Title: ~(40)Ar/~(39)Ar GEOCHRONOLOGY OF SUPERGENE K-BEARING MANGANESE OXIDES AND ITS PALEOCLIMATIC IMPLICATIONS
Abstract: Laterites and supergene manganese ore deposits, which have been generally formed under warm and humid environments, are rich in potassium-bearing Mn-oxide minerals. Therefore, direct and precise ~(40)Ar/~(39)Ar dating on these K-Mn oxides will provide important constraints not only on the timing and process of continental weathering and supergene enrichments of metals, but also on the paleoclimatic evolution of regional significance. Integrated studies of transmission electron microprobe, thermogravimetric analysis, cation exchange experiments, and ~(40)Ar/~(39)Ar isotopic analysis suggest that chalcophanite, lithiophorite, birnessite, and pyrolusite, which are characterized by layered or 1×1 tunnel structure, are not suitable for ~(40)Ar/~(39)Ar dating. However, cryptomelane, hollandite, and coronadite, which have the 2×2 tunnel structure being very thermally stable and argon retentive, are ideal minerals for ~(40)Ar/~(39)Ar dating. The suitability of the 2×3 tunnel structured romanechite and the 3×3 tunnel structured todorokite to ~(40)Ar/~(39)Ar dating is still to be proven, as these minerals possess large tunnel size and are less thermally stable. Previous studies demonstrated that, using laser incremental heating ~(40)Ar/~(39)Ar method, the ~(39)Ar_K recoil loss, the overgrowth of multiple generation Mn-oxides, the hypogene contaminants in supergene phases, and the excess argon would not cause the problems in obtaining meaningful weathering ages. Available data indicate that there were four major stages for the formation of thick weathering crusts and supergene manganese ore deposits, including the latest Cretaceous, the late Eocene to the early Oligocene, the Miocene, and the middle Pliocene. This reflects possibly episodic weathering facilitated by alternating paleoclimates.
Publication Year: 2004
Publication Date: 2004-01-01
Language: en
Type: article
Access and Citation
AI Researcher Chatbot
Get quick answers to your questions about the article from our AI researcher chatbot