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Partial dissolution of glauconitic samples: implications for the methodology of K-Ar and Rb-Sr dating

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Abstract:

The K-Ar dating of glauconite has been used as an important stratigraphic tool for many decades. The application of this technique is limited to pure glauconites, free of detrital contamination by K-bearing phases, often not easy to detect. This study extends the application of isotope dating to the contaminated glauconites and offers a precise technique for detecting the detrital contamination of glauconites.

The most common K-bearing detrital contaminants have smaller (K-feldspars, Al-rich dioctahedral micas) or greater (trioctahedral micas) dissolution rates than glauconite in extremely low pH solutions. The differences in the dissolution rates can be applied to evaluate the purity of the glauconite and its crystallization age.

The interlaboratory GLO glauconite standard and grain-size fractions separated from glauconitic sandstones of the Paleogene (sample GL) and Jurassic (sample GW8) ages were treated with acid (3 M HCl, at 99±2°C) for different reaction times (0.5–7 h) and measured for their apparent isotopic ages.

Microporous amorphous silica with large specific surface area is the solid product of the reaction and its content increases with reaction time. The K-Ar dates (apparent ages) of the solid residues increase significantly with reaction time: from 44.6 to 107 Ma for the GL sample and from 125.7 to 394.7 Ma for GW8. The increase is negligible in the case of the GLO standard. The Rb-Sr data of the GL sample were modeled using initial 87Sr/86Sr ratios of 0.707–0.709, which resulted in a 29.9–35.8 Ma date for the untreated portions of GL, and ∼42.6 Ma after 7 h of treatment.

The increase of isotopic K-Ar date with increasing time of dissolution is interpreted to be a result of increasing concentration of detrital, acid-resistant, K-bearing minerals, observed also with the electron microscope and X-ray diffraction. Probabilistic modeling based on single (K-Ar) or double (K-Ar and Rb-Sr) isotopic systems evaluated the isotopic ages of the detrital and authigenic minerals, and their K2O and Rb concentrations. The crystallization ages computed using these two methods are: 24.0, 26.5, and 32.3 Ma for the GL material, and 117.3–121.8 Ma for the GW8 series.

The proposed method based on partial dissolution is a potential tool for evaluating the reliability of glauconite dating.

Keywords: ACID DISSOLUTION; AUTHIGENIC MINERALS; DETRITAL MINERALS; GLAUCONITE; GLO; K-AR DATING; MIXED AGES; RB-SR DATING

Document Type: Research Article

DOI: http://dx.doi.org/10.1346/CCMN.2009.0570503

Publication date: October 1, 2009

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  • The JOURNAL publishes articles of interest to the international community of clay scientists, including but not limited to areas in mineralogy, crystallography, geology, geochemistry, sedimentology, soil science, agronomy, physical chemistry, colloid chemistry, ceramics, petroleum engineering, foundry engineering, and soil mechanics. Clays and Clay Minerals exists to disseminate to its worldwide readership the most recent developments in all of these aspects of clay materials. Manuscripts are welcome from all countries.

    Clays and Clay Minerals is the official publication of The Clay Minerals Society.

    The Editor-in-Chief is Professor Joseph W. Stucki jstucki@illinois.edu

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