Mass-dependent and mass-independent sulfur isotope fractionation (未³⁴S and 未³³S) from Brazilian Archean and Proterozoic sulfide deposits by laser ablation multi-collector ICP-MS

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• 作者：Bernhard Bü ; hn ; ^berbu@unb.br ; Roberto V. Santos ; Marcel A. Dardenne¹ ; Claudinei G. de Oliveira
• 关键词：Sulfur isotopes ; Neoarchean ; Mass-independent fractionation ; Laser ablation ICP-MS ; Brazil
• 刊名：Chemical Geology
• 出版年：2012
• 期刊代码：12_00092541
• 类别：ear
• 出版时间：18 June, 2012
• 卷：312-313
• 期：Complete
• 页码：163-176
• 文件大小：1316 K

摘要

Mass-independent sulfur isotope fractionation (MIF) has been observed in rocks of the geological record older than about 2.45 Ga, 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 2.7 Ga. Sulfur isotopes were analysed by in鈥恠itu 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鈥?(1 s) for 未³⁴S and 0.40-0.60鈥?(1 s) for 未³³S, and an accuracy of ~ 0.30鈥?for 未³⁴S and 未³³S was achieved. The standard measurements define a 未³³S/未³⁴S relationship of 未³³S = 0.513 * 未³⁴S + 0.149, with R虏 = 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鈥?for 未³⁴S (1 s), and 0.10-0.15鈥?(1 s) for 未³³S determinations. Including all precision and accuracy data, we arrive at the following 1 s error limits to which sulfur isotope analyses for MIF studies obtained with this method are reliable: 0.32, 0.34 and 0.46鈥?for pyrite (未³⁴S, 未³³S and 螖³³S determinations, respectively), 0.34, 0.42 and 0.54鈥?for pyrrhotite, and 0.34, 0.50 and 0.58鈥?for chalcopyrite.

Pyrites and pyrrhotites from 2200 Ma old sediments and volcanics associated with the Pilar greenstone belt, Goi谩s, have 未³⁴S from 鈭?#xA0;15.0 to + 2.0鈥? and 未³³S from 鈭?#xA0;8.0 to + 1.0鈥? with 螖³³S being slightly but not significantly positive. Pyrites from supposed ca. 2060 Ma old clastic sediments of the 脕guas Claras Formation (Caraj谩s, Par谩), have 未³⁴S = 9.5-12.5鈥?and 未³³S = 5.0-6.5鈥?with 螖³³S close to zero. Chalcopyrites from the Salobo and Sequerinha sulfide deposits, also Caraj谩s, are genetically related to 2570 Ma old A-type alkaline granites with endogenic sources. They have 未³⁴S = 1.6-2.9鈥?and 未³³S = 1.0-1.5鈥? and 未³⁴S = 0.2-0.9鈥?and 未³³S = 0.65鈥? respectively, with 螖³³S ranging from 鈭?#xA0;0.15 to 0.56. The only 螖³³S values of this study which clearly suggest sulfur isotope MIF come from the mineralized banded iron-formations of the Quadril谩tero Ferr铆fero, Minas Gerais. Pyrites from the Mina de Cuiab谩 deposit (2700 Ma) have 未³⁴S = 3.8-5.1鈥?and 未³³S = 2.9-4.8鈥?with 螖³³S = 0.9-2.2鈥? while pyrites and chalcopyrites from S茫o Bento (2720 Ma) have 未³⁴S = 1.6-3.2鈥?and 未³³S = 1.5-2.8鈥?with 螖³³S = 0.2-1.5鈥? 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 螖³³S values depends on the age and provenance of the sulfur.