A Review of Advances in the Study of Non-Traditional Antimony Isotopes in Earth Science

BRIEF REPORTAs human activities intensify, energy demand surges, and global environmental changes grow more pronounced, non-traditional stable isotope tracing has emerged as a cutting-edge interdisciplinary research direction[1]. Studies have shown that antimony isotopes offer advantages including mass differences, moderate fractionation magnitudes, a simple isotopic system, and high sensitivity to redox conditions. They exhibit stronger targeting and compatibility in multi-scenario applications, making them a key tracer for elucidating material cycling across Earth’s spheres and anthropogenic disturbances. However, current analytical methods suffer from poor applicability to low-antimony-content samples, low recovery rates, limited capacity for complex matrix processing, and an incomplete tracing application system—leaving their full potential unexploited.　　Since the breakthrough in multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS) at the start of the 21st century, researchers worldwide have achieved major advances in the analytical accuracy and practical applications of antimony isotopes[2]. Nevertheless, existing research still faces critical challenges: the absence of an international standardized system and unified reference materials, leading to poor interlaboratory data comparability; unclear fractionation mechanisms in complex environmental media and incomplete quantitative models for multi-process coupling; and insufficient research on multi-isotope synergistic tracing, with coupled fractionation patterns yet to be systematically established.　　1. Analytical methods for antimony isotope detection　　Antimony isotope analysis centers on the synergistic optimization of separation/purification and mass spectrometric detection techniques. Separation methods are matrix-dependent: the thiol cotton fiber (TCF) method is suitable for simple matrices (e.g., seawater) with an 82% recovery rate[4]. Ion exchange chromatography, employing AG50-X8 and Amberlite resins in combination, achieves high recovery rates of >94% for complex samples such as ancient glass[40]. The thiol silica gel and thiol-cellulose powder (TCP) methods enable efficient enrichment for medium-concentration matrices (e.g., mine water)[3,30,69]. MC-ICP-MS is the primary mass spectrometric technique, with analytical precision enhanced by coupling hydride generation (HG) systems or metal doping (Sn/In)[23,71-72]. However, the analysis of samples with low Sb contents is constrained. Matrix effects and cumbersome pretreatment remain key challenges[70].　　2. Antimony isotope fractionation mechanisms　　Antimony isotope fractionation is primarily controlled by redox, adsorption, biological, and evaporation processes, and its direction is closely related to valence state, bonding environment, and kinetic factors. The coupling of multiple processes in nature provides a theoretical basis for interpreting antimony migration and transformation.　　Redox processes: In redox reactions, Sb3+ is enriched in light isotopes, whereas Sb5+ is enriched in heavy isotopes. Previous experimental results demonstrate that when Sb5+ is reduced by sulfide, the reaction rate significantly affects the magnitude of fractionation[60]. During stibnite oxidation, ε123Sb varies by up to 2.5‰, confirming that valence state transitions induce fractionation[43].　　Adsorption: Mineral type is the critical factor controlling adsorption fractionation; goethite exhibits the strongest adsorption fractionation for Sb5+, followed by ferrihydrite[78]. Light isotopes are preferentially adsorbed onto iron (oxyhydr)oxide surfaces, leaving the solution enriched in heavy isotopes. This adsorption involves Fe–O–H participation that alters the bonding environment, with Sb–Fe bond lengths being longer than Sb–O bonds[78,87-88]. Simultaneously, this process is jointly controlled by multiple factors including mineral surface redox state, solution pH, and coexisting ions.　　Biological processes: When anaerobic bacteria methylate antimony to produce (CH3)3Sb, heavy isotopes are enriched[81]. When Pseudomonas sp. J1 oxidizes Sb3+, the extracellular oxidation pathway dominates the fractionation, involving bond cleavage and kinetic processes[76]. Simultaneously, plant roots selectively absorb light isotopes, leading to the enrichment of heavy isotopes in the surface soil layer[35].　　Evaporation: Evaporation causes the residual liquid phase to be enriched in heavy isotopes, whereas light isotopes—due to their lower mass and faster diffusion—preferentially enter the vapor phase[94]. The degree of fractionation is positively correlated with HCl concentration, temperature, and the number of evaporation cycles, but the fractionation factor remains unaffected, consistent with the Rayleigh fractionation model[82,94]. The enrichment characteristics of 123Sb in the vapor phase of smelting products confirm the existence of natural evaporation fractionation, providing a tracer for tracking antimony migration[22].　　3. Applications of antimony isotopes in Earth science　　3.1 Environmental pollution source tracing　　Antimony isotopes can be used to effectively distinguish between natural and anthropogenic sources, enabling precise tracing of pollution from mining and smelting activities. For example, studies of antimony mines in Japan have identified anthropogenic inputs through differences in isotopic composition[22]; the Gardon River in France has quantified tributary pollution contributions through isotopic gradient variations[3]. During antimony migration, transport is regulated by redox interfaces, and the conservative transport behavior of Sb supports long-distance pollution tracing[2], providing a basis for environmental remediation.　　3.2 Paleoenvironmental studies of sedimentary rocks　　Antimony isotopes can be used to reconstruct paleo-oceanic redox conditions, particularly in black shales. Antimony content shows a positive correlation with total organic carbon (TOC)[102], and organic matter enriches antimony through processes such as adsorption, complexation, and microbial redox, potentially inducing fractionation. In weathering profiles, ε123Sb values increase with the degree of weathering, presumably influenced by redox changes and adsorption onto Fe/Al oxides[103], providing a new proxy for paleoenvironmental evolution.　　3.3 Petroleum reservoir research　　Paleo-oil reservoirs are closely associated with antimony mineralization, with hydrocarbon gases promoting stibnite precipitation through thermochemical sulfate reduction (TSR). Spatiotemporal correlations in the Qinglong antimony deposit, Guizhou, demonstrate that the paleo-oil reservoir formed earlier than the antimony mineralization[105]. Experimental results confirm that crude oil exhibits antimony extraction rates as high as 60%–80% in Na2S medium[104]. Acting as a reducing agent, methane releases S2- under high-temperature (150–180℃) and acidic conditions, which combines with Sb3+ to form ore minerals, revealing the coupling mechanism between hydrocarbon and metal mineralization.　　3.4 Ore deposit genesis　　Antimony deposits commonly form in hydrothermal systems (e.g., orogenic belts), where isotopes can be used to trace the hydrothermal source and flow paths. Studies of the Xikuangshan deposit confirm its genesis as metamorphic-hydrothermal, with ε123Sb values higher at the orebody margin than in the core, reflecting fractionation effects in the distal parts of the hydrothermal system. In hydrothermal systems, sulfide precipitation triggers Rayleigh fractionation, leading to elevated isotopic ratios in the residual fluid[39]. Isotopic variations also reveal multi-stage mineralization and the mixing of deep-seated and shallow sources, deepening the understanding of complex ore deposit genesis.　　3.5 Sources and isotopic studies of antimony in coal　　Antimony isotopes in coal can be used to trace hydrothermal input and post-depositional fixation processes[5]. Antimony isotopes in coals from southwestern China are similar to those of hydrothermal origin, suggesting derivation from Sb-bearing hydrothermal fluids and subsequent immobilization by pyrite; antimony in coals from northeastern China is associated with groundwater-volcanic hydrothermal fluids[122], whereas that in northern China represents mixed input from terrigenous and hydrothermal sources. Organic matter and Fe-Mn oxides in coal exhibit strong adsorption capacity, and magmatic-hydrothermal volatiles result in antimony enrichment in contact zones[124]. The fractionation effect of antimony during gas–solid phase evaporation–condensation provides a new perspective for tracing supergene environments.　　4. Conclusions　　Non-traditional antimony isotopes undergo significant fractionation under different geological and environmental conditions, enabling effective tracing of antimony sources, migration pathways, and geochemical processes. This fractionation is influenced by redox, adsorption, biological, and evaporation processes, with 121Sb preferentially enriched in low-valence species. The magnitude of fractionation is regulated by reaction rates, temperature, and other factors. Analytically, MC-ICP-MS combined with TCF, ion exchange chromatography, and other techniques, has substantially improved analytical precision, providing support for research on complex matrices. Antimony isotopes can be used to trace pollution sources in environmental monitoring and can be applied to paleoenvironmental studies of sediments and other fields in Earth science. In the future, combining antimony isotopes with multi-element isotopes to construct models will provide support for resource exploration and pollution control.　　5. Future perspectives　　Future research needs to make breakthroughs in three major directions: (1) combining multi-element isotopes to construct comprehensive tracing models; (2) deepening the study of fractionation mechanisms in complex matrices and constructing quantitative models for multi-process coupling; and (3) optimizing analytical techniques to adapt to the detection of low-content samples, thereby providing more precise technical support for deep-earth resource exploration and global pollution control.