Mass-dependent and mass-independent sulfur isotope fractionation (δ34S and δ33S) from Brazilian Archean and Proterozoic sulfide deposits by laser ablation multi-collector ICP-MS
Mass-dependent and mass-independent sulfur isotope fractionation (δ34S and δ33S) from Brazilian Archean and Proterozoic sulfide deposits by laser ablation multi-collector ICP-MS
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DOI：
10.1016/j.chemgeo.2012.04.003
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发表时间：
2012-06
期刊：
影响因子：
3.9
通讯作者：
B. Bühn;R. Santos;M. Dardenne;Claudinei Gouveia de Oliveira
中科院分区：
文献类型：
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作者：
B. Bühn;R. Santos;M. Dardenne;Claudinei Gouveia de Oliveira
在地质记录的岩石中观察到了不依赖于质量的硫同位素分馏（MIF），该特征被认为与新太古代大气中的过程有关。因此，记录MIF效应的样品必须含有外源硫循环的硫，而内源硫不应显示这种效应。巴西六个矿床中硫化物的年龄为1.9 ~ 2.7Ga，既有外生硫源，也有内生硫源。通过原位激光烧蚀MC-ICP-MS分析硫同位素。使用国际和内部同位素标准品进行了一系列实验，这些实验在各种条件和设置下运行。其中包括参考材料IAEA-S1、IAEA-S3、NBS 123（闪锌矿）、NBS 127（重晶石）以及内部标准BSB-py（黄铁矿）和BSB-cpy（黄铜矿）。在6天的分析中，硫化物分析的内精密度δ 34 S为0.10-0.15‰（1 s），δ 33 S为0.40-0.60‰（1 s），δ 34 S和δ 33 S的准确度为~0.30‰。标准测量定义的δ 33 S/δ 34 S关系为δ 33 S =0.513*δ 34 S +0.149，其中R²=0.9997，这接近于质量相关分馏的理论关系。天然黄铁矿的分析误差最大，δ 34 S（1 s）批内精密度为0.05-0.15‰，δ 33 S（1 s）批内精密度为0.10-0.15‰。包括所有精密度和准确度数据，我们得出以下1 s误差限值，在该限值范围内，用该方法获得的MIF研究的硫同位素分析是可靠的：黄铁矿为0.32、0.34和0.46‰磁黄铁矿的δ 34 S、δ 33 S和Δ 33 S分别为0.34、0.42和0.54‰;黄铜矿的δ 34 S、δ 33 S和Δ 33 S分别为0.34、0.50和0.58‰。与戈亚斯皮拉尔绿岩带有关的2200 Ma老沉积物和火山岩中的黄铁矿和磁黄铁矿的δ 34 S为−15.0至+2.0‰，δ 33 S为−8.0至+1.0‰，Δ 33 S为轻微但不显著的正值。从假定的钙黄铁矿。2060 Ma的Águas Claras组（Carajás，Pará）碎屑沉积物的δ 34 S =9.5-12.5‰，δ 33 S =5.0-6.5‰，Δ 33 S接近于零。来自Salobo和Sequerinha硫化物矿床（也是Carajás）的黄铜矿与2570 Ma古老的A型碱性花岗岩具有内生来源。它们的δ 34 S =1.6-2.9‰和δ 33 S =1.0-1.5‰，δ 34 S =0.2-0.9‰和δ 33 S =0.65‰，Δ 33 S介于-0.15至0.56之间。本研究中唯一明确表明硫同位素MIF的Δ 33 S值来自米纳斯吉拉斯的QuadriláFerrífero的矿化带状铁建造。来自Mina de奎亚巴存款（2700 Ma）的黄铁矿δ 34 S =3.8-5.1‰和δ 33 S =2.9-4.8‰，Δ 33 S =0.9-2.2‰，而来自São Bento（2720 Ma）的黄铁矿和黄铜矿δ 34 S =1.6-3.2‰和δ 33 S =1.5-2.8‰，Δ 33 S =0.2-1.5‰。硫化物代表后生矿化，来自表壳岩，构成外生硫循环的一部分。研究表明，原位激光烧蚀ICP-MS是足够灵敏的检测MIF在新太古代岩石的影响。这些样品是否显示与MIF相关的Δ 33 S值取决于硫的年龄和来源。
Mass-independent sulfur isotope fractionation (MIF) has been observed in rocks of the geological record older than about 2.45Ga, a characteristic which is thought to be related to processes in the Neoarchean atmosphere. Samples recording a MIF effect therefore have to contain sulfur of the exogenic sulfur cycle, while endogenic sulfur should not show this effect. The sulfides analysed from six Brazilian deposits represent either exogenic or endogenic sulfur sources, with supposed ages ranging from about 1.9 to 2.7Ga. Sulfur isotopes were analysed by in‐situ laser ablation MC-ICP-MS. A range of experiments were conducted using international and in-house isotope standards, which were run under various conditions and set-ups. These include the reference materials IAEA-S1, IAEA-S3, NBS123 (sphalerite), NBS127 (barite), and in-house standards BSB-py (pyrite) and BSB-cpy (chalcopyrite). During six days of analysis, an internal precision of sulfide analyses of 0.10–0.15‰ (1s) for δ34S and 0.40–0.60‰ (1s) for δ33S, and an accuracy of ~0.30‰ for δ34S and δ33S was achieved. The standard measurements define a δ33S/δ34S relationship of δ33S=0.513*δ34S+0.149, with R²=0.9997, which is close to the theoretical relationship for mass-dependent fractionation. The by far best analytical errors were obtained for natural pyrite, reaching a within-run precision of about 0.05–0.15‰ for δ34S (1s), and 0.10–0.15‰ (1s) for δ33S determinations. Including all precision and accuracy data, we arrive at the following 1s error limits to which sulfur isotope analyses for MIF studies obtained with this method are reliable: 0.32, 0.34 and 0.46‰ for pyrite (δ34S, δ33S and Δ33S determinations, respectively), 0.34, 0.42 and 0.54‰ for pyrrhotite, and 0.34, 0.50 and 0.58‰ for chalcopyrite. Pyrites and pyrrhotites from 2200Ma old sediments and volcanics associated with the Pilar greenstone belt, Goiás, have δ34S from −15.0 to +2.0‰, and δ33S from −8.0 to +1.0‰, with Δ33S being slightly but not significantly positive. Pyrites from supposed ca. 2060Ma old clastic sediments of the Águas Claras Formation (Carajás, Pará), have δ34S=9.5–12.5‰ and δ33S=5.0–6.5‰ with Δ33S close to zero. Chalcopyrites from the Salobo and Sequerinha sulfide deposits, also Carajás, are genetically related to 2570Ma old A-type alkaline granites with endogenic sources. They have δ34S=1.6–2.9‰ and δ33S=1.0–1.5‰, and δ34S=0.2–0.9‰ and δ33S=0.65‰, respectively, with Δ33S ranging from −0.15 to 0.56. The only Δ33S values of this study which clearly suggest sulfur isotope MIF come from the mineralized banded iron-formations of the Quadrilátero Ferrífero, Minas Gerais. Pyrites from the Mina de Cuiabá deposit (2700Ma) have δ34S=3.8–5.1‰ and δ33S=2.9–4.8‰ with Δ33S=0.9–2.2‰, while pyrites and chalcopyrites from São Bento (2720Ma) have δ34S=1.6–3.2‰ and δ33S=1.5–2.8‰ with Δ33S=0.2–1.5‰. The sulfides represent an epigenetic mineralization, derived from supracrustal rocks making part of an exogenic sulfur cycle. The study shows, that in-situ laser ablation ICP-MS is sufficiently sensitive for detecting MIF effects in Neoarchean rocks. Whether or not such samples show MIF-related Δ33S values depends on the age and provenance of the sulfur.