receiving growing interest in their capacity to record the variation of the oceanic biological pump through geological timescales. Nonetheless, only little is known about the continental sources and processes that can drive the Ba flux and isotope composition to the ocean. Whereas the role of processes such as secondary phase formation, sorption and biological uptake on Ba isotopes has been recently investigated, the potential of various rock sources in producing a range of Ba isotope compositions remains poorly known. In particular, black shales often exhibit enrichment in Ba that might potentially be associated with various Ba isotope signatures.In this study, we report the isotope composition of Ba in rivers with different relative contribution of black shale rocks in their catchments, located in the Mackenzie Basin (Northwest Canada). Both the dissolved and solid loads of the Mackenzie tributaries are enriched in heavy Ba isotopes with respect to the continental crust, which is consistent with an additional source to the weathering of igneous and siliciclastic rocks. Although the dissolved Ba abundance is partially driven by weathering processes, the river dissolved Ba isotope composition rather reflects a binary mixing between 1) a “classical” siliciclastic source (with a Ba isotope composition close to that of the upper continental crust δ138Ba ∼ 0.15 ± 0.05‰) and 2) an isotopically heavier source especially present in the Mackenzie Mountains (with a δ138Ba ∼ 0.40‰). The positive relationships between dissolved δ138Ba and radiogenic osmium isotope ratios, as well as with sulphuric acid production, indicate that the heavy Ba source is tightly linked to the weathering of black shale. Although the exact reason for such heavy Ba isotope signatures remains elusive, this study emphasises the potential of black shale weathering in shifting the Ba isotope composition of the dissolved flux to the ocean towards heavier values.

Multi-colony calibration of barium isotopes between shallow-water coral skeletons and in-situ seawater: Implications for paleo proxies

Barium incorporated in coral skeletons has been widely used as paleo proxies to study terrestrial inputs, upwelling and anthropogenic activities in marine environments. However, these applications often face the challenges of poor replication in the coral skeletal Ba/Ca records from multiple coral colonies and complex Ba sources in coastal environments. Recent studies of Ba isotopes in seawater and deep-sea corals have demonstrated the potential to trace Ba sources and water mixing in the ocean, but there is still a lack of calibrations for Ba isotopes in shallow-water corals. In this study, we present the first multi-colony Ba isotope calibration from three shallow-water Porites lutea corals with the contemporaneous in-situ seawater data from the Singapore Strait. We also report the Ba isotope data in the regional water masses around the study area (e.g. South China Sea, Malacca Strait and Johor River estuary). Singapore water Ba concentrations and isotope compositions show a strong influence of terrestrial inputs, following seasonal salinity and monsoon-driven water-mass mixing between the Malacca Strait and the South China Sea. The coral skeletal δ138/134Ba results are generally consistent between the three coral colonies and time-series data closely follow the seasonal δ138/134Ba variations in seawater. Despite the partition coefficient of Ba having a large uncertainty (DBa=0.91±0.29), the Ba isotope fractionation between the coral skeletons and seawater is relatively constant (Δ138/134Bacoral-sw=−0.28±0.06‰) and shows no significant difference between these corals. The mechanism controlling Ba incorporation and isotope fractionation in coral skeletons remains unclear, but the evidence clearly indicates that the coral skeletal Ba is originated from dissolved Ba in seawater. The constant offset of Ba isotopes between coral skeletons and seawater allows for reliable records of seawater δ138/134Ba values. Ba isotopes in coral skeletons could be used to reconstruct surface water salinity variability in the Singapore Strait, reflecting monsoon driven changes in regional water mass mixing.

Re-visiting barium isotope compositions of mid-ocean ridge basalts and the implications

Barium (Ba) isotopes can be used as potential tracers for crustal material recycling in the mantle. Determination of the Ba isotope composition of the depleted mantle is essential for such applications. However, Ba isotope data for mantle-derived basalts are still rare. In this study, we reported high-precision Ba isotope data of 30 oceanic basalts including 25 mid-ocean ridge basalts (MORBs) from geochemically and geologically diverse mid-ocean ridge segments and five back-arc basin basalts. The δ<sup>138/134</sup>Ba values of these samples varied from −0.06‰ to +0.11‰, with no systematic cross-region variation. Together with published data, we constrained the average δ<sup>138/134</sup>Ba of global MORBs to +0.05‰±0.09‰ (2 standard deviation, <i>n</i> = 51). Based on depleted MORBs that have (La/Sm)<sub>N</sub> < 0.8, low <sup>87</sup>Sr/<sup>86</sup>Sr (< 0.70263), and low Ba/Th < 71.3, we estimated the average δ<sup>138/134</sup>Ba of the depleted MORB mantle (DMM) as + 0.05‰ ± 0.05‰ (2SD, <i>n</i> = 16) that is significantly lower than the DMM (≈ 0.14‰) reported previously. If a new estimation of the DMM is applied, it is unreasonable to infer that the Ba isotope signatures of the “enriched-type” MORBs (E-MORBs) could be attributed to pervasive sediment recycling in the upper mantle. We, therefore, conclude that the Ba isotope compositions of the E-MORBs could be sourced from the incorporation of subducted altered oceanic crust and/or sediments depending on the Ba isotope composition and other geochemical information of the local mantle.

Behavior of barium isotopes during high-pressure metamorphism and fluid evolution

Barium (Ba) and its stable isotopes are powerful geochemical tracers for the recycling of subducted materials into the convective mantle. However, this requires a good understanding of the behavior of Ba isotopes during subduction-zone fluid processes. Here we present the first integrated study on Ba isotopes for two eclogite-vein systems (Ganghe and Hualiangting) that formed at high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic conditions from the Dabie orogen, central China. The Ganghe system displays constant δ138/134Ba (−0.01‰ to 0.03‰) in the UHP eclogites but relatively high δ138/134Ba (0.17‰) in the HP vein. In contrast, in the Hualiangting system, δ138/134Ba of the UHP eclogites vary from −0.14‰ to 0.05‰, while δ138/134Ba of the multiple HP veins and the retrograded amphibolites range from −0.17‰ to 0.34‰ and from −0.11‰ to 0.46‰, respectively.The combined results suggest that Ba isotopes can be considerably fractionated during subduction-zone fluid processes. HP-UHP metamorphic dehydration of the eclogites during the early stages of exhumation can produce an isotopically heavy initial vein-forming fluid due to the preferential leaching of epidote enriched with heavy Ba isotopes from the host eclogites. However, such processes have limited effects on the bulk eclogites because the Ba-rich phengite exists as a residual phase. During vein formation, light Ba isotopes are preferentially enriched in minerals relative to the fluids, resulting in the residual fluids with high δ138/134Ba. In addition, the fluids released from the gneisses have negative δ138/134Ba which is different from the eclogites, indicating that Ba isotopes can be used to trace the source of the metasomatic fluids. This study demonstrates that the UHP metamorphic rocks and HP veins can record Ba isotope fractionation during subduction-zone metamorphism and fluid-rock interaction, and recycling of the metamorphic rocks could modify Ba isotopic composition of the mantle wedge and arc volcanics.

A single column separation method for barium isotope analysis of geologic and hydrologic materials with complex matrices

The increasing significance of barium (Ba) in environmental and geologic research in recent years has led to interest in the application of the Ba isotopic composition as a tracer for natural materials with complex matrices. Most Ba isotope measurement techniques require separation of Ba from the rest of sample prior to analysis. This paper presents a method using readily available materials and disposable columns that effectively separates Ba from a range of geologic and hydrologic materials, including carbonate minerals, silicate rocks, barite, river water, and fluids with high total dissolved solids and organic content such as oil and gas brines, rapidly and without need for an additional cleanup column. The technique involves off-the-shelf columns and cation exchange resin and a two-reagent elution that uses 2.5 N HCl followed by addition of 2.0 N HNO3. We present data to show that major matrix elements from almost any natural material are separated from Ba in a single column pass, and that the method also effectively reduces or eliminates isobaric interferences from lanthanum and cerium.

Barium isotope evidence for the role of magmatic fluids in the origin of Himalayan leucogranites

As an important post-collisional magmatic product in the orogenic belt, the Himalayan leucogranites are the critical host rocks for a number of rare-metal mineralization (such as Li, Be, Cs, Rb, Nb, Ta, and Sn). However, there is still a lack of good understanding on the formation and evolution of the leucogranites. Particularly, the role of the magmatic fluids in transporting and enriching the rare elements is not clear. Here we measure Ba isotope compositions for leucogranites from the Kampa Dome of the Himalayan belt to understand the fluid activity and behavior of fluid-mobile elements during leucogranite formation. Our results show that the δ138/134Ba of leucogranites range from −1.32‰ to +0.12‰, much lower than the literature values for S-type granites and various sedimentary materials, suggesting that the Ba isotope compositions of the leucogranites does not reflect the sedimentary source signatures. Instead, their low δ138/134Ba is accompanied by non-charge-and-radius-controlled (CHARAC) twin-element (such as Nb/Ta) behaviors, clearly showing the involvement of magmatic fluids during magma evolution. Experimental studies suggest that the low δ138/134Ba of the magmatic fluids most likely results from exsolution from a large deep magma reservoir. Such fluids not only modified Ba isotope compositions of the leucogranites, but also transported many fluid-mobile metal elements which may help form the rare metal ore deposits. Therefore, Ba isotope data provide new insights into formation and evolution of magmatic fluids and exploration of the rare-metal mineralization.

Barium isotope evidence for recycled crustal materials in the mantle source of continental basalts

To investigate the potential of using Barium (Ba) isotopes to trace recycled crustal materials, we report high-precision Ba isotopic compositions for 22 well-characterized samples of Cenozoic basalts from the Taihang Mountains, North China Craton. The basalts have δ138/134Ba [= (138Ba/134Basample/138Ba/134BaSRM3104a – 1) × 1000] of −0.06‰ to +0.03‰(±0.05‰，2SD), lower than the depleted upper mantle (+0.03‰ to +0.15‰). Because Ba isotope fractionation during basaltic magmatic differentiation is negligible, the light Ba isotopic compositions likely reflect the involvement of isotopically light crustal materials in the mantle source. The Ba isotopic variations, combined with the data of elements and SrNd isotopes of the basalts, indicate that the light Ba isotopic compositions mainly resulted from marine sediments rather than altered oceanic crust. The quantitative modeling using Sr-Nd-Ba isotopic compositions suggests that about 0.5% to 3% of marine sediments were involved in generating the continental basalts. Our study here highlights the potential of using Ba isotopes to trace recycled materials in the mantle.

Dramatic changes in the carbonate-hosted barium isotopic compositions in the Ediacaran Yangtze Platform

Barium (Ba) isotopic compositions (δ138Ba) have been advocated as a novel paleo-productivity proxy, because of the strong control of biological productivity on the distribution of the Ba isotopic composition in the modern oxic oceans. However, the ocean was dominantly anoxic throughout the majority of the Earth’s history, and the biogeochemical cycle of Ba in anoxic oceans may have been different from the case in oxic ones. It is not clear whether δ138Ba can be used to trace biological productivity in ancient oceans. The Ediacaran Period was marked by a dramatic evolution in the climate, atmospheric and oceanic oxygen level, marine carbon cycle, and biosphere, which have been documented in multiple successions of the Ediacaran sedimentary rocks. This provides an opportunity to investigate variations in biogeochemical Ba cycle during the crucial period. In this study, we find large δ138Ba variations in carbonate rocks from the Ediacaran Doushantuo and Dengying Formations in the Yangtze Gorges area, South China. We suggest that detrital contamination and potentially post-depositional alteration cannot explain the δ138Ba variations. The post-Marinoan carbonate rocks record generally negative carbonate-hosted δ138Ba (δ138Bacarb) around −0.3‰, reflecting the existence of a replete and homogeneous reservoir of light Ba isotopes in the postglacial ocean. Although the Shuram/Wonoka correlative carbonate rocks have documented a prominent negative δ13Ccarb excursion, the δ138Bacarb merely shows minor fluctuations between 0.00‰ and 0.35‰, possibly due to local anoxia/euxinia inhibiting the biogeochemical cycle of Ba. The late-Ediacaran carbonate rocks show a gradual increase in δ138Bacarb systematically with enhanced negative cerium anomalies, suggesting the establishment of the modern-ocean-like cycle of Ba in shallow waters of the Yangtze Platform, which may result from the replacement of cyanobacteria by eukaryotic macroalgae as the dominant primary producer. We suggest that the biogeochemical Ba cycle in ancient marine system is strongly controlled by local redox conditions, and thus Ba isotopic compositions could be utilized to trace variations in paleo-productivity, especially in oxic environments.

Barium stable isotopes as a fingerprint of biological cycling in the Amazon River basin

Abstract. The biological cycle of rock-derived nutrients on the continents is a major component of element transfer between the Earth's surface compartments, but its magnitude currently remains elusive. The use of the stable isotope composition of rock-derived nutrients, which can be fractionated during biological uptake, provides a promising path forward with respect to quantifying biological cycling and its overall contribution to global element cycling. In this paper, we rely on the nutrient-like behaviour of the trace element barium (Ba) and use its elemental and stable isotope compositions in dissolved and sediment load river samples to investigate biological cycling in the Amazon Basin. From these measurements, we show that dissolved Ba mainly derives from silicate rocks, and a correlation between dissolved Ba and K abundances suggests that biological cycling plays a role in the Ba river budget. Furthermore, the isotope composition of Ba (δ138Ba) in the dissolved load was found to be significantly different from that of the parent silicate rocks, implying that dissolved Ba isotopic signatures are affected by (i) the precipitation of soil-forming secondary phases as well as (ii) biological uptake and release from dead organic matter. Results from an isotope mass balance method applied to the river dissolved load data indicate that, after its release to solution by rock weathering, Ba is partitioned between the river dissolved load, secondary weathering products (such as those found in soils and river sediments), and the biota. In most sub-catchments of the Amazon, river Ba abundances and isotope compositions are significantly affected by biological cycling. Relationships between estimates of Ba cycled through biota and independent metrics of ecosystem dynamics (such as gross primary production and terrestrial ecosystem respiration) allow us to discuss the role of environmental parameters such as climate or erosion rates on the biological cycling of Ba and, by extension, the role of major rock-derived nutrients. In addition, catchment-scale mass and isotope budgets of Ba show that the measured riverine export of Ba is lower than the estimated delivery of Ba to the Earth surface through rock alteration. This indicates the existence of a missing Ba component, which we attribute to the formation of Ba-bearing particulate organics (possibly accumulating as soil organic matter or currently growing biomass within the catchments) and to organic-bound Ba exported as “unsampled” river particulate organic matter. Given our findings on the trace element Ba, we explore whether the river fluxes of most major rock-derived nutrients (K, Mg, Ca) might also be significantly affected by biological uptake or release. A first-order correction of river-derived silicate weathering fluxes from biological cycling shows that the carbon dioxide (CO2) consumption by silicate weathering at the mouth of the Amazon could be several times higher than the previously reported value of 13 × 109 mol CO2 yr−1 (Gaillardet et al., 1997). Overall, our study clearly shows that the chemical and isotope compositions of rivers in the Amazon – and most likely in other large river basins – bear a biological imprint, thereby challenging common assumptions made in weathering studies.

Barium isotope evidence for crystal-melt separation in granitic magma reservoirs

Crystal-melt separation is important for understanding the evolution of granitic magma and the connection between plutonic and volcanic rocks. Crystal accumulation and melt extraction are the two key factors in the separation process. However, the existing geochemical indicators for crystal-melt separation in many granitic plutons are largely concealed by the remaining interstitial melt. Here, we report Ba isotope compositions of well-characterized Huili granitic pluton and associated mineral separates from the Jiaobei Terrane in the North China Craton. The Ba isotopes are significantly fractionated between the two types of granites in the Huili pluton. The more differentiated albite granites (AG) have heavier isotope compositions (δ138/134BaAG = 0.50–0.95‰) than the K-feldspar granites (KG) (δ138/134BaKG = −0.14 to 0.16‰). The δ138/134Ba of the whole-rocks change in the same manner as multiple element contents (such as K, Ba, and Sr). Trace element modeling suggests that such variations can be explained by K-feldspar-controlled crystal-melt separation in a K-rich granitic magma.As the K-feldspar granites represent the residual crystal mush, the Ba isotope data of the coexisting Ba-bearing minerals can impose critical constraints on crystal-melt separation. The K-feldspar shows the highest Ba content (387–1465 μg/g) and the lowest δ138/134Ba (−0.23 to 0.01‰) among all the investigated minerals, suggesting that its crystallization should drive the interstitial Na- and Si-rich melt towards an isotopically heavy composition. The apparent fractionation values of coexisting biotite and muscovite (Δ138/134BaBiotite-Muscovite) vary from −0.98 to 1.01‰, indicating that the coexisting minerals are not in Ba isotope equilibrium. Combined with microstructural features, the Ba isotope disequilibrium signatures of coexisting minerals most likely reflect crystal accumulation and repacking during crystal-melt separation, providing a novel geochemical indicator for identifying the granitic plutons that represent residual crystal mush in the mid- to upper-crust. This study indicates that Ba isotopes are useful for deciphering petrogenetic links between intrusive and extrusive rocks.

Constraining barium isotope fractionation in the upper water column of the South China Sea

Enabled by the success in the determination of stable barium (Ba) isotopic compositions in seawater, Ba isotopes have been suggested as a novel tool to study physical and biogeochemical processes in the present and past ocean. However, a better understanding of the fractionation of Ba isotopes during particle-seawater interactions is a prerequisite for such applications. In this study, we use an extensive data set of concentrations and isotopic compositions of dissolved Ba (DBa and δ138BaDBa) and bulk particulate Ba (pBabulk and δ138BapBabulk) collected in the northern South China Sea (NSCS) to constrain Ba isotope fractionation in the upper ocean. Seawater and suspended particle samples for Ba isotope measurements were collected in January 2010 along a transect from the outer shelf to the lower slope. The water column profiles reaching depths of 1000 m are characterized by a general decrease of δ138BaDBa and an increase of DBa with depth. However, δ138BaDBa signatures are essentially constant at +0.6‰ in the upper 150 m of the entire study area. The corresponding δ138BapBabulk, which primarily represents the isotopic compositions of oceanic or excess particulate Ba (pBaxs), is consistently lower than δ138BaDBa but also constant at values of +0.1 to +0.2‰. This suggests that the same Ba isotope fractionation process prevails above 150 m on the NSCS outer shelf and slope resulting in a constant in situ fractionation factor of −0.5‰. This value is consistent with previously reported values of −0.4 to −0.5‰ in the upper 200 m of the open ocean and a lake. Moreover, we observe significant differences of pBaxs distributions from those of particulate calcium, particulate organic carbon and nitrogen, and biogenic silica indicating that passive adsorption onto particles rather than active biological utilization is most likely the primary process inducing Ba isotope fractionation in the upper NSCS. The constant δ138BapBabulk signatures suggest that particulate Ba isotopes integrate reliable information during transformation of DBa to pBaxs and are thus a more robust proxy for total particle fluxes than pBaxs concentrations, which show variable values potentially affected by other processes (e.g., particle sinking and/or zooplankton grazing) and thus reflects “snapshot” processes in the water column. We contend that biological productivity plays only a subordinate role in regulating the surface Ba isotopic composition of bulk suspended particles. The extent to which Ba isotopes may nevertheless be a reliable proxy for present and past export productivity requires further analyses of the δ138Ba signature of specific Ba carriers such as barite throughout the water column and in the sediments.

Leaching Methods for Ba Isotope Studies of Carbonates.

Barium (Ba) stable isotopes in carbonate rock have great potential to provide valuable information on environmental change and the biogeochemical cycles of oceans in the past. Ba in carbonate rock can exist in various phases, such as adsorbable and silicate-bound Ba. However, only the carbonate-bound phase is considered to record the Ba isotopic compositions of ambient seaweater. Here, we designed a two-step leaching experiment to obtain the carbonate-bound Ba in two typical carbonate rocks: limestone and cap dolostone. The results showed that after leaching by 1 mol L-1 ammonium acetate, the carbonate-bound Ba extracted by mixed solution of 1.5 mol L-1 acetic acid and 1 mol L-1 ammonium acetate in each studied sample have indistinguishable isotope ratios in leaching time conditions between 12 and 72 h. More importantly, the carbonate-bound Ba isotope ratios were quite different from those of the residue (up to 10 times of analytic uncertainty, 2SD ≤ ±0.04‰) after leaching in three out of four leaching experiments, indicating that noncarbonated fraction could overprint a primary seawater signal. Our sequential leaching techniques could improve targeting of carbonate-bound Ba isotope signatures in various carbonate rocks to trace the Ba cycling in the oceans.

Barium isotope systematics of subduction zones

Subduction zones are the focal points of mass transfer between the surface and deep Earth. Despite their significance, there remains substantial debate regarding the specific mechanisms of material transport from the slab to the overlying magmatic arc. Broadly, models accounting for slab material transport focus on the relative sequence of events promoting arc volcanism and, in particular, whether mobilization of the down-going slab leads or lags mixing with the mantle wedge. To address these uncertainties, we outline the utility of barium (Ba) isotope mass balance in subduction zones as a means to test different slab material transport models. Barium is a highly fluid-mobile element that is significantly enriched in arc magmas and is thus thought to be a sensitive tracer of slab material transport in arcs.We also present qualitative Ba isotopic mass balances for two well-characterized subduction zones—the Aleutian and Ryukyu magmatic arcs—by analyzing the Ba isotope systematics of their respective subduction inputs and outputs. Despite the narrow (and similar) Ba-isotope range of slab inputs to both systems, we find that erupted magmas exhibit systematic variations indicative of a small negative isotope fractionation during Ba mobilization (≈20–40 ppm AMU−1). We suggest that AOC (altered oceanic crust) is not the principal source of these negative isotope values using other geochemical parameters (e.g., Rb/Ba, Pb isotopes), and infer that the Ba isotope composition of AOC—though contributing a minor amount of Ba in these systems—is isotopically heavier than the overlying sediment package and the depleted mantle. Altogether, these findings are significant as they indicate that the magnitude of isotope fractionation associated with Ba mobilization is small relative to the likely isotopic contrast between subduction inputs in other subduction zones, such as beneath areas of strong ocean upwelling (e.g., South Sandwich, Kamchatka). Thus, we propose that the Ba isotope composition of erupted arc magmas holds great promise for constraining the importance of different slab components, which could help address uncertainties regarding the mechanism of slab material transport in subduction zones.

Rapid determination of Ba isotope compositions for barites using a H2O-extraction method and MC-ICP-MS

This study presents a rapid and simple method of high precision Ba isotope measurement for barite using H2O extraction.

Accurate Determination of Barium Isotopic Compositions in Sequentially Leached Phases from Carbonates by Double Spike-Thermal Ionization Mass Spectrometry (DS-TIMS).

Recent studies have proposed barium isotopes as a novel proxy for studying primary productivity in paleo-oceangraphical studies and elements cycling through the critical zone. Pristine marine carbonates are generally assumed to preserve Ba isotope compositions of ancient seawater. However, Ba incorporated in or adsorbed on detrital minerals such as clays in impure carbonates may limit the accurate application of the Ba isotope proxy for paleo-ocean environmental reconstruction purposes. We present here a sequential extraction procedure and show that a considerable range of Ba concentrations can be associated with the four operationally defined sequential leaching fractions (water-soluble, exchangeable, carbonate, and oxidizable fractions). Chemical separation of Ba from these leachates is achieved with a recovery of >98.6% by our modified ion exchange procedure. Potential instrumental mass bias effects and barium isotope fractionation during the chemical separation are corrected using a carefully optimized 130Ba-135Ba double-spike method. A long-term reproducibility better than ±0.03‰ (2SD) for δ137/134Ba has been achieved by using the double spike-thermal ionization mass spectrometry (DS-TIMS) in this study. We demonstrate that significant variations of δ137/134Ba in the analyzed leachates suggest a considerable Ba isotope fractionation between carbonate mineral phase and noncarbonate phases of marine carbonate rocks. The barium isotope distribution in a set of standard reference materials and natural geological samples under various geological settings has been presented. When utilizing Ba isotopes as a proxy for primary productivity and the biogeochemical cycling of Ba, our new findings from sequential Ba extraction as well as our modified precise DS-TIMS analytical setup should be taken into account.

Seasonal riverine barium isotopic variation in the middle Yellow River: Sources and fractionation

Barium (Ba) isotopes have been used to trace water mass mixing and export productivity in the oceans. However, the sources, isotopic signature and seasonal variation of dissolved Ba in large rivers remain poorly constrained. In order to improve our understanding of sources and fractionation of riverine Ba isotopes at the continental scale, weekly sampling of river water was carried out in the middle Yellow River over the full hydrological year of 2013. Dissolved Ba was mainly sourced from silicate dissolution, whose flux was correlated with physical erosion rate in this arid to semi-arid basin, largely covered by loess. More than half of the annual dissolved Ba flux was transported during the monsoon season (June to mid-September), in particular during a storm event period (accounting for 4% of the annual dissolved Ba flux in 4 days). The dissolved Ba isotopic composition (δ138Barw) ranged from +0.17‰ to +0.46‰, all higher than δ138Ba of loess (0.00 ± 0.04‰). The seasonal variation in δ138Barw is best explained via an adsorption model of light Ba isotopes onto solids in the river, which is sensitive to erosion of loess, in particular during the storm event and the spring-time ice melting intervals, with a potential role of barite precipitation in groundwater during the dry seasons. With one to three orders of magnitude higher particulate matter fluxes during the monsoon seasons in rivers such as the Yellow River, desorption of Ba could result an unaccounted strong variation of Ba concentration and isotopic composition in coastal environments, which further study requires.

Barium Isotopic Compositions in Thirty‐Four Geological Reference Materials Analysed by MC‐ICP‐MS

Measurement of Ba isotope ratios of widely available reference materials is required for interlaboratory comparison of data. Here, we present new Ba isotope data for thirty‐four geological reference materials, including silicates, carbonates, river/marine sediments and soils. These reference materials (RMs) cover a wide range of compositions, with Ba mass fractions ranging from 6.4 to 1900 µg g−1, SiO2 from 0.62% to 90.36% m/m and MgO from 0.08% to 41.03% m/m. Accuracy and precision of our data were assessed by the analyses of duplicate samples and USGS rock RMs. Barium isotopic compositions for all RMs were in agreement with each other within uncertainty. The variation of δ138/134Ba in these RMs was up to 0.7‰. The shale reference sample, affected by a high degree of chemical weathering, had the highest δ138/134Ba (0.37 ± 0.03‰), while the stream sediment obtained from a tributary draining carbonate rocks was characterised by the lowest δ138/134Ba (−0.30 ± 0.05‰). Geochemical RMs play a fundamental role in the high‐precision and accurate determination of Ba isotopic compositions for natural samples with similar matrices. Analyses of these RMs could provide universal comparability for Ba isotope data and enable assessment of accuracy for interlaboratory data.

Experimental and theoretical modelling of kinetic and equilibrium Ba isotope fractionation during calcite and aragonite precipitation

Barium isotope fractionation during calcite and aragonite inorganic precipitation was studied in mixed flow reactors as a function of precipitation rate at 25 °C and pH = 6.3 ± 0.1. The measured Ba isotope fractionation that occurs between calcite and the forming fluid in the investigated range of calcite growth rates (10-8.0 ≤ Rp(calcite) ≤ 10−7.3 mol/m2/s) is insignificant. Barium isotope fractionation between aragonite and the fluid decreases with increasing precipitation rate from Δ137/134Baaragonite-fluid = +0.25 ± 0.06‰ for Rp(aragonite) ≤ 10−8.7 mol/m2/s to −0.10 ± 0.08‰ for Rp(aragonite) = 10−7.6 mol/m2/s, thus reflecting preferential incorporation of either heavy or light Ba isotopes in aragonite at slow and fast growth rates, respectively. The dependence of Ba isotope fractionation on aragonite growth rate is well described by the surface reaction kinetic model developed by DePaolo (2011) when the values +0.27‰ and −2.0 ± 0.2‰ are used for the equilibrium and kinetic Ba isotope fractionation factor, respectively. The enrichment of aragonite in the heavier Ba isotopes is consistent with the equilibrium fractionation factor of +0.34‰, calculated here between Ba-substituted aragonite and Ba2+ (aq), from first-principles calculations. This positive fractionation is related to a shorter average BaO bond length in the aragonite structure while the coordination number does not change much (i.e. 9). The lack of isotope fractionation between the Ba aquo ions and the 6-coordinated Ba in calcite likely suggests that the coordination reduction required for the incorporation in the lattice of Ba adsorbed at calcite growing sites proceeds without isotope fractionation with the fluid. Otherwise the precipitated calcite should have been enriched in heavy isotopes by ∼0.17‰, as predicted by first-principles calculations. These results are the first experimental measurements of Ba isotope fractionation during inorganic calcite and aragonite mineral formation and set the basis for understanding the mechanisms controlling Ba isotope composition in CaCO3 minerals that is an essential perquisite for application of this isotopic system in natural samples.