Mechanisms of cadmium isotopic fractionation during the substitution of cadmium for zinc in sphalerite (ZnS)

IF 5 1区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Geochimica et Cosmochimica Acta Pub Date : 2025-09-01 DOI:10.1016/j.gca.2025.08.042

Chuanwei Zhu, Lisheng Gao, Xinran Yan, Hui Yin, Haifeng Fan, Yuxu Zhang, Guangshu Yang, Lei Zhang, Hanjie Wen

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Abstract

The chalcophile character of Cd controls its hyper-enrichment in Zn–Pb ore deposits via substitution for Zn in sphalerite (up to 13.2 wt%) and, as such, these deposits form Earth’s richest Cd reservoirs. Cadmium isotopes can provide insights into the origins of these deposits and the behavior of Cd in hydrothermal systems. However, Cd isotopic variations in sulfides are affected by mineral precipitation and the initial composition of the fluid, which remain poorly constrained for sphalerite precipitation. This hinders the application of Cd isotopes to ore-forming systems. In this study, we investigated Cd isotopic fractionation during absorption into sphalerite under varying pH, ionic strength, initial Cd2+ content, and mineral crystallinity. Using extended X-ray absorption fine structure (EXAFS) spectroscopy and spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM), we demonstrate that Cd is incorporated into the ZnS lattice rather than being adsorbed onto its surface. Isotopic equilibrium is achieved after 18 h in pure water and 10 % NaCl at pH 5.5, with lighter Cd isotopes preferentially partitioning into the solid phase. The magnitude of isotopic fractionation (Δ114/110Cdsolid–solution ≈ –0.5 ‰) is largely unaffected by the initial Cd2+ concentration, mineral crystallinity, and reaction pH in 10 % NaCl, but is higher than that in pure water (–0.7 ‰ to –0.9 ‰). This is probably due to Cl– complexation favoring the lighter Cd isotopes, leaving isotopically heavier Cd2+ to be absorbed into the sphalerite. Monte Carlo simulations using the obtained fractionation factors indicate the composition of the ore-forming fluid is the primary control on the Cd isotopic variations in sphalerite. These results advance our understanding of Cd isotopic systematics in hydrothermal systems and during supergene sulfide weathering, and provide a framework for interpreting Cd isotopic data in natural environments.

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闪锌矿中镉代锌过程中镉同位素分馏机理的研究

Cd的亲铜性控制了其在锌-铅矿床中的超富集，通过取代闪锌矿中的Zn（高达13.2 wt%），因此，这些矿床形成了地球上最丰富的Cd储层。镉同位素可以深入了解这些矿床的起源和镉在热液系统中的行为。然而，硫化物中的Cd同位素变化受到矿物沉淀和流体初始组成的影响，而闪锌矿沉淀对这些影响的限制仍然很差。这阻碍了Cd同位素在成矿系统中的应用。在本研究中，我们研究了不同pH值、离子强度、初始Cd2+含量和矿物结晶度下闪锌矿吸收过程中的Cd同位素分馏。利用扩展x射线吸收精细结构（EXAFS）光谱和球面像差校正扫描透射电子显微镜（Cs-STEM），我们证明Cd被纳入ZnS晶格而不是被吸附在其表面。在pH 5.5的纯水和10%的NaCl中放置18 h后达到同位素平衡，较轻的Cd同位素优先分配到固相。在10% NaCl条件下，同位素分馏的幅度（Δ114/110Cdsolid-solution≈-0.5‰）基本不受初始Cd2+浓度、矿物结晶度和反应pH的影响，但高于纯水条件下的分馏幅度（-0.7‰~ -0.9‰）。这可能是由于Cl -络合作用有利于较轻的Cd同位素，使得同位素较重的Cd2+被吸收到闪锌矿中。利用所得分选因子进行蒙特卡罗模拟表明，成矿流体的组成是控制闪锌矿Cd同位素变化的主要因素。这些结果促进了我们对热液系统和表生硫化物风化过程中Cd同位素系统的认识，并为自然环境中Cd同位素数据的解释提供了一个框架。

本文章由计算机程序翻译，如有差异，请以英文原文为准。

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来源期刊

Geochimica et Cosmochimica Acta 地学-地球化学与地球物理

CiteScore

9.60

自引率

14.00%

发文量

437

审稿时长

6 months

期刊介绍： Geochimica et Cosmochimica Acta publishes research papers in a wide range of subjects in terrestrial geochemistry, meteoritics, and planetary geochemistry. The scope of the journal includes: 1). Physical chemistry of gases, aqueous solutions, glasses, and crystalline solids 2). Igneous and metamorphic petrology 3). Chemical processes in the atmosphere, hydrosphere, biosphere, and lithosphere of the Earth 4). Organic geochemistry 5). Isotope geochemistry 6). Meteoritics and meteorite impacts 7). Lunar science; and 8). Planetary geochemistry.