Constraining the alteration history of a Late Cretaceous Patagonian volcaniclastic bentonite–ash–mudstone sequence using K–Ar and 40Ar/39Ar isotopes
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- 作者：L. N. Warr ; H. Hofmann ; B. A. van der Pluijm
- 关键词：Smectite age dating ; K–Ar and 40Ar/39Ar isotopes ; Cretaceous ash ; Bentonite ; Tuffaceous mudstone ; Detrital mica ; Neuquén Basin ; Argentina
- 刊名：International Journal of Earth Sciences
- 出版年：2017
- 出版时间：January 2017
- 年：2017
- 卷：106
- 期：1
- 页码：255-268
- 全文大小：
- 刊物类别：Earth and Environmental Science
- 刊物主题：Geology; Geophysics/Geodesy; Sedimentology; Structural Geology; Mineral Resources; Geochemistry;
- 出版者：Springer Berlin Heidelberg
- ISSN：1437-3262
- 卷排序：106
文摘
Smectite is typically considered unsuitable for radiometric dating, as argon (40Ar) produced from decay of exchangeable potassium (40K) located in the interlayer sites can be lost during fluid–rock interaction and/or during wet sample preparation in the laboratory. However, age analysis of Late Cretaceous Argentinian bentonites and associated volcaniclastic rocks from Lago Pellegrini, Northern Patagonia, indicates that, in the case of these very low-permeability rocks, the radioactive 40Ar was retained and thus can provide information on smectite age and the timing of rock alteration. This study presents isotopic results that indicate the ash-to-bentonite conversion and alteration of the overlying tuffaceous mudstones in Northern Patagonia was complete ~13–17 my after middle Campanian sedimentation when the system isotopically closed. The general absence of illite in these smectite-rich lithologies reflects the low activity of K and the low temperature (<60 °C) of the formation waters that altered the parent ash.