Greenalite-Chamosite composition, geothermometry and oxygen fugacity variations in pisolitic ironstone and carbonates of the Chilpi Group: implication on Paleoproterozoic seawater chemistry

IF 1.2 4区地球科学 Q4 MATERIALS SCIENCE, MULTIDISCIPLINARY Physics and Chemistry of Minerals Pub Date : 2023-11-16 DOI:10.1007/s00269-023-01258-1

Sarada P. Mohanty, Prasanta K. Mishra

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Abstract

Iron-rich rocks of Orosirian Period in the Chilpi Group on the northern margin of the Bastar Craton, Central India, contain an association of hematite-magnetite-greenalite-chamosite-quartz in oxide-silicate facies. Additionally chert (quartz) and siderite occur in chert and carbonate facies. Presence of these mineral assemblages was investigated to infer the redox state of the depositional basin. The results have indicated formation temperature variation of 116–255 °C (average 198 °C) and log P_(O2) between – 37 and – 60 (average –44). A ferruginous state of the shallow water depositional environment, having oxygen content of 10^–2 to 10^–5 times the present atmospheric level, is inferred. The variations in composition of greenalite-chamosite association indicate development of the mineral phases from the reaction involving kaolinite-illite and magnetite-siderite as end-members. Thermodynamic requirements for the formation of the rare association of magnetite-greenalite-cronstedtite indicate the precipitation of the mineral phases from seawater with enhanced activities of Fe²⁺, Al, Si, Mg and C compared to the level in the present day seawater. The results indicate a steep fall in the atmospheric oxygen content immediately after the Great Oxidation Event of 2400–2000 Ma.

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七皮群泥质铁石和碳酸盐绿绿岩-绿辉岩组成、地球温度和氧逸度变化:对古元古代海水化学的启示

印度中部Bastar克拉通北缘Chilpi群奥罗世富铁岩石中含有赤铁矿-磁铁矿-绿绿岩-绿辉石-石英组合，呈氧化硅酸盐相。另外，燧石(石英)和菱铁矿产于燧石相和碳酸盐相。研究了这些矿物组合的存在，以推断沉积盆地的氧化还原状态。结果表明，地层温度变化范围为116 ~ 255°C(平均198°C)，测井P(O2)在- 37 ~ - 60(平均- 44)之间。推断浅水沉积环境为含铁状态，含氧量为当前大气水平的10-2至10-5倍。绿绿石-绿辉石组合组成的变化表明，以高岭石-伊利石和磁铁矿-菱铁矿为端元的反应形成了矿物相。形成磁铁矿-绿绿石-长角辉石矿组合的热力学要求表明，海水中沉淀的矿物相与现在海水中的Fe2+、Al、Si、Mg和C的活性相比有所增强。结果表明，在2400-2000 Ma的大氧化事件发生后，大气中氧含量急剧下降。

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来源期刊

Physics and Chemistry of Minerals 地学-材料科学：综合

CiteScore

2.90

自引率

14.30%

发文量

审稿时长

3 months

期刊介绍： Physics and Chemistry of Minerals is an international journal devoted to publishing articles and short communications of physical or chemical studies on minerals or solids related to minerals. The aim of the journal is to support competent interdisciplinary work in mineralogy and physics or chemistry. Particular emphasis is placed on applications of modern techniques or new theories and models to interpret atomic structures and physical or chemical properties of minerals. Some subjects of interest are: -Relationships between atomic structure and crystalline state (structures of various states, crystal energies, crystal growth, thermodynamic studies, phase transformations, solid solution, exsolution phenomena, etc.) -General solid state spectroscopy (ultraviolet, visible, infrared, Raman, ESCA, luminescence, X-ray, electron paramagnetic resonance, nuclear magnetic resonance, gamma ray resonance, etc.) -Experimental and theoretical analysis of chemical bonding in minerals (application of crystal field, molecular orbital, band theories, etc.) -Physical properties (magnetic, mechanical, electric, optical, thermodynamic, etc.) -Relations between thermal expansion, compressibility, elastic constants, and fundamental properties of atomic structure, particularly as applied to geophysical problems -Electron microscopy in support of physical and chemical studies -Computational methods in the study of the structure and properties of minerals -Mineral surfaces (experimental methods, structure and properties)